Download Starfish (Sea Stars): Asteroidea

Starfish (Sea Stars): Asteroidea
General body plan
The Asteroids are free-living echinoderms, with radial symmetry and moving on their
oral surface. Asteroids consist of a central disc with the mouth in the middle of the
undersurface (oral side) and anus in the centre of the upper surface (aboral side). Ray-like
extensions, called rays or arms (usually five, sometimes many) radiate laterally from the
disc, though not always distinct (Anseropoda, Culcita and others are shaped like
pentagons). The outer surface is rough, warty, tuberculate or spiny and the arms may be
fringed with spines.
Pentamerous symmetry is the norm and five arms are common, though the number differs
depending on species. The arms define five radii of symmetry. With the starfish held oral
surface uppermost and with the arm opposite the madreporite labelled A, the other
arms/radii are labelled clockwise in alphabetical order. Interradii are between the arms.
Dimensions vary, the largest starfish are some 60 cm from arm tip to arm tip, and the
smallest are about 1 cm when fully grown.
The arms may be separate for much of their length (e.g. the Common Starfish, Asterias
rubens) or joined for most of their length as in Cushion-stars or Starlets (e.g. Asterina
gibbosa) or arms may be absent (as in the pentagonal Culcita). The colour of sea stars
varies from yellow to orange, red, green, blue, gray and brown and they may be
patterned. The aboral surface is generally more intensely coloured while the oral surface
is generally paler.
The body wall
The outer surface is covered by monociliated (flagellated?) and non-ciliated epithelial
cells, mucus cells and ciliated sensory cells. The mucus traps detritus which is swept
away by the cilia so keeping the animal’ surface clean. Minute pincer-like movable
pedicellariae assist in this function. These often surround the spines (and are themselves
modified spines) and may be stalked or non-stalked (sessile). These jaw-like pincers
remove small animals and larvae that settle on the starfish. At the base of the singlelayered epithelium is a layer of nerve cells forming the subepidermal plexus. The
epithelium rests upon a basement membrane.
Below the integument is the thick dermis, which contains skeletal plates, called ossicles.
Each ossicle is a single crystal of magnesium-rich calcite (6(Ca,Mg)CO3) and together
the ossicles form an endoskeleton. In burrowing starfish the centre of each aboral ossicle
may be raised in a parasol-shaped paxilla, which may be crowned by small movable
spines. Adjacent paxillae create a protected space above the integument through which
respiratory and feeding currents may flow even when the animal is buried. Ossicles are
bound together by connective tissue.
Ambulacral Grooves
A single groove, the ambulacral groove, runs down the oral surface of each arm. Rows
of tube-feet lie within these grooves. The ambulacral grooves are supported by a definite
arrangement of ossicles: two rows of rod-shaped ambulacral ossicles form the V-shaped
ambulacral groove itself. Where these meet they form the prominent ambulacral ridge on
their inner surface. Each ambulacral ossicle forms half of a pore through which a tubefoot protrudes between each serial pair of ossicles in each row resulting in two rows of
tube-feet per arm, one on each side of the groove. (The pores may zigzag giving the
impression of four rows of tube-feet per ambulacral groove). Lateral to the ambulacral
ossicles are the adambulacral ossicles bearing movable spines. These spines may be
lowered across the ambulacral groove or raised by pairs of antagonistic muscles.
Nutrition
Most asteroids are carnivorous and predate slow-moving or sedentary animals and also
weak fish. They will also scavenge from carcasses. In those with long, flexible arms the
prey is held by the arms while the stomach is everted onto the prey, releasing enzymes
that digest the soft tissues, which are then sucked into the digestive tract. When feeding
on bivalves (e.g. muscles, oysters) these seastars will prize the valves apart (using their
tube-feet suckers to gain a hold) until they open by as little as 0.1 mm and then they will
evert their stomach through this gap and digest their prey! Of course the bivalve will
resist opening using its adductor muscle to try and keep the valves shut, however, the
starfish usually gains an opening within 5-25 minutes, though the whole process from
opening the bivalve to complete digestion has been seen to take up to 10 hours. The force
required to open a bivalve is considerable and it is debatable whether or not the starfish
can win by brute force, or whether it utilises toxins. Stomach contents of some starfish
are known to have cardio-toxic effects on oysters.
Many starfish swallow their prey whole rather than everting their stomach (cardiac
stomach). They generally eat small animals, but may have very distensible mouths and
may consume bivalves, snails, crustaceans, polychaetes, and other echinoderms,
including young starfish.
Non-predaceous starfish may feed by everting their stomach over the sea-bottom,
digesting any organic matter encountered. Some species catch small fish and crustaceans
with their pedicellariae, if these animals come to rest on top of the seastar.
Some starfish are ciliary mucous feeders: plankton, detritus or mud that contacts the
body surface is trapped in mucus then transported to the ambulacral grooves by the
epidermal cilia and then along the mouth. In non-ciliary mucous feeders the same mucusciliary mechanism serves to remove debris from the animal.
Whilst some starfish have very restricted diets, others are generalists and feed on
whatever is available, though they may have preferences. They detect and locate prey by
chemicals released into the water. Some can detect buried prey and then burrow down
into the substratum to reach it. Finally some starfish feed using a combination of the
above methods.
The Digestive System
The alimentary canal is short and straight connecting the ventral mouth to the dorsal
anus. The mouth opens in the centre of the peristomial membrane and is provided with a
sphincter. The mouth leads into a short, wide oesophagus that connects to the stomach.
The stomach is often divided by a constriction into the oral voluminous and folded
cardiac stomach and the smaller flattened aboral pyloric stomach. Connected to the
pyloric stomach, via pyloric ducts, are ten glands: the pyloric caeca (digestive glands,
brachial caeca, hepatic caeca), two of which run, more or less, the length of each arm).
Each pyloric caeca is attached to the aboral wall of each arm by two longitudinal
mesenteries. Two mesenteries also attach the cardiac stomach to each ambulacral ridge
(gastric ligaments). Other mesenteries connect the stomach to the disk walls and to the
interbrachial septa. A very short intestine connects the pyloric stomach to the anus.
Rectal or intestinal caeca may be attached to the intestine. The intestine distal to the
caeca is sometimes called the rectum. One or more of the anus, intestine and intestinal
caeca are absent in some families.
Digestion is largely extracellular, the stomach wall and pyloric caeca secreting enzymes.
Ciliary currents carry digested particles from the stomach into the pyloric ducts and into
the pyloric caeca where they are further digested (extra- and intracellularly) and
absorbed. Products of digestion may be stored in the caeca or passed into the coelom for
distribution around the seastar. Waste is passed from the pyloric caeca to the rectum, via
the pyloric ducts, and expelled through the anus. If rectal caeca are present, then these
aid expulsion by pumping.
The Coelom
The general body cavity: the perivisceral coelom of the disc is a single cavity continuous
with the arm cavities. The tubular coelomic systems comprise the well-developed watervascular, haemal and perihaemal systems. Coelomic fluid is similar to sea water, but has
a slightly higher K+ content and lower Mg+ content and contains protein and
coelomocytes (phagocytic amoeboid cells) and is less alkaline (pH 6 - 8.1) than sea water
(pH 8.2+).
Coelomic fluid is kept circulating by the ciliated lining of the coelom. Generally it may
flow towards the arm tips aborally and then back to the disc along the ventrolateral
surfaces. These currents ensure thorough mixing of the coelomic fluid.
Water-vascular System
The water-vascular system is a system of water-carrying tubes that function to supply
fluid to the hydraulically operated tube-feet (podia, sing. podium). This system is unique
to echinoderms. The internal fluid is similar to sea water except that it contains
coelomocytes, some protein and has an elevated potassium content. Cilia drive water
around this system, but are assisted by ampullae or sac-like contractile pumps.
The madreporite is a circular, grooved plate situated on an interradius and the only
external structure breaking the radial symmetry. The bottom of each groove contains
many pores by which the madreporite connects the water-vascular system to the external
environment, but its specific function is uncertain. It may simply function to allow
external and internal hydrostatic pressures to equilibrate. A cranny in its inner surface
contains the madreporic ampulla and the dorsal sac. The madreporite leads vertically
down into the stone canal (so-called because its walls are strengthened with calcareous
spicules). The stone canal contains a scroll-shaped projection into its lumen from one of
its inside walls, which facilitates water circulation (towards the mouth inside the rolls of
the scroll and away from the mouth outside the scroll). At its oral end the stone canal
opens into the circum-oral water ring and connects to the madreporic ampulla at its aboral
end. This ampulla contracts cyclically and acts as a pump assisting fluid flow. From the
circum-oral canal one radial canal passes along each arm, giving off side-branches to
each tube-foot (via an ampulla at the base of each podium) and terminating in a modified
terminal tube-foot, which lacks an ampulla and is sensory in function.
Tiedemann’s bodies Five pairs of interradial glands arising from the wall of the circumoral ring (one may be missing where the stone canal joins the circum-oral ring, leaving 9
bodies). It is thought that these glands may synthesise coelomocytes.
Polian vesicles Interradial muscular sacs born on the circum-oral ring in some Asteroids
(absent, for example, in the common Asterias). Probably function to maintain pressure in
the system.
Mode of Operation of the Tube-feet
Tube feet (podia) are supported by a hydraulic and connective-tissue skeleton under the
control of antagonistic muscle pairs systems. The essential components are:
1. A fluid-filled cavity. Contraction of the ampulla pump by ampulla muscles increases
the pressure in the ampulla cavity, which forces its fluid into the fluid-filled cavity of
the podium. The resultant increase in pressure within the podium causes tube-foot
extension.
2. Connective tissue skeleton. Tube-foot extension under pressure is permitted and
limited by radial hoops of connective tissue fibres arranged in series down the length
of the podium, and connected by a longitudinal or axial series of hinges. The hinges
allow the hoops to move together when the podium retracts and to separate by a finite
amount when the podium extends. The hoops prevent wasteful radial extension of the
podium when it is under pressure.
3. Retractor muscles. These longitudinal muscles shorten the podium by contracting,
when the ampulla muscles relax. The resultant pressure moves fluid back into the
ampulla. The retractor muscles and ampulla muscles are antagonistic.
4. Orienting or postural muscles. These work in antagonistic pairs to move the tube foot
forwards and backwards. When the podia are attached to the substrate, co-ordinated
movements forwards or backwards will propel the echinoderm.
5. Sucker. The terminal sucker of the podium enables the podium to attach to the
substrate and hence to apply force to the substrate.
6. Disc levator muscles. Contraction of these muscles breaks the sucker seal, enabling
the adhered tube foot to detach from the substrate.
7. Terminal plate. A skeletal plate in the centre of the disc, to which the disc levator
muscles attach.
Haemal System
This fluid-transport system is enclosed in coelomic spaces (perihaemal spaces/sinuses)
and is not readily apparent except in serial sections. The oral haemal ring (enclosed in a
septum in the perihaemal or hyponeural ring sinus) gives off a radial haemal sinus
(enclosed in the septum in the hyponeural radial sinus) into each arm. These radial
sinuses run oral to the radial water canals. The aboral haemal ring (running around the
rectum inside the aboral or genital coelomic sinus) gives off branches to the gonads
within each arm (inside the coelomic branches to the gonads). The pyloric haemal ring
surrounds the pyloric stomach and gives off branches, called the gastric haemal tufts, to
the walls of the cardiac stomach and the hepatic haemal strands to the walls of the
hepatic caeca of each arm. Perihaemal sinuses do not enclose these parts of the haemal
system. Products of digestion enter the haemal system.
Axial gland Contains the axial haemal sinus and terminates in the contractile dorsal sac,
which acts as a pump for the haemal system. The axial gland, the gastric haemal tufts and
the aboral haemal ring are also reported to be contractile. The axial gland is rich in
coelomocytes.
Excretion
Between the ossicles sac-like or wart-like vesicles protrude from the external surface of
the starfish. These are called papulae and their fluid-filled interiors are continuous with
the coelom. These are formed from two ciliated epidermal layers – the external ciliated
epidermis covering the starfish and the internal ciliary epithelium lining the coelom
cavities – with a thin layer of connective tissue sandwiched in-between. As fluid flows
through the coelom, driven by ciliary currents, it gives rise to eddies inside the papulae.
Coelomocytes trapped inside the eddies accumulate inside the papulae, where they may
form a clot. Coelomocytes ingest foreign materials and non-soluble waste products and
then collect in the tips of the papulae, which are pinched off. Other coelomocytes migrate
to the outside through the epidermis, especially on the tube feet, and hence remove waste
from the starfish. Waste-laden coelomocytes also exit via the pyloric caecae and
madreporite. The pyloric caeca may also directly absorb and expel waste from the
coelomic fluid. Nitrogenous ammonium diffuses out through tube feet and papulae.
Osmoregulation
The coelomic fluid is similar to sea water and there is no power of osmoregulation and
the body wall is permeable to salts and water. Starfish can adapt to a range of salinities,
however.
Respiration
Gas exchange occurs across the podia and papulae. This is aided by ciliary currents on
the outer and inner epithelia of the papulae. In burrowing starfish, branched papulae are
protected by the paxillae and ventilating currents flow through the channels underneath
the paxillae.
Nervous and Sensory Systems
The nerve centre consists of a pentagonal circumoral nerve ring, in the peristomial
membrane just beneath the peristomial epidermis. This gives off five sensory radial
nerve cords which travel the length of each arm in the bottom of the ambulacral groove
just interior to the epidermis and separated from the hyponeural sinus on its interior side
by a thin dermis and the coelomic epithelium. Each cord terminates in a sensory cushion
aboral to the base of the terminal tentacle. The radial nerves are V-shaped in crosssection. The radial nerves are continuous with the subepidermal plexus, which covers
the whole surface and is concentrated around body-wall appendages, which it innervates.
These appendages include the podia. In the outer margins of each ambulacral groove the
subepidermal plexus is thickened into marginal nerve cords. A pair of these marginal
nerve cords innervates each arm with motor neurons. They give off a pair of lateral motor
nerves to each ambulacral ossicle, innervating the lateral transverse interambulacral
muscles.
Lange’s nerve is a nervous sheet in the lateral part of the oral wall of the hyponeural
sinus. These nerves are primarily motor and extend to the peristomium. They are
separated from the radial nerves by a thin connective tissue layer.
At the end of each arm is a modified podium, the terminal tentacle, which has a sensory
function. At the oral base of the terminal tentacle is the optic cushion (a red spot): a
cluster of pigment-cup ocelli, which may be covered by lenses. Up to 200 ocelli may
cluster in one optic cushion. Some species lack ocelli and some deep water species lack
photoreceptors altogether. When starfish move they often curve the tips of their arms
upwards to expose the ocelli to the light.
Sensory cells are scattered over the epidermis and concentrated on the surface of the
podial suckers, the bases of spines and pedicellariae and along the adambulacral region
(up to 7 x 104 per mm2) and on the terminal tentacles.
Most starfish are negatively phototactic: avoiding light and preferring shade. Many
however prefer light, though this may depend on the light intensity: moderate light may
be favoured, but direct sunlight avoided. Burrowing seastars may emerge under suitable
moderate levels of light.
Starfish will right themselves if placed upside-down. When inverted, the starfish will be
still for a moment and will then curve its arm-tips aborally until the podia gain a grip on
the substratum. Usually two of the arms will then walk underneath the animal, recruiting
more podia and raising the disc, which eventually flips over and is lowered (a slow
somersault, which takes from 20s up to 90 min). Whether the stimulus is loss of podial
contact, gravity or some other stimulus is uncertain. Isolated arms are also capable of
righting. At least some starfish are known to be responsive to gravity, though this may be
a response to the direction of pull on the podia.
Starfish, and isolated arms of starfish, respond to touch. The podia retract if touched, and
the retraction may spread along the arm and then to the whole animal, followed by podia
re-extension. Touching the aboral surface may evoke the dorsal reflex: dorsal flexure of
one or more arms. If the side of an arm is touched, podia may extend towards the
stimulus.
Muscular System

The coelomic side of the body wall contains an outer circular and an inner
longitudinal muscle layer.

The longitudinal muscle is thickened into a median aboral line that runs from the disk
along each arm.

An upper transverse muscle and a lower transverse muscle connect each pair of
ambulacral ossicles. Contraction of the upper causes the ambulacral groove to widen,
whilst contraction of the lower narrows the groove.

Upper and lower longitudinal ambulacral muscles connect adjacent ambulacral
ossicles, contraction of which shortens the ambulacral groove.

Longitudinal muscles between adjacent ambulacral ossicles aid in sideways
movements of the arms.

Dorsolateral arm muscles in Benthopectinids may cause thrashing movements of the
arms allowing these starfish to swim.
Locomotion
Coordinated action of the tube feet brings about slow creeping locomotion. One arm
temporarily dominates and leads the way, according to which arm receives the strongest
positive stimulus (induced arm dominance). Alternatively, in some starfish one particular
arm may dominate most of the time (intrinsic arm dominance).
Life-Cycle
1. Reproduction
Asexual reproduction occurs in some species. Most starfish seem to have great
regenerative powers and will regrow lost arms and repair damage to the disc. Often only
one arm with a small piece of disc attached to it is all that is required for complete
regeneration. Regeneration may require a year to complete, however. Uniquely Linckia is
able to reproduce by forcibly casting off whole arms (autotomy): the arm regenerated
into a complete starfish. (Regenerating forms are known as comets when they have only
small regenerating arms at the base of the old arm). Spontaneous fission is common in
some genera: the disc splits in two along a pre-determined line that leaves the arms intact.
Each half subsequently regenerates into two new starfish.
Sexual reproduction. Most asteroids are dioecious and have ten gonads: two in each
arm. These normally occupy a small volume near the base of the arm, but almost
completely fill the arm when full of eggs or sperm. Each gonad opens via a gonopore (or
cluster of gonopores) usually located between the bases of the arms (sometimes on the
oral surface). Some seastars are hermaphrodite, this depends on species and also varies
within a species. Some individuals of a normally dioecious species may have one or more
mixed gonad. Once a year the gametes are shed into the sea. A single female may shed
2.5 x 106 eggs. Fertilisation occurs in the sea. Starfish will aggregate together prior to
shedding gametes, so maximising the chances of fertilization. Some may pair off, with
the male sitting on top of the female with his arms alternating with hers. The presence in
nearby water of gametes of the opposite sex will stimulate gamete shedding. Gamete
shedding generally occurs in spring in the Northern Hemisphere and may be a response to
rising temperatures.
Sexes are generally visibly indistinguishable, though there may be slight colourdifferences in some species and minor morphological differences (statistical differences
in body shape and/or size). In many hermaphrodites the sex may change as the starfish
grows.
Generally parental duties end once the gametes are shed into the sea; however, some
mother starfish brood their eggs. The strategy depends on species and the different modes
of brooding are as follows:
1. The starfish may arch upwards on its arms to form a brooding chamber.
2. Eggs may be kept in the pouches of the cardiac stomach.
3. The paxillae of some cushion stars may support a supradorsal membrane with the
enclosed space being ventilated as water enters through incurrent spiracles and exits
through the excurrent osculum in the membrane. Eggs may be brooded in this
chamber.
4. The bases of the arms may swell and their ossicles interlock to form a basket. Each
such interradial container may hold 5-9 eggs.
Brooding species produce fewer eggs (a few hundred at most) and larger, yokey eggs.
2. Embryology
Radial cleavage produces a hollow blastula. Gastrulation leads to a gastrula. The
embryo becomes free swimming at some point between the blastula and gastrula stage.
Initially the entire embryo surface is ciliated, but later the cilia become defined to a
locomotor band as the gastrula develops into a dipleurula larva. The dipleura larva has a
circumoral ciliary band, which distinguishes it from trochophore larvae possessing an
equatorial ciliary tract (the prototroct). Arms develop from the body surface and the
ciliary bands extend into these arms, thereby increasing their effective surface area. The
larva is now a bipinnaria larva. The ciliary bands are used in locomotion and feeding. The
bands transport fine suspended particles and phytoplankton to the mouth.
Three additional short flexible arms develop at the anterior end and the larva becomes a
brachiolaria larva. The coelom is continuous with these three arms and the arm tips
contain adhesive cells. These adhesive arms temporarily anchor the larva to the substrate
when it settles out of the water column about two months after the beginning of the
brachiolaria stage. Between the bases of these three arms is an adhesive sucker, which
subsequently forms a more permanent adhesion.
Metamorphosis proceeds in about a day. (Development via metamorphosis is called
indirect development). The anterior of the larva degenerates into an attachment stalk,
while the rounded posterior end develops into the adult starfish. The left side becomes the
oral surface and the right side the aboral surface. The adult arms appear as extensions of
the body. Most of the larval gut, including the mouth and anus degenerate and are formed
anew. Tube-feet form and eventually pull the body away from the remains of the larva
and a new adult is formed; though less than 1 mm in diameter development is still far
from complete. Some starfish grow for five years before reaching maturity and starfish
may live for up to 10-35 years, depending on the species.
Ecology: the Role of Starfish in Marine Ecosystems
Starfish are important benthic predators and are of economic importance as predators of
oysters in commercial oyster beds. Starfish are common down to abyssal depths, but are
rare in the hadal zone, where holothurians dominate the benthic fauna.
A number of starfish parasites are known, including protozoa, crustaceans (copepoda,
amphipoda) including the strange barnacle Dendrogaster (found in the coelom), and
snails. The latter includes Melanella equestris, which lives on the external surface of
Stellaster equestris, puncturing the body wall with its proboscis to draw nutriment.
Some potential starfish prey organisms have developed elaborate escape reflexes,
triggered by the touch of a starfish. The snail Nassa reticulata leaps violently to escape
from the touch of the podia of Asterias rubens. Similarly, the queen scallop (Chlamys
opercularis) swims, by jet propulsion, to escape from Asterias rubens by clapping its
valves together. The snail Natica catena draws a fold of its foot over its shell when
Asterias rubens touches it with its podia, which presents a slippery surface the podia can
not grip.
Serpent Stars (Brittle Stars): Ophiuroidea
General body plan & External features
Five (rarely 6-7) symmetrically placed, long, slender or spiny jointed arms radiate from a
small flattened disk (10-30 mm diameter). The disk may have a circular, pentagonal or
scalloped contour and the arms are sharply demarcated from it. The arms are long
compared to the disc, typically 3-6 times the disk diameter, sometimes much more. The
serpentine appearance and movement of the arms give the ophiuroids their name of
serpent stars. Basket Stars are ophiuroids (F. Gorgonocephalidae) with highly branched
arms!
The aboral disk surface may be smooth and leathery, granulated, spiny, scaly or the
embedded plates may be visible. The arms lack ambulacral grooves and have joints
composed of internal ossicles (vertebral ossicles or vertebrae).
The disks tend to have duller colours than many Asteroids, including: cream, yellow,
green, olive, gray, brown, maroon, purple, black and may have spots or bands. The arms
are often a different colour to the disk.
The body wall
An epidermis may be lacking over much of the body surface, and when present it may be
syncytial. Cilia (flagella?) are restricted to the areas around the bursal slits and in some
species they also occur on the oral surface (of the disk and arm bases). There are no
pedicellariae and no papulae. The dermis contains the endoskeletal ossicles.
Endoskeleton
Superficial ossicles form shields. The most prominent are the radial shields at the base
of each arm, which may be spoke-like. Each arm is covered by a series of arm shields:
aboral and oral arm shields (often much reduced) and prominent lateral shields. The
aboral shield may be split-up into a mosaic of small plates. The lateral shields are
equivalent to the adambulacral ossicles of asteroids and may have spines (0-15 spines per
shield) which may be glandular, and are possibly poisonous.
Deeper ossicles form the vertebral ossicles (vertebrae) in the arms. These are disc-like
with lateral wings for the attachment of muscles. They form elaborate joints with each
other, either of the peg and pit variety or of opposing 'hourglass articulations’. Two
abutting hourglass articulations at right angles forming an articulation.
The nature of the vertebral joints and the range of movement that they allow form the
basis for the classification of the ophiuroids into two orders:
Order 1. Ophiurae
 Arms unbranched ;
 Arms move horizontally;
 Pit/projection joints between vertebral ossicles;
 Arms cannot twine around objects.
Order 2. Euryalae
 Arms may be branched;
 Hourglass articulations between vertebral ossicles;
 Arms can move vertically;
 Arms can entwine around objects.
The vertebral ossicles are thought to have derived from fused ambulacral plates, which
became internalised with the closing over the ambulacral grooves, which became an
internal canal (with its roof formed by a notch in the oral edge of each vertebra). This
canal caries the radial nerve, water canal, and the haemal/perihaemal radial sinuses.
The mouth frame is comprised of five wedge-shaped interradial jaws (modified plates)
bearing teeth (modified spines). The jaws are formed by the fusion of two sets of plates,
and each such ‘half-jaw’ has two podial pores for the buccal podia or buccal tentacles to
extrude through. The presence of tube feet on the jaws indicates that they are formed, at
least in part, by fusion of ambulacral plates.
Podia
The podia are reduced to small papillae (tentacles) and there is one pair per arm joint on
the oral surface. These papillae are often protected by immovable tentacle scales. The
papillae have adhesive gland cells.
Transport systems
The water-vascular, haemal and perihaemal systems are similar to those of asteroids,
except that the madreporite is on the oral surface.
Excretion
The bursae may be the main centre for waste removal, including waste-laden
coelomocytes.
Respiration
The bursae are 10 sac-like invaginations in the oral disk wall alongside the arm bases,
occupying the spaces between the stomach pouches. They open by the prominent
elongated bursal slits on the oral side of the disk. (Except in Ophioderma, which has two
pores in place of each slit). They are flanked by the genital shields. The bursae may fuse
into a single chamber in some species, and are absent in some species. Respiratory water
currents circulate through the bursae, and in some species movements of the aboral disk
may pump the system. Gametes are also shed through the bursal slits, and the bursae may
act as brooding chambers.
Nervous system
As in asteroids, the nervous system consists of a circumoral nerve ring and radial nerves.
These systems are double: the outer thick ectoneural system is sensory and motor, and the
inner thin hyponeural system is motor only.
Sensory systems
Some ophiuroids show strong reactions to light and most ophiuroids are negatively
phototropic. Ophiocoma wendtii is one such species. The photoreceptors were for a long
time thought to be diffuse epidermal photoreceptors, but recent research suggests the
possible presence of compound eyes with calcite microlenses. The dorsal (aboral) arm
plates (and the dorsal regions of the lateral arm plates) have arrays of hemispheres (each
40-50 mm) on their external surface. These transparent hemispheres resemble lenses in
section and can focus light. Within the mesh of the ossicle (stereom) nerve bundles are
found at the correct depth and it has been hypothesised that these nerves are the
photoreceptors. Pigment is also found in the stereom in chromatophores. It is thought that
these chromatophores regulate the intensity of light reaching the photoreceptors by
extending their pigment-filled processes to cover the lens during the day and retracting
them at night. This corresponds to the diurnal colour changes in O. wendtii: from
homogeneous dark brown during the day to banded grey and black at night. O. pumila
shows little reaction to light, no diurnal colour change, and lacks the calcite lens-like
structures.
Podia and spines also have sensory functions. Like sea stars, ophiuroids can detect food
at a distance, presumably via chemical cues.
Luminescence
Serpent stars are the only echinoderm group in which luminescence is known to occur
with certainty. Several ophiuroid species are known to be luminescent. The luminescence
is limited to the arms, being strongest in the spines and spine bases, and is absent from
the disc. Luminescence is restricted to the arm tips or the lateral shields or oral shields in
various species. Podia are not luminescent. The luminescence is yellow or greenishyellow. It is triggered by mechanical (and electrical and chemical) stimuli and spreads via
the radial and ring nerves. Some species luminesce in the light, but others will do so only
after a length of time in the dark. The process requires oxygen. The luminescence is
thought to originate in specialised gland cells.
Locomotion
Ophiuroids are the most mobile echinoderms. When they move, the disk is raised above
the substrate and with two arms pointing forwards and one or two arms trailing, the two
lateral arms perform rapid ‘rowing’ movements against the substratum, propelling the
animal in a gliding motion. The spines on the arms provide traction. There is no preferred
dominant arm (as there is in some asteroids). A few species creep slowly on their podia,
as do asteroids.
Burrowing species excavate mucus-lined tubular burrows by undulating arm movements,
accompanied by lateral digging movements of the podia. Arm undulation ventilates the
burrow.
Nutrition
Ophiuroids are carnivores, scavengers, deposit feeders or filter feeders. Most use several
of these feeding modes. Ophiocomina nigra uses all four, but is predominantly a
suspension feeder).
In deposit and suspension feeding, plankton and detritus adhere to mucous strands strung
between the arm spines. The captured food particles are transported to the tentacular
scales, by ciliary currents and/or by the tube feet. In filter feeding, feeding arms are
elevated with the oral surface facing the water current. The podia extend beyond the
spines to form combs. Food particles stick to the podia and are periodically wiped onto
the spines and collected by other tube feet. In either case, these tube feet compact the
particles into a bolus and transport this growing bolus to the mouth in a wave-like fasion
along the midoral line of the arm towards the mouth.
In scavenging the looping motion of an arm sweeps food into the mouth. The teeth or oral
tube feet are used to browse on algae and carcasses. The alimentary canal is simple; there
is no anus or intestine and no diverticula extending into the arms.
Reproduction
Most serpent stars are dioecious, and the sexes are indistinguishable, unless the more
intense colour of the female gonads shows through (this may also account for the slight
colour differences seen between the sexes of some asteroids). Four species are exceptions
in exhibiting sexual dimorphism. In these species dwarf males will cling to the larger
females, often mouth to mouth or on the aboral surface.
Many species are hermaphroditic. This may involve changes of sex from male to female
(protandry, also occurs in some asteroids). All hermaphroditic and some dioecious
species brood their young. Usually gametes are shed into the sea, through the bursal slits,
but in brooding species the eggs may be shed into the bursal sacs where they hatch whilst
inside the female (oviparity) and then the young live in the bursal sacs, which become
brooding chambers. The young may attach to the bursal wall by a stalk (which is part of
the bursa). The wall of the bursa and the egg yolk provide nourishment. Many species are
truly viviparous, the eggs being retained in the ovaries until they hatch. The young may
then reside in the brooding chamber of the bursa until quite large. Usually only 1-2 young
occupy each bursa, but as many as 200 embryos have been seen in a single bursa. At least
one species of serpent star is known to attach 20-2000 yolky eggs underneath stones or
seaweed, etc.
The gonads are sacs attached to the coelomic walls of the bursae, near to the bursal slits.
There is typically 1-2 gonads per bursa, but sometimes there may be clusters or rows of
several thousand per bursa.
Asexual reproduction may occur by fission, in six-armed species, especially in young or
small ophiuroids (< 3 mm disk diameter). There is no preformed fissure plane. Serpent
stars are able to regenerate lost arms, and actually cast-off arms if handled, or if the arm
is trapped. This gives rise to their common name of ‘brittle stars’. The disk needs at least
one arm to survive and regenerate.
Embryology
The first fully developed larval stage is the pluteus, which has four arms with ciliated
bands and a mouth. After about 18 days, this develops into an ophiopluteus, which has 8
arms, with ciliated bands, supported by skeletal rods, and one pair of epaulettes). The
arms continue to lengthen, until after about three weeks the late ophiopluteus resorbs or
discards its arms and sinks to the bottom as its heavy skeletal system develops, and
develops into a young brittlestar.
Deviations on this theme occur. Species with large yolky eggs may hatch a cylindroid
larva with 4 ciliated circles and no arms (similar to a doliolaria larva).
Ecology
Ophiuroids are found in all seas, at all latitudes and on all types of substrate, from
intertidal to Abyssal depths (6000 m).
Serpent stars are very numerous on tropical reefs. They may form large aggregations of
1000-2000 serpent stars per m2. Some serpent stars are epizoic. The only commensal
echinoids are species of ophiuroid. Some of these live inside the water-canals of sponges,
others are commensal on corals, while others live on the oral surface of feather stars, and
some live on the undersurfaces of sand dollars.
Sea Lilies and Feather Stars: Crinoidea
General body plan and external features
Of about 630 extant species of crinoid, about 80 are stalked crinoids or sea lilies, the
remainder are non-stalked feather stars (comatulids). There are more than 5000 species
of extinct crinoid. Crinoids have a jointed or scaly appearance. Sea lilies are divided into
the stem (stalk or column), which has a cylindrical or pentagonal contour and a jointed
appearance, and a crown or corona, which bears the arms.
The stem may be up to 50 cm long (up to 21 m in extinct forms) and has an attachment
disk or digitated root-like attachment organ. In some extinct forms the distal end of the
stem sometimes possessed a grappling hook or an end-bulb for attachment, or the stem
was slender and prehensile and in some forms possibly ended in a float.
The stem may possess cirri or jointed appendages. At the base of the stem these may also
provide additional anchorage. The cirri are arranged in regularly spaced whorls (2, 3 or 5
cirri per whorl, usually 5). Comatulids lose their stem during embryonic development and
usually have cirri (0-80 cirri). The cirri may be equipped with aboral spines and may have
terminal claws.
The crown consists of a central, rounded, oval, hemispherical or discoidal mass
(containing the viscera) and attached arms (brachia) arranged pentamerously. The aboral
surface forms a cup or saucer (calyx or dorsal cup) roofed by an oral membrane (tegmen,
disk, or vault). The mouth is at or near to the tegmen centre. Five ambulacral grooves
extend from the mouth to the arm bases. The anus is excentric at the tip of the anal cone
(anal tube). When present, the stem is attached to the aboral base of the calyx, so that the
mouth faces upwards.
The plane bisecting the mouth and anus (or alternatively mouth and hydropore) forms ray
A, the anterior ray or radius. Proceeding clockwise, as seen with the oral surface facing
the observer, the other four radii are labelled B, C, D and E. The anus occupies
interradius CD. When the mouth is displaced peripherally, only the ambulacral grooves
on the same side of the mouth may remain.
The tegmen is perforated by 500-1500 tiny pores, which form entrances to water canals
(ciliated funnels) leading into the coelom. The brachia sprout from the calyx-tegmen
boundary. These arms are also jointed or scaly in appearance. There are five arms, but
these may fork into ten and may branch 8-9 times to give 40-200 branches. The arms may
be long and slender or short and broad. Warm temperatures favour long branching arms,
whilst cold temperatures favour fewer shorter arms. The arms may vary from 10 mm to
300 mm in length.
A row of short, jointed side branches, known as pinnules, occur on each side of each
arm. The proximal or oral pinnules are tactile and protective, they have no ambulacral
grooves, no podia and are long, rigid and spine-like and exhibit limited movements. They
may be equipped with a terminal comb of teeth of unknown function. The middle
pinnules are the genital pinnules and contain the gonads, which become swollen at
maturity (at other times these pinnules resemble the distal pinnules). These pinnules are
short and slender and may possess podia and grooves. The distal pinnules are long,
slender and possess podia and grooves. These grooves branch from the main arm
grooves. The most distal 2-4 pinnule joints always bear aboral hooks. Pinnules may or
may not possess spines.
The colour of crinoids tends to decrease with depth. Comatulids are especially colourful
and may be: white, cream, yellow, orange, green, olive, bright red, wine red, maroon,
purplish red, purple, violet, brown or black. Some have two or more colours and some
species have numerous colour variants.
Body wall
Epidermis may be incomplete or absent over most of the body surface, and may be
syncytial. The epidermis lacks a basement membrane and is ciliated only on water canals
and ambulacral grooves.
Endoskeleton
Ossicles occur in the dermis, as in all echinoderms. The ossicles have a fenestrated mesh
structure and are: 83-91% calcium carbonate, 7-13% magnesium carbonate, 0.02-5.7%
silicon dioxide, and < 1% metallic oxides and contain traces of calcium phosphate.
The stalk ossicles form a single row of rounded or pentagonal diskoid or cylindrical
columnals (5 rows ancestrally, became fused??). If present, then whorls of cirri attach to
columnals called nodes, which possess articulatory facets for the columnals. The other
columnals form the internodes (1-45 columnals per internode). The stem grows by
adding new columnals to the top, just below the calyx, and by interpolation of columnals
into internodes. The maximum number of internodal columnals is typical of the species.
The columnals articulate by lock-and-key style articulations, which may be
pentametrously arranged.
The stem lacks muscles, but has elastic fibres connecting adjacent columnals. However,
the stem is capable of some movement, enabling it to position the crown against the
current (see ‘nutrition’ below). Rigid joints, called syzygies, connect nodes to internodes.
These are connected by very short elastic fibres. The stem is most prone to break at these
junctions. A central canal runs through the columnals (a small central hole) and carries
coelomic canals and nerves.
The cirri contain skeletal ossicles called cirrals (15-50 cirrals per cirrus). These also
contain central canals continuous with those of the column.
The calyx contains 2-3 alternating pentamerous cycles of skeletal plates. Monocyclic
forms have two cycles (with stem-angles at radii): the aboral basal plates and oral radial
plates. Dicyclic forms have three cycles (stem-angles interradial): additional infrabasal
plates are aboral to basal series. Additional perisomatic plates may occur between the
radials and arm bases (interradials and interambulacrals) and connected with the anal
cone (e.g. radianal plates). Plates within a cycle may fuse to give 1-4 plates. Extant
species are all either monocyclic or pseudomonocyclic. The latter appear cyclic because
the infrabasals are much reduced.
In comatulids, the basals are reduced or internalised in comatulids, forming a decagonal
disk. The centrodorsal ossicle is the top columnal and is retained to form a major part of
the calyx and bears the cirri (if present). The radials form a radial pentagon. Comatulids
possess a skeletal structure called the rosette, formed by fusion of the greatly reduced
basals. The rosette has a hole in its centre and forms an oral roof to the centrodorsal. The
oral face of the centrodorsal usually has 5 radiating interradial ridges or grooves between
which the radials fit. Sometimes five rods, the basal rays, extend from the rosette along
the radial grooves or ridges of the centrodorsal.
Brachials are the ossicles in the arms. Primibrachs are ossicles in the five main arm
trunks, secundibranchs occur in the ten forks and tertibrachs or palmars form the third
branches and postpalmers form the other smaller branches. The brachials may be
uniserial or may zig-zig in alternate fashion to give a biserial or intermediate
uniserial/biserial appearance.
Brachial ossicles may form movable articulations with flexor muscles opposing elastic
ligaments, or immovable joints. Of the immovable joints synarthrie are flecible, bound
together by elastic ligements, and syxygies are rigid and connected by short elastic
filaments. These can be seen as wavy lines joining a distal epizygal ossicle to a proximal
hypozygal ossicle.
The tegmen may or may not contain endoskeletal ossicles. The ancestral type of five
deltoid plates may be retained, or there may be many small tegmen plates or microscopic
calcareous inclusions.
The pinnules contain ossicle called pinnulars. These are moved by muscle/ligament
antagonistic articulations as in the brachials. Muscle contraction causes flexion, which
stretches the elastic ligaments, which hence contract when the muscles relax. The cirri are
held together by elastic ligaments only, not muscles, but can still move, suggesting that
these ligaments have some innate contractile powers.
The ambulacral grooves reside in deep depressions on the oral surface of the brachials
and pinnulars. The edges of the grooves may be raised to form repeating lappets, which
have a scalloped contour, alternating on both sides of the groove. Lappets may close over
the groove and podia for protection. Calcareous bodies (spicules, rods, fenestrated plates,
etc.) may occur in the soft tissues.
Tube feet
A ring of 20-25 oral or labial podia encircles the mouth (borne on the outer rim of a
pentagonal ambulacral depression). Tube feet also occur in the ambulacral grooves of the
arms and are considered under ‘nutrition’. Appendages called saccules: small spherical
bodies containing protein, also border the ambulacral grooves.
Coelom
The coelom is lined by a cuboidal epithelium. Rather than a single large cavity the
coelom is divided by strands, webs and membranes of connective tissue that fill much of
the body. (These may contain calcium carbonate inclusions). The coelomic axial sinus
surrounds the oesophagus and is enclosed by intestinal coils. This sinus, and other
coelomic spaces, continues into the arms and pinnules. (There are five coelomic canals
per arm – one aboral canal, two subtentacular canals, a genital canal and a tiny fifth
canal between the water canal and ectoneural nerve band). Cilia are lacking except in 2-6
ciliated pits in each pinnule, sometimes these occur in the arms.
The chambered organ is aboral to the axial sinus (in the cavity between the rosette and
inner surface of the centrodorsal plate in comatulids, and in a similar position in sea
lilies). This organ consists of 5 coelomic cavities pentamerously arranged (interradial in
monocyclic forms, radial in dicyclic forms). These chambers terminate orally, but
continue as canals into the cirri at each node. The chambers are ensheathed in nervous
tissue.
Water-vascular system
This system has no direct contact with the outside in crinoids and is coelomic. Canals run
along the arms, under the ambulacral grooves, and along the pinnules to the podia. They
also run along five main radial grooves beneath the ambulacral grooves of the tegmen. A
ring canal (pentagonal) encircles the mouth beneath a pentagonal ambulacral depression.
The ring canal gives off canals to the labial podia. About 30 stone canals per interradius
open to the coelom.
Haemal system
The perioesophageal haemal plexus innervates the spongy organ, which may be the site
of coelomocyte production. A subtegminal plexus supplies the genital tubes.
Axial gland
The axial gland resides inside the coelomic axial sinus and accompanies the spongy
organ. Aborally the axial gland passes through the central hole of rosette in comatulids,
and enters the central canal or cord of the chambered organ. Orally it terminates near the
mouth, close to haemal plexi. The lack of a discharge duct for the axial gland suggests
that it has an endocrine function.
Nutrition
Crinoids are suspension filter feeders. When feeding, the arms and pinnules are held
outstretched with the tentacle-like podia erect. The podia possess mucus-secreting
papillae along their length. The podia are arranged into triplets. The primary podium of
each triplet is long and when it contacts a food particle it whips into the ambulacral
grooves and is wiped against the ciliary current or against the short tertiary podia or
between adjacent lappets. The secondary podia function like the primary podia, but are
shorter. The food particles are subsequently transported down the food groove by the
ciliary current.
The principal food depends on species, but includes zooplankton and detritus (including
bacteria). The filter-fan is oriented into the current by bending of the stalk. The fan is
often at 90o to the current, depending on lift forces, but some crinoids form a vertical
collecting funnel to catch sedimenting particles. In crinoids concealed in protective
crevices the arms may simply extend in several directions. The pinnules and podia form a
tight mesh and an efficient filter. The total ambulacral length may be as much as 80 m!
The mouth opens into a short oesophagus that connects to the intestine. The intestine
undergoes a complete turn inside the calyx and may be enlarged and may possess lateral
outpouches. In those forms with an excentric (exocyclic) mouth at the periphery of the
disk, the intestine may form four coils, which are no wider than the oesophagus. The
intestine opens through the anus in the anal cone via a short rectum. The anus discharges
large, compact, mucus-cemented balls that sediment from the water column.
Nervous System
There are three interconnected nervous subsystems:
1. The oral/superficial/ectoneural system: comprises a neural band beneath the
ambulacral grooves of the arms and cirri, immediately beneath the epidermis, and
innervates the podia. The five main bands converge to the mouth and form a nerve
sheath along the wall of the digestive tract.
2. The deeper oral or hyponeural system: comprises a pentagon in the connective tissue
of the tegmen, lateral to the water-vascular ring canal. It gives out nerves to the
tegmen podia, the anal cone, the internal organs and ten radial nerves to the arms.
These ten nerves fork to give two nerves per arm. These innervate the water-vessels,
pinnules and podia.
3. The aboral or entoneural system: the main part of the nervous system in crinoids.
(The oral system dominates in other echinoderms). It forms a cup-shaped mass in the
apex of the calyx cavity and gives out nerves to the cirri (in comatulids) or to the
column and hence also to the cirri at the nodes (in sea lilies). It gives out 5 brachial
nerves to the arms. These contain ganglia that innervate the flexor muscles. The
aboral nerve mass also gives out 5 lateral trunks, which fork into ten and are united
by a pentagonal commissure (concentric with the main nerve mass) in the radial
plates of the calyx.
Locomotion
Comatulids swim by moving the arms up and down in alternate sets of 5. During the
upstroke the pinnules are folded and during the power stroke the pinnules extend.
Maximum speeds are about 5 m / min. and a maximum distance of 3 m can be covered in
a burst, before rest is required. When gaining initial lift the arms may beat as fast as 100
rpm. Comatulids can also move by creeping, using the arms to pull themselves along at
about 40 m / h.
Reproduction
The gonads are contained in the arms or genital pinnules. Sex cell masses fill the genital
cavities. Crinoids are dioecious, and the sexes are indistinguishable except for the
presence of brood chambers in the females of some species. The genital canal is a
coelomic canal in each arm, containing a genital cord or strand of cells that transport the
primordial sex cells to the gonad. Sometimes the genital cord is contained within a
genital tube, which is possibly a haemal sinus. The sex cells are liberated by rupture of
the pinnule wall. Some crinoids brood their eggs, Antedon attaches the eggs to the
pinnules, while some Antarctic comatulids retain the eggs in pinnule brood chambers, in
which the larva develops to the stalked stage. Male spawning has been observed to
trigger female spawning. Some crinoids are viviparous, though the mechanism of sperm
entry to achieve internal fertilisation is not understood.
Embryology
Development has been well studied in the comatulid Antedon. The egg undergoes
indeterminate, holoblastic and radial cleavage to form a coeloblastula. After gastrulation
the pore closes, cutting off the archenteron (after about 36 h), which later forms the gut
and coelom. The first larval stage is the pelagic doliolaria, with 4 ciliary rings. After
attachment to the substrate, the doliolaria metamorphoses into a stalked cystidean larva
(resembling a cystoid). After a few days the mouth opens and 5 arms are produced,
resulting in a pentacrinoid larva, which gives rise to the adult.
Ecology
Comatulids occur in depths of  1500 m, whilst sea lilies are only found in deep waters at
depths of 200 – 5000 m (occasionally up to 50 m). Crinoids support a range of parasites,
inc. copepods, isopods, snails and the unusual myzostome polychaetes. The myzostomes
barely resemble worms and curiously they induce the formation of galls in infected
crinoid tissue. The form and position of these galls varies, and one is reminded of oak
tree galls. As already discussed, crinoids are efficient filter feeders, consuming
sedimenting organic matter and detritus bacteria.
Sea Cucumbers: Holothuroidea
External features
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Holothuroids are generally cucumber shaped, but some are almost spherical and
others are vermiform.
The mouth and anus are at opposite poles.
A tail may be present (which may be postanal).
Sea cucumbers lack arms.
Ambulacral and interambulacral axes are arranged meridianally around the polar axis.
The polar axis is greatly lengthened.
The mouth and anus may be terminal, ventral or dorsal.
Sea cucumbers lie with one side of the body against the substrate. This becomes the
ventral surface, which may be flattened into a sole. The mouth and anus may then be
dorsal. The dorsal surface may be arched. This ventral-dorsal differentiation imposes
bilateral symmetry on top of the axial radial symmetry.
Buccal podia form a circle of 10-30 tentacles around the mouth. These tentacles may
all be the same size, or some may be dwarfed. The latter may comprise an inner
circlet of tentacles. The tentacles may branch (described as dendritic / arborescent),
and they may be pinnate, peltate or digitate. The tentacles are retractile and the body
wall can close over them. (The body wall may also form an introvert in
Dendrochirota, which can also be withdrawn). A tentacular collar is present.
The skeletal plates reduced to microscopic ossicles.
Interradius CD forms the middorsal line. Radius A is midventral, radius B is on the
animal’s right and radius C is on the left.
Genital papilla may lie between or posterior to the tentacles. A hydropore or
madreporic plate may occur close to the genital papilla.
Sea cucumbers typically range in length from a few cm to 30-50 cm. Stichopus variegates
may be up to 1 m long and 21 cm in diameter. Vermiform Apoda may be up to 2 m long
when fully stretched.
Most are dull coloured – gray, brown, olive or black. The sole may be a lighter hue –
white, yellow, pink, rose or terra cotta. Deeps-sea Elasipoda are often purple, maroon, or
violet. Some holothuroids are transparent, or brightly coloured – rose, pinkish, orange,
violet, yellow or red and may have spots or stripes.
Body wall
The epidermis is non-ciliated and covered by a thin cuticle. It is often thick and leathery
and slimy, but it may be thin and transparent. It is often covered by warts, papillae or
tubercles.
The podia may be arranged in rows (more or less), but dorsal podia often lose their
suckers and become reduced to sensory papillae. If present, the creeping sole embraces
three ambulacral radii (E, A, B) and is known as the trivium. The dorsal surface (radii D,
C) is the bivium. The tube feet may be radially arranged, or they may cover the sole.
Endoskeleton
The body wall may possess armoured calcareous endoskeletal plates (ossicles). The anus
is terminal or dorsal or ventral and may be surrounded by 5 calcareous teeth, plates or
papillae. A calcareous ring of calcareous plates encircles the beginning of the pharynx
(analogous to Aristotle’s lantern in echinoids). This supports the pharynx, nerve ring and
water vessels and is the point of insertion of longitudinal muscle bands and pharynx
retractor muscles. These muscles retract the tentacles and pharynx.
Movement
Most sea cucumbers are sluggish, benthic animals that crawl amongst seaweed, rocks,
over sand or live inside burrows. Depending on species the time required to excavate a
burrow is from 5 min to 4 hours, and is achieved by muscle contractions of the body and
digging movements of the tentacles. The maximum rate of burrowing is 2-3 cm / min.
Sea cucumbers tend to be nocturnal, outstretching their tentacles to feed at night and
exhibit both diurnal and annual activity cycles.
Locomotory podia, or pedicels, may be present. In forms with a flattened ventral sole,
pedicels may be present on the sole, but may be lacking elsewhere or reduced to warts or
papillae. The creeping sole may also apply suction to the substrate, and these animals can
climb vertical glass surfaces. Muscular waves in the sole may assist locomotion. These
sea cucumbers may travel 1 m in 15 min. The elasipods (elasipoda) are deep-sea
holothuroids with enlarged podia for walking. This helps to keep them clear of the muddy
substrate.
Apodous sea cucumbers lack tube feet and have poor locomotory powers. Most are
burrowers. They can locomote, however, by using the tentacles to pull the animal along,
assisted by muscular contractions of the body.
Pelagic sea cucumbers have floating or swimming devices, comprised of papillae webbed
together to form sails and fins. It is estimated that about half of sea cucumber species are
capable of swimming. The ends of the body may be thrashed together in a U-shaped
motion. Peniagone diaphana floats vertically with the tentacles uppermost.
Nutrition
Dendrochirotes are plankton feeders. The tentacles fan-out into the water or sweep across
the substrate. Their mucous secretions collect particles and one-by-one the tentacles bend
over into the pharyngeal lumen. The mouth closes and, as the tentacle is pulled out, the
food particles are wiped off into the pharynx. The food consists of microorganisms,
organic particles, small crustaceans, nematodes, etc.
Non-dendrochirotes use their tentacles to shovel the substrate into their mouths. They
pass 6-8 g of matter per hour.
The mouth is in the centre of the buccal membrane, which is encircled by tentacles and
equipped with a sphincter muscle. The pharynx leads through the calcareous and water
rings and may give rise to a short oesophagus. The pharynx or oesophagus lead into the
stomach and hence into the intestine. The intestine is looped within the coelom and is 2-3
times the length of the body. First it descends posteriorly along the middorsal and then
ascends anteriorly along the left side. This whole descending and ascending section forms
the small intestine. Finally the intestine descends posteriorly along the midventral, as the
rectum or large intestine, to the anus. The anus opens into the cloaca, along with the
respiratory trees. The gut is supported by 1-3 mesenteries.
Digested food particles are absorbed by amoebocytes, which enter the circulation via the
intestinal rete.
Nervous System
A supepidermal plexus innervates the body wall. The main neural centre consists of a
circular or pentagonal nerve ring in the buccal membrane close to the tentacle bases. Just
inside and adjacent to this nerve ring is a circular coelomic cavity – the peribuccal sinus.
The nerve ring sends out a ganglionated nerve into each tentacle and nerves to the buccal
membrane and pharynx and puts out radial nerves along the ambulacra in the dermis, just
external to the radial water vessels.
The nerve ring and the ectoneural radial nerves constitute the ectoneural system. This
system innervates the podia and the body wall. The hyponeural system is mostly, if not
entirely, motor and supplies the body wall muscles. There is no aboral system in contrast
to the crinoids.
Sense Organs
The body wall nerve plexus innervates scattered epidermal sensory cells. Podia are also
very sensitive. In synaptids, which lack podia, sensory buds or warts occur over the
surface. The tentacle stalks of apodous forms also contain 1-30 ciliated sensory pits per
tentacle. Statocysts are found in holothuroids. In Synaptids there is one pair along each
radial nerve. These statocysts are hollow spheres of flattened nonciliated epithelium
enclosing 1-20 lithocytes. The lithocytes are vacuolated cells containing an inorganic
material. Other sea cucumbers may have up to 100 statocysts around the nerve ring.
The dermis is sensitive to light. Some species possess a pair of ‘eyes’ at the base of each
tentacle.
Coelom
The coelom is a spatious fluid-filled cavity between the body wall and digestive tract.
Hyponeural sinuses, peripharyngeal sinuses, peribuccal sinuses and the perianal sinus
(coelomic ring) are all coelomic cavities.
Some synaptids have a pulsatile rosette – rounded coelomic projections near the
calcareous ring. These rosettes pulsate a few times every minute. Other synaptids have
vibratile clubs (ciliated clubs projecting into coelom) along the longitudinal muscle
bands. The functions of rosettes and vibratile clubs are unknown.
Coelomic fluid recirculates at 10-15 mm/s, due to action of the ciliated epithelium.
Excretion
In the Apoda, ciliated urns or funnels act as excretory organs. These usually occur on
mesenteries or on the body wall. Waste laden coelomocytes accumulate here, and at the
general body wall. Waste clumps (brown bodies) fall from the funnels into the coelom. In
species with no urns the respiratory trees are the principal excretory organs, and laden
coelomocytes will accumulate here. The gonads and intestines also act as excretory exits
for coelomocytes.
Water-Vascular System
The ciliated water ring (ring canal) encircles the pharynx, posterior to the calcareous
ring. It gives off five radial canals and connects to 1-12 expansion chambers, or polian
vesicles and typically a single stone canal. The stone canal may open to the exterior via a
hydropore or madreporite or may be closed off and terminate in a madreporic swelling
that may hang free in the coelom.
Haemal System
The haemal system is well developed and is complex in the larger forms. It consists of a
haemal ring around the pharynx, which gives off 5 radial sinuses, which innervate the
tentacles and podia. Ventral and dorsal sinuses run along the intestine. The dorsal sinus
supplies the gonad and also forms a rete mirabile (‘wondrous blood network’) supplying
the intestine. Coelomocytes are manufactured in large haemal channels. The blood spaces
are sinuses rather than true vessels, since they lack an epithelial lining, but instead the
lumen is lined by connective tissue, backed by muscle, which in turn is backed by
coelomic lining.
The dorsal sinus is contractile, pulsing 1-12 times / min. It delivers blood to the anterior
of the animal.
Axial Complex
This may disappear in adult forms (or is much reduced?).
Respiratory Trees
These arborescent tubes spring from the anterior part of the cloaca. They are digestive
tract evaginations that descend anteriorly in the coelom for much of the animal’s length.
The respiratory trees are contractile and take-up oxygen. They are in close association
with the rete mirabile of the haemal system and probably transfer their oxygen to it.
Tubules of Cuvier
These white, pink or red tubules are attached to the bases of the respiratory trees
(especially the left respiratory tree). They have a defensive function. When the animal is
sufficiently irritated, these tubules are emitted from the anus (through a rupture in the
cloacal wall) towards the source of irritation. They rapidly elongate into sticky or toxic
threads and detach from the animal. They then regenerate.
Reproduction
Most holothurians are dioecious. The sexes are indistinguishable, unless the female is
brooding her young. Some species are hermaphrodite. Unlike all other echinoderms,
holothurians do not have a pentamerous reproductive system. There is a single gonad,
situated in the anterior part of the coelom in interradius CD. It opens to the exterior (near
the madreporite when this is present) along interradius CD, via a ciliated gonoduct and
gonopore. The gonad may consist of numerous tubules, and may be branched.
During spawning, the sex cells leave the gonopore in a slow stream to be dispersed by
tentacular movements. Each spawning lasts from 15 min. to 4 or more hours, and there
may be several spawnings during spring and summer.
Some holothuroids brood their young, using one of the following strategies:
1. The ovaries may incubate the eggs (viparity).
2. The young hatch and adhere to the external surface of the mother, for example on the
creeping sole.
3. The eggs may be stored on the tentacles and then later ejected.
4. The young may develop in pockets or depressions on the body surface (on the back or
sole, sometimes beneath large dorsal scales).
5. The eggs may be caught by the tentacles, which form a mesh, and transferred to the
dorsal surface of the animal. Two very extensile podia near the gonopre may help this
transfer and dorsal podia transport the eggs to brood pockets.
6. The eggs may rupture from the gonads into the coelom. Sperm may enter the coelom
through the anus and then through pores in the intestine.
Development
Holoblastic, radial cleavage gives rise to a coeloblastula. Gastrulation gives rise to a
gastrula. The flagellated embryo later escapes either as a doliolaria larva (cf. crinoids) or
it develops (after about 3 days) into an auricularia free-swimming, pelagic larva (0.5-1
mm across). The auricularia is characterised by the possession of continuous flagellated
bands (including the preoral and anal loops). The auricularia develops into a doliolaria.
The doliolaria possesses a gut (vestibule, or oral region) and 3-5 flagellated rings. The
doliolaria undergoes torsion – the vestibule rotates to the anterior end – resulting in a
pentactula larva, which posses 5 primary tentacles and 1-2 podia. The pentactula
develops into a young sea cucumber.
Asexual Reproduction
Asexual reproduction may occur by transverse fission. Constriction, twisting and pulling
breaks the animal into 2-3 separate pieces, which regenerate.
Regeneration
Regeneration is known to occur after evisceration and transverse fission.
Ecology
Although some float or swim, most holothuroids are benthic animals. They inhabit all
seas and all depths. They are found from the littoral zone down to the Hadal depths. They
are generally sluggish and may not locomote at all if conditions are suitable.
Holothurians dominate Hadal depths below 8000 m, where they comprise 98% of
animals and more than 99% of wet weight animal samples. Deep sea holothuroids are
sometimes seen in herds, aggregating perhaps to reproduce or gathering around
particularly rich feeding sites.
Holothurians, as sluggish animals, support a wide range of commensals and parasites,
including protozoa, rotifers, turbellarians, polychaete annelids, crustaceans and gastropod
and bivalve molluscs. Some bivalve parasites have been found in the descending
intestine, robbing the host of food. Some parasitic snails are still recognisable as such.
Some have no shell, but some parasitic snails of holothuroids are changed beyond
recognition. They may be tubular or worm-like organisms, with no shell, attached at one
end to a haemal sinus or intestine. Some may reach 130 cm in length and hollow, lacking
all systems except the reproductive organs. These snails penetrate the host by boring
through the body wall with their proboscis, during which an anterior fold grows over the
visceral hump of the snail as a pseudopallium, which forms a brood chamber. Body
systems, except the reproductive system, degenerate. Gasterosiphon deimatis attaches at
one end to the inner wall of the sea cucumber and at the other to the host intestine and has
a sac-like pseudopallium containing embryos.
The pearl fish, Carapus, is a small, very slender fish up to 15 cm long, with a pale body
and long, slim tail. It occupies the main stem of one of the respiratory trees, with its head
protruding from the anus. They enter through the anus, and although the sea cucumber
may resist by closing the anal aperture, it must eventually open the anus for respiration.
The fish finds its own food (small crustaceans) when it leaves its host at night and only
users the host for shelter and refuge.
Sea Urchins, Sand Dollars and Sea (Echinoids)
There are about 750 species of extant echinoid.
External Characteristics
Urchins are usually plain and dark shades of green, olive, brown, purple, or black. Some
are pale, almost white or red. The spines may be cross-banded or with contrasting tips.
Colour may depend on age.
Regular Echinoids: external features
Regular urchins are a regular shape: globose, sometimes flattened at poles, some are
ovoid. They move upon their oral surface, which may be more or less flattened or
concave. The aboral surface is arched. The test may be up to 15 cm in diameter
(excluding the spines). Some deep-sea forms are as much as 32 cm across (exc. Spines).
They possess an armature of thickly placed spines. The spines may be short or long (up to
30 cm long). The spines may be all the same size, or the oral and aboral spines may be
shorter than the equatorial / lateral spines. Often small and large spines are intermingled.
The podia are arranged in a pentaradiate manner: there are 5 double rows of podia
extending from the oral region to the apex along the five ambulacra. They may or may
not cross the peristome (the membrane containing the mouth). The five interambulacra
are usually wider than the ambulacra. The podia usually possess terminal discs, though
these suckers are sometimes lacking on the aboral surface. The podia are very flexible
and extensible and can protrude beyond the spines.
The mouth is in the centre of the oral surface, surrounded by a soft membrane – the
peristome. There may be a thickened rim or lip around the mouth. Five pairs of buccal
podia may encircle the mouth. These are short, stout podia and may be chemoreceptive.
The peristome contains embedded plates and it may have small spines and may have
pedicellariae. Around the edge of the peristome, situated in the interambulacra, there may
be 5 pairs of small bushy gills, which may sit in gill slits (gill cuts) and are presumably
respiratory.
The anus may is either in the centre of the aboral surface, or excentric and sits on a soft
centric membrane – the periproct. This may also have small spines and may have
pedicellariae.
The shell, or test, is the body wall containing immovably fused calcareous plates, which
support the spines. However, in some echinoids the test may be leathery and flexible. The
ambitus is the equatorial circumference / contour of the test and is usually circular or
slightly pentagonal or oval.
Regular echinoids: Test Morphology
The test is made-up of 20 curved rows of plates. There are 5 ambulacra with 2 plate rows
each and 5 interambulacra, also with 2 plate rows each. Pores for the podia pass through
and not between the ambulacral plates. Spines are mounted on the tubercles of the test.
Primary tubercles form meridional rows and are largest at the ambitus, smaller at the
poles. Smaller tubercles occur between the rows of primaries. The tubercles consist of a
basal boss (a low truncate cone) and a terminal knob (mamelon) which articulates with
the spine. The areole is the bare area encircling the boss and is the site of spine muscle
attachment. A ring of secondary tubercles may surround the areole (the scrobicular
tubercules). These bear small secondary spines (scrobicules).
The plates are largest at the ambitus and smaller at the poles. They are 5-sided and
elongated horizontally. The plates of each double row alternate, forming a zigzag line
where they meet. The outer edges are straight. The plates are held together by ligaments.
Aboral plates surrounding the periproct form the apical system. This consists of 5 larger
genital plates (interambulacral) each pierced by a gonopore and 5 smaller terminal or
ocular plates (ambulacral) each pierced by a small pore for a modified terminal podium.
One of the genital plates is enlarged into a multiporous madreporite.
The perignathic girdle is a clacareous ridge on the inner side of the peristomial perimeter
for the attachment of the masticatory apparatus. In cidaroids the ridge is well developed
at each interambulacrum, forming a pair of apophyses. In all other echinoids the ridge is
best developed at each ambulacrum where a pair of projections called auricles are
formed. These auricles may meet, forming an arch over each ambulacrum.
The test is usually rigid, however, in the Echinothurridae the test is comprised of
overlapping plates and is flexible. It is moved by a special set of body-wall muscles.
In echinoids two canals connect each podium to its ampulla, so there is one pore pair per
podium. Primary plates contain one pore pair per plate (this is considered the original
condition) whilst compound plates, which are composed of several merged primary
plates, have more pores. Compound plates may be oligoporous (2-3 pore pairs per plate)
or polyporous (>3 pore pairs per plate).
There are several different arrangemenst of the ambulacral plates, depending on species.
These are:
1. The diademoid condition in which there are 3 full-sized primary plates per
ambulacrum, as in Diadema.
2. In the arbacioid condition a full-sized median primary plate flanked by 2 short
demiplates forms each ambulacrum, as in Arbacia.
3. In the echinoid condition each ambulacrum is formed from 2 primary plates with one
short demiplate between their outer ends, as in Echinus.
4. Insertion of more demiplates into these three types gives several polyporous types.
Irregular Echinoids: external features
Irregular echinoids have an oval to cordiform to circular ambitus. The body is flattened
orally and may be arched aborally or flattened, as in sand dollars.
The periproct and anus are excentric and displaced along an interambulacrum. This
introduces an axis of bilateral symmetry with anterior and posterior ends. The anterior is
ambulacrum D (in Carpenter’s system) or ambulacrum III (in Lovén’s system) and the
posterior is interambulacrum AB or V. The mouth and peristome may be displaced
anteriorly.
The ambulacra are petal-shaped and hence called petaloids and are equipped with
respiratory podia. The ambulacra continue over the ambitus to the peristome. Irregular
urchins lack gills.
Irregular echinoids are divided into two groups: the spatangoids or heart urchins and the
clypeastroids or sand dollars, cake urchins and sea biscuits.
Spatangoids (heart urchins)
Spatangoids may be up to 18 cm long. The heart urchins have an oval or cordiform
ambitus with an arched aboral surface. The three anterior ambulacra are short and form
the trivium. The two posterior ambulacra are longer and form the bivium. The anterior
ambulacrum (D) is non-petaloid. The oral ends of the bivial ambulacra expand into a
petal-like shape, the phyllode, around the peristome. The phyllode podia are modified.
In spatangoids the labrum is a posterior lip bordering the peristome. A few large subanal
podia form a single row in each posterior ambulacrum. The podia between the phyllodes
and petaloids are much reduced. There are no locomotory podia. The plastron or
sternum is the wide part of the posterior interambulacrum on the oral / ventral surface
that is enclosed by the two long and narrow posterior ambulacra. It may have special
spines and extends from the labrum in front to the periproct behind.
The spines are small to moderate and usually curved and held parallel to the body surface
(they are ‘combed back’). Clavules are narrow bands of dense minute spines, heavily
ciliated basally and shaped like tennis rackets. They maintain water currents that remove
sand grains from the test. The clavules are grouped into tracts called fascioles. The
peripetalous fasciole encircles the petaloids and crosses the anterior ambulacrum. The
internal fasciole encloses the aboral apex and much of the anterior ambulacrum. The
subanal fasciole encloses the posterior part of the plastron anterior to the periproct and
encircles the subanal podia. The anal fascioles run from the angles of the subanal fasciole
along either side of the periproct. The lateral or lateroanal fascioles extend from the
posterior angles of the peripetalous fasciole backward toward the posterior. Not all
fascioles are present in any one species; many have 2-3 with the subanal and peripetalous
fascioles being the most common.
The Pourtalesiidae family of spatangoids are very different in appearance. Some are
triangular, pyramidal or bottle-shaped. The periproct is on the aboral surface of the
narrowed end. The peristome is on the oral surface of the broad end. They lack petaloids
and phyllodes. The subanal fasciole is present. They inhabit deep waters and have fragile,
transparent tests.
Clypeastroids (sand dollars, cake urchins, and sea biscuits)
Some sand dollars are as little as 10 mm in diameter. Clypeastroids usually have an oval
or circular ambitus and are greatly flattened orally-aborally, though some are arched
dorsally. They are covered in a fur of short spines. The central aboral apex is surrounded
by 5 petaloids. The peristome is in the centre of the oral surface and the periproct is
usually oral. There are no phyllodes and no fascioles. Some species possess two or more
round to elongated holes called lunules, for example the keyhole urchins.
Irregular urchins: Test morphology
The test of irregular echinoids lacks conspicuous tubercles and is covered with
innumerable small or minute tubercles.
Spatangoid test: Larger tubercles occur between the petaloids in some spatangoids. The
ambulacral plates are all simple primary plates, each bearing one pore-pair in the
petaloids, and a single pore elsewhere. The reduced podia and penicillate podia are
uniporous. The ambulacra are often very narrow in spatangoids. The labrum is a plate
forming the posterior border of the peristome. In some irregular urchins, single
interambulacral plates meeting the peristome between the phyllodes form swollen
prominences (bourrelets) which form a flower-like figure called a floscelle. The test of
spatangoids is often thin.
Clypeastroid test: In clypeastroids the test may be very thick and equipped with internal
beams and columns for support. Pore pairs are limited to the petaloids. Numerous small
suckered podia emerge through single pores, on both ambulacra and interambulacra of
both surfaces. Primary plates and demiplates alternate in the petaloid ambulacra.
Nutrition
The mouth opens into the buccal cavity, which leads into the pharynx or oesophagus. The
oesophagus passes through the centre of Aristotle’s lantern and leads to the intestine.
Aristotle’s lantern is the masticatory apparatus and is found in regular and some irregular
echinoids. The intestine turns anticlockwise (viewed aborally) and then turns clockwise.
The anticlockwise portion is termed the small intestine or stomach. The clockwise
portion is the large intestine and leads into the rectum, which opens to the outside via the
anus. A blind pouch or caecum is opens into the gut at the oesophagus / intestine
junction.
Food, e.g. seaweed, is held by the podia and spines and gnawed by the teeth. Food
contacting the aboral surface is moved to the mouth by the spines and podia.
Pedicellariae may immobilise and help to hold food. Echinoids may be carnivores, eating
weak, sluggish or sessile animals, or they may be herbivores, but most will eat almost
anything. Many are scavengers and some ingest bottom ooze. Shells are also chewed-up
and ingested.
Irregular urchins live in sandy bottoms in burrows lined by mucus secreted by the spines.
Curved lateral spines are used for burrowing. The plastron spines ventilate the burrow.
The pencillate podia of the phyllodes are thrust out through the surface hole and probe
the surface, collecting particles by way of an adhesive secretion. The podia then retract,
delivering food to the spines of the upper lip and labrum and hence to the mouth. Food
consists mostly of diatoms, foraminifera and tissue fragments.
In keyhole scutellids, mucous strands with trapped particles are moved toward the mouth
by ciliary action.
Lantern of Aristotle
This is a complex of muscles and calcareous pieces that chew food. It is pentamerous and
conical. It has an apex of 5 teeth, usually seen protruding from the mouth. There are five
main interradial pyramids, each consisting of two half-pyramids joined by a suture. The
short spaces between pyramids are filled with interpyramidal or comminator muscles
(transverse fibres) by which the pyramids can be rocked upon each other. The aboral end
of each pyramid forms a bar or epiphysis – two per pyramid, which may be sutured
together. The aboral end of the lantern forms the base of the cone. In line with the
comminator muscles there are 5 slender radial compasses and 5 stouter rotules. Each
compass is formed from two pieces – inner and outer halves. The outer half is often
forked at the end. The pyramids support the teeth, which are long calcareous bands in the
interior spaces of the pyramids. The hard oral ends of the teeth project into the buccal
cavity. The softer, often curled, aboral ends of the teeth are enclosed in the dental sac.
This sac is a coelomic cavity (formed by evagination from the pharyngeal cavity) from
which the teeth grow continuously.
In total the lantern consists of 40 skeletal pieces: 5 teeth, 10 half-pyramids, 10 epiphyses,
5 rotules and 10 compass pieces. The protractor muscles are a pair of flat bands that
extend from the epiphyses to the perignathic girdle at the interambulacra. These push the
lantern outward, exposing the teeth. Retractors pull the lantern back and open the teeth.
These originate on the auricles of the ambulacra and insert on the lower ends of the
pyramids.
Small external and internal rotular muscles connect the epiphyses with the rotules. These
transmit movements of the epiphyses to the teeth. The compasses and their associated
muscles are part of the respiratory apparatus.
The whole lantern and its muscles are enclosed in a coelomic membrane, forming a
coelomic cavity around the lantern. This cavity is continuous with the gill lumina. The
compass elevator muscle raises the compasses, drawing fluid out of the gills. Two
depressor muscles per compass depress the compass, forcing fluid into the gills. Thus, the
gills are mechanically ventilated by the piston action of the compasses. The elevator
muscle is a flat pentagonal muscle encircling the oesophagus and attached to the
compasses. The depressor muscles attach the outer end of the compass to the outer
surface of the lantern protractors and originate on the perignathic girdle of the
interambulacra.
Body wall
The glandular epidermis is single-layered, cuboidal to columnar and ciliated, except on
the podial suckers / discs and other exposed places (which may be covered by a cuticle).
The epidermis also covers the spines. The epidermis is underlain by the dermis,
containing the embedded ossicles or skeletal plates. This is underlain by a flagellated
coelomic lining.
Pedicellariae
There are several types of pedicellariae found in echinoids:
1. Tridentate or tridactyle pedicellariae, are the largest and are very common. They have
a head of 3 elongated jaws or blades, often with serrated edges. Usually the jaws only
meet distally. Rostrate tridentate pedicellariae have shorter curved jaws, and occur in
spatangoids.
2. Triphyllous / trifoliate pedicellariae. These are small and have short broad jaws that
do not meet distally. Bidentate / biphyllous pedicellariae have 2 jaws and are common
in sand dollars (clypeastroids). Quadridentates are 4-jawed and occur in the family
Saleniidae, Quinquedentates are 5-jawed and occur in the clypeastroid family
Laganidae.
3. Ophiocephalous pedicellariae. These are found mostly on the peristome. They have
short inwardly concave jaws with blunt tips. They have a basal arc or handle that
interlocks, holding the grip when the jaws are closed.
4. Globiferous pedicellariae are equipped with poison glands. The jaws are armed with
one or more teeth. They are absent in spatangoids. The claviform type contains stalk
glands, the head having atrophied, and consist of a stalk only with 3 poison sacs. The
dactylous variety occurs in the echinothuriidae. These have 4-5 long, narrow jaws
with terminal discs and poison glands.
Cidaroids lack triphyllous and ophiocephalous pedicellariae and so have two types only.
Globiferous pedicellariae act in defence, and release toxin in response to a chemical
stimulus. The heads may detach and embed in attacking starfish. The tridentate and
ophiocephalous pedicellariae are less toxic. The body fluids and axial glands of echinoids
may also be toxic.
Sphaeridia
These are minute glassy, transparent, hard, solid, oval or spherical bodies on the
ambulacral areas (except in cidaroids). They are usually stalked and there is one to many
per ambulacrum. In irregular echinoids they sit in cavities, depressions or grooves. They
are thought to be organs of equilibrium.
Gills
All regular urchins except the cidaroids possess gills. Gills are absent in irregular
echinoids. There is one pair at the oral beginning of each interambulacrum. Their lumens
open into the peripharyngeal cavity.
Podia
Regular urchins have 5 double rows of podia on each ambulacrum from the peristome to
the periproct. They may continue over the peristome to the mouth edge (when there are
no buccal podia). Specialised buccal podia may occur around the mouth. Most of the
podia are locomotory, each with a terminal sucker supported by a ring of internal
calcareous pieces. Calcareous spicules support the stalk.
The aboral podia may lack suckers and are sensory papillate podia. Cidaroid podia have
terminal suckers, but locomotion is due primarily to the spines. In spatangoids
locomotion is also due to the spines.
Spatangoid podia
The aboral petaloids have large, thin-walled, leaf-like, lobulated branchial podia, which
lack skeletal support. These may function as gills. The frontal podia of the non-petaloid
anterior ambulacrum are tapering or topped with a scalloped or stellate disc.
The phyllode podia are penicillate (resemble a fruiting Penicillium mould) with an
expanded end covered with erect club-shaped projections, each supported by an internal
skeletal rod. They are chemoreceptive and assist in food capture.
From the phyllodes along the ambulacra in the aboral direction up to the petaloids, the
podia decline to very small, slender forms, except for a few large subanal podia enclosed
by the subanal fasciole. These resemble penicillate podia or frontal podia.
Clypeastroid podia
There are two main types of podia in the clypeastroids: large simple or lobulated brachial
podia on the petaloids and very numerous, small, suckered podia, which cover much of
the test, both ambulacra and interambulacra. These small suckered podia assist the spines
in locomotion and gather food.
Musculature
Due to the rigid body wall, body musculature is absent, except in the echinothuriidae,
which have soft deformable tests. Muscles move the moveable appendages (spines,
pedicellarise) and the lantern.
Echinoids: nervous system
The nervous system is similar to that in holothuroids. The main ectoneural system
consists of the circumoral nerve ring, radial nerves and the subepidermal plexus. The
radial nerves ascend along the midline of the ambulacra along the inner surface of the
body wall. The deeper oral or hyponeural system is present in those echinoids with an
Aristotle’s lantern. Five plaques of radial nervous tissue on the aboral surface of the
nerve ring send nerves to the lantern (and its muscles?).
Sense organs
Spines, podia and spines are all sensory. There are also dispersed epidermal sensory cells.
Sphaeridia may function as organs of balance. In the diadematidae, bright blue spots on
the genital plates, in rows along the interambulacra, often on the peristome and
sometimes on the ambulacra. Sometimes these spots are fused into stripes. These are
thought to be compound eyes. In Astropyga radiata stalked oral blue spots are thought to
be photoreceptors.
The subepidermal plexus is photosensitive and echinoids are negatively phototactic. The
aboral surface may be covered with pieces of plants, shells, small stones, etc. held by the
podia, as a protection from light. Shadows passing over urchins ellicit a defencive spine
erection reflex, which involves radial nerve activity. The spines may converge and point
towards the object casting the shadow. Eyes are not necessary for this reflex; the test
surface (not the spines) contains the photoreceptors. Keyhole scutellids undergo diurnal
colour changes, lightening in the dark.
Coelom
There is a spacious major cavity. In addition there are several minor cavities: the
peripharyngeal cavity, periproctal sinus, perianal sinus and the genital sinus. The
peripharyngeal cavity encloses the lantern and may give off 5 radial sacs (Stewart’s
organs) which may function as expansion chambers.
The coelomic fluid contains about 4 x 103 coelomocytes per mm3. These are amoebocytes
and a small number of flagellated cells. The flagellated cells may be peritoneal cells
detached from the coelomic lining. They are thought by some to give rise to
amoebocytes, though the dermis is also known to be a site of coelomocyte production.
Some of the amoebocytes are phagocytic with either pointed pseudopods (filopods) or
petallate pseudopods. Others may contain inclusions, including clear granules that give
rise to melanin on cell breakdown. Some amoebocytes contain red echinochrome
pigment.
Water-vascular system
This has the usual echinoderm plan. A main water ring around the digestive tube (where
it emerges from the lantern) gives rise to a stone canal, which ascends to the madreporic
plate. An axial gland accompanies the stone canal. Interradial branches connect to five
polian vesicles or spongy bodies. Alternatively there may be a continuous spongy ring in
some clypeastroids. Five radial water canals follow inside the ambulacra, giving off
branches to the ampullae. Each radial canal ends in a terminal tentacle.
Axial gland
This accompanies the stone canal and is comprised of spongy tissue. There is no axial
sinus in echinoids. The axial gland is hollow, due to an internal coelomic cavity, and is
well supplied by the haemal system. The axial gland is thought to be a point of
communication between the haemal and water-vascular systems. It consists of a
meshwork containing coelomocytes.
Haemal system
A haemal ring encircles the oesophagus on the aboral surface of the lantern. This gives
rise to interradial spongy bodies and radial haemal sinuses. Each haemal sinus passes
down the outer surface of the pharynx, inside the lantern, to the peristome and then along
the test radius, giving off branches to the podia. There is an inner or ventral marginal
sinus supplying the large and small intestines, and a smaller outer or dorsal marginal
sinus supplying the small intestine. Presumably these marginal sinuses take-up the
products of digestion from the intestine.
Excretion
Waste-laden coelomocytes accumulate in the gills and body wall or deposit granules in
the body wall. The axial gland is also involved; waste-laden coelomocytes accumulate
here and exit via the stone canal and madreporite.
Locomotion
Locomotion may be affected by podia, spines or both. Spine powered locomotion is
faster, reaching 25-35 mm / s compared to a maximum of 150 mm / min. for podial
locomotion. The lantern may also be used out of water: the urchin lurches forward by
pushing with its teeth and spines. Righting movements occur in 1.5 to 2 minutes in
regular urchins, but may take an hour in sand dollars. Righting in sand dollars is slower
without the sphaeridia, but these are not essential. There may be a slightly preferred
leading ray in some regular species. Irregular urchins only move with their anterior end
forward.
Sand dollars move with Loven’s ray III forward, turning rather than reversing. They are
propelled at up to 18 mm / min. on top of the substrate, primarily by spine movement.
They burrow by first building a mound of earth into which they thrust themselves. This
whole process takes 15-20 minutes.
Keyhole scutellids burrow in about 15 minutes by rotating their test from side-to-side,
during which sand is driven through the anterior lunules by spine action. Some species
can not right, but the inverted position is unstable and easily righted by wave action.
Irregular urchins live in mucus-lined burrows. The curved lateral spines are used for
burrowing. Spines secrete the mucus, while plastron spines ventilate the burrow.
Reproduction
Regular echinoids have 5 interambulacral gonads are fastened by mesenteries. Each leads
to an external gonopore, via a gonoduct, on the apical genital plate. Most irregular
echinoids have 4 gonads (gonad along interambulacrum AB is missing), but some have 3
(CD and AB missing) and others have 2 (CD, AB, DE missing).
Sea urchins are strictly dioecious, but rare anomolous hermaphroditic individuals do
occur. A few species exhibit sexual dimorphism. In Psammechinus milers the male
gonopores occur on short papillae. In Echinocyamus pusillus the male genital papillae are
longer. In brooding spatangoid species the female has deeper petaloids than the male.
Fertilisation is external. Echinoids may aggregate to spawn, which may take place at full
moon. Some pair off for spawning. Spawning occurs around spring and summer in the
Northern Hemisphere.
The gonads have an outer coelomic epithelium, with muscle fibres and connective tissue
underneath and an inner germinal epithelium. Contraction of the muscle fibres causes
spawning. This may be in response to a chemical stimulus since the gonads have no
apparent nerve supply.
Brooding is most common in the cidaroids and spatangoids, especially in the Antarctic.
These species produce large yolky eggs, which develop on the peristome or around the
periproct (sometimes in an annular groove). Brooding spatangoids may have deepened
petaloids, which form a brooding chamber roofed by criss-crossed spines.
Development
The eggs float or sink, depending on species. Cleavage is holoblastic and initially equal,
giving rise to an 8-celled stage with 4 vegetal cells, which give rise to small micromeres
and large macromeres by unequal cleavage, and an animal pole of mesomeres. An 8001000 cell coeloblastula is formed, with small cells at the vegetal pole. After about 12
hours post-fertilisation, a free-swimming flagellated blastula is produced, which develops
into a pluteus larva (an echinopluteus).
The echinopluteus has 4 arms, one pair of postoral arms and one pair of shorter
anterolateral arms. These increase the area of ciliary loops used in feeding on microplankton. As it develops it may develop extra arms, up to a total of 4,5 or 6 pairs. These
are the posterodorsal arms and preoral arms and optionally the anterodorsal and
posterolateral arms. The posterolateral arms may only be present as short processes. The
resultant larvae can have very variable shape and structure. Calcareous rods support the
arms.
Diadema larvae have 4 arms, and the horizontal postoral arms greatly elongate into the
so-called plutei transversi. Spatangoid plutei develop an aboral spike (median posterior
projection).
After 4-6 weeks metamorphosis produces a young urchin (takes about one hour) less than
1 mm in diameter. Spines, podia, test, etc. continue to develop. Urchins may live for 8
years or more.
Ecology
Echinoids are all marine and benthonic. They inhabit all seas and all types of bottom,
from the intertidal zone to about 5000 m. They are apparently absent from the deepest
abysses. They are most common in the littoral zone. Regular urchins tend to prefer hard
botttoms, irregular urchins soft sandy bottoms. Deeper water forms live on the bottom
ooze.
Rock-boring urchins excavate burrows in rocks for protection against waves. The urchin
may become permanently trapped in the burrow as it grows. Boring is accomplished by
the rotary motion of spines, assisted by the teeth. These urchins cause damage to steel
pilings.
Colobocentrotus (an echinometrid) has large strong oral podia and an ambitus fringed by
broad flat spines, which act as a sucker for firm attachment. The aboral spines serve to
break the force of the waves.
Crabs, sea stars, large fish, mammals and birds may eat urchins. Birds may fly up and
drop them onto rocks to crack them open. Crustaceans and small fish may seek shelter
among the spines of poisonous echinoids. The spines of cidaroids are devoid of a living
surface and are usually overgrown by algae, sponges, hydroids, zoanthids, anemones,
bryozoans, brachiopods, tubiculous polychaetes, and barnacles, etc. Commensal ciliates
commonly occur in the digestive tract. Other protozoa may live in the coelomic fluid and
flatworms may live in the intestine or gonads. Nematodes up to 150 cm long have been
found in the coelom. A peculiar sea cucumber, Taeniogyrus cidaridis, lives on cidaroids,
with the posterior part of its body coiled around the urchin’s spines. Ophiuroids have
been found attached near the mouth. Some bivalves and snails may also parasitise
echinoids. These snails may bore into the base of a cidaroid spine, producing a gall,
which contains one or more snails. Crustaceans may also parasitise echinoids and may
induce galls.