Download Phylum Echinodermata

Phylum
Echinodermata
Peter Shaw
Invertebrate Phyla: BIOH20.043
I hereby salute the echinoderms as a noble
group especially designed to puzzle zoologists
Libbie Henrietta Hyman 1955
Introduction
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Echinodermata are all marine,
triploblastic unsegmented
coelomates
Phylum has 3 unique features:
pentagonal symmetry (bilateral in
larvae)
 calcite spicules embedded in the
skin, often partly fused
 Tube feet (podia)
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Affinities
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The only connected phylum is our
own, the chordates - based on
embryological evidence.
No chordate stores calcite skeletally we use calcium phosphate
No common ancestor of echinoderms
and chordates has been found; if the
link is indeed valid the separation
occurred early in metazoan evolution.
An unhurried phylum..
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No echinoderm moves fast, apart
from a very few deep sea
holothurids which swim actively
Crinoids are sessile, the others
crawl at a rate of mm / minute
During one Antarctic marine survey
a starfish was tagged. A year later
the same animal was in the same
exact spot, having apparently done
nothing at all!
Anatomical basics:
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There is no cephalisation, hence notions of
anterior/posterior are inapplicable here
There is a meaningful gradient in all echinoderm
bodies: one surface has the mouth and tube feet
(ORAL or AMBULACRAL), while one does not
(ABORAL)
The anus is often, but not always, aboral.
Originally…
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It seems that the ancestral
echinoderm was a sessile
filter-feeder, extending its oral
surface upwards to capture
food
Uniquely, this sedentary
design has evolved into motile
forms where the feeding
surface faces downwards
Functional groups 1:
nerves
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Echinoderms have a diffuse
nervous system with no “brain”
There is a 5-radial circum-oral
nerve ring, and a superficial net
running close to ectoderm
In addition to the ectoneural net,
there is an endoneural net
(muscles between ossicles) and a
hyponeural net (local motor ganglia
for podia)
Hydraulics
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These are far more complex than
the nervous system!
Main hydraulic systems are
derived from the coelom, although
separate sections of the coelom
also surround viscera
The podia are operated by a
hydraulic system called the watervascular system
Mutable connective tissue
(also called catch connective tissue).
This is another echinoderm uniqueness! Their skeleton,
consisting of calcite ossicles, is embedded in a collagenous
connective tissue whose properties can be modified under
nervous control. The degree of cross-linking of the collagen can
be increased (making it tougher and stiffer) or decreased (making
it soft, in extreme cases able to flow into new shapes). It can
actively contract, like a slow muscle. Thus the body plan relies
heavily on extra-cellular materials (calcite and collagen) which
are low maintenance, allowing these animals to persist on a low
nutrient/energy budget.
5-radial layout
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Many organ systems in the
echinoderms follow the same basic
structure as the water-vascular and
nervous systems: a 5-radial circumoral ring
These rings give rise to 5 radial
branches (canals in the case of the
WVS)
A few asteroids have 7, 10, 11 arms in which case 7,10, 11 radial branches
Hydraulics, contd.
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Each radial canal of the WVS supplies
water to tube feet, each with its
ampulla
There is one asymmetric element: a
single tube (the “stone canal”)
running from the oral WVS ring to the
outside via the madreporite
The oral ring also has 4 (sometimes 9
if paired) water-storage sacs
“Tiedman’s bodies” or “Polian
vesicles”. Why not 5 or 10? 1 is the
stone canal.
Surface features
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Echinoderm skin has several
distinctive sets of organs
protruding from their skin:
 Tube
feet (podia)
 Spines
 Pedicillaria
Tube feet..
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Podia are not scattered
haphazardly over the body surface
They lie in 10 rows (5 pairs), the
ambulacral grooves
Each tube foot + its ampulla is
isolated from the WVS by a valve
Tube feet vary - starfish have
muscular suction cups, other forms
have sticky tips.
Crinoids are different - primitive
Tube feet..
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Originally began as outgrowths of
the WVS. In crinoids and
ophiuroids these remain essentially
as tentacles.
In other radiations, notably
asteroids, these have evolved a
highly specialised suction cup used
for locomotion and prey capture.
Tube feet..
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Have retractor muscles and can
bend, but no extensors
To extend, muscles around the
ampulla contract
Acts as a local effector unit
working on a hydraulic skeleton
Each podium has a nervous arc to
its branch of the hyponeural
system - a an individual motor
centre
Role of WVS
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Hydraulics
Respiration - O2 is exchanged
between ampulla and perivisceral
coelomic fluid
Probably (?) this was the ancestral
function of the WVS, with tubes +
podia lining arms to exploit ciliary
current already used in food
collection
Pedicillaria
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…Are defensive organs, assumed to protect
against encrusting organisms
Are active, independent local effector units able to
inject toxins on contact
Madreporite
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Is stated to allow pressure equalisation and top up
water supply to the WVS
There is something of a mystery here - the
madreporite shows a continual water influx, but
animals in which it is experimentally blocked appear
to function and move normally
Is absent in crinoids
Haemal system
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This consists of spongy tissue
enclosed in coelomic tissue,
running up the body in 5 strands
from a circum-oral ring.
Despite its name this is nothing to
do with respiration (can be
surgically removed without
affecting metabolism), and seems
to be involved in immune defences
Gonads
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Lie as 10 (2N) paired structures at the base of
ambulacral grooves.
Sexes are separate, and discharge gametes
into the sea water in response to chemostimulus of other gametes.
There are 2N gonopores, ie 2 per arm in
asteroidea.
Gonads can be large - echinoid gonads almost
fill the test, and can be eaten as a delicacy.
Ophiuroidea
Brittlestars
Crinoidea
Crinoids - feather stars
Echinoidea
Sea urchins
Asteroidea
starfish
Holothuridea
sea cucumbers
Concentricycloidea
discovered 1986
Living forms
Phylum Echinodermata
Living forms only
Asteroidea
starfish
Ophiuroidea
Brittlestars
Holothuridea
sea cucumbers
Crinoidea
Crinoids - feather stars
Echinoidea
Sea urchins
Concentricycloidea
discovered 1986
Sadly...
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Of the 13 classes of echinoderms
known, 7 are extinct.
Echinoderms were dominant forms
in Carboniferous seas, but have
suffered a long-term decline in
phyletic richness
Concentricycloidea
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A little known abyssal class,
described in 1986
These are circular but with 5radiate symmetry, and live on
wood remains at great depths
One genus: Xyloplax
Crinoidea
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Crinoids or feather stars - almost
certainly close to the ancestral
form of the phylum
These are mainly abyssal filter
feeders, though in previous
geological periods were dominant
in shallow waters
Some Carboniferous fossil beds
are made of crinoid ossicles
Crinoidea
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Feather stars / sea lilies - are
poorly studied due to habitat +
difficulty of keeping alive in aquaria
5000 fossils spp, 620 living
of 620 living spp, 80 are sessile (4
subclasses)
The remainder are comatulids: free
living spp of shallow waters
All have many commensals, inc
annelids
Crinoidea
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Body is mainly made of ossicles
10 arms have podia (no ampullae)
lining ciliated grooves feeding
particles to the mouth. Podia
seem to catch large particles
Arms can move, thanks to muscles
between arm ossicles
Mouth and anus are both on oral
side (!)
Comatulids
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Free living crinoids - “feather stars”
Have >10 arms, often migrating
vertically to filter feed in shallow
waters at night, usually by crawling
Can be common: > 70 /m2 in Red
Sea reefs
Typical genus: Antedon: A. bifida is
found in UK waters. This can swim
actively.
Asteroidea
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“Starfish” - one of best known invert groups
Most are active predators, usually on sessile
prey (bivalves), using suction cups on podia to
pull open the shells with forces of up to 5kg
The stomach is eversible, and can be partially
inserted inside prey’s shell (enzymes but no
toxins)
Odd asteroidea
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Acanthaster plankii attacks coral
polyps, causing widespread diebacks.
A few spp are ciliary filter feeders,
using mucus.
The ciliation in stomach of Porania
& Henricia resembles the gutsorting mechanisms of ciliaryfeeding molluscs
Giant sand star Oreaster is a
facultative microphage
Echinoidea
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Recipe: take a starfish and roll its 5
arms together into a ball, then fuse
and calcify with an external armour
The armour is called the test, and is
primitively roughly spherical
Note that this design greatly reduces
the aboral surface
Also ponder how tissues outside the
continuous test get food from inside
the gut… (active uptake from
seawater seems to be involved).
Echinoidea
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Are all herbivores, preferring macroalgae so are mainly found in sunlit
waters.
They can be highly effective grazers,
creating “urchin barrens” devoid of
algae
The mouthparts are unique, 5-radiate
(of course!), known as Aristotle’s
Lantern. This involves 5 continually
growing chisel teeth, each with 8
supporting skeletal pieces. This gives
the teeth remarkable versatility in
their action.
Irregulars
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At least twice echinoids have
evolved a 2ndry bilateral symmetry
- these are the irregular echinoids
All are sand burrowing, and have
re-evolved an antero-posterior axis
The heart urchin Echinocardium
has an anterior mouth and no
lantern; sand-dollars (Clypeaster)
are more flattened with a lantern
inside a ventral mouth.
Noli tangere
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Many echinoids have wickedly sharp
spines, which break off in your skin.
Beware Diadema!
Only a few fish, trigger fish, persist in
attacking long-spined spp.
Jacques Cousteau was more scared
of spiny urchins than of sharks
Spines are under muscular control,
and can be used to locomote
(metachronal rhythms in Diadema)
Noli tangere
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There are a very few echinoids that
are lethal to touch - they have
pedicillaria that inject a neurotoxin
which happens to paralyse
mammalian muscles.
Toxopneustes is known and feared by
pearl divers
Doubtless this is an unlucky
coincidence, and the real target is
some marine predator or fouling
organism
Ophiuridae - brittle
stars
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These resemble bony starfish in
general appearance, but have
arms sharply demarcated from the
body disc.
The internal structure of the arms
involves interlocking internal
ossicles, confusingly called
vertebrae. (Thus Ophiuroids are
invertebrates with vertebrae!)
Ophiuroids
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.. Are primarily detrital or filter feeders, raising their arms in a
current to capture particulates
Can be locally highly abundant, with densities of 100s per
square metre recorded (in Atlantic off W coast Ireland).
Ophiocoma wendtii has been found to be covered in
thousands of perfectly formed microscopic lenses (calcite of
course) which focus light on a bed of nervous tissue 0.05mm
below the surface: these animals change colour between day
and night, and it seems that their whole body surface is a
light-receptive organ.
Holothuridae- Sea
Cucumbers
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These animals have converted the basic
echinoderm design into something akin
to a sausage roll
They have no calcitic skeleton, except
for spicules embedded in a leathery skin
Most are immobile, and lie on the sea
bed rolling back and forth with the swell.
Some have limited mobility using their
tube feet.
Despite retaining 5-radiate anatomy, they
have re-evolved bilateral symmetry along
their long axis (the oral-aboral)
Holothuridae
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They mainly feed on detritus, collected
by oral tentacles which are derived from
tube feet.
Oxygen exchange is performed using
gills inside their anus
They have 2 odd defensive strategies:
 Squirting a stick goo from cuverian
glands. (It is hard to be sticky in
seawater, but this is!)
 Voiding their entire intestines - a
drastic procedure from which they will
recover.
Holothuridae - odd
snippets
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Are the basis of a commercial
fishery: “Beches de mere”. This is
intensively exploitative, and
operates by stripping an area then
moving on.
A few deep sea holothurids are
pelagic, swimming constantly a few
metres above the bottom Pelagothuria.