Download Phylum Arthropoda (Jointed Animals)

MARINE
Invertebrates
BIOL 505
Understanding Marine
Invertebrates,
Their Environments
and Processes
Phylum Arthropoda (Jointed Animals)
Defining Characteristics:
1. Epidermis produces segmented, jointed, hardened
(sclerotized) chitinous exoskeleton, with internal
musculature between individual joints of appendages.
2. Complete loss of motile cilia in adult and larval stages.
Phylum Arthropoda (Jointed Animals)
General Characteristics
>75 % of all animals described to date belong to this phylum.
Thus arthropod body plan is most represented in all animal
kingdom.
Like annelids, arthropods basically metameric, with new
segments arising from special budding zones at posterior
of animal as larva develops.
However, in most extant spp, the metamarization is masked
when segments fuse and modify for specialized function
(tagmatization), also seen in polychaetes, but reaches
greatest extent in arthropods.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Two of the major arthropod groups (Insecta and Crustacea)
have 3 distinct tagmata: head, thorax, and abdomen.
Arthropods lack cilia, even in larval stages.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Exoskeleton.
A hard, external, protective covering.
However, unlike molluscan hard outer
covering. Covering in the 2 phyla
produced by very different processes,
have different chemical composition,
different physical properties, and
perform different functions.
Molluscan covering serves mainly as
protection for soft body within.
Arthropod covering does this, but also
functions as locomotory skeleton.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Exoskeleton
Exoskeleton secreted by epidermal cells.
Outermost layer (epicuticle) usually waxy, made of
lipoprotein layer, underlain with layers of lipids.
Advantage: epicuticle is impermeable to water – so water
loss minimized through body surface.
Disadvantage: body surface can’t be used for gas exchange.
Most of exoskeleton made up of the endocuticle, or
procuticle, mainly made of the polysaccharide chitin,
associated with several other proteins.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Phylum Arthropoda (Jointed Animals)
General Characteristics
Exoskeleton
Chitin is used in protection, support, and movement, and
provides rigid skeletal system.
The procuticle is strengthened by several hardening
elements. In the crustaceans, hardening is partly achieved
by depositing CaCO3 in some procuticle layers.
“Tanning” the procuticle’s protein component also helps
hardening. This process (sclerotization) involves forming
cross-links between protein chains.
The procuticle varies in thickness, and does not harden
uniformly over the entire body. This is where the main
functional significance lies.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Exoskeleton
In many regions of body, procuticle is thin and flexible in
different directions, forming the joint.
Appropriate musculature joined to exoskeleton provides for
jointed appendages that function in much the same way
as vertebrate skeleton, with pairs of muscles that
antagonize through system of rigid levers.
The jointed, flexible exoskeleton is the secret of arthropod
success.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Phylum Arthropoda (Jointed Animals)
General Characteristics
The Hemocoel
For animals encased in suit of rigid plates, coelom can play
no role in locomotion.
Thus, for arthropods, coelom is greatly reduced.
Instead, main body cavity is hemocoel, part of the blood
circulatory system, as in molluscs.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Unlike the shell of molluscs, arthropod skeletons do not
grow gradually. Instead, secreted over all regions of body
simultaneously.
Once hardened, arthropod is encased in it’s shell, except
where sensory hairs protrude and gland openings occur.
Major regions of hind and foregut also lined with cuticle.
To grow, arthropods must shed cuticle (including that lining
gut), grow, the harden new cuticle around larger,
sometimes morphologically different, body.
Old cuticle is degraded by enzymes, and split before molting.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Old cuticle splits by uptake of water and increased blood
pressure that cause body to swell.
Ecdysis (Gk. “escape”, or “slipping out”) is process of
removing old cuticle.
New cuticle is secreted before old on removed, so animal
can stay partially active.
Temporarily “soft” crabs rely on high blood pressure in
hemocoel to maintain locomotory function, so hemocoel
acts as internal ???
hydroskeleton, until new exoskeleton hardens.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Proecdysis (premolt): the period before molting when the
new exoskeleton is deposited below the old one. Lost
appendages will begin to regenerate as limb buds that
will unfold at the time of molting. Crab actively storing
salts which will be necessary in the shedding process as
well as storing water and food reserves.
Ecdysis (molt): active shedding of exoskeleton. During
this stage which is the shortest in duration of the four,
salts and water used by crab’s hemolymph to build up
hydrostatic pressure and crack the exoskeleton to be
able to withdraw from it. Land hermit crabs molt while
in their shell which acts as a mold for the soft crab.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Metecdysis (postmolt): freshly molted crab begins to
harden and recover movement ability. The crab will
consume his exoskeleton to recycle necessary minerals
and salts to aid in the calcification process.
Anecdysis (intermolt): the longest period during which the
exoskeleton will begin to bulk up as calcium and
minerals are consumed and deposited. As the crab
grows, this phase gradually increases in length.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Although increases in size discontinuous, tissue growth
(biomass) continuous process.
If number of epidermal cells increases continuously (as in
some arthropods), additional tissue becomes folded into
pleats until molt, and increase in size takes place.
Ecdysis and formation of new exoskeleton are under neural
and hormonal control.
The Y-organ (gland located in head of crustaceans) produces
ecdysteroid hormones that stimulate molting.
Production of EH inhibited during intermolt by 2nd hormone.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Second hormone produced by neurosecretory complex (Xorgan) in the eyestalks.
When X-organ ceases, Y-organ activity no longer inhibited
and ecdysone produced.
Or, X-organ secretions may not turn Y-organ off, but may
inhibit action of ecdysone directly.
Eyestalk oblation results in premature production of
ecdysone  ecdysis.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Molting
Several other functions under neurohormonal control:
1. Regulation of reproductive cycle
2. Regulation of body fluid osmolarity
3. Migration of light-screening pigments in eye
4. Movements of pigment granules within
chromatophore cells  gradual color changes of
body.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Nerves and Muscles
Arthropod nervous system different than both vertebrates
and other invertebrates.
In contrast to vertebrate muscle contraction, in arthropods,
strength of contraction depends on rate at which
impulses delivered to muscle fibers.
A single fiber may be innervated by up to 5 different types
of neurons.
Contraction type (slow, long vs fast, short), partly depend
on source of stimulation.
Also, some neurons are inhibitory.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Nerves and Muscles
Additionally, arthropods have muscle fiber types that differ
physiologically and functionally, so rate of contraction
is partly function of individual muscle fiber.
Thus, fine control of movement depends on both on types
of muscle fibers stimulated and interaction of several
neuron types on single muscle fiber.
Also, single neuron may innervate many muscle fibers, so
a given muscle may be innervated by only a few
neurons.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Nerves and Muscles
Arthropod musculature differs from other invertebrates in
that arthropod muscle is ALL striated (most other
inverts have mainly (or all) smooth muscle.
Striated muscle can contract much more quickly.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Circulation
Circulation begins when blood is collected directly from
the hemocoel into the heart through ostia (holes in the
heart).
Blood leaves heart through closed vessels (anterior and
posterior aorta), that dump the oxygenated blood into
the hemocoel.
Thus, the circulatory system is “open” with blood moving
through a series of sinuses.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Circulation
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Arthropod vision uses one of two forms:
1. Ocelli
2. Compound eyes
An ocellus is a cup with a light-sensitive surface backed by
light-absorbing cells.
Cup is often covered with lens (corneal lens).
Photosensitive pigment underlying cup is derivative of
vitamin A combined with a protein. Stimulation by light
causes chemical change in pigments (rhabdom), sending
signal down neurons. Not usually image forming.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Compound eyes can form images. This eye system well
represented among crustaceans and may occur along with
ocelli.
Composed of many individual units (ommatidia) that are
each oriented in slightly different direction from the
other, due to eyes convex shape.
Each ommatidia has 1) a fixed-focus lens (cornea) that has
depth of field from 1 mm – several m all in focus at
receptor; 2) underlying gelatinous crystalline cone that
acts as lens in most crustaceans; 3) series of up to 8
photoreceptors (retinular cells) each with light-sensitive
pigment;
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
4) Cylindrical cells (collars) containing shielding pigment,
that isolates every ommatidium from it’s neighbor; and
5) a neural cartridge at the basal end that is a cluster of
neurons receiving information carried by retinular cells,
and sending action potentials to optic ganglia for
processing.
Crustacean eyes sensitive to polarized light. Polarized light
known to be used by some arthropods as navigational
cue.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Light-sensitive pigments of retinular cells contained in
thousands of rhabdomeres, fine microvilli that fold out
from retinular cell walls.
Rhabdomeres in each omatidium form distinct, ordered
association (a central shaft) called the rhabdom.
Thus, the rhabdom is not really a structure, but a central area
formed by the microvilli of the retinular cells.
Rhabdom records light intensity at center of image that falls
on its tip; not the entire image. So, tip of single rhabdom
analogous to single rod in mammal eyes.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Remainder of cylindrical rhabdom acts to guide light for this
segment of the image down to the neural cartridge at the
base of the rhabdom.
The brain then constructs the complete image from the
thousands of omatidia. However, image is not put
together as single image, but as many images from
slightly different angles.
Each omatidium functionally isolated from neighbor by
shielding pigments, or reflective trachioles.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Basic compound eye called apposition eye, because each
lens positioned directly apposed to receiving rhabdom.
Since each lens is very small, each rhabdom receives only
small amount of light. Thus apposition eyes work best
with high light intensity.
To work well in low light, each neural cartridge must receive
light from more than one ommatidium.
If screening pigment absent between ommatidia, many facets
can combine light received into single image on retina.
This type eye is superposition eye.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Superposition eye
Apposition eye
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
In superposition eye, ommatidium has large space between
distal end of crystalline cone and rhabdom.
Pigments that typically inhibit light passage, migrate out of
the way, and allow light to pass from several lenses into
one rhabdom.
This produces signal of greater intensity than received
through single lens.
Pigment migration under hormonal control, so superposition
eye takes time to light- or dark-adapt.
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Sharpness of image formed depends on:
1. How much the light hitting rhabdomere enters at angle
parallel to long axis of that ommatidium.
2. How much light from other ommatidium impinge on
receptor pigments of an ommatidium (reduced
resolution).
3. Amount of difference adjacent ommatidia are oriented
(decreased angle = increased resolution).
4. Number of ommatidia/eye (more = better resolving
power).
Phylum Arthropoda (Jointed Animals)
General Characteristics
Visual System
Sharpness of image formed depends on:
5. Complexity of brain receiving impulses from ommatidia.
Even most complex and largest compound eye forms
somewhat coarse-grained image, since image is formed
from collaborative effort of from 6 – few thousand neural
cartridges.
Phylum Arthropoda (Jointed Animals)
Subphylum Chelicerata
Defining Characteristics:
1. No antennae
2. Body divided into two distinct sections (prosoma and
opisthosoma)
3. First pair of appendages (chelicerae) on prosoma
adapted for feeding.
Phylum Arthropoda (Jointed Animals)
Subphylum Chelicerata
The only arthropods without antennae.
First anterior segment has no appendages at all.
Second anterior segment has pair of clawed appendages
(chelicerae), next to mouth, for grabbing and tearing
food material.
Chelicera also have no mandibles (appendages found next
to mouth in other arthropod groups, used for chewing
and grinding food during digestion).
Phylum Arthropoda (Jointed Animals)
Subphylum Chelicerata
Class Pycnogonida
Defining Characteristics:
1. Body NOT divided into tagmata (distinct regions).
2. Unique proboscis with opening at tip, located at anterior
of animal.
3. Number of walking legs varies among spp.
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Small group of ~ 1000 spp.
Known as “sea spiders”, all spp are marine and have
obviously long legs; from 3 – 16 times the length of the
body.
Body length ranges from few mm (shallow spp) to >10 cm
(deep water spp).
Majority of the body is prosoma. Abdomen (opisthosoma)
reduced to short stump.
Found in all world oceans and have lengthy fossil record
(Ordovician).
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Unlike true spiders, pycnogonids lack special respiratory or
excretory systems.
Do have complete digestive system with sucking mouth at
anterior tip of proboscis.
Both digestive system and gonads extend well into the legs.
Most spp have 4 pairs of walking legs, posterior to pair of
chelicerae and pair of palps.
Some spp have 5 or 6 pairs of walking legs.
The head also holds pair of ovigers used by both sexes
(gonochoristic) to groom other legs and trunk.
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Ovigers also used by males to carry eggs after fertilization.
Unlike most other arthropods, juvenile pycnogonids grow
during both the molt and intermolt stages. The thin
flexible joint membranes stretch as tissue mass increases.
Adults mostly free-living, but extremely slow moving.
Larvae, if leave egg sac before completing development,
grow as parasites on cnidarians.
Many adults and juveniles are parasitic in, or on, or are
commensal with other marine invertebrates, including
gastropods, bivalves, echinoderms and scyphozoans.
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Most adults carnivores, feeding on bryozoans, colonial
hydozoans, poriferans, and anthozoans.
One well-studies sp, Pycnogonum litorale, can be starved for
several months.
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Anoplodactylus evansi
Phylum Arthropoda (Jointed Animals)
Class Pycnogonida
Unidentified fluorescent pycnogonid with fluorescent cyanobacteria
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Defining Characteristics:
1. Appendages on third head segment modified as
mandibles for chewing/grinding food.
2. Retinula of compound eyes contains 8 cells.
3. All members have mandibles on the third head segment
for feeding.
4. Group contains animals with both uniramous and
biramous appendages.
5. Mandibulata have 3 classes: Myrapoda (centipedes);
Insecta; Crustacea.
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Defining Characteristics:
1. Head has 5 pairs of appendages, including 2 pairs of
antennae.
2. Development contains a triangular larval form
(nauplius) with 3 appendages and a single middle eye.
Even spp that hatch at later stage of development pass
through nauplier stage.
~ 45,000 crustacean spp
Most crustaceans divided into 6 subclasses:
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Crustacean Subclasses:
1. Malacostraca
2. Branchiopoda
3. Ostracoda
4. Copepoda
5. Pentastomida (all parasitic)
6. Cirripedia
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
Defining Characteristics:
1. Thorax with 8 segments, abdomen with 6 – 7 segments
plus telson.
2. Appendages on sixth abdominal segment, flattened to
form uropods.
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
This subclass contains ~ 60% of all described crustacean
spp.
Includes the decapods, euphausiids, stomatopods, isopods,
and amphipods.
Basic body plan is tripartite, with head, thorax, and
abdomen.
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
A. Euphuasiida
B. Isopoda
C. Amphipoda
D. Stomatopoda
E. Decapoda
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
Unlike insect head that is separated from thorax by flexible
joint, crustacean head is almost always fused with
thorax, and may be covered by a carapace extending
posteriorly from head.
May function as a cephalothorax.
In some spp carapace may have anterior projection
(rostrum).
Have large stalked compound eyes, as well as 2 pair of
head appendages; 1st (antennules) and 2nd (antennae).
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca
Phylum Arthropoda (Jointed Animals)
Subphylum Mandibulata
Class Crustacea
Subclass Malacostraca