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Biology of Animals
ZOO 2040
Topic 11.2
Arthropods II: Crustaceans
Subphylum Crustacea
Mandibulata
■ 
About 45,000 spp. Main distinguishing characteristic is 2 pairs of antennae:
• The head also has a pair of mandibles and two pairs of maxillae.
• There is one pair of appendages on each of the additional somites, but some somites
may lack appendages.
• All appendages, except perhaps the first antennae, are biramous (they have two main
branches).
• Primitive crustaceans may have up to 60 somites; derived crustaceans have fewer
segments.
The tagmata are usually a head, thorax and abdomen, but they are not
homologous across taxa.
■  The arrangement of tagmata in!
Malacostraca is the ancestral plan:
■ 
• The head has 5 fused somites, the!
thorax has 8 and the abdomen has 6.
• The anterior end is a non-segmented!
rostrum.
• The telson, with the last abdominal!
somite and its uropods, forms a ‘tail’.
■ 
Two “Subphyla”: Crustacea (biramous appendages) and Uniramia
Body Form and Function
Form and Function
External Features
Appendages
The secreted cuticle is!
made of chitin, protein!
and calcareous material.
■  Heavy plates have!
more calcareous!
deposits; the joints!
are soft and thin,!
allowing flexibility.
■  A dorsal tergum and!
ventral sternum are!
plates on each somite!
lacking a carapace.
■  The telson is not a somite; it bears the anus and may be homologous to the
pygidium.
■  In some species, the telson may bear a pair of processes, the caudal furca.
■  Gonopores may be at the base of appendages, at the tail, or on somites
without legs.
■ 
■ 
Members of Malacostraca and Remipedia have!
appendages on each somite:
• Other classes may not bear appendages on the abdominal somites.
■ 
Specialization of appendages is based on the basic!
biramous plan:
• The maxilliped has a basal protopod, a lateral exopod and a medial endopod.
• On crustacean limbs, endites are!
medial processes, exites are lateral!
processes, and the epipod is an!
exite on the protopod (Table 19-1).
■ 
Circulation
■ 
There is a dorsal covering (‘carapace’)!
that may cover most of the body or just!
the cephalothorax.
Crayfish appendages represent!
serial homology; they have!
evolved a wide variety of!
walking legs, mouthparts,!
swimmerets, etc. from!
modifications of the basic!
biramous appendage.
Circulation and Respiration
The major body space in!
arthropods is not a coelom!
but a blood-filled hemocoel:
Afferent sinus chambers carry hemolymph to the gills (if present) for
oxygen and carbon dioxide exchange.
Smaller crustaceans may exchange!
gases across thinner areas of cuticle!
on their legs, etc.
■  Larger crustaceans use featherlike!
gills for gas exchange.
■  The decapod carapace overlaps the!
gill cavity, leaving anterior and!
posterior openings.
■  The “bailer” of the second maxilla!
draws water over the gill filaments.
■  Gills may project from the pleural!
wall, the articulation of the thoracic legs, or thoracic coxae.
■  Hemocyanin (a copper-based) and/or hemoglobin are respiratory pigments
(in solution in the blood).
■  Hemolymph is typically colorless. It contains at least two types of!
ameboid cells that may help prevent disease and excess bleeding.
■ 
■ 
• Vestigial coelomic sacs are lost!
within the space between!
mesoderm, ectoderm and yolk.
• The spaces that develop as hemocoel are not!
lined by mesodermal peritoneum.
• In crustaceans, coelomic compartments remain as!
end sacs of excretory organs and space around the gonads.
In contrast with annelids, crustaceans and other arthropods have an open
circulatory system; there is no venous system or separation of blood from
interstitial fluid.
■  Dorsal heart is a single-chambered sac of!
striated muscle:
■ 
• Hemolymph (blood) leaves the heart by arteries!
(valves in the arteries prevent backflow), washes!
through the hemocoel and returns to pericardial!
sinuses (spaces) before returning to the heart.
• Small arteries empty into tissue sinuses that!
discharge into the large sternal sinus.
Page 1
Biology of Animals
ZOO 2040
Topic 11.2
Arthropods II: Crustaceans
Excretory System
Neuromuscular Systems
Antennal or maxillary glands open at the base!
of those structures.
■  Decapods have antennal glands called green glands:
Muscular and nervous!
systems and segmentation!
show the metamerism of annelid-!
like body.
■  Striated muscles make up a!
major portion of the crustacean body.
■  Most muscles are arranged as antagonistic!
groups; flexors draw a limb toward the body, extensors move it away:
■ 
■ 
• The end sac of an antennal gland has a small vesicle and a!
spongy labyrinth.
• Labyrinth connects by an excretory tubule to a dorsal!
bladder that opens to the exterior.
• Hydrostatic pressure within a hemocoel provides a force!
for filtration of fluid into the end sac.
• Abdominal flexors of a crayfish allow it to swim backward.
Resorption of salts, amino acids, glucose and some water occurs as the
filtrate passes through the excretory tubule and bladder, and is finally
excreted as urine.
■  Nitrogenous wastes, mostly ammonia, are excreted across thin areas of
cuticle in the gills.
■  Freshwater crustaceans are constantly threatened with over-dilution
with water. Gills must actively absorb Na+ and Cl–. This is a MAJOR
energy investment.
■  Marine crustaceans have urine that is isosmotic with the blood.
■ 
Strong muscles located on each side of the stomach control the
mandibles.
■  The nervous system of crustaceans has more fusion of ganglia than is
found in other arthropods:
■ 
• A pair of supra-esophageal ganglia connects to the eyes and the two pairs of antennae.
• Neuron connectives join this brain to the subesophageal ganglion, a fusion of at least 5
pairs of ganglia that supplies nerves to the mouth, appendages, esophagus, and
antennal glands.
• The double ventral nerve cord has a pair of ganglia for each somite and nerves to
control the appendages, muscles, and other body parts.
Sensory Organs
Reproduction
Crustaceans have more developed sense organs than do annelids.
■  Eyes and statocysts are the largest sensory organs:
■ 
■ 
• A statocyst opens at the base of each first antenna in crayfishes (lined with
sensory hairs that detect the position of grains of sand inside).
■ 
Eyes are compound, made of many units called ommatidia (Fig.
19.8):
• These larvae have uniramous first!
antennae, biramous second!
antennae, and mandibles (all of!
which aid in swimming).
• Appendages and somites are!
added in a series of molts.
• Crayfishes develop directly without a larval form.
• In some ostracods, males are scarce and reproduction is by!
parthenogenesis.
• Each ommatidium behaves like a tiny eye. Each ommatidium only sees a
restricted area of vision, forming a mosaic, in bright light (like an LED diplay
on a cell phone or TV).
• A transparent layer of cuticle (cornea) divided into many squares or hexagons
(facets) focuses light down the columnar ommatidium.
• Distal retinal, proximal retinal and reflecting pigment cells form a sleeve
around each ommatidium. In dim light, the distal and proximal pigments
separate and produce a continuous image.
Most crustaceans hold their eggs in ‘brood sacs‘ attached to the!
abdomen or in ‘brood chambers’ (e.g., terrestrial isopods).
■  Barnacles are monoecious but generally cross-fertilize.
■ 
Tactile hairs occur on the body, especially on the chelae, mouthparts
and telson.
■  Chemical sensing of taste and smell occurs in hairs on the antennae
and mouthparts.
■ 
• Metamorphosis of a barnacle proceeds from a free-swimming nauplius!
to a larva with a bivalve carapace called a cyprid, and finally turns into!
a sessile adult.
Ecdysis
Once initiated, ecdysis!
is irreversible and!
needs no further!
hormonal control
Molting is necessary for a crustacean to!
increase in size as the exoskeleton does!
not grow.
■  The physiology of molting affects!
reproduction, behavior and many!
metabolic processes.
■  The underlying epidermis secretes the cuticle:
■ 
Epidermal cells enlarge!
before ecdysis.
■  They secrete a new!
epicuticle and begin!
secreting a new exocuticle.
■  Enzymes released into!
the area above new!
epicuticle dissolve the!
old endocuticle.
■  Some calcium salts reabsorbed into the blood!
and are are stored in gastroliths in the walls!
of the stomach.
■  When only the old exocuticle and epicuticle!
remain, the animal swallows water to expand.
■  The soft new cuticle (Mmm! Soft-shelled!
crabs!) stretches and then hardens with the!
redeposition of inorganic salts.
■ 
• An outermost epicuticle is very thin lipid-impregnated protein.
• Most of the cuticle is several layers of the procuticle.
• The exocuticle is beneath the epicuticle and contains protein, calcium salts and
chitin.
• The endocuticle has a heavily calcified principal layer and an uncalcified
membranous layer next to a layer of epithelial cells.
■ 
Most crustaceans have a!
primitive nauplius larva that is!
unlike the adult in body form!
and ecology, and which undergoes!
metamorphosis:
Molting occurs often in young animals, and may cease in adults:
• Temperature, day length or other stimuli trigger the central nervous system to begin
ecdysis.
• The central nervous system decreases production of molt-inhibiting hormone by a
group of neurosecretory cells (X-organ) in the medulla terminalis of the brain.
• This promotes release of molting hormone from the Y-organs (which are beneath the
epidermis neat the adductor muscles of the mandibles), initiating ecdysis.
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ZOO 2040
Biology of Animals
Topic 11.2
Arthropods II: Crustaceans
Crustacean Diversity
Other Endocrine Functions
Removing eyestalks accelerates molting and prevents color changes to
match the substrate (hormones from neurosecretory cells in the eyestalk
control dispersal of cell pigments).
■  Neurosecretions from the pericardial organs cause an increase in heartbeat.
■  Androgenic glands in male amphipods stimulate expression of male
characteristics.
■ 
Class Remipedia
A small class of 10 recently-described species; all are found in!
caves connected to the sea in the Bahamas.
■  Primitive features include 25-38 segments with similar, paired,
biramous, swimming appendages. Antennules are also biramous.
■  Maxillae and maxillipeds are prehensile and specialized for!
feeding.
■  Shape of swimming legs are similar to copepods and!
cephalocarids, but they are directed laterally rather than ventrally.
■ 
Feeding Habits
The same basic mouthparts are adapted to a wide array of feeding habits.
Suspension feeders generate!
water currents in order to eat!
plankton, detritus, and bacteria.
■  Predators consume larvae,!
worms, crustaceans, snails!
and fishes.
■  Many are scavengers.
■  Crayfishes have a two-part!
stomach; a gastric mill!
grinds up food in the!
first compartment.
■ 
■ 
Class Cephalocarida
Another small class (9 spp.).
Live in coastal bottom sediments from intertidal!
zones to 300 m depth.
■  Thoracic limbs and second maxillae are very similar.
■  Lack eyes, a carapace or abdominal appendages.
■  They are true hermaphrodites and unique in discharging eggs and
sperm through the same duct.
■ 
■ 
Crustacean Diversity
Class Ostracoda (“mussel shrimps”)
Class Branchiopoda
■ 
About 6,000 spp.
Body enclosed in a bivalve carapace,!
resemble tiny (0.25-10 mm long) clams.
■  Considerable fusion of trunk somites:!
thoracic appendages are reduced to!
one or two.
■  Development is by gradual!
metamorphosis..
■  Widespread in both marine and freshwater habitats; important
in aquatic food webs.
■  Most live on the bottom or climb onto plants, but some are!
planktonic, parasitic, or burrowing.
■  Most are diecious, but some are are parthenogenetic. Males of
some species emit light flashes to attract female.
■  More than 10,000 fossil spp. are known and are important
‘index fossils’ for locating petroleum deposits.
800 spp. In four orders:
■ 
■ 
• Order Anostraca includes fairy shrimp!
and brine shrimp that lack a carapace.
• Order Notostraca includes tadpole shrimp;!
the carapace forms a large dorsal shield.
• Order Conchostraca includes clam shrimp!
enclosed by a bivalved carapace.
• Order Cladocera includes water fleas. The!
carapace encloses the body but not the head.
■ 
All have flattened and leaf-like legs that are the!
main respiratory organs:
• Legs assist in suspension feeding and, except for cladocerans, function in
locomotion.
■ 
Most are freshwater. Cladocerans are important freshwater
zooplankton:
• Similar to rotifers, many use parthenogenesis to rapidly boost summer
populations and then use fertilized eggs, which are resistant to cold and dryness
and are critical for winter survival of the population.
Subclass Copepoda
Class Maxillopoda
■ 
Includes a number of groups that are descended
from a common ancestor:
■ 
■ 
• The general body plan has five cephalic, six thoracic and
four abdominal somites plus a telson. Reductions from this
body plan are common; there are no appendages on the
abdomen.
• When present, the eye of the nauplius is unique in structure
and is called a maxillopodan eye.
Small (only a few mm in length).
Lack a carapace and retain the simple, median,!
nauplius eye in the adult:
• A major joint separates the posterior from the anterior,!
appendage-bearing portion of the body.
• Single pair of uniramous maxillipeds and four pairs of!
flattened, biramous, thoracic swimming appendages.
• Antennules are often longer than other appendages.
■ 
Free-living copepods may be the dominant!
primary consumers on Earth:
• The marine copepods (Calanus spp.) are estimated to be the most abundant
organism in the zooplankton by biomass.
• Cyclops and Diaptomus form important elements of freshwater plankton.
Subclass Mystacocarida
Tiny crustaceans (<0.5 mm!
long live in interstitial water!
between sand grains.
■ Ten species have been!
described worldwide.
■ 
■ 
■ 
■ 
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Parasitic forms are highly modified and reduced, often unrecognizable
as arthropods.
Some free-living copepods are intermediate hosts of animal and human
parasitic tapeworms and nematodes.
Development is indirect and some highly modified parasites have
unusual metamorphoses.
ZOO 2040
Biology of Animals
Topic 11.2
Arthropods II: Crustaceans
Subclass Pentastomida (“five-mouth”)
Subclass Tantulocarida
Only recently been described; 12 spp.
■  Tiny copepod-like ectoparasites of deep-sea!
benthic crustaceans. There are no head!
appendages except for one pair of antennae in!
sexual females.
■  They likely alternate between a parthenogenetic!
cycle and a bisexual cycle with fertilization.
■  Tantalus larvae penetrate the cuticle of the host by a mouth tube.
■  Abdomen and all thoracic limbs are lost during metamorphosis to adult.
“Tongue worms”. About 90 spp. Formerly considered!
to be a separate phylum, and another ‘missing!
link’ between annelids and arthropods.
■  Range from 1- 13 cm in length.
■  Five protuberances on the anterior end. Four!
have pairs of chitinous claws; the 5th has the!
mouth and two pairs of hooks for attachment.
■ 
■ 
Body covered by a non-chitinous cuticle tha!
is regularly molted (rings produce a!
segmented appearance).
■  Striated muscles are arranged metamerically.
■  Large hemocoel and larvae often with 3 pairs of!
appendages, but no leg joints (adults are legless).
■  Parasites of vertebrate respiratory systems:
■ 
Subclass Branchiura
Lack gills (they breathe through their!
legs (hence their name); mostly parasites!
of marine and freshwater fishes (‘fish lice’).
■  5-10 mm long, with a broad, shield-like!
carapace, compound eyes, four biramous!
thoracic swimming appendages, and a!
short unsegmented abdomen. 2nd!
maxillae are modified as suction cups.
■  Development is direct with no nauplius stage.
■ 
• Most infect lungs and nasal cavities of snakes, lizards, and crocodiles.
• A few infect air sacs of birds or lungs of mammals.
• One species is known to infect humans.
■ 
Pentastomida (cont.)
Subclass Cirripedia
Includes barnacles in order Thoracica and three!
orders of burrowing or parasitic species. Adults!
are sessile and attach directly (acorn barnacles)!
or by a stalk (gooseneck barnacles). Nearly 1000!
spp. All marine.
■  Carapace surrounds the body and secretes a set!
of calcareous plates. Intertidal barnacles may be!
exposed to drying; the plates close to protect them.
■  Head reduced, abdomen absent, and long thoracic legs have hair-like setae:
■ 
Simple straight digestive system is adapted for sucking blood. No
circulatory, excretory, or respiratory organs.
■  Nervous system has paired ganglia along the ventral nerve cord
■  Dioecious: females are larger than males.
■  Female produces millions of eggs.
■  Encapsulated embryos are lost with the host’s nasal secretions and saliva,
or pass up the host’s trachea, are swallowed and leave the host with its
feces.
■  Larvae are oval-shaped, with 4-6 stumpy legs and a tail.
■  Most life cycles require an intermediate vertebrae host, usually a fish or
reptile. Insects can act as obligate intermediate hosts.
■  After ingestion by the correct intermediate host, the larva completes
development, tears through the capsule wall, penetrates the intestine, and
migrates to a specific tissue, where it undergoes up to 9 molts before until it
changes into a nymph.
■  Nymph eventually becomes encapsulated and remains dormant until the
host is eaten, emerging as a juvenile.
■  Juvenile finds its way to the lung, feeds on blood and tissue, and matures.
■  These parasites (with insects as intermediate hosts), have been found to
limit the population growth of geckos and other lizards.
■ 
•  Many-jointed cirri that bear the setae are extended from the plates to feed on small
particles.
■ 
Order Isopoda
The only truly terrestrial crustaceans. Also live in the!
oceans and freshwater. >10,000 spp.
■  Common land forms include the sow bugs!
and pill bugs (AKA, “woodlice”):
■ 
Largest class of crustaceans (75% of all crustacean species) and is the most
diverse: 3 subclasses, 14 orders, many suborders, infraorders, and
superfamilies:
• Contains the only wholly terrestrial species (isopods).
• Decapods include many economically important species: lobsters, crabs, shrimps, and
crayfish.
• Krill are an essential food for many marine species.
■ 
Hermaphroditic and metamorphic:
•  Most hatch as nauplii and become cyprid!
larvae with a bivalve carapace and!
compound eyes.
•  Attach to the substrate by their first antennae!
and adhesive glands. They then secrete!
calcareous plates, lose eyes and change!
swimming appendages to filtering cirri.
•  Parasitic forms may have a kentrogon!
stage that injects cells into the host’s!
hemocoel.
Class Malacostraca
■ 
More common in tropical regions, but also in SE North America, Europe
and Australia.
• Armadillidium vulgaris has been introduced in!
many parts of the world.
• Accumulate high levels of metals.
They are dorsoventrally flattened, lack a!
carapace and have sessile (non-stalked)!
compound eyes.
■  The first pair of thoracic limbs are maxilli-!
peds; remaining thoracic limbs lack exopods.
■  Abdominal appendages bear gills, except!
for the uropods.
■  Cuticle lacks the protection of insect cuticle; must live in!
moist conditions.
■  Some are highly modified as parasites of fishes, cephalopods,!
or crustaceans.
■  Development typically direct but may be metamorphic in!
parasitic forms. In some, males live attached to larger females.
■ 
Enormous diversity of forms, but basic body plan is tripartite : head, thorax,
and abdomen, and appendages are generally biramous:
• Head and thorax nearly always fused as a cephalothorax, generally covered by a
carapace.
• Often the head has a pointed anterior projection (rostrum).
• Generally with large, stalked compound eyes.
• Head has 2 pairs of antennae (1st pair are antennules, 2nd pair are antennae), and 3
pairs of feeding and/or respiratory appendages: mandibles (for crushing food) and 1st
and 2nd maxillae (generate water currents and manipulate food).
• 8 thoracic segments, usually with 1st, 2nd, and 3rd maxillipeds (also manipulate food)
and 5 pairs of walking legs (pereopods), the first 3 pairs of which may be chelate.
• 6 abdominal segments, each with pair of appendages (pleopods) for swimming and/or
generating respiratory currents and brooding eggs in females. Last pair of appendages
are the uropods (which lie on either side if the telson, forming a tail).
Page 4
ZOO 2040
Biology of Animals
Topic 11.2
Arthropods II: Crustaceans
Order Amphipoda
■ 
Order Decapoda
Resemble isopods; they lack a!
carapace, have sessile compound!
eyes, and one pair of maxillipeds:
Decapods have three pairs of maxillipeds and five!
pairs of walking legs, the first forming chelipeds.
■  They range from a few millimeters to the largest!
arthropod, a Japanese crab with a 4 meter leg span.
■  About 10,000 species of decapods in 100 families!
are known, including: crayfishes, lobsters, crabs, and true shrimps.
■  Crabs are particularly diverse, have a broader cephalothorax and reduced
abdomen, compared to crayfish or lobsters:
■ 
• Compressed laterally with gills in!
the thoracic position.
• Abdominal and thoracic limbs are grouped for jumping and swimming.
Many are marine; others are beach-dwelling, freshwater, or parasitic. 6,000
spp.
■  Development is direct.
■ 
Order Euphausiacea
■ 
• Many species are largely terrestrial need only remain near!
salt water to reproduce.
• Giant crabs of some mid-Pacific Islands cut down and!
eat coconuts.
• Hermit crabs have elongated abdomens with hooks that!
anchor their posterior in gastropod shells.
• Decorator crabs camouflage their exoskeletons with!
detritus or marine life (including sponges and anemones).
Only about 90 spp., but includes!
important marine zooplankton (“krill”):
• Krill form a major part of the diet of baleen whales, many seabirds (e.g., penguins),
fishes (including whale sharks, basking sharks manta rays), and some seals.
Carapace does not completely enclose the gills.
■  Lack maxillipeds; all limbs have exopods.
■  Most are bioluminescent with a light-producing!
organs called photophores that contain light-!
producing bacteria.
■  Eggs hatch as nauplii and development is direct.
■ 
■ 
• Shrimp trawls kill 10 kg of marine life for each kg!
of shrimp that are caught.
• Shrimp aquaculture damages coastal ecosystems.
Phylogeny and Adaptive Radiation
■ 
True shrimps are popular foods, but consuming!
them has environmental consequences:
The monophyly of crustaceans is controversial:
• Crustacea have been allied with uniramians in the Mandibulata because both
have mandibles.
• Critics consider the mandibles to be different and not homologous.
• Both, however, share similar ommatidia, a tripartite brain and a five-somite head.
Remipedia appear to be the most primitive of Crustacea.
Fossils of an arthropod from the Mississippian are likely the sister
group to remipedians.
■  One theory is that each modern somite represents two ancestral somites
that fused together.
■  Crustaceans are unquestionably the dominant arthropods in marine
environments. They also share dominance in freshwater environments
with the insects
■  The class Malacostraca is most diverse, but members of the Copepoda
are most numerically abundant.
■  Copepods are particularly successful as parasites of both vertebrates
and invertebrates.
■ 
■ 
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