Crustaceans
Chapter 20
Subphylum Crustacea
 Crustaceans,
subphylum
Crustacea typically
have biramous,
branched,
appendages that are
extensively
specialized for
feeding and
locomotion.
Subphylum Crustacea
 Crustacea is
divided into 5
classes.
 Current
molecular
phylogenies
do not
support the
monophyly of
all classes.
Subphylum Crustacea - External
Features
 Secreted cuticle is made of chitin, protein, and
calcareous material.
 Heavy plates have more calcareous deposits joints are soft and thin, allowing flexibility.
 Dorsal tergum and ventral sternum are plates
on each somite lacking a carapace.
 Anterior end is a nonsegmented rostrum and
the posterior end is the unsegmented telson.
Subphylum Crustacea
 Crustaceans are the only
arthropods that have two pairs
of antennae.
 They also have a pair of
mandibles (jaw-like
appendages) and two pairs of
maxillae on the head.
 Each body segment usually has
one pair of appendages.
 Ancestrally biramous except for the
first antennae.
Subphylum Crustacea - Appendages
 Members of Malacostraca and Remipedia have
appendages on each somite.
 Other classes may not bear appendages on abdominal
somites.
Subphylum Crustacea - Appendages
 Appendages have become specialized by
evolving into a wide variety of walking legs,
mouthparts, swimmerets, etc. from modification
of the basic biramous appendage.
Subphylum Crustacea
 The ancestral condition in arthropods is to have many
body segments.
 Fewer segments and increased tagmatization is the
derived condition.
Subphylum Crustacea - Internal
Features
 Muscular and nervous systems and segmentation
exhibit metamerism of annelid-like ancestors.
 Hemocoel - persistent blastocoel that becomes filled
with blood.
Subphylum Crustacea - Muscular
System
 Striated muscles make up a major portion of
crustacean body.
 Most muscles arranged as antagonistic groups.
 Flexors draw a limb toward the body and extensors
straighten a limb out.
Subphylum Crustacea - Muscular
System
 Abdominal flexors of a crayfish allow it to swim
backward.
 Strong muscles located on each side of stomach
control the mandibles.
Subphylum Crustacea - Respiratory
System
 Smaller crustaceans
may exchange
gases across thinner
areas of cuticle.
 Larger crustaceans
use featherlike gills
for gas exchange.
 “Bailer” of 2nd
maxilla draws water
over gill filaments.
Subphylum Crustacea - Circulatory
 Open circulatory system
 Dorsal heart - singlechambered sac of striated
muscle.
 Valves in the arteries prevent
backflow of hemolymph.
 Hemolymph conducted to
gills, if present, for oxygen
and carbon dioxide
exchange.
 Hemolymph may be colorless, reddish, or bluish.
 Hemocyanin (blue) and/or hemoglobin (red) are
respiratory pigments.
Subphylum Crustacea - Excretory
System
 Antennal or maxillary glands are
called green glands in decapods.
 Labyrinth connects by an
excretory tubule to dorsal bladder
that opens to exterior pore.
 Resorption of salts and amino
acids occurs as the filtrate passes
the excretory tubule and bladder.
 Mainly regulates the ionic and
osmotic composition of body
fluids.
Subphylum Crustacea - Excretory
System
 Nitrogenous wastes are excreted across thin areas of
cuticle in the gills.
 Freshwater crustaceans constantly threatened by overdilution with water.
 Gills must actively absorb Na+ and Cl-.
 Marine crustaceans have urine that is isosmotic with
blood.
Subphylum Crustacea - Nervous
System
 Pair of supra-esophageal ganglia connects to eyes and
two pairs of antennae.
 The subesophageal ganglion supplies nerves to mouth,
appendages, esophagus, and antennal glands.
 Double ventral nerve cord has a pair of ganglia for each
somite to control appendages.
Subphylum Crustacea - Sensory
System
 Eyes and statocysts are the largest sensory
organs.
 Tactile hairs occur on the body, especially on
chelae, mouthparts and telson.
 Chemical sensing of taste and smell occurs in
hairs on antennae and mouth.
 Statocyst opens at base of first antenna in
crayfish.
 Statocyst lined with sensory hairs that detect position
of grains of sand.
Subphylum Crustacea - Sensory
System
 Compound eyes are
made of many units
called ommatidia.
 Cornea focuses light
down the columnar
ommatidium.
 Each ommatidium detects a restricted area of objects, a
mosaic, in bright light.
 In dim light, the distal and proximal pigments separate
and produce a continuous image.
Subphylum Crustacea - Diversity of
Reproduction
 Barnacles are monoecious but generally crossfertilize.
 In some ostracods, males are scarce and
reproduction is by parthenogenesis.
Subphylum Crustacea - Diversity of
Reproduction
 Most crustaceans brood eggs in brood chambers, in
brood sacs attached to the abdomen, or attached to
abdominal appendages.
 Crayfishes develop directly without a larval form.
Subphylum Crustacea - Diversity of
Reproduction
 Most
crustaceans
have a larva
unlike the adult
in form, and
undergo
metamorphosis.
 Gulf shrimp
Subphylum Crustacea - Diversity of
Reproduction
 The nauplius is a common larval
form with uniramous first antennae,
and biramous second antennae and
mandibles that all aid in swimming.
 Appendages and somites are added in a
series of molts.
 Metamorphosis of a barnacle
proceeds from a free-swimming
nauplius to a cypris larva with a
bivalve carapace and finally to a
sessile adult with plates.
Subphylum Crustacea Ecdysis
 Ecdysis is necessary
for a crustacean to
increase in size – the
exoskeleton does not
grow.
 Physiology of molting
affects reproduction,
behavior, and many
metabolic processes.
 Underlying epidermis
secretes cuticle.
Subphylum Crustacea Ecdysis
 Molting animals grow in the intermolt phases, or
instars.
 Soft tissue increases in size until there is no space
within the cuticle.
 Molting occurs often in young animals and may cease
in adults.
Subphylum Crustacea Ecdysis
 Hormonal Control of Ecdysis:
 Temperature, day length, or other stimuli trigger central
nervous system to begin ecdysis.
Subphylum Crustacea - Endocrine
Functions
 Removing eyestalks accelerates molting and prevents
color changes to match background.
 Hormones from neurosecretory cells in eyestalk control
dispersal of cell pigment.
Subphylum Crustacea - Feeding
Habits
 Same fundamental mouthparts in various crustaceans
are adapted to a wide array of feeding habits.
 Suspension feeders generate water currents in order to
feed on plankton, detritus ,and bacteria.
 Predators consume larvae, worms, crustaceans, snails,
and fishes.
 Scavengers eat dead animal and plant matter.
Subphylum Crustacea - Feeding
Habits
 Crayfishes have a two-part stomach.
 Gastric mill grinds up food in 1st compartment.
Oligostraca
 Clade
Oligostraca
includes
 Mystacocarida
 Ostracoda
 Branchiura
 Pentastomida
Class Ostracoda
 Ostracods are enclosed in a two part carapace and
look a bit like a clam.
 Marine or freshwater.
 Mostly benthic.
Class Maxillopoda – Subclass
Branchiura
 Members of the
subclass Branchiura
lack gills.
 Most are
ectoparasites of
marine and
freshwater fish.
 5–10 mm long.
 Development is
direct.
Class Maxillopoda – Subclass
Pentastomida
 Subclass Pentastomida tongue worms.
 Consist of about 90
species of parasites of
vertebrate respiratory
systems.
 Most infect reptile lungs, a
few infect air sacs of birds
or mammals.
 Range from 1 to 13 cm in
length.
 Chitinous cuticle regularly
molted.
Xenocarida
 Clade
Xenocarida
includes
 Remipedia
 Cephalocarida
Class Remipedia
 Only 10 described species in
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Class Remipedia.
All found in caves connected
to the sea.
Primitive features include 25–
38 segments with similar,
paired, biramous, swimming
appendages.
Antennules also biramous.
Maxillae and maxillipeds are
prehensile and specialized for
feeding.
Swimming legs are directed
laterally rather than ventrally
as is found in copepods and
cephalocarids.
Class Cephalocarida
 Only 9 species described
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in Class Cephalocarida.
Live in coastal bottom
sediments from intertidal
zones to 300 meters depth.
Thoracic limbs and 2nd
maxillae are very similar.
Lack eyes, a carapace,
and abdominal
appendages.
True hermaphrodites and
unique in discharging eggs
and sperm through same
duct.
Vericrustacea
 Clade
Vericrustacea
includes
 Branchiopoda
 Copepoda
 Thecostraca
 Malacostraca
Class Branchiopoda
 Includes three orders:
 Anostraca – fairy
shrimp and brine
shrimp, no carapace.
 Notostraca – tadpole
shrimp, carapace forms
a large dorsal shield.
 Diplostraca – water
fleas – carapace
encloses body but not
head.
Class Branchiopoda
 Phyllopodia – legs that serve as respiratory organs.
 Legs may be used for filter feeding and locomotion as
well.
 Mostly freshwater forms.
Class Branchiopoda
 Water fleas (like Daphnia) produce females
parthenogenetically in summer. Males are produced
when unfavorable conditions arise and overwintering
fertilized eggs are produced that are resistant to cold
and desiccation.
Class Copepoda
 Planktonic crustaceans
include many species of
copepods which are among
the most numerous of all
animals.
 They lack a carapace.
 Retain the simple
maxillopodan eye in adults.
 Antennules used in
swimming.
 Very diverse.
Class Copepoda
 Parasitic forms highly modified and reduced - often
unrecognizable as arthropods.
 Free-living copepods may be the dominant
consumer.
 Marine copepod Calanus is most abundant
organism in zooplankton by biomass.
 Cyclops and Diaptomus important elements of
freshwater plankton.
 Some free-living copepods are intermediate hosts
of human parasitic tapeworms and nematodes.
Class Thecostraca
 Barnacles – class Thecostraa– are a group of
mostly sessile crustaceans whose cuticle is
hardened into a shell.
Class Thecostraca
 Their legs are long,
many jointed cirri
that extend out
through the
calcareous plates to
filter feed.
Class Thecostraca
 Barnacles are hermaphroditic.
 Most hatch as a nauplius
larva then become a cyprid
larva (resembles the ostracod
Cypris).
 Cyprids attach to the
substrates and begin secreting
calcareous plates.
Class Thecostraca
 Parasitic forms may have a kentrogon stage that injects
cells into the hemocoel of host.
Class Malacostraca
 Largest and most diverse class of Crustacea with over
20,000 species.
 Contains three subclasses, 14 orders, and many
suborders.
Class Malacostraca
 Malacostracans
usually have a
head with 5 fused
segments, a thorax
with 8 segments
and an abdomen
with 6.
 Anterior rostrum
 Posterior telson
Class Malacostraca – Order Isopoda
 Order Isopoda – including pill bugs.
 Only truly terrestrial crustaceans.
 Also have marine and freshwater forms.
 Dorsoventrally flattened, lack a carapace, and
have sessile compound eyes.
 Compressed dorsoventrally.
Class Malacostraca – Order
Amphipoda
 Order Amphipoda – many marine, terrestrial &
freshwater forms.
 Amphipods resemble isopods:
 Lack a carapace, have sessile compound eyes, and
one pair of maxillipeds.
 However, they are compressed laterally.
 Development is direct.
Class Malacostraca – Order
Euphausiacea
 Order Euphausiacea contains approximately 90
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species.
Includes important ocean plankton called krill.
Most are bioluminescent with a light-producing organ
called a photophore.
Form a major component of the diet of baleen whales
and of many fishes.
Eggs hatch as nauplii.
Class Malacostraca
 Decapods – order decapoda – are all
relatively large crustaceans and include
lobsters, crabs, crayfish, and shrimp.
 3 pairs maxillipeds & 5 pairs walking legs.
Class Malacostraca
 Harder, heavy plates in larger crustaceans due
to calcareous deposits in addition to chitin.
 The carapace covers much or all of the
cephalothorax.
Phylogeny
 Remipedia appear to be the most primitive of
Crustacea.
 Two pairs of uniramous limbs on each segment.
 One theory is that each modern somite represents two
ancestral somites that fused together, forming the
biramous appendage.
 Recent genetic studies show that modulation of genes
determine the location of distal ends of arthropod limbs
such that ancestral crustaceans were biramous while
uniramous is derived state
Adaptive Diversification
 Crustaceans are unquestionably the dominant
arthropod in marine environments.
 They also share dominance in freshwater
environments with the insects.
 The class Malacostraca is most diverse and
members of Copepoda are most abundant.
Classification
 Class Remipedia
 Class Cephalocarida
 Class Branchiopoda
 Order Anostraca
 Order Notostraca
 Order Cladocera
Phylogeny and Adaptive
Diversification
 Class Ostracoda
 Class Maxillopoda
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Subclass Copepoda
Subclass Tantulocarida
Subclass Branchiura
Subclass Pentastomida
Subclass Cirripedia
Phylogeny and Adaptive
Diversification
 Class Malacostraca
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Order Isopoda
Order Amphipoda
Order Euphausiacea
Order Decapoda