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Biology 320
Invertebrate Zoology
Fall 2005
Chapter 19 – Phylum Arthropoda,
Subphylum Crustacea
Introduction to Crustaceans
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42,000 species
Crabs, shrimps, lobsters, and
barnacles
Important ecologically
• Zooplankton
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Mainly marine, but some FW and
a few terrestrial varieties
Mandibulates, along with
myriapods and insects
Two pairs of antennae
Body Form
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Two tagma in most
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Two pairs of antennae
• 1st and 2nd antennae
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Mandibles
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Two pairs of maxillae
• 1st and 2nd maxillae
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Lateral compound
eyes
• Often on eyestalks
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Pereopods
• Walking legs
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Pleopods
• Swimmerets
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Uropods
• Make up tail fin
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Exoskeleton
typically
mineralized
• Calcified
Nutrition and Digestive System
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Many strategies; feed on
suspended particles,
detritus, carrion, plants
and/or animals
Possess several
mouthparts
• Usually have gnathobases
• Coxae of appendages
forms a food groove
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Many use a structure called
a setal comb to filter feed
• Often located on feeding
appendages
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Most have J – shaped
gut
Often have a grinding
stomach called a
gastric mill
Midgut has digestive
ceca termed the
hepatopancreas
Intestines form and
remove wastes
• Also reclaim water
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Anus located at base
of telson
Other Organ Systems
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Small species don’t require
complex organ systems
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Hemal
• Typical of arthropods
• Hemocyanin or hemoglobin
dissolved in plasma
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Respiratory
• Gills located in branchial
chamber
• Inhalant and exhalant
chambers
• Gill bailer (scaphognathite)
ventilates
• Many strategies for cleaning
gills and removing sediment
from branchial chamber
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Excretory system
• Paired saccate nephridia
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Located in segment with 2nd antennae or 2nd maxillae
Called antennal, maxillary, or green glands
Nephridiopores open near 2nd antennae/maxillae
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Most marine species are osmoconformers
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Freshwater species are osmoregulators
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Ammonotelic, so nitrogen diffuses across
gills
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Nervous system is typical
of arthropods
• Eyes
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Often compound with as
many as 14,000 ommatidia
Often at the end of a
movable eyestalk, which
greatly increases the field
of view
Some can distinguish colors
• Many use chromatophores
to match color of
surroundings
• Statocysts and
chemosensors
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Often associated with
antennae
Animals frequently clean
their antennae
Reproduction
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All but barnacles are
gonochoric
Usually copulation
with internal
fertilization
Penis or gonopods
transfer sperm
Appendages such as
2nd antennae are
often used to grasp
female
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Sometimes males
cannot mate with
females until after
she molts
Most brood eggs
attached to
appendages or in
brood chambers
Direct or indirect
(nauplius larva)
development
Class Anostraca
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Fairy shrimp and brine shrimp
(sea monkeys)
Live in ephemeral pools
• Typically saline
• Lack fishes
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15 – 30 mm, but some grow as
large as 10 cm
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Lack carapace
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Sexually dimorphic
• Males have large 2nd antennae
for grasping females
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Stalked compound eyes
Possess setal combs for
suspension feeding
Swim upside down
Many tolerate a wide
range of salinities
Sperm transfer is direct
• Internal fertilization
Class Phyllopoda
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800 species
Inhabit freshwater
habitats that are
similar to those of
anostracans
Two main types
• Large - tadpole
shrimps
• Small - water fleas
Tadpole Shrimps
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Triops is most famous genus
• Ten species altogether
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Enormous shield-like carapace
covers most of the animal
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Most are small, but some reach
10 cm in length
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Amazing physiology
• Dr. Carl Reiber’s lab has
conducted research using Triops
as a model
• Survive in hypoxia and anoxia
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Hemoglobin subunit recruitment
• Produce two types of eggs
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Summer eggs- thin shelled and
hatch rapidly
Winter eggs - undergo diapause
Water Fleas
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Also called cladocerans
• Daphnia is most famous genus
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Often live in ponds and lakes
where they make up a large
portion of FW zooplankton
community
Swim using enlarged 2nd
antennae as oars
Undergo cycles of
parthenogenesis and bisexual
reproduction
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Typically brood eggs
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Two types of eggs produced
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Undergo cyclomorphosis
• Seasonal changes in
morphology
• Head shape, spines, neck
teeth produced
• Evidence suggests these
changes are inducible
responses
Class Malacostraca
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Many important
orders of crustaceans
Crabs, crayfish, and
shrimps
23,000 species
Tagmatization is
standardized
Order Stomatopoda
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300 species of mantis
shrimps
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Predators of fish, crabs,
shrimps, and molluscs
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Raptorial claws are
extended and retracted
rapidly to capture prey
Best developed compound
eyes of any crustacean
• Some even have depth
perception
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5 cm – 36 cm long
Can be brightly
colored
Many live in burrows
Often pair up with one
mate for life
Squilla is most famous
genus
Order Decapoda
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10,000 species of
shrimps,
crayfishes,
lobsters, and crabs
Important
ecologically and
economically
All have 10 legs,
hence the name
Shrimp-like Decapods
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Laterally compressed
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Thin flexible exoskeleton
(uncalcified)
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Muscular abdomen that
can be used for
escaping (tail flip)
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Large pleopods for
swimming
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Benthic
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Penaeus is a common
genus
Snapping shrimp
• Synalpheus
• Has one extremely
large cheliped
• One movable finger is
cocked
• When released the force
generated can stun
prey or crack a clam
shell
• Also makes a loud noise
Lobster-like Decapods
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More dorsoventrally
flattened
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Heavier legs
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Chelipeds
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Muscular abdomen
• Tail flips
• Small pleopods
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Homarus and
Procambarus are famous
genera
Crab-like Decapods
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Most dorsoventrally
compressed of the three
groups
Abdomen is greatly
reduced
• Hermit crabs have a large
abdomen that is soft and is
housed in a shell
• Periodically need to find
larger shells
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Typically walk sideways
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Some like Callinectes can
swim
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Eggs brooded
between the
cephalothorax
and abdomen
Types of crabs:
• Alaskan king
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Probably the
largest crab
• Mole crabs
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Lack chelipeds
Burrow in sand
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Japanese spider crab
• Greatest leg span of any
arthropod
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Decorator crabs
• Have hooked setae
• Attach things to its
exoskeleton
• Aids in protection and
camouflage
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Dungeness crab
• Cancer magister
• Probably the most popular
food crab
Freshwater Decapods
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Crayfish
• The most successful with 400
species, worldwide
• Many live under stones and
some burrow
• Procambarus is an invasive
species in Western US
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Shrimps
• Most are tropical
• A few species in the US
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Crabs
• Usually must return to the sea
at some point to release larvae
• Chinese mitten (or river) crab is
an invasive in CA
Terrestrial Crabs
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Adapted for life on land
• Morphological adaptations
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Gills converted into a
modified lung
Uricotelic
• Behavioral “adaptations”
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Typically live near oceans
or in other high humidity
habitats
Visit water to wet body
and respiratory surfaces
Typically burrow and are
nocturnal
Some brood embryos on
land and make mass
migrations to release
larvae
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Terrestrial hermit
crabs (Coenobita)
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Coconut crabs
(Birgus)
• Climb trees and
open coconuts,
which they eat
• Drink sea water
• Can reach one
meter in length
• Some can lift 28 kg
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Christmas Island
red crabs
• Make mass
migrations from
forests to the sea
every year to mate
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Fiddler crabs are
considered semiterrestrial
Uca lives on sand or
mud beaches or in
mangrove swamps
Emerge from burrows
when the tide is out
• Foraging
• Mating
• Fighting
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Dependent on tide to
bring new food and
remove wastes
The Decapod Hemal System
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Heart is not tubular
• Rectangular with three pairs of ostia
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Well developed vasculature
• Seven major arteries leaving heart
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McGaw and Reiber have studied many
parameters of decapod circulation
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Circulation takes around 40 sec in large species
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Hemocyanin transports around 90% of oxygen
Neurosecretory Organs of Decapods
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Eyestalks are the most important neurosecretory organs
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Hormones synthesized in neuron cell bodies of eyestalk
control:
• Molting
• Chromatophores
• Reproduction
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Sinus gland in eyestalk stores and releases hormones into
blood
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There are a few endocrine organs outside of nervous
system, but these are under nervous control
• Y-organ
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Secretes ecdysone
Located in anterior cephalothorax
Chromatophores
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Located in connective tissue,
deep to the epidermis
Color change is apparent in
areas where the cuticle is thin
or transparent
Used for:
• Thermoregulation – blanching
in fiddler crabs
• Concealment – many shrimps
change color to match
surroundings
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Astaxanthin is a carotenoid
pigment found in decapods
• Is blue in live animals, because
it is bound with a protein
• Boiling denatures the protein
and the animal turns red
Decapod Reproduction
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Copulation
• Hermit crabs have to
partially exit shell
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Sperm transfer is
indirect
• Spermatophore extruded
from penes, onto
gonopods
• Transferred by gonopods
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Most anterior pair of
pleopods, that are modified
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Aquatic species attract each other using pheromones
• Tactile cues are also important
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Visual and sometimes auditory cues are important to
terrestrial species
• Combat between male fiddler crabs
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Egg mass is known as a sponge, and is brooded on
pleopods
Autotomy
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Limbs occasionally get
autotomized (removed)
• Predators
• Self amputation
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Little bleeding
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Limb bud forms
• New limb regenerates
inside
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Limb bud unfolds during
next molt
• Removing a limb bud
delays molting until a
new bud forms and limb
is regenerated
Class Malacostraca, Order Euphausiacea
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85 species of krill
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Pelagic, shrimp-like animals found
throughout oceans
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Small (about 3 cm long), but found
in enormous densities
• 60,000 / m3
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Chief food for many marine animals
• Blue whale may consume four tons
per day
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Bioluminescence functions in
schooling and courtship behaviors
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Can rapidly molt and leave exuvia
behind as decoys
Order Amphipoda
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6000 species of scuds
• Gammarus is most famous
genus
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Small (5 – 15 mm)
• Giant Pacific scuds can reach
28 cm and live 5300 m deep
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Some FW species, and
some semi-terrestrial such
as beach hoppers
• Can leap many times body
length
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Laterally compressed
Order Isopoda
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4000 species
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Mostly marine
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Pill bugs (wood lice) are
the most successful
terrestrial crustaceans
Most are 5 -15 mm, but
the deep sea isopod
(Bathynomus) reaches
42 cm
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Dorsoventrally compressed
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Nutritional strategies
• Some bore into wood
• Some are carnivorous
• Some parasitize fishes
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Have enrollment muscles (pill
bugs)
Terrestrial species specialized
structures to collect water and
channel it to gills
Class Copepoda
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12,000 species
Most marine, but FW varieties
dominate zooplankton
communities
Some are parasitic
Free-living varieties are small (1 17 mm), but some parasites reach
25 cm
Long first antennae
• Used by males to grasp females
• Usually branch 90° angles, relative
to longitudinal axis
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Possess a median nauplius
eye
Form oil droplets for
buoyancy, which can lead
to petroleum deposits
Some undergo DMV of
around 300 m
• One researcher figured out
that this is the greatest
movement of biomass on
the planet
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Famous genera:
• Cyclops
• Tisbe
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Swarm on small fish and
eat at their fins
After fish is immobilized,
it is devoured
Class Cirripedia
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Barnacles
Secrete and inhabit a rigid
calcareous shell that is capable
of growth
Sessile animals that encrust on
rocks, timber, ships, and even
whales
Fouling ability is amazing
• May reduce speed and fuel
efficiency of a ship by 30%
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Abundant intertidally, and may
be found in distinct patters of
zonation
• Due to interspecific competition
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Larvae settle and
cements to substrate
using glands on its
head
• Ventral aspect is up
• Six pairs of cirri on
thorax are used for
filter feeding
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Hermaphroditic
• Highly extensible
penis deposits sperm
in a neighboring
barnacle
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Calcareous shell is
secreted by exoskeleton
There are two pairs of
ventral, movable
calcareous plates that form
an operculum
• Scuta – anterior plates
• Terga – posterior plates
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Muscles control aperture
diameter
Ecdysis occurs but
calcareous shell / plates
aren’t shed
Shell is added on to with
new secretions
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Diversity
• Acorn barnacles
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Balanus
Those on West coast
of South America
can be 23 cm high, 8
cm wide, and are
eaten locally
• Stalked barnacles
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Have a peduncle and
capitulum
Lepas can grow to
75 cm in height
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Some do not produce shells
• Bore into rocks
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Others parasitize crabs
• Barnacles alter crab’s behavior, physiology, and
anatomy
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Molting suppressed
Reproduction suppressed, in fact most are sterilized
Male crabs become feminized and develop a larger
female abdomen, into which barnacle places its own
brood pouch
• Betters suits the barnacle’s needs
Class Ostracoda
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6000 species of seed
shrimps that resemble
miniature clams
• Have a bivalved carapace
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0.25 – 25 mm
(Gigantocypris)
Some have cephalic silk
glands and spinnerets that
secrete silk threads used
for:
• Draglines
• Building shelters for molting
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Carapace is an
extension of head
• Is shed with each molt
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Some have
bioluminescence
• Bluish light
• Flashed for periods of 1
to 2 seconds
• Used to attract mates
Class Branchiura
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200 species of fish lice
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Ectoparasites of marine and FW
fishes, and tadpoles
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5 – 30 mm in length
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Structural adaptations for
attaching to host
• First antennae have claws
• Mandibles are toothed
• First maxillae are suckers
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Produce digestive enzymes to
erode host tissue, and feed on
blood and mucus
Infestations in fish farms can
eradicate entire populations