Crustaceans-DrR

Diversity and Characteristics
Characteristics
Source: www.coastalstudies.org/ research/calving.htm
• Over 30,000 living
species
• Together, insects and
crustaceans compose
over 80% of all named
species
• Members of the copepod
genus Calanus are most
likely the most abundant
animals in the world
• Crustaceans and uniramians differ
enough to separate them at the
subphylum level
• Arthropodization may have occurred
more than once, but there are basic
similarities
Subphylum Crustacea
Distinquishing Features
• Two pairs of antennae.
• Head also has a pair of mandibles and 2
pairs of maxillae.
• One pair of appendages on each of the
additional somites; Some may lack them.
• Appendages (except 1 st antennae in some)
are biramous (two main branches).
More Distinquishing Features
• Few (derived) to as many as 60 (primitive)
somites.
• Tagmata are usually head, thorax, and
abdomen—not homologous across all taxa.
• Caridoid facies arrangement of tagmata is
the ancestral plan.
• Dorsal covering is the carapace; May cover
most of body or just the cephalothorax.
Figure 19-1
Form and Function
External Features.
• Cuticle made of chitin, protein, and
calcareous material.
• Joints soft and thin—flexible.
• Telson is not a somite, bears anus.
• Gonopores may be at the base of
appendages, at the tail, or on somites
without legs.

Internal Features
• Metamerism of annelid-like ancestors
shown in muscular and nervous systems.
• Hemocoel
• Major blood-filled body space—not a coelom.
• Vestigial coelomic sacs are lost between
mesoderm, ectoderm, and yolk.
• Is not lined with mesodermal peritoneum.
• In crustaceans, coelom remains as end sacs of
excretory organs and gonads.
Muscular System
• Striated muscles make up most of the body
• Most muscles arranged as antagonistic
groups of flexors and extensors
• Abdominal flexors in crayfish allow it to swim
backwards
• Mandibles are controlled by strong muscles
located on either side of stomach

Respiratory System
• Smaller crustaceans gas exchange occurs
across leg cuticle
• Larger crustaceans use gills
• “Bailer” of 2 nd maxilla draws water over
the gill filaments
Circulatory System
• “open” system; no veins to separate blood from
interstitial fluid.
• Hemolymph leaves the heart by arteries but washes
through a hemocoel to return to the heart via
sinuses.
• Contrasts with annelids which have a closed system.
• Hemolymph colorless; contains ameboid cells that
may help to prevent clotting.
• Hemocyanin and/or hemoglobin are respiratory pigments.
Excretory System
• Antennal (green glands in decapods) or
maxillary glands.
• Hydrostatic pressure provides force for
filtration.
• Nitrogenous wastes (NH 3 ) excreted across gill
cuticle.
• Freshwater crustaceans have problem with
over-dilution; gills actively absorb Na + and Cl - .
• Marine crustacean urine is isosmotic with
blood.
Nervous System
• More fusion of ganglia than is found in other
arthropods.
• Double ventral nerve cord has a pair of ganglia for
each somite to control the appendages.
• Crustaceans have more-developed sense organs
than annelids.
• Eyes (compound-with ommatidia) and statocysts
are largest sense organs.
• Chemical (taste, smell) sensing occurs in hairs on
antennae and mouth.

Reprodution, Life Cyles, and
Endocrine Function
• Great diversity in reproductive strategies
• Monoecious (barnacles) Parthenogenesis
(ostracods), Dioecious (most)
• Development may be direct (crayfish) or
indirect (most)
• Nauplius is the common larval form with
uniramous 1 st antennae, and biramous 2 nd
antennae and mandibles that aid in swimming.
Molting (Ecdysis) -- Steps
• Epidermis secretes inactive form of enzymes at
the base of the skeleton. Epidermis then
detaches from skeleton and secretes new
epicuticle.
• The inactive enzymes now become activated
and digest the untanned endocuticle. Ca and
proteins from old cuticle are reabsorbed.
• New procuticle secreted beneath new epicuticle.
• Longitudinal rupturing of old cuticle along dorsal
or lateral sides of body. Animal pulls self out.
Molting
• Growth possible because new cuticle is
soft and pliable.
• Animal takes in water or air to expand it.
• Gradually grows new tissues to fill new
armor. Growth is therefore gradual.
• Instars or stadia are growth periods
between molts. Segments are added and
pairs of appendages.
• Metamorphosis occurs at various molts.
Molting
• Disadvantage.
• Animal vulnerable. Dead white and
conspicuous until the cuticle hardens (soft
shell crabs). Movement restricted because
of soft skeleton.

Behavioral Changes Associated
With Molting.
Neuro-endocrine Controls in
Crustaceans.
• Most hide.
• Occasionally reverse normal reflex patterns (e.g.
light gradients).
• Land crabs need privacy or won't molt. Hormones
get fouled. Sensory input from disturbing factors
increases secretion of inhibitory hormone from
brain and prevents secretion of molting hormone.
Other crabs eat molting crabs. Can't even trust
your relatives.
Hormonal Control Summary
• X-organ in eye stalk produces neurosecretory
hormone which inhibits molting and is stored in
the sinus gland.
• Y gland located in antennary or 2nd maxillary
segments produces molting hormone.
Degenerates in animals which reach terminal
stage and no further growth occurs.
• When the CNS receives a stimulus that it is time to
molt, the sinus gland stops releasing the molt
inhibitor and the Y-organ is free to release its
ecdysone and the molting process begins.
• Temperature, day length or other stimuli may act
as stimulus to begin ecdysis.
Other Endocrine Functions
• Removing eyestalks accelerates molting and
prevents color changes to match the
background.
• Hormones from neurosecretory cells in eyestalk
control dispersal of cell pigment.
• Functions unrelated to ecdysis
• Regulation of heartbeat
• Expression of male characteristics in amphipods