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23. Arthropoda: Crustaceans
and Pycnogonids
S. T. Ahyong
N CRUSTACEA
The Crustacea is the largest group of marine arthropods,
earning it the title ‘insects of the sea’. Being arthropods,
crustaceans have a tough, chitinous, segmented exoskeleton, but are distinguished from other arthropods
by having two pairs of antennae at some stage in their
life cycle (antennae are lost in adult barnacles). Crustaceans are primarily marine species, but also occur in
many terrestrial and freshwater habitats. They range in
size from microscopic copepods to the Japanese giant
spider crab (Macrocheira) with a 4 m leg span. The bestknown crustaceans are the crabs, shrimps and lobsters
for their large size and culinary qualities.
The crustacean body generally comprises three regions: the cephalon, thorax and abdomen (or pleon).
Each region usually bears paired biramous limbs. The
cephalon bears the compound eyes and two pairs of
antennae. The limbs of the thorax form the mouthparts,
grasping and locomotory legs; and those of the abdomen typically form pleopods (swimmerets) and uropods. Many crustaceans breathe through gills located
either at the base of the thoracic limbs (e.g. decapods)
or abdominal limbs (e.g. stomatopods).
Crustaceans continue to grow throughout their lives,
unlike insects, which stop growing at maturity. The
hard exoskeleton, however, cannot stretch or expand to
accommodate growth. To increase in size crustaceans
must periodically shed their old exoskeleton in a process known as moulting or ecdysis. In the lead-up to a
moult, calcium and various other minerals are resorbed
into the body from the old exoskeleton, beneath which
a new, soft skin develops. The old exoskeleton ‘loosens’
at key locations and fractures along various defined
points (called moult sutures). The animal extracts itself
from the old exoskeleton, often within only a few minutes. Recently moulted animals, being soft and extremely vulnerable to predation, hide and remain
inactive. At this time, calcium and other minerals from
the old exoskeleton and surrounding seawater are
slowly redeposited in the new soft exoskeleton. Within
24 to 48 hours, the new exoskeleton hardens and the
animal can resume normal activity.
Sexes are usually separate in crustaceans and most
have planktonic larvae. Planktonic larvae pass through
many stages and can spend months in the plankton before settling on the bottom. Crustacean larvae are a significant component of the zooplankton.
On the GBR, crustaceans are abundant in all habitats, with around 1300 species recorded. In general,
back-reef sites on mid-shelf reefs have the highest overall crustacean diversity, but most habitats await detailed faunal study. The most conspicuous crustaceans
23. Arthropoda: Crustaceans
and Pycnogonids
S. T. Ahyong
N CRUSTACEA
The Crustacea is the largest group of marine arthropods,
earning it the title ‘insects of the sea’. Being arthropods,
crustaceans have a tough, chitinous, segmented exoskeleton, but are distinguished from other arthropods
by having two pairs of antennae at some stage in their
life cycle (antennae are lost in adult barnacles). Crustaceans are primarily marine species, but also occur in
many terrestrial and freshwater habitats. They range in
size from microscopic copepods to the Japanese giant
spider crab (Macrocheira) with a 4 m leg span. The bestknown crustaceans are the crabs, shrimps and lobsters
for their large size and culinary qualities.
The crustacean body generally comprises three regions: the cephalon, thorax and abdomen (or pleon).
Each region usually bears paired biramous limbs. The
cephalon bears the compound eyes and two pairs of
antennae. The limbs of the thorax form the mouthparts,
grasping and locomotory legs; and those of the abdomen typically form pleopods (swimmerets) and uropods. Many crustaceans breathe through gills located
either at the base of the thoracic limbs (e.g. decapods)
or abdominal limbs (e.g. stomatopods).
Crustaceans continue to grow throughout their lives,
unlike insects, which stop growing at maturity. The
hard exoskeleton, however, cannot stretch or expand to
accommodate growth. To increase in size crustaceans
must periodically shed their old exoskeleton in a process known as moulting or ecdysis. In the lead-up to a
moult, calcium and various other minerals are resorbed
into the body from the old exoskeleton, beneath which
a new, soft skin develops. The old exoskeleton ‘loosens’
at key locations and fractures along various defined
points (called moult sutures). The animal extracts itself
from the old exoskeleton, often within only a few minutes. Recently moulted animals, being soft and extremely vulnerable to predation, hide and remain
inactive. At this time, calcium and other minerals from
the old exoskeleton and surrounding seawater are
slowly redeposited in the new soft exoskeleton. Within
24 to 48 hours, the new exoskeleton hardens and the
animal can resume normal activity.
Sexes are usually separate in crustaceans and most
have planktonic larvae. Planktonic larvae pass through
many stages and can spend months in the plankton before settling on the bottom. Crustacean larvae are a significant component of the zooplankton.
On the GBR, crustaceans are abundant in all habitats, with around 1300 species recorded. In general,
back-reef sites on mid-shelf reefs have the highest overall crustacean diversity, but most habitats await detailed faunal study. The most conspicuous crustaceans