23 – Arthropoda: Crustaceans and Pycnogonids
271
and probably and adaptation to living on level soft sediments, is their very flat carapace that extends sideways, covering their legs.
Astacidea: clawed lobsters and freshwater crayfish
The Astacidea is the large group containing the marine
clawed lobsters and freshwater crayfish. The most obvious feature of the astacideans, differentiating them
from the palinurid lobsters, is the possession of at least
one pair of enlarged, powerful chelipeds. In almost all
cases, the large, first pair of chelipeds is followed by
two pairs of smaller chelipeds. Freshwater crayfish,
such as the yabby and marron (Cherax) are common on
the mainland, and marine counterparts, such as scampi
(Metanephrops, Nephropsis and Thaumastochelopsis) occur in deep, outer shelf waters. However, only a few
species of astacideans live on the reef itself: the dwarf
reef lobsters of the genus Enoplometopus (Fig. 23.3D)
and family Enoplometopidae. Enoplometopidae, containing about 20 species worldwide, possibly represents the last remnant of a lineage of lobsters that
otherwise went extinct in the Lower Jurassic Period.
Species of Enoplometopus do not exceed 20 cm in length
(more often less than 10 cm) but have vivid orange, red
and purple colouration. Enoplometopids are very distinctive, but they are seldom seen because of their shy,
nocturnal habit and lower reef slope habitat.
Thalassinidea: mud shrimps, ghost shrimps,
marine yabbies
Thalassinideans are shrimp-like decapods closely related to the Anomura and Brachyura. They typically
have at least one pair of enlarged chelipeds, small eyes,
a much-reduced first abdominal segment, but long
abdomen, and tailfan. The carapace and abdominal
exoskeleton of thalassinideans is usually thin, translucent and somewhat soft, so not surprisingly, they rarely
leave their burrows. Most thalassinideans burrow in
sand and mud, but some burrow into sponges, live in
holes or under coral rocks. Those burrowing in soft
sediments, especially of species of the Callianassidae
and Upogebiidae, are important bioturbators, often
creating tall mounds of excavated sand and mud
around the burrow entrances. The species most likely
to be seen on the reef flat is the orange Strahlaxius plectorhynchus, living in vertical, rubble lined burrows on
the inner reef flat. Strahlaxius glyptocercus (Fig. 23.3E)
and Trypaea australiensis (Fig. 23.3F) occur in muddier
or more fine-grained habitats, especially closer to the
mainland coast. At least 38 species in nine families are
known from the GBR. Thalassinids are usually difficult
to capture because of their deep burrows, and no doubt
many more species than presently recorded occur in
the region.
Anomura: hermit crabs, squat lobsters,
porcelain crabs and allies
The anomurans are the nearest relatives of the Brachyura, the true crabs, and include the hermit crabs
(Paguroidea), squat lobsters and porcelain crabs
(Galatheoidea) and mole crabs (Hippoidea). Unifying
features of the Anomura include the very small, reduced last walking legs, and presence of an uncalcified groove along the sides of the carapace (the linea
anomurica). The last pair of walking legs is so reduced,
that at first sight, most anomurans appear to have
only six instead of eight walking legs. The last pair of
walking legs are usually folded under the abdomen or
edges of the carapace, and used for grooming. Anomurans are abundant on the GBR, with the most obvious
being the hermit crabs. Most hermit crabs have a soft,
vulnerable abdomen, and usually use a gastropod
shell for protection. Hermit crabs (Fig. 23.3G) are especially well adapted to occupying gastropod shells;
the asymmetrical, coiled abdomen and asymmetry of
walking legs and claws enable the animal to fit snugly
inside. Species range from less than 10 mm to almost
300 mm in length. Common hermit crabs on the GBR
fall into one of three major groups: the coenobitids, or
terrestrial hermit crabs; the diogenids (the left handed
hermit crabs), and pagurids (the right handed hermit
crabs). Coenobitids, represented by Coenobita on the
GBR, live on beaches above the tide line where they
forage among flotsam and jetsam. To avoid desiccation, Coenobita carries water in its gastropod home
and shelters in the shade or under driftwood during
the hottest part of the day. Though they spend their
entire adult life out of water, coenobitids must return
271
and probably and adaptation to living on level soft sediments, is their very flat carapace that extends sideways, covering their legs.
Astacidea: clawed lobsters and freshwater crayfish
The Astacidea is the large group containing the marine
clawed lobsters and freshwater crayfish. The most obvious feature of the astacideans, differentiating them
from the palinurid lobsters, is the possession of at least
one pair of enlarged, powerful chelipeds. In almost all
cases, the large, first pair of chelipeds is followed by
two pairs of smaller chelipeds. Freshwater crayfish,
such as the yabby and marron (Cherax) are common on
the mainland, and marine counterparts, such as scampi
(Metanephrops, Nephropsis and Thaumastochelopsis) occur in deep, outer shelf waters. However, only a few
species of astacideans live on the reef itself: the dwarf
reef lobsters of the genus Enoplometopus (Fig. 23.3D)
and family Enoplometopidae. Enoplometopidae, containing about 20 species worldwide, possibly represents the last remnant of a lineage of lobsters that
otherwise went extinct in the Lower Jurassic Period.
Species of Enoplometopus do not exceed 20 cm in length
(more often less than 10 cm) but have vivid orange, red
and purple colouration. Enoplometopids are very distinctive, but they are seldom seen because of their shy,
nocturnal habit and lower reef slope habitat.
Thalassinidea: mud shrimps, ghost shrimps,
marine yabbies
Thalassinideans are shrimp-like decapods closely related to the Anomura and Brachyura. They typically
have at least one pair of enlarged chelipeds, small eyes,
a much-reduced first abdominal segment, but long
abdomen, and tailfan. The carapace and abdominal
exoskeleton of thalassinideans is usually thin, translucent and somewhat soft, so not surprisingly, they rarely
leave their burrows. Most thalassinideans burrow in
sand and mud, but some burrow into sponges, live in
holes or under coral rocks. Those burrowing in soft
sediments, especially of species of the Callianassidae
and Upogebiidae, are important bioturbators, often
creating tall mounds of excavated sand and mud
around the burrow entrances. The species most likely
to be seen on the reef flat is the orange Strahlaxius plectorhynchus, living in vertical, rubble lined burrows on
the inner reef flat. Strahlaxius glyptocercus (Fig. 23.3E)
and Trypaea australiensis (Fig. 23.3F) occur in muddier
or more fine-grained habitats, especially closer to the
mainland coast. At least 38 species in nine families are
known from the GBR. Thalassinids are usually difficult
to capture because of their deep burrows, and no doubt
many more species than presently recorded occur in
the region.
Anomura: hermit crabs, squat lobsters,
porcelain crabs and allies
The anomurans are the nearest relatives of the Brachyura, the true crabs, and include the hermit crabs
(Paguroidea), squat lobsters and porcelain crabs
(Galatheoidea) and mole crabs (Hippoidea). Unifying
features of the Anomura include the very small, reduced last walking legs, and presence of an uncalcified groove along the sides of the carapace (the linea
anomurica). The last pair of walking legs is so reduced,
that at first sight, most anomurans appear to have
only six instead of eight walking legs. The last pair of
walking legs are usually folded under the abdomen or
edges of the carapace, and used for grooming. Anomurans are abundant on the GBR, with the most obvious
being the hermit crabs. Most hermit crabs have a soft,
vulnerable abdomen, and usually use a gastropod
shell for protection. Hermit crabs (Fig. 23.3G) are especially well adapted to occupying gastropod shells;
the asymmetrical, coiled abdomen and asymmetry of
walking legs and claws enable the animal to fit snugly
inside. Species range from less than 10 mm to almost
300 mm in length. Common hermit crabs on the GBR
fall into one of three major groups: the coenobitids, or
terrestrial hermit crabs; the diogenids (the left handed
hermit crabs), and pagurids (the right handed hermit
crabs). Coenobitids, represented by Coenobita on the
GBR, live on beaches above the tide line where they
forage among flotsam and jetsam. To avoid desiccation, Coenobita carries water in its gastropod home
and shelters in the shade or under driftwood during
the hottest part of the day. Though they spend their
entire adult life out of water, coenobitids must return