The Great Barrier Reef
272
species occasionally enter the water column as hitchhikers on jellyfish or among floating algae or debris.
On beaches above the tideline, the fast running
stalk-eyed ghost crab (Ocypode; Fig. 23.3J) is conspicuous, particularly early and late in the day. Ghost crabs
scavenge and opportunistically prey on whatever they
can find on the beach, including the occasional turtle
hatchling. Though ghost crabs live high up on the
beach, their burrows usually reach down to the water
table. On the reef flat, the slow moving xanthoid crabs,
such as Actaea (Fig. 23.4A), Atergatis, Carpilius, Chlorodiella, Eriphia, Etisus, and the poisonous Lophozozymus
(Fig. 23.3K) are common in and under crevices of coral
and rocks. Xanthoids are well adapted to living in and
among corals and rock, and are the most abundant and
diverse of coral reef crabs. The xanthoid body is typically compact, often covered in small spines, bumps, or
bristles, and the legs are relatively short and stout. The
claws of many xanthoids have dark fingertips, hence
the common name ‘dark fingered crabs’. The parthenopoids also occur among the rubble of the reef flat,
and common genera include Actaeomorpha (Fig. 23.3L)
and Daldorfia. Parthenopoids usually have a somewhat
triangular or rectangular carapace, with the whole surface of the body and claws covered in tubercles knobs
and pits, effectively simulating their rubbly habitat.
Unlike the xanthoids and parthenopoids, the swimming crabs (family Portunidae) are fast moving and aggressive. The hind legs of portunids form swimming
paddles and the other legs are flattened to assist swimming and digging. The most common swimming crabs
on the reef flat are of the genus Thalamita (Fig. 23.4B, C),
recognisable by their widely spaced eyes. Species of
Thalamita are powerful and aggressive, and can be seen
foraging in the open over sand, rubble and seagrass.
The mud crab, Scylla serrata (Fig. 23.4D), is the largest known swimming crab, and is recreationally and
commercially fished throughout tropical Australia. It
usually lives along the coast in mangrove swamps, but
can also be found on the soft offshore sediments between reefs, and amongst mangroves that sometimes
occur on or near reefs flats. The claws are particularly
powerful and can cause serious injury if mishandled. A
much smaller, more ‘gentle’ crab of the mangroves is
to the sea to breed. Diogenids, with the left cheliped
generally larger than the right, and the pagurids, with
the opposite pattern, are common on all parts of the
reef, from the intertidal beach rock to the reef flat,
slope, and inter-reef areas. Hermit crabs are generally
scavengers or herbivores, and large numbers can often be seen at night foraging on the reef flat or on the
reef slope.
Other anomurans, such as the colourful squat lobsters and porcelain crabs, are common in most parts of
the reef. Squat lobsters (Galatheidae) resemble a small,
flattened lobster with numerous transverse groves on
the carapace, the tail tucked under the body and chelipeds pointing forwards. Common galatheids include
species of Munida that live in deeper, inter-reefal waters, species of Galathea, that live among corals, on bryozoans and sponges, and the conspicuously striped
Allogalathea elegans (Fig. 23.3H) that lives in pairs on crinoids. Porcelain crabs (Porcellanidae) have a short,
smooth or spiny carapace and chelipeds protruding
sideways. Common porcellanid genera include the free
living Petrolisthes and Polyonyx (Fig. 23.3I), and anemone associated Neopetrolisthes. Not more than 30 species
each of Porcellanidae and Galatheidae are presently
known from GBR waters.
The mole crabs, Hippoidea, are highly specialised
for burrowing in sand. Their elongate bodies are flattened or oval in shape, and the legs are flattened into
digging spades. They can burrow extremely rapidly
and are difficult to detect, let alone capture. Most hippoids live in shallow water, but some live at depths beyond 200 m.
Brachyura: crabs
The Brachyura, the true crabs, are possibly the most
successful of all decapods, with more the 6500 described species in more than 50 families worldwide.
Almost 500 species in about 40 families occur on the
GBR. The shallow water crabs of the region are relatively well documented, though many new species
continue to be discovered. The name Brachyura, means
‘short-tail’ and refers to the characteristic feature of
crabs—a short abdomen that is tucked beneath the
body. Crabs occur in every reef habitat, and some
272
species occasionally enter the water column as hitchhikers on jellyfish or among floating algae or debris.
On beaches above the tideline, the fast running
stalk-eyed ghost crab (Ocypode; Fig. 23.3J) is conspicuous, particularly early and late in the day. Ghost crabs
scavenge and opportunistically prey on whatever they
can find on the beach, including the occasional turtle
hatchling. Though ghost crabs live high up on the
beach, their burrows usually reach down to the water
table. On the reef flat, the slow moving xanthoid crabs,
such as Actaea (Fig. 23.4A), Atergatis, Carpilius, Chlorodiella, Eriphia, Etisus, and the poisonous Lophozozymus
(Fig. 23.3K) are common in and under crevices of coral
and rocks. Xanthoids are well adapted to living in and
among corals and rock, and are the most abundant and
diverse of coral reef crabs. The xanthoid body is typically compact, often covered in small spines, bumps, or
bristles, and the legs are relatively short and stout. The
claws of many xanthoids have dark fingertips, hence
the common name ‘dark fingered crabs’. The parthenopoids also occur among the rubble of the reef flat,
and common genera include Actaeomorpha (Fig. 23.3L)
and Daldorfia. Parthenopoids usually have a somewhat
triangular or rectangular carapace, with the whole surface of the body and claws covered in tubercles knobs
and pits, effectively simulating their rubbly habitat.
Unlike the xanthoids and parthenopoids, the swimming crabs (family Portunidae) are fast moving and aggressive. The hind legs of portunids form swimming
paddles and the other legs are flattened to assist swimming and digging. The most common swimming crabs
on the reef flat are of the genus Thalamita (Fig. 23.4B, C),
recognisable by their widely spaced eyes. Species of
Thalamita are powerful and aggressive, and can be seen
foraging in the open over sand, rubble and seagrass.
The mud crab, Scylla serrata (Fig. 23.4D), is the largest known swimming crab, and is recreationally and
commercially fished throughout tropical Australia. It
usually lives along the coast in mangrove swamps, but
can also be found on the soft offshore sediments between reefs, and amongst mangroves that sometimes
occur on or near reefs flats. The claws are particularly
powerful and can cause serious injury if mishandled. A
much smaller, more ‘gentle’ crab of the mangroves is
to the sea to breed. Diogenids, with the left cheliped
generally larger than the right, and the pagurids, with
the opposite pattern, are common on all parts of the
reef, from the intertidal beach rock to the reef flat,
slope, and inter-reef areas. Hermit crabs are generally
scavengers or herbivores, and large numbers can often be seen at night foraging on the reef flat or on the
reef slope.
Other anomurans, such as the colourful squat lobsters and porcelain crabs, are common in most parts of
the reef. Squat lobsters (Galatheidae) resemble a small,
flattened lobster with numerous transverse groves on
the carapace, the tail tucked under the body and chelipeds pointing forwards. Common galatheids include
species of Munida that live in deeper, inter-reefal waters, species of Galathea, that live among corals, on bryozoans and sponges, and the conspicuously striped
Allogalathea elegans (Fig. 23.3H) that lives in pairs on crinoids. Porcelain crabs (Porcellanidae) have a short,
smooth or spiny carapace and chelipeds protruding
sideways. Common porcellanid genera include the free
living Petrolisthes and Polyonyx (Fig. 23.3I), and anemone associated Neopetrolisthes. Not more than 30 species
each of Porcellanidae and Galatheidae are presently
known from GBR waters.
The mole crabs, Hippoidea, are highly specialised
for burrowing in sand. Their elongate bodies are flattened or oval in shape, and the legs are flattened into
digging spades. They can burrow extremely rapidly
and are difficult to detect, let alone capture. Most hippoids live in shallow water, but some live at depths beyond 200 m.
Brachyura: crabs
The Brachyura, the true crabs, are possibly the most
successful of all decapods, with more the 6500 described species in more than 50 families worldwide.
Almost 500 species in about 40 families occur on the
GBR. The shallow water crabs of the region are relatively well documented, though many new species
continue to be discovered. The name Brachyura, means
‘short-tail’ and refers to the characteristic feature of
crabs—a short abdomen that is tucked beneath the
body. Crabs occur in every reef habitat, and some