The Great Barrier Reef
274
the soldier crab (Mictyris; Fig. 23.4E), which forages in
large numbers on receding tides. Soldier crabs are easily
recognised by their somewhat spherical, bluish body,
and forward instead of sideways walk. They may march
in the thousands, but at the first sign of danger, bury
themselves in the sand and mud in a quick ‘corkscrew’
motion leaving only a small pock mark on the surface
where they previously stood. Other conspicuous crabs
on the open reef flat include the box crabs (Calappa;
Fig. 23.4F). Box crabs hide, buried in the sand, and
emerge at low tide to feed on molluscs, especially
gastropods. Box crabs are often called ‘shame-faced
crabs’ because the chelipeds are broad and high, almost
entirely hiding the mouth and eyes. A peculiar feature
of box crabs is a special ‘peg-like’ tooth on the side of
the moveable finger of the right cheliped. The box crab
uses this special tooth on the right cheliped like a can
opener to literally ‘peel’ open gastropod shells.
Some crabs are very specific about their habitat.
These include the turtle-grass crab (Caphyra rotundifrons), which lives only in the green alga Chlorodesmis;
the swimming crab (Lissocarcinus orbicularis), which
lives on and inside holothurians; and coral crabs (Trapezia (Fig. 23.4H) and Tetralia of the families Trapeziidae
and Tetraliidae, respectively), which live in the branches
of scleractinian corals such as Acropora and Pocillopora.
Trapeziids and tetraliids feed mostly on coral mucous,
and though not more than 20 mm wide, will attempt to
defend their host against intruders including crown-ofthorns starfish (Acanthaster planci). The pinnotherid
crabs, commonly called pea crabs, usually make their
homes in another type of host, in most cases, bivalve
molluscs. Species of Nepinnotheres and Arcotheres are
common in pearl oysters, the rare Durckheimia lochi
(Fig. 23.4D) lives in file shells (Limidae), and Xanthasia
murigera lives in tridacnid clams. Spider crabs (Majidae;
Fig. 23.4I) and sponge crabs (Dromiidae) are common
on the reef, but they are seldom obvious because they
move slowly and are well camouflaged. As their name
implies, sponge crabs carry a sponge, held against the
carapace by the last two pairs of legs. Spider crabs usually adorn themselves with algae, coral and sponge that
are attached to Velcro-like hooked setae; they are often
known as ‘decorator crabs’. Over time, the sponge, algae or coral grows over the surface of the crab to the
extent that the crab can hardly be detected unless it
moves. As a small, mobile piece of ‘reef’, other hitchhikers including worms, amphipods and even other small
crabs can be found on large majids. Numerous species
of crabs occur on the soft sediments between reefs, particularly swimming crabs of the genera Charybdis and
Portunus. Among these is the edible blue swimming
crab, Portunus armatus (previously known as P. pelagicus) (Fig. 23.4J) and mud crab, Scylla serrata (Fig. 23.4G).
N PYCNOGONIDA
Sea spider is an apt name for the pycnogonids
(Fig. 23.4K, L), for they superficially resemble true spiders (Araneae), albeit with a more slender body and
sometimes more than four pairs of legs (up to six pairs
in some Antarctic species). The body of pycnogonids is
small and slender. The head and thorax is fused to form
the prosoma, and the abdomen (or opisthosoma) is
small and unsegmented. Pycnogonids are also unusual
in having the mouth placed at the end of a proboscis
that may be as long as half the body. The pycnogonids
are an ancient group dating back to the Devonian,
probably representing the nearest relatives of the Chelicerata (spiders, mites, scorpions), or possibly the nearest relatives of the remaining Arthropoda. Around 1500
species of pycnogonid are known worldwide with
about 30 species in 17 genera known from the GBR and
adjacent Queensland coast. Some deep-sea pycnogonids have a leg span of 750 mm, but species on the GBR
are much smaller, at less than 20 mm across. All are
cryptic and slow moving, so detection can be difficult.
Pycnogonids prey on bryozoans, polychaetes, hydroids, xoanthids and even scleractinian corals.
ADDITIONAL READING
Ahyong, S. T., and O’Meally, D. (2004). Phylogeny of
the Decapoda Reptantia: resolution using three
molecular loci and morphology. Raffles Bulletin of
Zoology 52(2), 673–693.
Arango, C. (2003). Sea spiders (Pycnogonida, Arthropoda) from the Great Barrier Reef, Australia: new
species, new records and ecological annotations.
Journal of Natural History 37, 2723–2772.
274
the soldier crab (Mictyris; Fig. 23.4E), which forages in
large numbers on receding tides. Soldier crabs are easily
recognised by their somewhat spherical, bluish body,
and forward instead of sideways walk. They may march
in the thousands, but at the first sign of danger, bury
themselves in the sand and mud in a quick ‘corkscrew’
motion leaving only a small pock mark on the surface
where they previously stood. Other conspicuous crabs
on the open reef flat include the box crabs (Calappa;
Fig. 23.4F). Box crabs hide, buried in the sand, and
emerge at low tide to feed on molluscs, especially
gastropods. Box crabs are often called ‘shame-faced
crabs’ because the chelipeds are broad and high, almost
entirely hiding the mouth and eyes. A peculiar feature
of box crabs is a special ‘peg-like’ tooth on the side of
the moveable finger of the right cheliped. The box crab
uses this special tooth on the right cheliped like a can
opener to literally ‘peel’ open gastropod shells.
Some crabs are very specific about their habitat.
These include the turtle-grass crab (Caphyra rotundifrons), which lives only in the green alga Chlorodesmis;
the swimming crab (Lissocarcinus orbicularis), which
lives on and inside holothurians; and coral crabs (Trapezia (Fig. 23.4H) and Tetralia of the families Trapeziidae
and Tetraliidae, respectively), which live in the branches
of scleractinian corals such as Acropora and Pocillopora.
Trapeziids and tetraliids feed mostly on coral mucous,
and though not more than 20 mm wide, will attempt to
defend their host against intruders including crown-ofthorns starfish (Acanthaster planci). The pinnotherid
crabs, commonly called pea crabs, usually make their
homes in another type of host, in most cases, bivalve
molluscs. Species of Nepinnotheres and Arcotheres are
common in pearl oysters, the rare Durckheimia lochi
(Fig. 23.4D) lives in file shells (Limidae), and Xanthasia
murigera lives in tridacnid clams. Spider crabs (Majidae;
Fig. 23.4I) and sponge crabs (Dromiidae) are common
on the reef, but they are seldom obvious because they
move slowly and are well camouflaged. As their name
implies, sponge crabs carry a sponge, held against the
carapace by the last two pairs of legs. Spider crabs usually adorn themselves with algae, coral and sponge that
are attached to Velcro-like hooked setae; they are often
known as ‘decorator crabs’. Over time, the sponge, algae or coral grows over the surface of the crab to the
extent that the crab can hardly be detected unless it
moves. As a small, mobile piece of ‘reef’, other hitchhikers including worms, amphipods and even other small
crabs can be found on large majids. Numerous species
of crabs occur on the soft sediments between reefs, particularly swimming crabs of the genera Charybdis and
Portunus. Among these is the edible blue swimming
crab, Portunus armatus (previously known as P. pelagicus) (Fig. 23.4J) and mud crab, Scylla serrata (Fig. 23.4G).
N PYCNOGONIDA
Sea spider is an apt name for the pycnogonids
(Fig. 23.4K, L), for they superficially resemble true spiders (Araneae), albeit with a more slender body and
sometimes more than four pairs of legs (up to six pairs
in some Antarctic species). The body of pycnogonids is
small and slender. The head and thorax is fused to form
the prosoma, and the abdomen (or opisthosoma) is
small and unsegmented. Pycnogonids are also unusual
in having the mouth placed at the end of a proboscis
that may be as long as half the body. The pycnogonids
are an ancient group dating back to the Devonian,
probably representing the nearest relatives of the Chelicerata (spiders, mites, scorpions), or possibly the nearest relatives of the remaining Arthropoda. Around 1500
species of pycnogonid are known worldwide with
about 30 species in 17 genera known from the GBR and
adjacent Queensland coast. Some deep-sea pycnogonids have a leg span of 750 mm, but species on the GBR
are much smaller, at less than 20 mm across. All are
cryptic and slow moving, so detection can be difficult.
Pycnogonids prey on bryozoans, polychaetes, hydroids, xoanthids and even scleractinian corals.
ADDITIONAL READING
Ahyong, S. T., and O’Meally, D. (2004). Phylogeny of
the Decapoda Reptantia: resolution using three
molecular loci and morphology. Raffles Bulletin of
Zoology 52(2), 673–693.
Arango, C. (2003). Sea spiders (Pycnogonida, Arthropoda) from the Great Barrier Reef, Australia: new
species, new records and ecological annotations.
Journal of Natural History 37, 2723–2772.