23 – Arthropoda: Crustaceans and Pycnogonids
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during the strike. ‘Spearers’ (Fig. 23.2A, B, C) strike with
an open dactylus, impaling prey—soft-bodied animals
like fish, cephalopods and shrimps—on a row of sharp
spines. ‘Smashers’ (Fig. 23.2D, E, F) strike with the dactylus closed against the propodus. The outer edge of
the dactylus is a calcified heel, used like a hammer to
smash open hard-bodied prey such as crabs and snails.
The power of the raptorial strike can be lethal—the
force of the impact may approach that of a small calibre
bullet. So, in ‘smashers’, the raptorial claws are used
not only for hunting, but also to avoid unnecessary confrontations. Many ‘smashers’ have evolved elaborate
threat displays that involve lunging and spreading the
limbs to display a coloured ‘meral spot’ on the inner
surface of the raptorial claw (Fig. 23.2F). The intensity
of the display and brightness of the ‘meral spot’ varies
between species, and gives a potential opponent some
indication of the ferocity that it might encounter.
The powerful raptorial claws of stomatopods are
complemented by finely tuned vision. Each eye can be
independently rotated and is divided in half by a central band of ommatidia. This provides the stomatopod
with binocular vision from each eye, and the central
band of ommatidia provides many of the reef species
with colour vision and polarising filters. The combination of excellent vision and powerful raptorial claws
make the stomatopod an efficient predator of both slow
and fast moving prey.
Almost 500 species of stomatopod are known
worldwide and more than 90 from GBR waters. Many
new species await scientific documentation, especially
small species that live deep inside rock and coral crevices. Around Lizard Island, almost 30 species are
known. The most common stomatopods among rubble
and corals are ‘smashers’ of the superfamily Gonodactyloidea. Common gonodactyloid genera on the reef
flat are: Gonodactylellus, Gonodactylus, Gonodactylaceus,
Chorisquilla and Haptosquilla. Haptosquilla glyptocercus
and H. trispinosa, both with striped antennae and not
exceeding about 40 mm length can be seen on the reef
flat peering out of circular holes in reef rock. ‘Spearers’,
primarily members of the Squilloidea and Lysiosquilloidea, burrow in the sand and sediment of the reef flat
and on the seabed of the lagoon and between reefs.
Several species of squilloid are commercially harvested,
taken as trawl bycatch (e.g. Harpiosquilla harpax and
Oratosquillina spp.). On the reef flat, the very large piscivore, Lysiosquillina maculata (up to 385 mm long), lives
in mated pairs in deep, vertical burrows, with burrow
entrances almost 100 mm across (Fig. 23.2B). Lysiosquillina maculata is the largest known stomatopod, with
a body marked by bold, light and dark transverse
bands. Pseudosquilla ciliata, another common spearer,
burrows in seagrass beds or under rocks on the reef
flat, and ranges in colour from bright yellow or dark
green to mottled green and grey (Fig. 23.2A, C). On the
reef slope, the number of species increases, and some of
the most spectacularly coloured stomatopods, those of
the ‘smashing’ genus Odontodactylus, can be seen walking out in the open (Fig. 23.2E).
Decapoda
The best known of all crustaceans are the decapods.
They include the largest arthropods and many commercial species such as the mud crab (Scylla serrata),
blue swimming crab (Portunus armatus (previously
known as P. pelagicus)), red-spot king prawn (Melicertus
longistylus), Moreton Bay bug (Thenus spp.) and spiny
lobsters (Panulirus spp.). Decapods have a welldeveloped carapace covering the thorax, and five
(rarely four) pairs of legs of which one or more pairs
usually forms a cheliped (pincer). At least nine decapod infraorders are recognised: Dendrobranchiata
(peneidean prawns), Caridea (shrimps), Stenopodidea
(coral shrimps), Polychelida (deep-sea blind lobsters),
Achelata (spiny-crayfish), Astacidea (clawed lobsters
and freshwater crayfish), Thalassinidea (mud-shrimps,
marine yabbies), Anomura (hermit crabs, squat lobsters, mole crabs), and Brachyura (true crabs). With the
exception of the deep-sea polychelids, members of the
other infraorders occur on the GBR.
Dendrobranchiata: prawns
The dendrobranchiates, collectively known as prawns
in Australia, are possibly the oldest lineage of the decapods. They have the familiar elongate body with a slender, muscular abdomen and three pairs of small
chelipeds. Unlike other decapods, female prawns do
not carry eggs until hatching, but spawn their fertilised
267
during the strike. ‘Spearers’ (Fig. 23.2A, B, C) strike with
an open dactylus, impaling prey—soft-bodied animals
like fish, cephalopods and shrimps—on a row of sharp
spines. ‘Smashers’ (Fig. 23.2D, E, F) strike with the dactylus closed against the propodus. The outer edge of
the dactylus is a calcified heel, used like a hammer to
smash open hard-bodied prey such as crabs and snails.
The power of the raptorial strike can be lethal—the
force of the impact may approach that of a small calibre
bullet. So, in ‘smashers’, the raptorial claws are used
not only for hunting, but also to avoid unnecessary confrontations. Many ‘smashers’ have evolved elaborate
threat displays that involve lunging and spreading the
limbs to display a coloured ‘meral spot’ on the inner
surface of the raptorial claw (Fig. 23.2F). The intensity
of the display and brightness of the ‘meral spot’ varies
between species, and gives a potential opponent some
indication of the ferocity that it might encounter.
The powerful raptorial claws of stomatopods are
complemented by finely tuned vision. Each eye can be
independently rotated and is divided in half by a central band of ommatidia. This provides the stomatopod
with binocular vision from each eye, and the central
band of ommatidia provides many of the reef species
with colour vision and polarising filters. The combination of excellent vision and powerful raptorial claws
make the stomatopod an efficient predator of both slow
and fast moving prey.
Almost 500 species of stomatopod are known
worldwide and more than 90 from GBR waters. Many
new species await scientific documentation, especially
small species that live deep inside rock and coral crevices. Around Lizard Island, almost 30 species are
known. The most common stomatopods among rubble
and corals are ‘smashers’ of the superfamily Gonodactyloidea. Common gonodactyloid genera on the reef
flat are: Gonodactylellus, Gonodactylus, Gonodactylaceus,
Chorisquilla and Haptosquilla. Haptosquilla glyptocercus
and H. trispinosa, both with striped antennae and not
exceeding about 40 mm length can be seen on the reef
flat peering out of circular holes in reef rock. ‘Spearers’,
primarily members of the Squilloidea and Lysiosquilloidea, burrow in the sand and sediment of the reef flat
and on the seabed of the lagoon and between reefs.
Several species of squilloid are commercially harvested,
taken as trawl bycatch (e.g. Harpiosquilla harpax and
Oratosquillina spp.). On the reef flat, the very large piscivore, Lysiosquillina maculata (up to 385 mm long), lives
in mated pairs in deep, vertical burrows, with burrow
entrances almost 100 mm across (Fig. 23.2B). Lysiosquillina maculata is the largest known stomatopod, with
a body marked by bold, light and dark transverse
bands. Pseudosquilla ciliata, another common spearer,
burrows in seagrass beds or under rocks on the reef
flat, and ranges in colour from bright yellow or dark
green to mottled green and grey (Fig. 23.2A, C). On the
reef slope, the number of species increases, and some of
the most spectacularly coloured stomatopods, those of
the ‘smashing’ genus Odontodactylus, can be seen walking out in the open (Fig. 23.2E).
Decapoda
The best known of all crustaceans are the decapods.
They include the largest arthropods and many commercial species such as the mud crab (Scylla serrata),
blue swimming crab (Portunus armatus (previously
known as P. pelagicus)), red-spot king prawn (Melicertus
longistylus), Moreton Bay bug (Thenus spp.) and spiny
lobsters (Panulirus spp.). Decapods have a welldeveloped carapace covering the thorax, and five
(rarely four) pairs of legs of which one or more pairs
usually forms a cheliped (pincer). At least nine decapod infraorders are recognised: Dendrobranchiata
(peneidean prawns), Caridea (shrimps), Stenopodidea
(coral shrimps), Polychelida (deep-sea blind lobsters),
Achelata (spiny-crayfish), Astacidea (clawed lobsters
and freshwater crayfish), Thalassinidea (mud-shrimps,
marine yabbies), Anomura (hermit crabs, squat lobsters, mole crabs), and Brachyura (true crabs). With the
exception of the deep-sea polychelids, members of the
other infraorders occur on the GBR.
Dendrobranchiata: prawns
The dendrobranchiates, collectively known as prawns
in Australia, are possibly the oldest lineage of the decapods. They have the familiar elongate body with a slender, muscular abdomen and three pairs of small
chelipeds. Unlike other decapods, female prawns do
not carry eggs until hatching, but spawn their fertilised