The Great Barrier Reef
266
200 species of isopod are known from the GBR, and
most obvious are the mostly free living flabelliferans,
particularly species of the families Cirolanidae (Fig.
23.1G) and Sphaeromatidae (Fig. 23.1H). Cirolanids
are efficient scavengers, emerging from the substratum to swim rapidly towards a food source such as a
dead or maimed fish. They can also be voracious predators, sometimes biting humans. Sphaeromatids are
perhaps less conspicuous than the cirolanids, but often
have a highly sculptured pleotelson and trailing uropods. Some isopods vary markedly from the ‘typical’
form. These include the slender valviferans and anthurideans, and most notably the endoparasitic bopyroids
that infect decapods. Bopyroids have lost many of the
features of free living isopods, and being internal parasites, are seldom seen. A rounded bulge on the surface
of the carapace of the decapod host is the only external
clue that a bopryroid lies beneath. Other parasitic isopods such as the cymothoids (Fig. 23.1I) infect the skin,
mouth, and gills, of fish, to which they cling with
sharp, hooked ‘feet’.
Amphipoda: sandhoppers, scuds and
skeleton shrimps
The amphipods are the most speciose of the peracarids
with more than 6000 species worldwide. They can be
recognised by their (usually) laterally compressed body,
biramous antennules and three pairs of uropods. On the
GBR, about 200 species are known from depths shallower than 50 m, of which 80% are yet to be formally
named, and this all from near Lizard Island in the far
northern section of the Reef. Thus, many more species
can be expected from the GBR overall. The three major
Suborders of Amphipoda (Gammaridea, Hyperiidea
and Caprellidea) each have a very distinctive appearance. The majority of amphipods are gammarideans
(Fig. 23.1J, K), commonly known as scuds and sandhoppers (for terrestrial species), and these are the most familiar form. Gammarideans have a relatively deep,
curved body that is obviously laterally compressed;
they are the most accomplished swimmers of the Amphipoda, and can ‘scuttle’, ‘skip’ and ‘flick’ their bodies
to escape capture. Gammarideans live in almost every
habitat on the GBR—among rubble, boulders, coral and
algae, and sometimes in association with hydroids, ascidians and sponges. Some are predatory, some scavenge, and others graze on algae and detritus. The
hyperiideans resemble gammarideans with a swollen
head and long slender legs. Unlike gammarideans, however, hyperiideans are seldom seen on the reef because
they are oceanic creatures that live inside salps or on jellyfish. The caprellideans (Fig. 23.1L), or skeleton shrimps,
are so named because of the very slender, stick-like body.
They can be seen standing upright with forelimbs spread
out whilst clinging by their posterior legs to algae or hydroids. They crawl with a somewhat looping motion by
alternately using their anterior and posterior limbs.
Though appearing very different from gammarideans,
caprellideans probably represent a highly specialised
lineage that arose from within the Gammaridea.
Stomatopoda
The mantis shrimps, Order Stomatopoda, are the most
flamboyant crustaceans, and only distant relatives of
the decapods. They are primarily shallow water tropical animals and are abundant throughout the GBR.
They are active, aggressive predators and often vividly
coloured. Adults range in size from less than 10 mm to
almost 400 mm. The body is elongate, with a short,
shield-like carapace that does not cover the last three
thoracic somites. Unlike decapods, which either broadcast spawn or incubate the eggs on the pleopods, stomatopod females carry the egg mass with their
mouthparts until hatching. The most characteristic
feature of stomatopods, however, is the greatly enlarged raptorial forelimbs used to catch prey. The raptorial claw of stomatopods resembles that of the insect,
the praying mantis, in which the last segment (dactylus) folds like a jackknife against the preceding segment (propodus). However, instead of grasping prey
like the praying mantis, stomatopods strike violently
with considerable speed and force. The raptorial strike
of the stomatopod is one of the fastest known animal
movements, being completed within five milliseconds.
Obviously, large stomatopods must be handled with
great care.
Stomatopods can be divided into two functional
groups depending on how they use their raptorial claws
266
200 species of isopod are known from the GBR, and
most obvious are the mostly free living flabelliferans,
particularly species of the families Cirolanidae (Fig.
23.1G) and Sphaeromatidae (Fig. 23.1H). Cirolanids
are efficient scavengers, emerging from the substratum to swim rapidly towards a food source such as a
dead or maimed fish. They can also be voracious predators, sometimes biting humans. Sphaeromatids are
perhaps less conspicuous than the cirolanids, but often
have a highly sculptured pleotelson and trailing uropods. Some isopods vary markedly from the ‘typical’
form. These include the slender valviferans and anthurideans, and most notably the endoparasitic bopyroids
that infect decapods. Bopyroids have lost many of the
features of free living isopods, and being internal parasites, are seldom seen. A rounded bulge on the surface
of the carapace of the decapod host is the only external
clue that a bopryroid lies beneath. Other parasitic isopods such as the cymothoids (Fig. 23.1I) infect the skin,
mouth, and gills, of fish, to which they cling with
sharp, hooked ‘feet’.
Amphipoda: sandhoppers, scuds and
skeleton shrimps
The amphipods are the most speciose of the peracarids
with more than 6000 species worldwide. They can be
recognised by their (usually) laterally compressed body,
biramous antennules and three pairs of uropods. On the
GBR, about 200 species are known from depths shallower than 50 m, of which 80% are yet to be formally
named, and this all from near Lizard Island in the far
northern section of the Reef. Thus, many more species
can be expected from the GBR overall. The three major
Suborders of Amphipoda (Gammaridea, Hyperiidea
and Caprellidea) each have a very distinctive appearance. The majority of amphipods are gammarideans
(Fig. 23.1J, K), commonly known as scuds and sandhoppers (for terrestrial species), and these are the most familiar form. Gammarideans have a relatively deep,
curved body that is obviously laterally compressed;
they are the most accomplished swimmers of the Amphipoda, and can ‘scuttle’, ‘skip’ and ‘flick’ their bodies
to escape capture. Gammarideans live in almost every
habitat on the GBR—among rubble, boulders, coral and
algae, and sometimes in association with hydroids, ascidians and sponges. Some are predatory, some scavenge, and others graze on algae and detritus. The
hyperiideans resemble gammarideans with a swollen
head and long slender legs. Unlike gammarideans, however, hyperiideans are seldom seen on the reef because
they are oceanic creatures that live inside salps or on jellyfish. The caprellideans (Fig. 23.1L), or skeleton shrimps,
are so named because of the very slender, stick-like body.
They can be seen standing upright with forelimbs spread
out whilst clinging by their posterior legs to algae or hydroids. They crawl with a somewhat looping motion by
alternately using their anterior and posterior limbs.
Though appearing very different from gammarideans,
caprellideans probably represent a highly specialised
lineage that arose from within the Gammaridea.
Stomatopoda
The mantis shrimps, Order Stomatopoda, are the most
flamboyant crustaceans, and only distant relatives of
the decapods. They are primarily shallow water tropical animals and are abundant throughout the GBR.
They are active, aggressive predators and often vividly
coloured. Adults range in size from less than 10 mm to
almost 400 mm. The body is elongate, with a short,
shield-like carapace that does not cover the last three
thoracic somites. Unlike decapods, which either broadcast spawn or incubate the eggs on the pleopods, stomatopod females carry the egg mass with their
mouthparts until hatching. The most characteristic
feature of stomatopods, however, is the greatly enlarged raptorial forelimbs used to catch prey. The raptorial claw of stomatopods resembles that of the insect,
the praying mantis, in which the last segment (dactylus) folds like a jackknife against the preceding segment (propodus). However, instead of grasping prey
like the praying mantis, stomatopods strike violently
with considerable speed and force. The raptorial strike
of the stomatopod is one of the fastest known animal
movements, being completed within five milliseconds.
Obviously, large stomatopods must be handled with
great care.
Stomatopods can be divided into two functional
groups depending on how they use their raptorial claws