23 – Arthropoda: Crustaceans and Pycnogonids
263
are the comparatively large and usually colourful decapods (crabs, shrimps, lobsters) and stomatopods (mantis shrimps), but small-bodied, speciose groups such as
peracarids, ostracods and copepods, are also common.
The major crustacean groups to be encountered are discussed below.
Ostracoda
Ostracods (Fig. 23.1A), commonly called seed shrimps,
have a bivalved carapace that fully encloses the body.
They range in length from less than 1 mm to about
20 mm, but most species seen on the GBR are 3–5 mm
long. Ostracods use their two pairs of antennae for
locomotion, either swimming or ‘creeping’, and their
thoracic limbs are reduced to two pairs. Ostracods are
divided into two major groups: the Podocopa and Myodocopa. Podocopans are typically elongate and peanut shaped whereas myodocopans are generally
rounded or ovate with a notch in the margin. Almost
any sediment or algal sample will contain ostracods;
species may be detritivores, carnivores or scavengers.
More than 40 species are known from Lizard Island
Elsewhere on the GBR, ostracods have been scarcely
studied but many more species almost certainly await
discovery.
Copepoda
Copepods are abundant on the GBR and form a major
component of zooplankton. Most are no longer than a
few millimetres, though some parasitic species reach
300 millimetres. Copepods lack a carapace, and most
free living species have a single median eye, a muchelongated pair of antennae that is held outwards, a
broadened trunk area and slender abdomen. The most
common copepods on the GBR are free living forms—
the compact harpacticoids and barrel-shaped calanoids
and cyclopoids. Harpacticoids are generally benthic,
living under rocks, in algae and sediment. Calanoids
and cyclopoids are common in the plankton where they
may form dense swarms. Free living copepods feed
variously on other zooplankton, diatoms, and algae,
and are an extremely important food source for larval
fish and decapods. Parasitic copepods feed on the
blood or tissue of their hosts. Examples include the
siphonostomatoids and poecilostomatoids that attach
to the skin of sharks and fish, and certain cyclopoids,
which have become worm-like as internal invertebrate
parasites.
Cirripedia
The sessile habit and interlocking shell plates of barnacles led early investigators to believe they were molluscs. Not until the 1830s, when barnacle larvae were
studied, were they correctly recognised as crustaceans.
Barnacles are more abundant in temperate waters than
in the tropics, but nevertheless range throughout the
GBR on all hard substrata, including the carapace of
sea turtles and the shells of other crustaceans. More
than 40 species and 20 genera of barnacles are presently
known from GBR waters. The body of most barnacles
is enclosed by a somewhat tubular or conical opened
topped chamber formed by a series of interlocking calcareous plates—usually known as the shell. The animal
is positioned on its back, and thrusts its modified,
feathery thoracic limbs, known as cirri, out of the shell
to strain food particles from the water.
Unlike most other crustaceans, barnacles are hermaphrodites, meaning that each animal possesses male
and female reproductive organs. The majority of barnacles in the reproductive phase develop a greatly elongated penis (up to 10 times their body length) that is
used to reach out of the shell to fertilise a receptive
neighbour. The fertilised eggs develop into freeswimming larvae that eventually settle on the substratum close to others of their species, identified by their
unique chemical signature.
Three major types of barnacles will be encountered
on the reef: balanomorphs (Fig. 23.1B), lepadomorphs
(Fig. 23.1C) and rhizocephalans. In balanomorph
barnacles, the so-called acorn barnacles, the shell is
firmly fixed to the substratum, be it rock, coral, mangroves or wharf piles. Common balanomorphs on
the reef flat rocks and boulders include species of
Balanus, Chthamalus, Megabalanus, Tesseropora, Tetraclita
and Tetraclitella. Not all barnacles are fixed to the substratum by the shell, however. The lepadamorphs, the
goose-necked barnacles, are attached to the substratum
by a flexible stalk, with common genera including Lepas
and Ibla. Conchoderma virgatum is unusual in using sea
snakes as a substratum. In contrast to the balanomorphs
263
are the comparatively large and usually colourful decapods (crabs, shrimps, lobsters) and stomatopods (mantis shrimps), but small-bodied, speciose groups such as
peracarids, ostracods and copepods, are also common.
The major crustacean groups to be encountered are discussed below.
Ostracoda
Ostracods (Fig. 23.1A), commonly called seed shrimps,
have a bivalved carapace that fully encloses the body.
They range in length from less than 1 mm to about
20 mm, but most species seen on the GBR are 3–5 mm
long. Ostracods use their two pairs of antennae for
locomotion, either swimming or ‘creeping’, and their
thoracic limbs are reduced to two pairs. Ostracods are
divided into two major groups: the Podocopa and Myodocopa. Podocopans are typically elongate and peanut shaped whereas myodocopans are generally
rounded or ovate with a notch in the margin. Almost
any sediment or algal sample will contain ostracods;
species may be detritivores, carnivores or scavengers.
More than 40 species are known from Lizard Island
Elsewhere on the GBR, ostracods have been scarcely
studied but many more species almost certainly await
discovery.
Copepoda
Copepods are abundant on the GBR and form a major
component of zooplankton. Most are no longer than a
few millimetres, though some parasitic species reach
300 millimetres. Copepods lack a carapace, and most
free living species have a single median eye, a muchelongated pair of antennae that is held outwards, a
broadened trunk area and slender abdomen. The most
common copepods on the GBR are free living forms—
the compact harpacticoids and barrel-shaped calanoids
and cyclopoids. Harpacticoids are generally benthic,
living under rocks, in algae and sediment. Calanoids
and cyclopoids are common in the plankton where they
may form dense swarms. Free living copepods feed
variously on other zooplankton, diatoms, and algae,
and are an extremely important food source for larval
fish and decapods. Parasitic copepods feed on the
blood or tissue of their hosts. Examples include the
siphonostomatoids and poecilostomatoids that attach
to the skin of sharks and fish, and certain cyclopoids,
which have become worm-like as internal invertebrate
parasites.
Cirripedia
The sessile habit and interlocking shell plates of barnacles led early investigators to believe they were molluscs. Not until the 1830s, when barnacle larvae were
studied, were they correctly recognised as crustaceans.
Barnacles are more abundant in temperate waters than
in the tropics, but nevertheless range throughout the
GBR on all hard substrata, including the carapace of
sea turtles and the shells of other crustaceans. More
than 40 species and 20 genera of barnacles are presently
known from GBR waters. The body of most barnacles
is enclosed by a somewhat tubular or conical opened
topped chamber formed by a series of interlocking calcareous plates—usually known as the shell. The animal
is positioned on its back, and thrusts its modified,
feathery thoracic limbs, known as cirri, out of the shell
to strain food particles from the water.
Unlike most other crustaceans, barnacles are hermaphrodites, meaning that each animal possesses male
and female reproductive organs. The majority of barnacles in the reproductive phase develop a greatly elongated penis (up to 10 times their body length) that is
used to reach out of the shell to fertilise a receptive
neighbour. The fertilised eggs develop into freeswimming larvae that eventually settle on the substratum close to others of their species, identified by their
unique chemical signature.
Three major types of barnacles will be encountered
on the reef: balanomorphs (Fig. 23.1B), lepadomorphs
(Fig. 23.1C) and rhizocephalans. In balanomorph
barnacles, the so-called acorn barnacles, the shell is
firmly fixed to the substratum, be it rock, coral, mangroves or wharf piles. Common balanomorphs on
the reef flat rocks and boulders include species of
Balanus, Chthamalus, Megabalanus, Tesseropora, Tetraclita
and Tetraclitella. Not all barnacles are fixed to the substratum by the shell, however. The lepadamorphs, the
goose-necked barnacles, are attached to the substratum
by a flexible stalk, with common genera including Lepas
and Ibla. Conchoderma virgatum is unusual in using sea
snakes as a substratum. In contrast to the balanomorphs