23 – Arthropoda: Crustaceans and Pycnogonids
265
and lepadomorphs that simply use the substratum as
an anchor point, the rhizocephalan barnacles have an
entirely different lifestyle as partially internal and partially external parasites of decapod crustaceans. Rhizocephalans have lost any resemblance to typical adult
barnacles, including loss of the calcareous plates. A
common rhizocephalan, Sacculina, infects crabs and is
visible externally only as a fleshy mass beneath the abdomen of the crab. Rhizocephalans interfere with the
reproductive cycle of crabs and through the effect of
hormones, cause male crabs to slowly develop female
characteristics, an effect known as ‘parasitic castration’.
Peracarida
The peracarids are a large group of malacostracan
crustaceans recognisable by the possession of the
lacinia mobilis (a blade like process on the mandible),
and direct development of young, which are brooded
in a pouch formed by lamellar outgrowths of the
bases of the thoracic limbs (with the exception of the
thermosbaenaceans, which brood their eggs under
the carapace). In most cases, peracarids are minute,
not exceeding 10 mm in length (many individual exceptions exist of course, such as the giant deep-sea
isopod, Bathynomus). Despite their small size, however, peracarids are highly speciose and abundant in
most habitats. Of the nine generally recognised peracarid orders, the most important on the GBR are
the Mysidacea, Cumacea, Tanaidacea, Isopoda, and
Amphipoda.
Mysidacea: opposum shrimps
The mysidaceans (Fig. 23.1D) resemble juvenile decapod shrimps, and usually do not exceed 20 mm in
length. Though mysids superficially resemble decapod
shrimps, an important distinction is the position of the
statocyst (balance organ resembling a circular pore).
The statocyst of mysids is located at the base of each
branch of the tailfan, instead of the base of the first pair
of antennae in decapods. Unlike other peracarids,
mysidaceans have a well-developed carapace and
stalked, instead of fixed, eyes. Most mysids are omnivorous suspension feeders, and form dense schools near
the bottom or in the water column. Some species, however, have more particular associations, such as the red
and white Heteromysis harpaxoides, which lives with
hermit crabs inside the gastropod shell. More than 800
species in more than 120 genera occur worldwide, with
around 50 species on the GBR.
Cumacea: comma shrimps
The cumaceans (Fig. 23.1E) are distinctive because of
their bulbous carapace and slender abdomen that terminates in a pair of stick-like uropods. They burrow in
reef sediments or seagrass beds, and feed on detritus,
algae and microbes. At night, cumaceans ascend into
the water column to moult and mate. Of the approximately 1300 species of cumacean worldwide, around
20 species are known from Queensland of which several are known from the GBR.
Tanaidacea
The tanaids (Fig. 23.1F), usually less than 10 mm
length, resemble a slender isopod (below) with a pair
of chelipeds. A unique feature of the tanaids is the fusion of the head with the first and second thoracic segments. Most tanaids fall into one of two major groups:
the apseudomorphs, which have a somewhat flattened
body, and tanaidomorphs, with a cylindrical body.
Though seldom seen in the open, tanaids are abundant
in reef sediments, seagrass, algae and hydrozoans
where they burrow or build tubes from sand and fine
particles. Despite their numerical abundance, however, the GBR tanaids are poorly known with only five
of more than 800 known species formally recorded
from the region.
Isopoda: slaters, pill bugs, fish lice
The isopods are a remarkably diverse group of peracarids that have colonised almost all habitats, from
terrestrial leaf litter to abyssal depths. Isopods are
characterised by fixed, immovable eyes, no carapace,
seven pairs of thoracic legs, five pairs of pleopods, and
a pleotelson (formed by fusion of the last two body
segments). Most isopods have the familiar, flattened,
oval body shape. The largest isopod, the deep-sea
Bathynomus giganteus, reaches 35 cm in length, but on
the GBR, most species are less than 10 mm long. Some
265
and lepadomorphs that simply use the substratum as
an anchor point, the rhizocephalan barnacles have an
entirely different lifestyle as partially internal and partially external parasites of decapod crustaceans. Rhizocephalans have lost any resemblance to typical adult
barnacles, including loss of the calcareous plates. A
common rhizocephalan, Sacculina, infects crabs and is
visible externally only as a fleshy mass beneath the abdomen of the crab. Rhizocephalans interfere with the
reproductive cycle of crabs and through the effect of
hormones, cause male crabs to slowly develop female
characteristics, an effect known as ‘parasitic castration’.
Peracarida
The peracarids are a large group of malacostracan
crustaceans recognisable by the possession of the
lacinia mobilis (a blade like process on the mandible),
and direct development of young, which are brooded
in a pouch formed by lamellar outgrowths of the
bases of the thoracic limbs (with the exception of the
thermosbaenaceans, which brood their eggs under
the carapace). In most cases, peracarids are minute,
not exceeding 10 mm in length (many individual exceptions exist of course, such as the giant deep-sea
isopod, Bathynomus). Despite their small size, however, peracarids are highly speciose and abundant in
most habitats. Of the nine generally recognised peracarid orders, the most important on the GBR are
the Mysidacea, Cumacea, Tanaidacea, Isopoda, and
Amphipoda.
Mysidacea: opposum shrimps
The mysidaceans (Fig. 23.1D) resemble juvenile decapod shrimps, and usually do not exceed 20 mm in
length. Though mysids superficially resemble decapod
shrimps, an important distinction is the position of the
statocyst (balance organ resembling a circular pore).
The statocyst of mysids is located at the base of each
branch of the tailfan, instead of the base of the first pair
of antennae in decapods. Unlike other peracarids,
mysidaceans have a well-developed carapace and
stalked, instead of fixed, eyes. Most mysids are omnivorous suspension feeders, and form dense schools near
the bottom or in the water column. Some species, however, have more particular associations, such as the red
and white Heteromysis harpaxoides, which lives with
hermit crabs inside the gastropod shell. More than 800
species in more than 120 genera occur worldwide, with
around 50 species on the GBR.
Cumacea: comma shrimps
The cumaceans (Fig. 23.1E) are distinctive because of
their bulbous carapace and slender abdomen that terminates in a pair of stick-like uropods. They burrow in
reef sediments or seagrass beds, and feed on detritus,
algae and microbes. At night, cumaceans ascend into
the water column to moult and mate. Of the approximately 1300 species of cumacean worldwide, around
20 species are known from Queensland of which several are known from the GBR.
Tanaidacea
The tanaids (Fig. 23.1F), usually less than 10 mm
length, resemble a slender isopod (below) with a pair
of chelipeds. A unique feature of the tanaids is the fusion of the head with the first and second thoracic segments. Most tanaids fall into one of two major groups:
the apseudomorphs, which have a somewhat flattened
body, and tanaidomorphs, with a cylindrical body.
Though seldom seen in the open, tanaids are abundant
in reef sediments, seagrass, algae and hydrozoans
where they burrow or build tubes from sand and fine
particles. Despite their numerical abundance, however, the GBR tanaids are poorly known with only five
of more than 800 known species formally recorded
from the region.
Isopoda: slaters, pill bugs, fish lice
The isopods are a remarkably diverse group of peracarids that have colonised almost all habitats, from
terrestrial leaf litter to abyssal depths. Isopods are
characterised by fixed, immovable eyes, no carapace,
seven pairs of thoracic legs, five pairs of pleopods, and
a pleotelson (formed by fusion of the last two body
segments). Most isopods have the familiar, flattened,
oval body shape. The largest isopod, the deep-sea
Bathynomus giganteus, reaches 35 cm in length, but on
the GBR, most species are less than 10 mm long. Some