The
crustacean order Tanaidacea (known as tanaids) make up a minor group within the
classMalacostraca. There are about 940 species in this order.
Description
Tanaids are small, shrimp-like creatures ranging from 0.5 to 120 millimetres (0.020 to 4.7 in) in adult size, with most species being from 2 to 5 millimetres (0.08 to 0.2 in). Their carapace covers the first two segments of the
thorax. There are three pairs of limbs on the thorax; a small pair of
maxillipeds, a pair of large clawed
gnathopods, and a pair of
pereiopods adapted for burrowing into the mud. Unusually among crustaceans, the remaining six thoracic segments have no limbs at all, but each of the first five
abdominal segments normally carry
pleopods. The final segment is fused with the
telson and carries a pair of
uropods.[1]
The
gills lie on the inner surface of the carapace. The thoracic limbs wash water towards the mouth, filtering out small particles of food with the mouthparts or maxillipeds. Some species actively hunt prey, either as their only food source, or in combination with filter feeding.[1]
Habitat
Most are marine, but a handful are also found in
freshwater. They live buried in bottom sediments, sometimes in self-built tubes.[2]
Life cycle
Tanaids do not undergo a true
planktonic stage. The early developmental period is spent while young are within the
marsupium of the mother. Subsequently, post-larvae, called
mancas, emerge as
epibenthic forms. Some species are
hermaphroditic.[1]
Taxonomy
The oldest representatives of the group are known from the
Carboniferous, with modern forms emerging during the
Mesozoic.[2]
The order Tanaidacea is divided into the following sub-orders, superfamilies and families:[3]