The crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

 order Tanaidacea (known as tanaids) make up a minor group within the class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Malacostraca

Malacostraca

Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds , mantis shrimp and many...

. There are about 940 species in this order.

Description

Tanaids are small, shrimp-like creatures ranging from 0.5 to 120 mm (0.0196850393700787 to 4.7 in) in adult size, with most species being from 2 to 5 mm (0.078740157480315 to 0.196850393700787 in). Their carapace covers the first two segments of the thorax

Thorax

The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

. 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

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 segments normally carry pleopods. The final segment is fused with the telson

Telson

The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

 and carries a pair of uropods.

The gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s 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.

Habitat

Most are marine, but some are also found in freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 coastal habitat

Habitat (ecology)

A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...

 or estuaries

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

. The majority of species are bottom-dwellers in shallow water environments, but a few live in very deep water, exceeding for some species 9000 metres (29,527.6 ft). In some deep sea environment, they represent the most abundant and diverse fauna to be found.

Life cycle

Tanaids do not undergo a true planktonic stage

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

. The early developmental period is spent while young are within the marsupium

Brood pouch (Peracarida)

The marsupium or brood pouch, is a characteristic feature of Peracarida, including the orders Amphipoda, Isopoda and Cumacea. It is an egg chamber formed by oostegites, which are appendices which are attached to the coxae of the first pereiopods...

 of the mother. Subsequently, post-larvae, called mancas, emerge as epibenthic

Benthos

Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

 forms. Some species are hermaphroditic

Hermaphrodite

In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

.

Taxonomy

The order Tanaidacea is divided into the following sub-orders, superfamilies and families:

Anthracocaridomorpha Sieg, 1980

• Anthracocarididae Schram, 1979
• Niveotanaidae Polz, 2005

Apseudomorpha Sieg, 1980

• Apseudoidea Leach, 1814
• Apseudellidae Gutu, 1972
• Apseudidae Leach, 1814
• Gigantapseudidae Kudinova-Pasternak, 1978
• Kalliapseudidae Lang, 1956
• Metapseudidae Lang, 1970
• Numbakullidae Gutu & Heard, 2002
• Pagurapseudidae
  Pagurapseudidae
  Pagurapseudidae is a family of crustaceans in the order Tanaidacea. Like hermit crabs, they inhabit empty gastropod shells, but can be told apart from hermit crabs by the lack of a carapace, revealing the segmentation of the thorax, and by the greater number of legs: hermit crabs have five pairs of...
  
   Lang, 1970
• Pagurapseudopsididae Gutu, 2002
• Parapseudidae Gutu, 1981
• Sphaeromapseudidae Larsen, 2011
• Sphyrapidae Gutu, 1980
• Tanzanapseudidae Bačescu, 1975
• Whiteleggiidae Gutu, 1972
• Cretitanaoidea Schram, Sieg, Malzahn, 1983
• Cretitanaidae Schram, Sieg, Malzahn, 1983
• Jurapseudoidea Schram, Sieg & Malzahn, 1986
• Jurapseudidae Schram, Sieg & Malzahn, 1986

Neotanaidomorpha Sieg, 1980

• Neotanaidae Lang, 1956

Tanaidomorpha Sieg, 1980

• Tanaoidea Dana, 1849
• Tanaidae Dana, 1849
• Paratanaoidea Lang, 1949
• Agathotanaidae Lang, 1971
• Anarthruridae Lang, 1971
• Colletteidae Larsen & Wilson, 2002
• Cryptocopidae (McLelland, 2008 MS) Bird & Larsen, 2009
• Leptochelidae Lang, 1973
• Leptognathiidae Lang, 1976
• Nototanaidae Sieg, 1976
• Mirandotanaidae Blazewicz-Paszkowycz & Bamber, 2009
• Paratanaidae Lang, 1949
• Pseudotanaidae Sieg, 1976
• Pseudozeuxidae Sieg, 1982
• Tanaellidae Larsen & Wilson, 2002
• Tanaissuidae Bird & Larsen, 2009
• Teleotanaidae Bamber, 2008
• Typhlotanaidae Sieg, 1986
• Tanaidomorpha incertae sedis
• Alavatanaidae Vonk & Schram, 2007