Ostracoda
Identification

Pelagic ostracods are collected live, and their taxonomy is based extensively on their limb morphology, unlike many benthic podocopids whose taxonomy, developed by palaeontologists, is based extensively on carapace characteristics (as for subfossil or fossil material). The use of carapace characters gives continuity between the systematics of living and fossil taxa, but confusion can arise between species which are either close siblings or show marked convergence. In the planktonic species, the sole use of carapace characters can lead to confusion between some closely related species, but an experienced field ecologist will be able to identify most adult specimens on the basis of external characters such as size, shape, ornamentation and the positions of the gland openings. Identification of juvenile instars is much more of a problem, and for the earliest three or four stages it is largely guesswork.

A key to the South Atlantic planktonic ostracods is not available. Identification of species should be carried out by comparison of family, generic and specific characters. Pelagic ostracods belong to the Order Myodocopida, which is devided into two suborders: the Halocypridina and the Myodocopina. First step in the identification is to distinguish between halocyprids (Suborder Halocypridina: Family Halocyprididae) and myodocopids (Suborder Myodocopina: Family Cypridinidae). In particular in shallow-living species the difference between halocyprids and myodocopids is readily made on the basis of the absence of obvious light receptors. In most myodocopids living at depths shallower than about 2000 m, compound eyes are clearly visible through the transparent carapace, situated close to the insertion of the adductor muscles. The adults of the mesopelagic genus Macrocypridina are immediately recognisable on the basis of the dark chocolate-brown colour of the carapace, with an unpigmented window in each valve overlying the compound eyes. The large globular bathypelagic species of Gigantocypris lack compound eyes, but have large and elaborate naupliar eyes with mirrored reflectors (Land, 1978). Many abyssal and trogloditic (cave-dwelling) myodocopids have also lost both compound and naupliar eyes. Halocyprids lack obvious visual receptors, but must have some light sensitivity because they undertake diel vertical migrations synchronised with the day/night cycle. The frontal organ, which is generally considered to be chemosensory, has nerve tissue with an ultrastructure that is suggestive of some light sensitivity (Anderssen, 1977).

Editor's note: the scheme below enables to navigat from this page between the different ostracod pages in the Higher Taxa module during the process of identification.

1a) Suborder Halocypridina: Family Halocyprididae
2a) Subfamily Halocyprinae (three genera, six species)
2b) Subfamily Conchoecinae (complex of 23 genera and/or arbitrary species groups—the latter based on key characters, 104 species)
2c) Subfamily Archiconchoecinae (monogeneric, 12 species)
2d) Subfamily Euconchoecinae (two genera, one South Atlantic species)
1b) Suborder Myodocopina: Family Cypridinidae (two genera, four species)