Members of the family Calanidae are medium to large-sized copepods: tropical species are slightly less than 2 mm long while Antarctic species are up to 6 mm long. The genera of this family were reviewed by Bradford & Jillett (1974) and Bradford (1988). It is still likely that not all species have been described in this family, especially in Calanoides and it appears there may be cryptic species in some genera (e.g. Bucklin 1998).

The best-known member of the Calanidae is probably Calanus finmarchicus. Many of the details of its anatomy, reproduction, development, life cycle, chemical composition, food and feeding, respiration, vertical migration and parasites are known (Marshall & Orr 1955) although these data have been supplemented by numerous, more recently published works (see Mauchline 1998). They are exclusively pelagic with an epipelagic to mesopelagic habit (Bradford-Grieve 1994). A number of genera and species are found in greatest numbers in open coastal regions although others are oceanic in their distribution.

Many species perform ontogenetic vertical migrations and are capable of over-wintering at mesopelagic depths. Reproduction is usually coupled to euphotic-zone primary production except for a number of species of Neocalanus where mating and reproduction occurs at mesopelagic depths in winter (Andrews 1966; Jillett 1968; Kitou & Tanaka 1969; Petit & Courties 1976; Matsuzaki 1978; Ohman et al. 1989). This downward migration may contribute substantially to carbon flux to the interior of the ocean in the case of Neocalanus species (Bradford-Grieve et al. 2001) although in the North Atlantic this downward flux is thought to be small (Longhurst & Williams 1992).

Members of this family have been classified as herbivores based on the morphology of their mouthparts (Itoh 1970), but are more likely to be omnivores (Arashkevich 1969; Arashkevich & Timonin 1970; Harding 1974; Landry; 1981; Greene & Landry 1988; Atkinson 1996). Chapman (1981) demonstrated that Neocalanus is capable of taking up glucose from seawater; this supplemental nutrition may be important during periods of reduced primary production.

The Calanidae are amongst the more recently evolved families of the Calanoida. Several morphological characteristics support this view. For example, the nerve axons of the Calanidae are myelinated (Davis et al. 1999; Lenz et al. 2000), a relatively rare occurrence among invertebrates (Weatherby et al. 2000) whereas, the Arietelloidea and Diaptomoidea do not exhibit such a derived character. The female genital apparatus is not separated into two parts on either side of the genital double somite. Rather, it retains a pair of seminal receptacles and has the gonopores situated together in the midline, covered by a common genital operculum. It is thought that the Calanidae arose in the Permian, 248-290 million years ago, when the oceans are thought to have become less turbid and were undergoing the first really deep ventilation (Bradford-Grieve 2002). Most of the Calanidae have short-lived, non-feeding males implying they evolved at a time when primary production was highly seasonal and the synchronising of the readiness to breed would have been readily achieved.

Identification

The following characters used principally to distinguish species in this family are for both sexes: whether or not there are teeth on the inner border of the coxa of leg 5, whether or not endopod segment 1 of leg 1 has an inner edge seta; female: the number of setae on the leg 5 endopod and on praecoxal endite 1 of the maxilla; male: whether or not the left leg 5 is prehensile, the number of setae on the endopods of leg 5, which segments of the antennule are fused, whether the mouth parts are reduced and whether or not exopod segment 3 of the right leg 5 has inner edge setae.