In copepods from the Family EUCALANIDAE Giesbrecht, 1892 the body elongates, especially anterior to antenna 2, they are more or less transparent, often with a triangular head. Head and pedigerous segment 1 are fused, pedigerous segments 4 and 5 are partially fused, the rostrum carries two long filaments. (modified from Bradford-Grieve, 1994 (Page 71).

The classification of species in the Eucalanidae has been confused because of the lack of distinctive sexually modified appendages and rigorous estimates of variability (Fleminger, 1973). On the basis of integumental organs, Fleminger (1973) has clarified relationships between species with the result that four groups (elongatus group, attenuatus group, subtenuis group, and pileatus group) have been identified. Later Geletin (1976) further clarified relationships between Eucalanus senso lato species and Rhincalanus based on the development of the abdomen. He identified four groups, characterised by E. hyalinus, E . attenuatus, E . subtenuis, and R. nasutus respectively. The following characters are those principally used to distinguish species in this family: the shape of the head, degree of fusion between abdominal segments, ornamentation of the urosome and metasome, the patterns of distribution of integumental organs, the presence and form of the fifth swimming leg, form and setation of the second antenna, the mandibular palp, first maxilla, and first swimming leg. It appears that the Eucalanidae feed chiefly on micro-algae (Arashkevich 1 969; Arashkevich & Timonin 1970; Itoh 1970; Samyshev 1970) although Mullin and Brooks (1967) showed an increasing preference for large animal particles as they matured. This key is specifically for the species which have remained in the genus Eucalanus after the most recent revisions. Taken from Bradford-Grieve, 1994 (Pages 71,74).