1 Total body length greater than 2 mm.....Calanus pacificus
1' Total body length less than 2 mm.....2
2 (1') Posterior corners of prosome produced as rounded spines, urosome asymmetrical.....Tortanus discaudatus
2' Posterior corners of prosome not angular or produced as spines; urosome of three segments......3
3 (2') Last segment of metasome rounded, with 3-5 marginal spinules.....Acartia clausi
3' Urosome of four segments......4
4 (3') Total body length less than 1 mm.....Microcananus pygmaeus pusillus
4' Total body length greater than 1 mm.....5
5 (4')Reduced 5th leg present and urosome relatively thick......Paracalanus parvus
6 (5')Genital segment swollen.....Pseudocalanus minutus
6' 1st antennae longer than the body.....Sinocalanus brevicaudatus
7 Urosome of 5 segments or more with 5th leg on 1 segment of urosome and short 1st antennae.....Oncaea borealis
Page author: Matthew P. Kweskin