An updated key, based on the females, to the eight species groups currently recognized within Caligus

1 Leg 4 with compound segment (2 & 3) with III apical spines, seta 4 of leg 1 last exopodal segment distinctly longer than terminal spines........................................................................ C. macarovi group

- Leg 4 with compound segment (2 & 3) with IV spines (I lateral and III apical spines)............................... 2

- Leg 4 with 3 segments armed with I, I, and III spines respectively............................................... 3

2 Leg 1 last exopodal segment inner margin without 3 pinnate setae or with only a vestige of a seta distally and leg 2 2 nd endopodal segment outer margin armed with long spinules............................................. C. productus group

- Leg 1 last exopodal segment inner margin with 3 pinnate setae................................................. 4

3 Leg 3 1 st exopodal segment with distolateral spine forming a large, recurved hook, apron with raised bifid cuticular rib and arrangement of denticles................................................................. C. confusus group

- Leg 3 without distolateral spine being a large, recurved hook and apron without rib and arrangement of denticles, leg 2 2 nd & 3 rd endopodal segments covered with fine denticles, distolateral spines of leg 2 2 nd and 3 rd exopodal segments align with longitudinal axis of rami................................................................ C. diaphanus group

4 Leg 2 2 nd endopodal segment outer margin with large denticles, distolateral spines on 1 st and 2 nd exopodal segments lying obliquely across axis of rami, distolateral spine on 3 rd segment at most halve length or smaller than adjacent (distal) spine.......................................................................................... C. bonito group

- Leg 2 2 nd endopodal segment outer margin with or without fine setules, only 1 small distolateral/distal spine on 3 rd exopod segment, no sternal furca................................................................. C. trichiuri group

- Leg 2 2 nd endopodal segment outer margin with or without fine setules, 3 rd exopodal segment with distolateral and distal spines of varying sizes, sternal furca present..................................................................... 5

5 Leg 2 3 rd exopodal segment distolateral spine markedly reduced, distal spine also relatively small, genital complex as long as wide without posterolateral lobes and as long as abdomen.................................. C. pseudorhombi group

- Leg 2 3 rd exopodal segment distolateral spine often halve length of distal spine which may also be reduced, genital complex longer than wide..................................................................... C. undulatus group