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Morphological differentiation between species of Pseudocalanus is very ambiguous, identification bordering impossible in juvenile stages. Adult P. acuspes can be distinguished from other species of the genus by the rounded cephalosome.
North Sea, Baltic Sea, Norwegian Sea, Spitzbergen, Arctic (Barents Sea, Nansen Basin, Amundsen Basin, Pechora Sea, Kara Sea, Laptev Sea, Canada Basin), Bering Sea
Oceanic, epi-bathypelagic
As differences between species are very subtle, for general morphological description, see genus diagnosis.
Female:
Cephalothorax stocky in lateral view, cephalosome rounded, although sometimes slightly flattened anteriorly and dorsally, scarcely extending anteriad of the rostrum. Shape of the cephalosome is somewhat dependent on body size – narrower in small specimens and more rounded in very large specimens. Posteroventral margins of the first 2 thoracic segments without, sometimes with small spiniform processes. The mediodorsal surface of one or more abdominal segments 1-3 with a short sensillum and integumental pore. Seminal receptacle small. The ratio of the lengths of the 1st and 2nd basipodites of P4 is less than 1.5.
Male:
Cephalothorax stocky, cephalosome slightly flattened between a row of long anterodorsal sensilla and 2 posterior sensilla. The mediodorsal surface of one or more abdominal segments 2-4 with a short sensillum. The 2nd abdominal segment less than 1.37 times longer than the 4th segment. The ratio of the lengths of the 1st and 2nd basipodites of P4 is less than 1.5.
