Morphological characters for distinguishing among Calanus sinicus, C. jashnovi and
C. pacificus are described for all copepodite stages using specimens collected from the western North Pacific. We applied species-specific polymerase chain reaction (PCR) to identify the three species. The three species differ morphologically either in
(i) prosome length (PL), (ii) length and width of caudal ramus, or (iii) shape and width of cephalosome. At copepodite stages III–VI female (CIII–CVI female), PLs of C. jashnovi were greater than those of both in C. sinicus and C. pacificus. At CIII, PL of C. jashnovi slightly overlapped with that of C. sinicus, but C. jashnovi also differed from the other species in caudal ramus length (CRL) and width (CRW). CIII–CVI females of C. sinicus and C. pacificus differed in CRL. At CI–CII, the C. sinicus cephalosome was oval-shaped, with a minor axis longer than that of C. pacificus, whereas
C. jashnovi had a rhomboid rather than oval cephalosome. Species composition of Calanus copepodites was investigated at a location in the Oyashio–Kuroshio mixed region using the aforementioned morphological characters. Co