First description of the female of Bradyidius angustus (Calanoida, Aetideidae) with descriptions of the male and copepodids

抄録

The aetideid copepod Bradyidius angustus was originally described 67 years ago based on two males collected from the Pacific coast of central Japan. This species has been the only one with the female unknown in the genus. Based on specimens from the Seto Inland Sea, Japan, this study describes the female of B. angustus for the first time, together with a complementary description of the male and a brief description of the later copepodids. The female has no seta on the medial margin of the exopod terminal segment of the antenna, which is a unique character within the genus. The distinction of the female from the currently known 18 congeners is also possible by a combination of a body length less than 2.0 mm and ornamentation on the legs, the antenna exopod or the mandible basis. The male is distinct from all congeners with regards to leg 5, the ornamentation of the endopod of legs 3 and 4, or the rostrum. The male specimens have some minor differences from the figures of the original description. The later copepodids (CIV and CV) have a stockier prosome than the adult, but are similar to the adult in some easily observable morphologies, by which they could be possibly identified to the species. This species is considered to be an epipelagic form. Discussion is also made on a past description of B. armatus from Japan.