A new genus and species of nicothoid copepod (Crustacea: Copepoda: Siphonostomatoida) parasitic on the mysid Siriella okadai Ii from off Japan.

Neomysidion rahotsu, a new genus and species of the siphonostomatoid family Nicothoidae Dana, 1849, is described from the marsupium of the mysid Siriella okadai Ii in the shallow waters of the Seto Inland Sea of Japan. This is the first record of nicothoids parasitic on mysids in Asian waters. The new genus is closely related to the three other mysid-infesting nicothoid genera, Aspidoecia Giard & Bonnier, 1889, Mysidion Hansen, 1897 and Hansenulus Heron & Damkaer, 1986, but is distinguishable by: the trunk of adult females being remarkably transformed at maturity; the male cephalosome covered with numerous rounded knobs posterodorsally; the absence of caudal rami in the female; three-segmented antennules in both sexes; two-segmented antenna in the male; remarkable sexual dimorphism in the maxillae; the presence of a pair of spinulate sclerites between the maxillae in both sexes; three- segmented maxillipeds in both sexes; the absence of legs 1 to 5 in the female; the presence of two pairs of legs in the male; and egg-sacs laid free.