Four new species of splanchnotrophid copepods (Poecilostomatoida) parasitic on doridacean nudibranchs (Gastropoda, Opistobranchia) from Japan, with proposition of one new genus

Abstract. Four new species of splanchnotrophid copepods are described based on specimens collected from 5 species of doridacean nudibranchs from coastal waters of Japan. They belong to 3 genera, one of which, Majimun gen. n., is new. The parasites and their hosts are as follows: Ceratosomicola japonica sp. n. ex Hypselodoris festiva (A. Adams); Splanchnotrophus helianthus sp. n. ex Thecacera pennigera (Montagu); Splanchnotrophus imagawai sp. n. ex Trapania miltabrancha Gosliner & Fahey; and Majimun shirakawai gen. et sp. n. ex Roboastra luteolineata (Baba) and Roboastra gracilis (Bergh). Ceratosomicola japonica sp. n. is the fifth species of Ceratosomicola and is characterized by the shape and armature of the prosome in females. Both Splanchnotrophus helianthus sp. n. and Splanchnotrophus imagawai sp. n. are differentiated from 4 known congeners by the absence of posterolateral processes or lobes on the prosome in females, and the females of these 2 new species are separated from each other by the shape and armature of the genito-abdomen, the mandible, and the swimming legs. Majimun gen. n. is distinguished from other splanchnotrophid genera by the segmentation of the antennule as well as the combination of the following characters in females: 2 postgenital somites and the shape of the antenna, the mandible and the swimming legs.