A new species of Peniculus (Copepoda: Siphonostomatoida) parasitizing mesopelagic myctophid fish: first discovery of colonization of the genus in deep water.

Peniculus hokutoae n. sp. is described on the basis of an ovigerous adult female parasitizing the caudal fin of the myctophid fish Symbolophorus evermanni (Gilbert, 1905), collected from Suruga Bay, Japan. This is the first record of parasitism by this genus on mesopelagic myctophid fish. The new species is easily distinguished from other congeners in: (1) the presence of a conical process anterior to the rostrum; (2) the secondary elongation of the first pedigerous somite; (3) the incorporation of the third and fourth pedigerous somites into the trunk; (4) the unilobate maxillule bearing two unequal apical setae; (5) the lack of any processes on the first segment of the maxilla. Four morphological patterns of the cephalothorax, neck and anterior parts of the trunk can be found in the genus. We infer that initial colonization of a mesopelagic myctophid fish as host is likely to have occurred when the diurnally-migrating myctophid host was feeding in near-surface waters at night and was exposed to infective stages of Peniculus.