Two remarkable nematodes from sharks in Japan

Abstract

Two species of nematodes, Acanthocheilus rotundatus (Rudolphi, 1819) and Metaleptus rabuka Machida, Ogawa and Okiyama, 1982, have been recorded from the stomach of Mustelus griseus Pietschmann and M. manazo Bleeker (Triakinidae, Carcharhiniformes) and from the intestine of Apristurus fedorovi Dolganov (Scyliorhinidae, Carcharhiniformes), respectively, from the North Pacific Ocean off Honshu, Japan. Both species are redescribed and their cephalic structures have been studied, for the first time, by scanning electron microscopy. A. fedorovi represents a new host record for M. rabuka. The morphology of M. rabuka, particularly the presence of a somewhat trilobed pseudolabia, an undivided oesophagus and the structure of the male caudal end, shows that it is necessary to transfer the monotypic genus Metaleptus Machida, Ogawa and Okiyama, 1982 from the Physalopteroidea to the Gnathostomatoidea, where a new subfamily, Metaleptidae subfam. n., is established to accommodate it. Undoubtedly, Metaleptus represents a very old and primitive form of nematode, the morphology of which indicates affinities also to physalopterids, cucullanids and acanthocheilids.