Copepod parasites of the fatheads (pisces, psychrolutidae) and their implication on the phylogenetic relationships of psychrolutid genera.

Nine species of psychrolutids kept in the Marine Zoology of Hokkaido University were found to carry eight species of copepod parasites. The parasites and their hosts are: Bobkabata kabatabobbus Hogans and Benz, 1990 on Malacocottus zonurus Bean; Chondracanthus parvus n. sp. on Eurymen gyrinus Gilbert and Burke; Chondracanthus yabei n. sp. on Dasycottus setiger Bean and M. zonurus; Ch. yanezi Atria, 1980 on Psychrolutes phrictus Stein and Bond; Caligus similis n. sp. on Neophrynichthys latus (Hutton); Clavella adunca (Strøm, 1762) on M. zonurus; Neobrachiella amphipacifica Ho, 1982 on Ambophthalmos angustus (Nelson), Cottunculus sp., D. setiger, Ebinania brephocephala (Jordan and Starks), E. vermiculata Sakamoto, and P. phrictus; and Naobranchia occidentalis Wilson, 1915 on D. setiger and M. zonurus. Chondracanthus parvus is closest to Ch. deltoideus Fraser, 1920, but differs from it in having only one pair of small knobs on the head and carrying a pair of lateral processes on the second pediger. Chondracanthus yabei resembles Ch. yanezi Atria, 1980, but can be distinguished from it by the presence of three, low protrusions on the mid-dorsal surface of the trunk; besides, maxillule also shows difference. As the name indicates, C. similis resembles several species of Caligus that bear a short abdomen and a formula of I; IV on the exopod of leg 4. However, it can be separated from them by the combination of the following characters: a genital complex distinctly smaller than the cephalothoracic shield, a pair of relatively short caudal rami, a smooth dentiform process on maxillule, and a pair of truncate tines on sternal furca. Analysis of the occurrence of Chondracanthus on the psychrolutid fishes shows that the phylogeny of Chondracanthus is in congruence with that of the Psychrolutidae. It implies that parasitism of Chondracanthus occurred after the Dasycottus clade diverged off the main stock of the Psychrolutidae and the occurrence of Ch. yabei on D. setiger is resulted from a later colonization.