Journal of Eish Diseases 1987, 10, 241-247 SHORT COMMUNICATION Ultrastructure and pathogenesis of Ichthyobodo sp. from wild common dab, Limanda limanda L., in the North Sea A. DIAMANT Department of Agriculture and Fisheries for Scotland, Marine Laboratory, Aberdeen, Scotland Increasing reports on the occurrence of ichthyobodiasis in the marine environment have aroused interest in the responsible agent, the bodonid flagellate Ichthyobodo necator (Hen- neguy, 1883). Initially, this ectoparasite was observed in sea water in Scotland on young cultured salmonids (Needham & Wootten 1978), and it was suggested that the parasite had persisted in infected fish transferred from fresh water (Elhs & Wootten 1978). Subsequently, it was found in marine flatfish: wild plaice, Pleuronectes platessa L., off the coast of west Scotland (Bullock & Robertson 1982) as well as in winter flounder, Pseudopleuronectes americanus (Walbaum), off Newfoundland, Canada (Cone & Wiles 1984). In all cases, the agent was identified as /. necator, although this species is normally considered incapable of survival at raised salinities and can be eliminated from culture facilities by bathing affected fish in a 1% NaCl solution for 30 min (Van Duijn 1973). The occurrence of the parasite on wild marine fish was, therefore, perplexing, and it was assumed to have originated from coastal freshwater sources and adapted to sea water. Morrison & Cone (1986) recently reported an Ichthyobodo infection on the gills of haddock, Melanogrammus aeglefinus (L.), caught in the north-west Atlantic, 120 km off the Nova Scotian coast. It was clear that the parasites in this case were part of a truly marine population and, in all hkelihood, represented a separate race or species of Ichthyobodo (Morrison & Cone 1986). They suggested that a similar marine Ichthyobodo sp. may have been responsible for the infections reported in plaice and winter flounder (Bullock & Robertson 1982; Cone & Wiles 1984). During a study of X-cell disease of common dab, Limanda limanda L., in the North Sea, trophozoites of a flagellate fitting the description of /. necator were found on some of the examined fish. As common dab is purely a marine fish, and the infections were found nearly 30 km offshore, it appeared that this was an additional case of a truly marine Ichthyobodo population. The infection was associated with gill pathology, and some ultrastructural features of the host-parasite relationship are reported here. Common dabs were collected on board FRV Goldseeker during May 1986 from depths of 40-100 m, 12 to 29 km off Buckie (57°50'N, 02°50'W) in the Moray Firth, north-east Scotland. Salinities in this region are reportedly 34-6-34-8%o during May, the annual mini- mum being about 34-2%o (Adams & Martin, in press). Smears and small pieces of tissue for electron microscopy were taken from 23 fish (TL Correspondence: Dr A. Diamant, National Center for Mariculturc, Israel Oceanographic and Limnological Research Ltd, PO Box 1212, Eilat 88112, Israel. 241