Short-term thymectomized (two months after thymectomy) adult rainbow trout, Oncorhynchus mykiss (Walbaum) infected with the flagellate Cryptobia salmositica Katz, 1951 responded well during primary infection with C. salmositica and recovered fish also showed secondary response (rapid production of complement fixing antibody after homologous challenge) five months after recovery. Complement fixing antibody was detected during primary and secondary responses and the titres of complement fixing antibody in thymectomized fish were significantly lower than those in infected intact fish. The parasitaemia in thymectomized infected fish was significantly lower than in intact infected fish. Both recovered thymectomized fish and intact fish were protected from cryptobiosis when they were challenged. Similarly, long-term thymectomized fish (nine months after thymectomy) vaccinated with an attenuated strain of C. salmositica were protected from cryptobiosis. There were no significant difference (P > 0.05) in parasitaemia, packed cell volume and complement fixing antibody titres between vaccinated/challenged thymectomized and vaccinated/challenged intact fish. Hence, thymectomy in adult rainbow trout did not decrease the detectable complement fixing antibody against C. salmositica in long-term thymectomized fish but reduced the detectable protective antibody in short-term thymectomized fish.
The distribution and coexistence of gill ectoparasites of 121 specimens of Cephalopholis argus Bloch et Schneider, caught between October 1994 and October 1995, were investigated. Adults of the monogenean Benedenia sp. and copepod Hatschekia sp., the larval caligid copepod Caligus sp. (copepodite and chalimus stages), and praniza larvae of the isopod Gnathia sp. were found. All species were aggregated within the host population. Infracommunities were poor, with only 40.5% of fish infected by two parasite species. Only two individual fish harboured all the parasite species observed at the component community level. Prevalences were less than 50% and mean intensities were low (less than 6 parasites/host). No dominant parasite species were observed in the host population. The spatial distribution of each parasite species was studied on different partitions of the gill arches. Adult parasite stages that are mobile showed much overlap in their distribution, whereas temporarily attached larvae of Caligidae were more site specific. Copepodite and chalimus larvae showed niche restriction that is probably due to gregarious behaviour. Positive associations between caligid larvae reflected intraspecific interaction for site and/or resources. Each of the Caligus sp. larval stages prefers specific sites, as do the adults, which occur exclusively in the buccal cavity of the host. Infracommunities were too poor and too few to induce processes of interspecific competition.