A novel association between an endemic stickleback and a parasitic dinoflagellate. 1. Seasonal cycle and host response

Abstract

We report the first case of a dinoflagellate infection of stickleback (Gasterosteidae), discovered on an endemic population of Gasterosteus that inhabits an acidic bog lake on the Queen Charlotte Islands, western Canada. A major difference between this and other dinoflagellate infections is that the autotrophic vegetative cyst rather than the parasitic trophont is the predominant stage on the fish. During peak infection in July, 99% of the fish were infected, and cysts often covered the entire fish including the eyes. Density of cysts was highest on the dorsal surface of the fish (to 68/mm); this is possibly associated with the photosynthetic ability of the cysts. There were no consistent differences in the infection among sizes classes of fish or between the sexes. Salmonids (Oncorhynchus kisutch and Salvelinus malma), which are uncommon in the lake, also harboured cysts, but at very low densities. Host reponse to the initial infection included extensive epithelial hyperplasia, producing a layer of cells over the entire fish that enclosed the dinoflagellates. Subsequent infections were covered by additional layers of epithelium, resulting in a thick gelatinous coating. Even in cases of extreme infection, the fish exhibited no obvious behavioral indicators of pathological responses to the infection.