Distribution ofCrassiphiala bulboglossa, a parasitic worm, in shoaling fish

Abstract

1. The distribution of transmittable parasites in group‐living host species has been investigated in a number of studies. However, little is known about the distribution of parasites with indirect life cycles in this context. This study investigates the relationship between parasitism and shoal composition by looking at the distribution ofCrassiphiala bulboglossa(Haitsma), a trematode worm which uses fish as an intermediate host, within and between fish shoals in a lake. In particular, tests aimed at establishing whether the parasite distribution was affected by parasite‐assortative shoaling behaviour, host group size, host habitat characteristics, and other factors (such as species identity and body length) that potentially covary with parasite infection.2.Overall, 34 fish shoals were caught. For each shoal member, the fish species was identified, its standard body length measured and its parasites (C. bulboglossa) counted. Three habitat characteristics were also recorded for each shoal: water depth and distances to shore and to vegetation.3.Crassiphiala bulboglossaprevalence and load varied with fish species. Only 6% of the golden shiners [Notemigonus crysoleucas(Mitchill)] were parasitized, whereas 24% of the killifish (Fundulus diaphanus(Lesueur)] and 98% of the white suckers [Catostomus commersoni(Lacépède)] were infected. Infected shiners and killifish had very low parasite loads, whereas suckers were heavily parasitized. Parasite prevalence within shoals was not correlated with the average body lengths of killifish and shiners in shoals (sample size was too small for testing in suckers).4.Killifish shoals showed parasite prevalences significantly different from those expected under a null hypothesis that assumes assortment of fish between shoals by size and species but not by parasitism. In shiners and suckers, no evidence for non‐random distribution of parasites among shoals was found, which may have been because of lack of statistical power.5.Parasite prevalence was related to the number of conspecifics in a shoal for shiners but not for killifish.6.There was no significant relationship between parasite prevalence and habitat characteristics in shiners and killifish.