Spatial and temporal variation of trematode infection in coexisting populations of intertidal gastropods Littorina saxatilis and L-obtusata in the White Sea

Trematode infection was studied in sympatric populations of the periwinkles Littorina saxatilis and L. obtusata in 2 regions of Kandalaksha Bay of the White Sea to assess host-parasite interactions at the population level. Tw enty-seven spatially separated populations were each surveyed in 1984-1994; 2 heavily infected populations were investigated annually over a 16 yr per iod. Ten trematode species were found in the periwinkle populations. The cl osest association in spatial distribution and temporal dynamics was observe d between 3 ecologically and morphologically similar trematodes of the 'pyg maeus' group: Microphallus piriformes, M. pygmaeus and M. pseudopygmaeus. F or these 3 species, the prevalences were closely associated in the 2 host s pecies when spatially separated sites from the 2 studied regions were consi dered, while in the 2 populations studied over the 16 yr period, a correlat ion was only observed between the infection levels of L. saxatilts and L. o btusata by either M. piriformes and immature microphallids. Likewise, withi n each host species, significant correlations were revealed between the pre valence of the different microphallids of the 'pygmaeus' groups. However, t hey were fewer and weaker when the long-term dynamics of infection in the 2 heavily infected populations were considered. Most other trematodes did no t show significant association in prevalence either within or between the 2 host species on spatial and temporal scales.