Differential drift and parasitism in invading and native Gammarus spp. (Crustacea: Amphipoda)

Abstract

Invading and native species often interact directly, such as by predation, producing patterns of exclusion and coexistence. Less direct factors, such as interactions with the broader abiotic and biotic environment, may also contribute to such patterns, but these have received less recognition. In Northern Ireland, the North American Gammarus tigrinus has invaded freshwaters populated with the native Gammarus duebeni celticus, with intraguild predation between the two implicated in their relative success. However, these species also engage in day and night “drifting”, an activity that subjects amphipods to intense predation from fish and wildfowl. Sampling of two rivers where the invader and native co‐occur showed that, compared with the benthos, G. tigrinus was underrepresented and G. d. celticus overrepresented in the drift. In addition, G. tigrinus were free from parasites, whereas some G. d. celticus harboured the acanthocephalans Polymorphus minutus and Echinorhynchus truttae and the muscle wasting microsporidian Pleistophora sp. (new species). Compared with the benthos, G. d. celticus parasitized with P. minutus were overrepresented and unparasitized individuals underrepresented in the drift. The opposite pattern was found with Pleistophora sp. In laboratory experiments, G. tigrinus were less positively phototropic and less “active” than G. d. celticus (unparasitized animals). Polymorphus minutus increased G. d. celticus positive phototropism and activity, while Pleistophora sp. increased positive phototropism but decreased activity. Previous studies show that the invader G. tigrinus is more disadvantaged by intraguild predation from the native G. d. celticus than vice versa. However, the native appears more disadvantaged with respect to drift, parasitism and the interaction of the two. These factors may mitigate direct interactions and help explain complex patterns of coexistence between these invader and native species.