Behavioural alterations in surface and cave populations of isopod crustacean Asellus aquaticus by Acanthocephalus anguillae.

Acanthocephalans are obligatory endoparasites that often alter the phenotype of their invertebrate intermediate host to facilitate trophic transmission to their final vertebrate host. Acanthocephalus anguillae, a widespread parasite of European freshwater fishes and isopod Asellus aquaticus, was recently discovered also in Postojna-Planina Cave System (Slovenia) parasitising olms (Proteus anguinus) and cave populations of A. aquaticus. This setting offers a unique opportunity to investigate potential fine-tuning of parasitic manipulations to the specifics of the highly divergent subterranean environment where some common phenotypic alterations lose functionality, but others might gain it. We measured three behavioural traits: movement activity, shelter-seeking, and response to light of infested and uninfested isopods from surface and cave populations. All behaviours were quantified from 1-h video-recordings via video-tracking isopod's movement in empty or custom modified (half-sheltered/half-illuminated) Petri dishes. Infested isopods of both populations spent significantly less time sheltering and were significantly less photophobic than uninfested ones, whereas the activity of isopods was not altered. However, we observed almost no cave-specific responses upon infestation in the two altered behaviours. It seems phenotypic alterations are not particularly fine-tuned to the subterranean environment and its hosts, and likely still reflect the parasite's surface origin.