Species-specific tidal locomotion linked to a parasitic infection in sympatric sea snails

Abstract

Parasites can play a critical role in mediating inter-species interactions. Potential effects induced by parasites can range from species-wide traits to functional alterations in host community structure. One of the most intriguing host–parasite interactions pertains to adaptative host manipulation, an evolutionary occurrence where parasites alter the phenotype of their host to increase their own fitness. This study aims to address this phenomenon in a marine setting by investigating the effect of a philophthalmid trematode, Parorchis sp., on the vertical upward movement and phototactic behaviour of their intermediate hosts whilst simultaneously addressing the host specificity of these effects. These behaviours could impact the odds of trematodes successfully transmitting from their intermediate snail host to their definitive shorebird host, a crucial step in the life cycle of these parasites. Most trematode species exhibit strong specificity for snail hosts, typically infecting only a single snail species. In this rare system, however, the trematode infects a pair of sympatric and congeneric littorinid sea snails found in the intertidal zone of New Zealand’s rocky shores: Austrolittorina cincta and A. antipodum. Precisely, experiments were conducted in a controlled, laboratory-based setting, extending over a period of six weeks, during which vertical displacement and response to light were measured. Our results demonstrate that vertical upward movement amongst infected snails increased for A. cincta, but not for A. antipodum. No difference in response to light between infected and uninfected groups was evident across either species. Our findings highlight the complex nature of parasitic infections, where trait-specific behavioural effects are dissimilar across even congeneric host taxa.