Altered microhabitat use and movement of littorinid gastropods: the effects of parasites

The distribution of organisms at small spatial scales and their use of microhabitats are important determinants of species-level interactions. In many ubiquitous rocky shore invertebrates, use of intertidal microhabitats has previously been studied with relation to thermal and desiccation stress, ontogenetic changes and predation. Here, the effects of parasitism on the microhabitat use and movement of two New Zealand littorinid hosts, Austrolittorina antipodum and A. cincta, were investigated by examining the effect of infection by a philophthalmid trematode parasite. Alterations in microhabitat use and movement of infected versus uninfected individuals were found during both field mark-recapture and laboratory experiments, carried out from August 2012 to March 2013 in Otago Harbour, New Zealand (45.83°S, 170.64°E). Specifically, a trend towards increased use of rock surface habitats and a reduction in the distance moved by infected snails was observed. In addition, decreased downward movement was observed for some infected individuals. This alteration in individual distribution is likely to increase the availability of infected individuals to predators, hence aiding the successful transmission of the trematode parasite. These results highlight the importance of including parasitism as a biotic factor in studies of gastropod movement and spatial distribution.