Reduced attachment strength of rocky shore gastropods caused by trematode infection

For rocky shore gastropods, attachment strength is a key determinant of survival, as getting dislodged by wave action or predators has negative consequences. Yet little is known of the factors that cause inter-individual variation in attachment strength among conspecifics. Here, we test the influence of trematode infection on the suction-mediated attachment strength of periwinkles from two New Zealand species, Austrolittorina cincta and A. antipodum. Using a standardised experimental protocol, we measured both the strength of attachment of individual snails to the substrate, and its repeatability, i.e. the consistency of measurements taken on different occasions on the same individuals. We then compared the attachment of snails infected with a philophthalmid trematode with that of their uninfected conspecifics. First, we found that for a given snail mass, infected snails were easier to detach from the substrate than the uninfected ones, although this pattern was only significant for A. cincta, the larger of the two snail species. Second, the repeatability of attachment strength measurements per individual snail did not differ between infected and uninfected conspecifics, for either of the two periwinkle species. Our findings show that parasitism can weaken snail attachment, and indirectly increase snail mortality, on exposed rocky shores, suggesting a new way in which parasites can affect host population dynamics.