Impact of parasitism on gastropod shell morphology

Background/Question/Methods

Ilyanassa obsoleta is a marine gastropod that can be infected by nine different species of trematode parasites.  These snails are thought to live longer than 20 years, and once they become parasitized, it is unlikely that they ever become uninfected.  With such long lifespans and with long-term infections, it is possible that parasites have a large effect on shell shape.  Many studies have examined I. obsoleta and their parasites, but no study has examined how parasitism might affect shell morphology.  I. obsoleta were collected from four different sites across Long Island, NY during the summers of 2013 and 2014.  Pictures were taken of each snail which were then dissected to determine infection status, and, if infected, to identify the species of parasite.  The photographs were then analyzed using geometric morphometrics.  Landmarks were placed on each picture at key places along the shell.  A Generalized Procrustes Analysis was used to transpose and scale the landmark data, which was than analyzed using a Procrustes ANOVA. 

Results/Conclusions

Preliminary results show that snails collected from different sites have different morphologies (p< 0.5) and that snails infected with different parasite species have slightly different shell shapes (p< 0.1).  The parasite species do affect morphology, but other site-specific factors have a larger impact on shell shape.  Since snails cannot be parasitized until they are adults, factors such as habitat and presence of predators will affect shell morphology before snails become parasitized.  Both habitat and chemical cues of predators are known to affect shell morphology in other gastropod species.  Snails that reside in habitats with strong waves usually have larger apertures so they can have a good grip on the substrate and not be tossed around by the waves.  Shell lip thickness and aperture shape can change depending on what predator a snail is exposed to.  These changes in shell shape make it harder for predators to break the shell or reach the snail.  Since habitat and predator prevalence vary at each site, it is understandable why the collection site would have a large influence on shell shape.