The effect of trematode parthenitae on the growth of Onoba aculeus, Littorina saxatilis and L. obtusata (Gastropoda: Prosobranchia)

To determine whether modern theories predict snail growth responses to trematode infestation a field growth study of Onoba aculeus, Littorina saxatilis and L. obtusata infected with two closely related Microphallidae trematodes was conducted in the White Sea. In each of the three host–parasite combinations studied trematode infection causes inhibition of snail reproduction. However, contrary to the classical interpretation of the gigantic growth of infected snails as a side effect of parasitic castration, the presented study failed to demonstrate that Microphallus piriformes causes gigantism in Littorina. The infection tended to stunt growth in L. obtusata and had no significant effect on growth rate of L. saxatilis. In contrast, gigantic growth was observed in O. aculeus infected with M. pseudopygmaeus. Considering that both trematode species are very similar biologically, the discrepancy is attributed to differences in the life history of the snail's hosts. Onoba aculeus is a relatively short-lived snail (2.5–3 y). The lifespan of L. saxatilis and L. obtusata is much longer (up to 9–11 y). These findings agree with a previously reported ‘energetic’ hypothesis that predicts growth alterations in accordance with life history variations of the snail species.