Manipulation of a mollusc by a trophically transmitted parasite: convergent evolution or phylogenetic inheritance?

We investigated the influence of infection by the trematode Curtuteria australis on the burrowing behaviour of its intermediate host, the bivalve Austrovenus stutchburyi. Laboratory experiments and field observations revealed that cockles, unable to bury completely or even partially under the sediment, have a reduced foot length compared with buried individuals. The ability to bury proved to be highly repeatable in field experiments: cockles found at the surface and transplanted to an experimental area did not bury themselves, and cockles found buried stayed buried when relocated. All metacercariae of C. australis were found strictly in the foot and for each of 3 samples collected in different sites, there was a negative and significant relationship between the relative length of the foot and the parasite load. A predation test conducted under natural conditions indicated that cockles with the stunted foot and the altered behaviour are significantly more susceptible to predation by aquatic birds than other cockles. Given that the definitive host of C. australis is an oystercatcher, we first discuss our results in the context of transmission strategy. Comparisons with other studies on more or less related trematode species parasitic in bivalves and evolving under similar constraints for their transmission, shed light on the origin of this adaptation in C. australis.