Effect of trematode parasites on cockle Cerastoderma edule growth and condition index: a transplant experiment

We quantified the effect of trematode parasites in cockles through a 303 d transplant experiment. Cockles were sampled in 3 different sites on the southwest coast of France characterized by 3 different trematode communities, i.e. Royan (RO): high Meiogymnophallus minutus abundance near cockle heart, South Arguin (SA): high Himasthla interrupta abundance in the mantle margin, and North Arguin (NA): almost free of trematodes. Sampled cockles were individually tagged and placed in different cages at Banc d’Arguin, Arcachon Bay. At the end of the experiment, RO cockles had lost their trematodes, highlighting the fact that trematode lifespan can be short, while SA cockles maintained their trematode load. When cockles were the second intermediate host, there was no effect of site of origin on cockle condition index and survival. The effect on growth rate was mostly due to initial cockle length. Within each treatment, the only significant effect of trematode infection on growth was found for H. interrupta at SA. The 23% growth deficit observed could be due to the disruption of cockle shell synthesis in the mantle margin. During the transplant experiment, cockles were also infected at an exceptionally high rate by Bucephalus minimus, a trematode which invades most of the tissues and uses the cockle as a first intermediate host. Surprisingly, there was an effect on the growth rate and condition index of cockles at only one of the sites under study. This field experiment succeeded in quantifying the moderately negative effect of trematode parasites on the growth and condition of their host and suggested the existence of interactions with as yet unknown factors.