MANIPULATION OF HOST BEHAVIOR BY PARASITES - ECOSYSTEM ENGINEERING INTHE INTERTIDAL ZONE

Understanding the influence of parasites on the community ecology of f ree-living organisms is an emerging theme in ecology. The cockle Austr ovenus stutchburyi is an abundant mollusc inhabiting the sheltered sho res of New Zealand. This species, which lives just a few centimetres u nder the surface, plays a key role for many benthic invertebrate speci es, because in these habitats the cockle shell is the only available h ard surface where invertebrates can establish. However, the behaviour of this cockle can be altered locally by a parasite, the trematode Cur tuteria australis. Indeed, heavily infected cockles are unable to bury perfectly and typically lie entirely exposed at the surface of the mu d. In this study we investigated the ecological consequences of this b ehavioural alteration for two invertebrate species commonly associated with cockles, the anemone Anthopleura aureoradiata and the limpet Not oacmea helmsi. A field study first demonstrated that in both infected and non-infected populations of cockles, there was a negative relation ship between the number of anemones and limpets found on cockles. In t he laboratory, we showed that predation of limpets by anemones is poss ible when they share the same cockle shell. In a heavily infected popu lation of cockles, limpets were significantly more frequent and more a bundant on cockles manipulated by C. australis than on cockles with a normal behaviour. A colonization test conducted in natural conditions demonstrated that the predominance of limpets on manipulated cockles r esults from a direct habitat preference. Conversely anemones were sign ificantly less frequent and less abundant on manipulated cockles than on cockles manipulated by C. australis. A desiccation test revealed th at, relative to limpets, they had a lower resistance to this physical stress. We discuss our results in relation to current ideas on ecosyst em engineering by organisms.