Facilitation on an intertidal mudflat: the effect of siphon nipping by flatfish on burying depth of the bivalve Macoma balthica.

During deposit feeding on benthic micro-algae, the siphon of buried tellinid bivalves like Macoma balthica is vulnerable to nipping by plaice Pleuronectes platessa and other flatfish. We experimentally tested the hypothesis that siphon nippers facilitate predation on the entire bivalve (by shorebirds, for example) by inducing a decrease in burying depth. Three experiments in May and in September, during which likely siphon nippers (juvenile plaice P. platessa) were allowed to feed on M. balthica siphons, demonstrated that the bivalves indeed lost siphon mass and came closer to the surface. However, the strength of the burying response and the speed of recovery of siphons after nipping varied greatly between experiments, partly as a consequence of body condition-related differences in initial burying depth. Our experimental study confirms that siphon nippers can enhance the availability of bivalve prey to probing predators, and will thus facilitate avian predation on intertidal flats.