Shell-boring polychaetes affect gastropod shell strength and crab predation

Crab predation may profoundly affect the structure of marine benthic mollusc populations and prey choice of crabs may be altered by organisms associated with their prey. We investigated effects of the shell-boring polychaetePolydora ciliataon shell strength of the periwinkleLittorina littorea, and the concomitant prey selectivity of one of its major predators, the crabCarcinus maenas. Shell strength of periwinkles measured as force required to cause cracking was significantly lower in snails infected withP. ciliatathan those without infection. In laboratory predation experiments,C. maenasconsumed more snails infected withP. ciliatathan uninfected periwinkles in a given size class. This was true when infected and uninfected snails were offered independently and simultaneously. AlthoughC. maenaspreferred small-sized (13 to 17 mm shell height) over medium (18 to 21 mm) and large (22 to 24 mm) periwinkles, consumption of large snails withP. ciliatawas twice as high as for medium-sizedL. littoreawithout polychaetes. Thus, this shell-boring polychaete causes crabs to shift their prey choice, and may even eliminate a size refuge for large infected periwinkles. We conclude thatP. ciliatamodifies predator–prey interactions, and we propose more generally that a high prevalence of shell-colonising organisms may exert a strong indirect effect on the dynamics and size distributions of mollusc populations.