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Escalated prey defenses occur within multi-species assemblages but are largely evalu- ated by considering only direct interactions between predator and prey. If additional species compro- mise prey defenses, they might indirectly facilitate predation. Since the introduction of green crabs Carcinus maenas in the northwest Atlantic, many intertidal mollusks including the dogwhelk Nucella lapillus have acquired shell traits that have been interpreted as escalated defenses against this shell- crushing predator. I examined whether 'parasite-like' spionid polychaetes (Polydora sp.), which bore into and reside in dogwhelk shells but feed externally, indirectly facilitated predation by crabs. Small collections of dogwhelks from 19 sites around the Gulf of Maine (USA) revealed Polydora infestations at 16 sites. Intensive collections at 1 site showed that Polydora disproportionately infested large dog- whelks that are normally most resistant to predation. Presenting paired non-infested and fully- infested (Polydora bore holes in shell spire and siphonal canal) dogwhelks to individual crabs re- vealed that fully-infested individuals had significantly shorter times until first observed handling and shorter survival durations. In trials where single dogwhelks survived, they were always non-infested and their shell size scaled positively with crab size. These results suggest that Polydora increased both the size-specific susceptibility of dogwhelks to predation and the critical size of snails vulnera- ble to a given sized crab. Although Polydora altered this predator-prey interaction, mechanical tests showed no significant effect of Polydora on shell strength, so mechanical tests alone are not a reliable way to evaluate defensive traits. Since prey defenses can be modified by additional species, evalua- tions of changing shell defenses must consider both direct and indirect interactions.

Parasite-like associations in rocky intertidal assemblages: implications for escalated gastropod defenses