Sneaky snails and wasted worms: kleptoparasitism by Trichotropis cancellata (Mollusca, Gastropoda) on Serpula columbiana (Annelida, Polychaeta)

Individuals of the marine snail Trichotropis cancellata can either suspension feed or steal food from tube-dwelling polychaete worm hosts (kleptoparasitism). Because kleptoparasitism is facultative in this case, the performance of both members can be evaluated separately and together. I used field experiments to compare the growth of suspension feeding snails with that of snails (either singly or in groups) on Serpula columbiana polychaete hosts. All sizes of T cancellata grew more quickly (up to 18 times faster) when parasitizing S. columbiana than when restricted to suspension feeding. Kleptoparasitism provided a proportionally greater growth benefit for small and medium-sized snails than for larger ones, suggesting that smaller gastropods are more food-limited when suspension feeding. Fecundity was positively correlated with snail size, so the growth benefits of kleptoparasitism are likely to be selectively advantageous. Increasing numbers of medium-sized snails on a host reduced individual snail growth rates, suggesting competition for the worm's food. All snails were equally affected; there was not a superior competitor among medium-sized snails. Parasitism was experimentally demonstrated because serpulid worms grew their tubes more slowly when supporting parasitic snails and were increasingly negatively affected by additional medium-sized parasitic snails. Adding large parasitic snails did not exacerbate the negative impact on worm tube growth, so I concluded that a single large snail reduces worm hosts to their minimal rate of tube extension. Suspension feeding T. cancellata gain additional resources when they switch to kleptoparasitism.