Predator-induced defenses have a significant influence on the expression of morphological and behavioral traits of marine species. In mussels, common responses to predators include thickening of the shell, enlargement of the adductor muscle and increases in byssus production. We hypothesize that predators with different feeding strategies have different effects on byssus production of the common intertidal mussels Perumytilus purpuratus and Semimytilus algosus in central Chile. Predators that dislodge prey mussels before killing them, such as crabs and seastars, should elicit increased byssus production in their prey compared to other predators such as whelks, which use different feeding mechanisms. Laboratory experiments with the seastar Heliaster helianthus, the crab Acanthocylus gayi, and the muricid gastropods Concholepas concholepas and Acanthina monodon showed that only A. gayi induced significant increases in byssus production, causing remarkably similar responses in both mussel species. Further experiments in which individual mussels with different attachment strengths were offered to A. gayi showed that the crabs first tried different mussels and then selected those with the weakest attachment, leading to consumption rates of weakly attached mussels that were 5 to 6 times higher than those of mussels with strong attachment to the substratum. Measurements of mussel attachment strengths in the field showed that, where A. gayi is abundant, both mussel species are more strongly attached than in habitats where this predator is scarce. While responses of mussels to crabs seem to be adaptive, the lack of a response to H. helianthus is intriguing, because it is one of the most important mussel predators in the system which can dislodge entire clumps of mussels at a time. It is possible that increased byssus production is ineffective in reducing predation by this large predator; however, this adaptive explanation requires further studies. These results highlight the predator-specific nature of many prey phenotypic responses and the importance of considering the multiplicity of predators typically present in most habitats.
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