Phylogenetically mediated anti-predator responses in bivalve molluscs

Sedentary, group-living taxa, such as bivalves, respond to waterborne predator cues by enhancing tissues that increase anti-predation attributes such as muscle and shell mass. The ability of bivalves to differentiate among cues generated by the consumption of different prey species that vary in phylogenetic relatedness is unknown. We exposed mussels to cues generated by crabs feeding on mussels, cockles and periwinkles to test whether there was a relationship between the magnitude of the induced response and the level of relatedness to the cue source. Mussels exposed to either effluents of unfed crabs or seawater only served as controls. Exposure to cues from conspecifics and cockles caused significantly reduced growth in shell size of mussels compared to control mussels. Shell growth of mussels exposed to cues from crabs feeding on periwinkles did not differ significantly from mussels exposed to effluents from unfed crabs only. Tissue dry weight increased least in mussels exposed to cues of crabs fed conspecifics, followed by mussels exposed to crabs fed cockles. Cues of crabs that were fed mussels caused increased byssus thread production, whereas the other treatments did not. Exposure to cues from crabs fed mussels and crabs fed cockles resulted in significantly thicker and heavier shells compared to unexposed control mussels. The results of this study indicate that alarm cues of progressively more closely related molluscs cause a higher degree of induced morphological anti-predator responses in mussels. The physiological and behavioural responses of mussels to these cues may therefore have a phylogenetic basis.