Predator-labeling effect on byssus production in marine mussels Perna viridis (L.) and Brachidontes variabilis (Krauss).

Mussels Perna viridis and Brachidontes variabilis were exposed to chemical cues from the predatory crab Thalamita danae maintained on different diets, and byssal thread production of the mussels was studied. P. viridis produced the highest number as well as the thickest and longest byssal threads when they were exposed to crabs maintained on a diet of P. viridis as compared with those exposed to crabs maintained on a diet of the top shell Monodonta labio, the rock oyster Saccostrea cucullata, or crabs that were starved. For B. variabilis, results were similar, in that a diet containing B. variabilis elicited the greatest response as compared with other treatments. This indicates that the mussels were able to discriminate chemical cues released from predators maintained on different diets, and respond accordingly to the level of predation risk. By increasing the strength of byssal attachment as a defensive trait, the chance of being dislodged and consumed by crabs is reduced. As energy cost involved in the induction of an antipredatory response is considerable, this defensive trait seems to be an advantage to the mussels in enhancing efficiency. The short response time in byssal thread production allows the mussels to increase resistance against predation by crabs at the time when predation pressure is the highest in a tidal cycle.