Phenotypic plasticity of blue mussels under threat from different predators

Abstract: My study confirms that there are phenotypic plastic traits in blue mussels (Mytilus edulis) inresponse to predatory pressure from shore crabs (Carcinus maenas) and sea stars (Asteriasrubens) and a combination of them. Blue mussels can sense predators by olfactory cues fromthe predators themselves, or by alarm cues from attacked conspecifics, and then developinducible defences. In this experimental study, blue mussels were exposed to either nopredator (control) or to enclosed predators in terms of two shore crabs, two sea stars or thecombination of one shore crab and one sea star, over a period of six weeks. According to previous studies a good defence against predation from shore crabs should be athicker and also a more circular mussel shell, and a good defence against sea stars should be astrong posterior adductor muscle. All three predatory treatments resulted in mussel individuals with significantly heavier shells.When exposed to sea stars, individuals grew less lengthwise and showed a significantly lowerend volume, as well as a significantly heavier posterior adductor muscle. These mussels alsoshowed a tendency to survive sea star predation better in the predation test. Thus, induciblephenotypic defences against predation by sea star predation, were clearly demonstrated. Theexposure to shore crabs resulted in a significantly higher height of the mussels, whencontrolling for mussel length. The mussels exposed to shore crabs also showed end volumessimilar to control mussels, whereas an exposure to a combination of a crab and a sea starresulted in intermediate end volumes. This supports a phenotypic plasticity in traits related topredator threat. Individuals in all three predatory treatments were harder attached by morebyssus threads at the end of the experiment. Control mussels and those exposed to a singlecrab and sea star were repeatedly found to be more aggregated (i.e. fewer solitary mussels)over the course of the experiment, whereas the mussels presumably exposed to more olfactorycues from two shore crabs or two sea stars were more often found solitary and attached bybyssus threads. This study demonstrated inducible defences in how blue mussel allocate their resources todifferent dimensions of growth, shell weight, adductor muscle weight, as well as aggregationand byssus attachment, depending on predatory threat.