Anti-predator behaviour in the green-lipped mussel Perna viridis: byssus thread production depends on the mussel's position in clump

Green-lipped mussels Perna viridis secrete a larger number of byssus threads to anchor themselves to the substrate more firmly when predation risks are high. These mussels also form clumps. As predators usually attack clumps from the periphery, the predation risks of solitary mussels are higher than those of mussels living along the edge of clumps, which are in turn higher than those of mussels living within the clumps. The byssus thread number, length and diameter were investigated in relation to the mussels' position in the clump. We set up chemical stimulus treatments with damaged conspecifics and heterospecifics, suggesting predation risks, as well as with intact conspecifics and heterospecifics, shrimp cues and control (no cue). The mean number of byssus threads produced was significantly affected by both the mussels' position in the clump, the treatment type, and the interactive effect between these 2 variables. In the treatment in which mussels were exposed to damaged conspecific cues, the mean number of byssus threads produced by the solitary mussels was greater than the number produced by edge mussels, which was, in turn, greater than the number produced by the mussels within the clump. Furthermore, the solitary and edge mussels exposed to damaged conspecifics secreted more byssus threads than their counterparts in the control, intact conspecifics, intact and damaged heterospecifics, and shrimp meat treatments. However, the mussels within the clumps exposed to damaged conspecifics did not secrete more byssus threads than their control counterparts. Results of the present study suggest that the anti-predator responses to risk may be related to the presence of damaged conspecifics and the mussels' position in the clump, both indicative of local predation pressure.