Predator-inducible defences and local intrapopulation variability of the intertidal mussel Semimytilus algosus in central Chile

Predator-inducible defences have a strong influence on the expression of morphological traits of intertidal invertebrates. For instance, mussels exposed to predators often have thicker shells than non-exposed. On the intertidal rocky shores of Chile, the mussel Semimytilus algosus is a preferred prey of many carnivorous invertebrates, including the snails Nucella crassilabrum and Concholepas concholepas, and the crab Acanthocyc1us gayi. Preliminary observations indicated that S. algosus exists as 2 morphotypes: a thick, smooth shell and a thinner, ringed shell. The thick-shell morphotype was found mostly on compact, rocky platforms, whereas the thin one was found on emergent rocks. We examined the role of invertebrate predators in determining the morphological differences observed in S. algosus as a process of defence induction. The density and size of mussel predators showed significant differences between habitats: A. gayi dominating the platforms and N. crassilabrum emergent rocks. C. concholepas did not show differences between habitats. Waterborne cue experiments demonstrated that the mussel shell thickness is increased by the presence of predators, especially A. gayi. Furthermore, in contrast to the other predators, A. gayi preferentially selects mussels of the thin-shell morphotype. We demonstrate the cause and effect connection between variation in mussel shell morphology in the laboratory and their associated spatial distribution in the field, as well as the ecological role played by predators. We propose that, at local scales, the distribution and abundance of predators in the field explain the inter-population morphological differences of the mussel S. algosus.