In this thesis I investigated whether the risk of predation can induce development of defensive traits in the blue mussel Mytilus edulis, a common species of the temperate seashores. I performed a series of field and laboratory experiments in which I exposed blue mussels to waterborne scents from its main predators, the starfish Asterias rubens, and the shore crab Carcinus maenas.

I found that predator scents can elicit a variety of responses in the mussels. In the short term the exposure caused activity reduction (decreased respiration, excretion, and shell-gaping rate). This behaviour probably serves to reduce the risk of detection by predators. Predator scents affected other behaviours as well, such that the exposed mussels aggregated more and strengthened their byssal attachment. To be aggregated and firmly attached is likely beneficial when attacked by predators. Furthermore, I found that the growth pattern changed in starfish-exposed mussels, within a few weeks. They grew less in outer size, but became more compact and strong, with relatively larger adductor muscle and thicker shell. In practical tests I found that the induced mussels survived significantly better, when attacked by starfish. I also discovered that predator-exposed mussels increased their gonad ratio, which suggests that they increased the reproductive output. To put more effort on reproduction and try to "escape as gametes" may serve as an "ultimate defence", that is effective even when mussels have small chances to survive the predator attacks.

All these inducible changes seem to be beneficial in situations when the predation risk increases. Inducible defences are not unique for blue mussels. In the recent decades, the evidence has grown, that predator-inducible plasticity is geographically and taxonomically widespread, both in freshwater and marine habitats. Inducible defences may evolve in environments with variable predation pressure. The prey benefits by developing the defences only when the predation risk is large, and disabling the defences and avoiding the costs when predators are absent, thereby increasing the competitive ability. In a comparative experiment with the related tropical mussel Perna viridis I did not detect any inducible morphological defences. In contrast to tropical environments, the temperate seashores are typically very dynamic, with harsh and variable conditions, leading to continuous changes in the biotic assemblages, and variable predation pressure. The organisms living here will likely benefit by being phenotypically plastic and adaptable.