Predators regulate the taxonomic structure of mixed Mytilus edulis L. and M.trossulus Gould settlements in the shallow waters of the White Sea

Cryptic mussel species Mytilus edulis (Me) and M.trossulus (Mt) form mixed settlements in shallow waters of the White Sea. The proportion of Mt and Me in local settlements varies in broad limits but the factors regulating taxonomic composition of mixed populations are poorly understood. Present study focuses on the assessment of the sea star (Asterias rubens) predation influence on the Mt-Me ratio in natural habitats. For this purpose, we’ve conducted a series of field experiments. For mussels placed on experimental units (ceramic plates submerged in starfish-infested habitats), we’ve estimated a probability of being eaten by starfish. We’ve constructed a linear model that includes several predictors which may influence the probability of being eaten (starfish biomass, mussel size, mussel species, prey abundance, Mt-Me ratio on experimental units and abundance of conspecifics). The analysis has revealed that the probability of being eaten expectedly increases with sea stars’ biomass increasing, but it is higher for Mt in comparison with Me. This indicates Mt as a preferable prey. Negative correlations between the probability of being eaten and shell size, number of conspecifics and mussel abundance were also revealed. No significant dependence on Mt-Me ratio was found. Additionally, we’ve taken field samples which revealed a significant decrease in the proportion of Mt in mussel patches after a massive starfish attack. This indicates that starfish ate out Mt as its preferable prey. These results show that predators can contribute to the regulation of taxonomic composition of mixed Mt-Me settlements in the White Sea.