Distribution of cryptic mussel species (Mytilus edulis and M. trossulus) along wave exposure gradients on northwest Atlantic rocky shores

Abstract

We determined the distribution of two cryptic mussel species (Mytilus edulis and M. trossulus) along full gradients of wave exposure in northwest Atlantic rocky intertidal habitats. Research was conducted in Nova Scotia (Canada) and Maine (United States) and species were determined using genetic analyses. In very sheltered habitats, only M. edulis occurred. In sheltered, exposed, and very exposed habitats, both species co-existed, M. edulis predominating in Maine and M. trossulus in Nova Scotia. Hybrids were absent or rare. The distribution of mussels regardless of species (Mytilus spp.) was remarkably consistent across levels of wave exposure in both regions. In very sheltered habitats, organisms were large (4–5 cm long, on average) and old (7–8 years, on average) and occurred in low densities. In sheltered, exposed, and very exposed habitats, organisms were small (< 1 cm, on average) and young (1–2 years, on average); density was low in sheltered and very exposed habitats, but high in exposed habitats. Abundance data for predatory snails (dogwhelks, Nucella lapillus) and canopy-forming algae (Ascophyllum nodosum and Fucus spp.) suggest that predation and facilitation may explain some of the observed changes in mussel population traits along the wave exposure gradients. Our results could be useful as baseline information to predict the effects of the progressive increase in wave action caused by climate change on intertidal mussel populations.