Award: OCE-1233868

Most species live within a distinct geographical range. Understanding the processes that determine where the boundary of a speciesÆ range is located can tell us a lot about whether these boundaries are likely to shift with a changing climate. The majority of coastal marine invertebrate animals have a larval stage that potentially disperses long distances. This stage is the primary source of movement among populations and the mechanism by which new populations become established. Range boundaries in such species are mainly thought to occur either because larvae encounter a barrier to dispersal and can not colonize habitat beyond a certain point, or because adults or larvae are unable to tolerate the physical conditions beyond that point. This project tested these two competing hypotheses for a northern blue mussel whose southern range boundary in eastern Maine corresponds to a region where the prevailing coastal current flowing along the shore (the pathway by which most larvae enter the region) turns away from the shore. The range boundary could occur in this location either because larvae in the coastal current are swept past the coast and can not reach the shore, or because the water in the coastal current is colder than the water in the bays inshore of that current. Sampling of mussel larvae along transects oriented perpendicular to shore showed that larvae were far more abundant offshore than inshore, thus supporting the hypothesis that limited mixing between the coastal current and bay waters inhibited the inshore movement of larvae. By contrast, neither transplant experiments with adults nor laboratory temperature manipulations with larvae showed that mussels experienced much temperature stress in the warmer waters inshore of the coastal current. Consequently, we conclude that the range boundary of this species is more likely to be established by constraints on larval dispersal than by changes in the physical environment. Our results suggest that climate change is not likely to shift the range boundary for this species via direct temperature effects. However, water movement patterns are also expected to vary with climate, so changes to coastal circulation could effect the location of the range boundary in more subtle ways by altering larval dispersal patterns. This project had the broader impacts of training seven undergraduate and two MS students in coastal oceanographic research. The mussel aquaculture industry has also been very interested in our results. Mussel aquaculture relies on a closely related southern mussel and avoids the study species because of its softer shell. Our results provide information on how the settlement of the less desirable species varies in space, enabling aquaculture operations to avoid it when seeding rope cultures. In addition, fisheries managers in the State of Maine are interested in applying our results to other commercially harvested bivalves (clams, oysters, and scallops). In the absence of any other information on how bivalve populations in the region are connected by larval dispersal, our data on mussels provide a reasonable first approximation for other harvested species. Last Modified: 06/05/2014 Submitted by: Philip O Yund