Award: OCE-0961325

Species of all organisms on earth inhabit only a portion of the geographic range that they could potentially occupy, and understanding the factors that set these "range boundaries" is crucial, particularly in this time of changing climate. Range boundaries for coastal marine species—many species at once—often exist where water masses collide, as when a coastal ocean current moves on or offshore, creating a sharp gradient in water temperature (e.g. Cape Hatteras and Cape Cod are important range boundaries for several species of fish and invertebrates). While temperature tolerance of organisms is likely in play, there are other physical barriers to consider as well, to explain these broad patterns. For the many marine species whose dispersal from location to location is mainly by planktonic larvae that ride passively on currents, we must consider ocean circulation patterns in the areas where water masses collide, since these may create strong dispersal barriers that interact with physiological tolerance to dictate where the range of a species ends. A good example of this scenario plays out along the coast of Downeast Maine near Machias Bay. This is the point where the Eastern Maine Coastal Current (EMCC), a cold tongue of water originating off the Scotian Shelf, diverges offshore. Near this point are the most southern North Atlantic populations of the blue mussel Mytilus trossulus, a species that is ecologically dominant in rocky intertidal environments along the east and west coasts of the North Atlantic and North Pacific. Our project aimed to understand how dispersal barriers and physiological tolerance together create this southern range boundary, and it had had four major components: (1) a study of the physical oceanography of the EMCC and coastal waters in this region (2) field studies of how M. trossulus larvae are distributed here, (3) transplant studies of growth and survival of juvenile mussels moved past their native range boundary, and (4) lab studies of growth and survival of larvae in different temperatures and food regimes. Physical oceanography studies confirmed the EMCC divergence zone to be a barrier to water flowing across the continental shelf, suggesting that larvae spawned upstream and carried along the EMCC would not often be often delivered to the coast where they could settle into mussel beds. Our field studies of M. trossulus larvae were challenging, since we needed to develop molecular techniques to identify these larvae and separate them from the related, co-occurring species M. edulis which is even more abundant in these waters. We found M. trossulus larvae to be more abundant in offshore areas near the EMCC than inshore, along transects. Larvae of M. edulis were also not homogeneously distributed, and were abundant near the coast and offshore, but not between these regions, again suggesting limits to their transport across the shelf by water currents. Together these results suggest a strong role for dispersal limitation. Studies of juvenile and larval performance gave mixed results. Transplants indeed showed that survival (not growth) was poorer for M. trossulus than for M. edulis controls moved southwest along the coast past the range boundary, but only when they were moved 80-200 km beyond Machias Bay. These experiments were short-term, however suggesting that physiological limitation might play a large role over the longer term, in populations. Our studies of larvae showed no species differences in mortality or growth as function of temperature, across the range they might encounter along this coastline (both inshore and towards the southwest). In fact, strong genetic (i.e. among-family) effects within species overwhelmed any discernible differences between species, and while diet interacted with temperature in revealing these family effects, it also did not reveal any species differences. The map in the attached Figure shows the cold-temperature signal o...