This project will test whether the southern range boundary of a northern blue mussel, Mytilus trossulus, is determined by limitations on the dispersal of larvae, or the physiological tolerance of larvae and/or juveniles. Mytilus trossulus and its sister species, M. edulis, co-occur throughout the Canadian maritime provinces and the northern Gulf of Maine. While the abundance of M. trossulus decreases abruptly south of the Canadian border, M. edulis ranges south to North Carolina. Work to date has demonstrated that: 1) Adult M. trossulus in northeastern Maine inhabit coastal sites, islands, and man-made structures that are within the colder water of the Eastern Maine Coastal Current (EMCC). 2) Drifters released in the EMCC rarely enter nearshore waters to the south, suggesting that across-shelf transport is extremely limited. 3) Larvae of the two species may differ slightly in thermal tolerance, and some evidence suggests that tolerance may also be affected by nutritional status. 4) Mytilus trossulus juveniles transplanted within the northeastern Maine region, but outside of the EMCC, have high survivorship, while transplants further to the southwest suffer high mortality. In combination, these results suggest that larval transport sets the proximate range boundary within northeastern Maine (on a scale of 10 km), but thermal tolerance would ultimately limit the distribution on a larger spatial scale (200 km).
This research project is designed to test this pair of hypotheses via a combination of field and lab experiments. Satellite drifters equipped with temperature loggers deployed in and out of the EMCC during the season of M. trossulus larval dispersal (mid-June to mid-August) will be used to quantify the physical flow fields and temperature regimes during larval dispersal. Drogues will allow us to assess whether larvae at different depths may experience different flow fields. Data from hydrographic surveys, combined with regular spatial and temporal sampling of mussel larvae and new settlers, will be used to assess possible associations between larval and post-settlement stages and different water masses. The physiological tolerance of new settlers will be assayed via transplants to sites in and out of the EMCC. Finally, laboratory growth and survival experiments will assay larval performance in different thermal and feeding regimes. The investigators will use molecular markers to identify the morphologically indistinguishable larvae and settlers of these sibling species.
Broader Impacts
This project will provide training for one MS and one PhD student, and several undergraduates. The PIs are at institutions that emphasize undergraduate and graduate research, and our project will provide numerous student opportunities for field and laboratory research in oceanography and benthic ecology. Such research opportunities are likely to attract a number of students who would otherwise pursue careers in biomedical research. The Gulf of Maine is home to a thriving Mytilis edulis aquaculture industry, and M. trossulus is a commercially inferior species ? growers are concerned about its possible spread. Hence a better understanding of the factors determining the range boundary of this species will help growers avoid M. trossulus spat. Results will be disseminated to enhance communication with the Maine aquaculture community. All of the fieldwork will be conducted in a region of Maine facing great educational challenges; teachers from area schools will be recruited to assist with summer field and laboratory work.
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 of the EMCC (darker blue water) from 25-year averages in June. Overlain is the % frequency of M. trossulus, which by eye (and when analyzed) correlates well with this colder water. Our results, however, urge caution when interpreting this pattern, since they suggest that the currents carrying this cold water can also have an effect by limiting delivery of larvae to the coast. The broader message is that studies of range boundaries in the sea—particularly as oceans warm—must consider the full range of physical and biotic factors that interact to limit the distributions of species.
