71.8 Friday, Jan. 6 Transplants of juvenile mussels show that thermal tolerance, alone, is unlikely to set the Atlantic southern range boundary of the northern blue mussel, Mytilus trossulus, MCCARTNEY, MA*; YUND, PO; Univ. of North Carolina, Wilmington; University of New England email@example.com
Thermal tolerance limits and thermal optima for growth and reproduction undoubtedly play a large role in establishing marine biogeographic boundaries. However, range limits often occur where water masses carried by ocean currents meet, forming water flow and temperature discontinuities that interact in unknown ways to limit dispersal across range boundaries. The more northern blue mussel Mytilus trossulus overlaps with the more southern species, M. edulis, throughout the Canadian Maritimes. Mytilus trossulus reaches its Atlantic southern range limit at Machias Bay Maine, very near where the swift, southwesterly flowing Eastern Maine Coastal Current (EMCC) departs the coast. Our studies of hydrography and plankton larval samples indicate that, near the range boundary, limited mixing between the EMCC and inshore waters is associated with a decline in bivalve larval abundance, suggesting a role for dispersal limitation. To begin to address thermal limitation, we have performed transplants of tagged juveniles from M. trossulus-native source populations in Maine to cages located at increasing distances beyond the range boundary. Results from 3 seasons show a decline in M. trossulus survival relative to M. edulis, but this pattern does not appear until transplants are moved well southwest of the range boundary. Growth rates of surviving M. trossulus individuals, relative to those of its southern congener, are not affected by transplanting. These results suggest a role for temperature, but the scale over which it limits M. trossulus cannot fully account for the geographic position of this sharp range limit near the site of EMCC divergence.