Project: Does larval transport or physiological tolerance set the southern range boundary of a northern blue mussel?

Collaborative Research: Does larval transport or physiological tolerance set the southern range boundary of a northern blue mussel?
Acronym "MuLTI" (Mussel Larval Transport Initiative)

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).

We will 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. We will use molecular markers to identify the morphologically indistinguishable larvae and settlers of these sibling species.