Mytilus edulis , blue mussel

Background

Blue mussel populations in the western North Atlantic are composed of two sometimes sympatric species, Mytilus edulis and M. trossulus and their hybrids. Until recent years these were considered as one species and literature addressing their thermal biology did not differentiate between them. This account makes no attempt to differentiate temperature values attributed to “ M. edulis ” which in fact may be M. trossulus or hybrids.

Mytilus edulis , the blue mussel, is circumpolar in the Arctic Ocean. In the western North Atlantic it ranges south to South Carolina. Newell 1989 gave a species profile of the blue mussel. This species inhabits hard substrate from the shore of brackish shallow estuaries to deep offshore waters of 499 m. It prefers bays and estuaries. Adults aggregate and can form dense beds. They have a limited capacity of movement using their byssal threads. Blue mussels are filter feeders. They have a variable spawning season governed more by food than by latitude and temperature. Fertilization of eggs occurs in the water column. Approximate early life stage durations are eggs five hours, planktonic larvae 15 – 35 days, with settlement in the littoral or shallow subtidal zone on algae, hydroids, or solid substrate where they live as juveniles for up to two years. Blue mussels are fished and/or cultured in the Maritime provinces, Newfoundland, and Quebec. In U.S. waters they are fished commercially in Maine, Massachusetts, and to a small extent in New York (http://www.st.nmfs.gov/st1/commercial/landings/ds_8850_bystate.html ).

Temperature limits, critical thresholds, vulnerability, and barriers to adaptation

Sea surface temperatures in the current distribution of the blue mussel range from a February minimum of -2.1 oC to an August maximum of 28.05 oC.

Thermal criteria for blue mussels are from Anonymous 2003, Newell 1989 and references therein, and http://govdocs.aquake.org/cgi/reprint/2004/802/8020500.pdf . In Canada blue mussels experience a wide range of temperatures from sub-zero to 25 oC. The southern range limit of the blue mussel is limited by water temperatures of approximately 27 oC. The lethal temperature has been estimated as 27o-29 oC. In experimental conditions more southern mussel populations can withstand higher temperatures than can more northern populations due to differences in summer temperature experience. In the north blue mussels tolerate long periods of low temperatures to the point of being frozen in ice for several months each year. Their northward limit is determined by the need for temperatures above approximately 5 oC for sufficient duration to allow growth and reproduction. However, growth may still occur in sub-zero temperatures such as from November to April provided sufficient food is available. Growth is maximized from 16 o-22 oC. Blue mussel larvae undergo normal development between 8 o-20oC, with maximum values lowering to 16 oC with increasing latitude. Tolerance to environmental variation is reported as lower in the earlier than later larval stages, and lower in the larvae than the adult. Smaller mussels can withstand

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short periods of elevated temperatures longer than larger mussels. Development of ovarian eggs is initiated by water temperatures falling below 21 oC. A temperature of 14 oC is required to induce spawning.

Blue mussels were of average rank in the sensitivity analysis. Their lack of mobility at most life stages was partially outweighed by their eurythermal nature, which may reduce the overall vulnerability of individuals and populations to potential climate change. There is little information to compare potential vulnerability of life stages based on thermal criteria. However, because tolerance to environmental variation may be lower in the earlier than later larval stages, and lower in the larva than the adult, the larval stage may be the most thermally critical.

Impacts

A 4 oC rise in global temperature will impact the future distribution of blue mussels in the western Atlantic. Results from all models and scenarios are generally similar and show a potential loss of thermal habitat from the southernmost part of the range northward perhaps to New Jersey. Model CCSR shows potential habitat loss in Labrador waters, as well. No gain of thermal habitat is predicted as the northern range limit extends well beyond the confines of this study. All other waters should remain thermally suitable for blue mussels. We predict that within suitably thermal habitat blue mussel populations will adapt to slowly increasing water temperatures. Commercial fishing should not be affected.

References

Anonymous. 2003. Profile of the blue mussel ( Mytilus edulis ). Gulf Region. Policy and Economics Branch, Gulf Region, Department of Fisheries and Oceans, Moncton, New Brunswick. Newell, R.I.E. 1989. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (North and Mid-Atlantic) - blue mussel. U.S. Fish and Wildlife Service Biological Report 82(11. 102 ). U.S. Army Corps of Engineers, TR El-82-4. 25 pp.

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