Seasonal variations of cold tolerance in intertidal mollusks and relation to environmental conditions in the St. Lawrence Estuary

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Abstract
Seasonal variations of cold tolerance of Mytilus edulis, Mya arenaria, Macoma balthica, and Acmaea testudinalis were examined. The lower median lethal temperature (MLT) of all species was low throughout the year, with reduced seasonal variation. High MLT values, between −10 and −15 °C, were observed for Acmaea testudinalis and Mya arenaria, while low values, between −12.5 and −20 °C, were observed for Macoma and Mytilus. Seasonal variations of MLT are partly explained by the seasonal variations of water salinity. Small (juveniles) Mytilus, Mya, and Macoma showed less cold tolerance than large (adult) Mytilus, Mya, and Macoma (MLT between −8.0 and −12.5 °C). The possibility of genetic adaptation due to the selection of cold tolerant individuals is discussed. Cold tolerance determinations of animals subjected to cyclic emersions of various durations to a "sublethal" temperature of −8 °C every 12.4 h (one tidal period) showed a cumulative and significant effect of repeated mild cold stress. The ecological significance of the results is discussed in relation to the conditions prevailing along the shores of the St. Lawrence Estuary.