Energetic consequences of single and repeated freezing in the intertidal mussel, Mytilus trossulus

The bay mussel, Mytilus trossulus, is a bivalve commonly found in intertidal zones along the west coast of North America which risks freezing during low tides in the winter. Previous work has shown that despite their freeze tolerance, freezing can still cause damage. Yet little is known about the energetic consequences of these freezing events. Here we measured the oxygen consumption of mussels before and after single and repeated freezing exposures across three seasons. We compared these responses to hypoxia exposures as tissues are not perfused while frozen which inhibits gas exchange. In general, we observed that mussels metabolic rates decreased immediately after a single freeze but recovers after 24 hours. By contrast, repeated freeze-thaws caused variable effects of metabolic rate, depending on the season of exposure. Overall, this suggests that the periods of recovery that occur between freeze-thaw cycles may mitigate freezing damage. We also found that hypoxia in general causes an increase in metabolic rate and may be associated with clearing oxygen debt. Therefore, the metabolic consequences after freezing are unlikely to be driven by hypoxia stress. Here we explore the energetic costs of freezing in M. trossulus informing future research directions into the mechanisms behind freeze tolerance.