1. 1. A physiological mechanism responsible for increasing the freezing tolerance of the bivalve Modiolus demissus (Dillwyn) following low-temperature acclimation was demonstrated.

  2. 2. The rates of oxygen consumption of M. demissus acclimated to temperatures between 0 and 24 °C were presented as an Arrhenius plot. A change in slope occurred at 10 °C, suggesting that temperature alone was not responsible for the increased decline in the rate of oxygen consumption below 10 °C.

  3. 3. Low-temperature acclimation had no effect on blood Na+ or K+ concentrations but did reduce the concentration of blood Mg2+ and, in addition, resulted in the accumulation of end-products characteristic of anaerobic metabolism - tissue alanine and proline, and blood Ca2+. Furthermore, maintenance of M. demissus under anaerobic conditions increased freezing tolerance.

  4. 4. Taken together, these data indicate that the increased freezing tolerance of M. demissus acclimated to low temperatures involves a conversion to anaerobic metabolism.

  5. 5. The increase in blood Ca2+ following low-temperature acclimation was associated with the increased freezing tolerance. Finally, Mg2+ simulated the effect of Ca2+ on freezing tolerance, but was only 20% as effective.

  6. 6. These results suggest that a Ca2+-dependent mechanism responsible for increasing the freezing tolerance of M. demissus exists, and that the increase in blood Ca2+ is due to a conversion to anaerobic metabolism.

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