1. 1.

    A time course of the changes in blood Ca2+ and freezing tolerance of Modiolus demissus (Dillwyn) demonstrated that increases in freezing tolerance parallel increases in blood Ca2+. The increases in freezing tolerance occurred rapidly, suggesting that Ca2+ affects freezing tolerance directly by its presence in the blood.

  2. 2.

    The presence of La3+ reduced the freezing tolerance of isolated foot muscle. Thus, Ca2+ appears to increase freezing tolerance directly by binding to cell membranes.

  3. 3.

    The loss of the contractile response of freeze-thawed foot muscle to Ach, KCl and caffeine and the continued response to CaCl2 suggested that cell membranes are the primary sites of freezing injury.

  4. 4.

    The increase in blood Ca2+ following low-temperature acclimation accounted for only 40% of the total change in freezing tolerance. Therefore, other mechanisms responsible for increasing the freezing tolerance of M. demissus following low temperature acclimation also exist.

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