The physiological basis for changes in the freezing tolerance of intertidal molluscs. I. Response to subfreezing temperatures and the influence of salinity and temperature acclimation

Abstract

AbstractAcclimation to either low temperature or high salinity increased the freezing tolerance of the intertidal mollusc, Modiolus demissus demissus (Dillwyn). Lethal freezing temperatures ranged from −5.11°C to −12.16°C when mussels were acclimated between 12.1o/oo and 33.9o/oo (15°C). Acclimation to 45.5o/oo, however, did not increase freezing tolerance above the value obtained at 33.9o/oo. Lethal freezing temperatures also ranged from −10.4°C to −13.76°C in winter mussels acclimated between 23°C and 0°C (34o/oo). Thermogram recordings of intact M. demissus exposed to various subfreezing temperatures indicated that freezing occurs extracellularly only. Consequently, freezing mortality apparently results from cell dehydration as a critical amount of tissue water is lost as ice. Calorimetry on adductor muscles isolated from M. demissus acclimated to various salinities showed that increasing salinity acclimation increases freezing tolerance by reducing the amount of tissue water frozen and, thus, preventing the attainment of a lethal dehydration level. These calorimetric studies also suggested that the lack of further increases in freezing tolerance above a specific salinity acclimation may result from salt concentrations reaching a toxic level in the frozen tissue. On the other hand, calorimetry showed that the amount of tissue water frozen is independent of temperature acclimation. Thus, low temperature acclimation does not act to increase freezing tolerance by reducing the amount of tissue water frozen but, rather, acts through an unspecified mechanism to increase cellular tolerance to greater levels of dehydration and, possibly, to greater concentrations of toxic salts.