A Comparative Study of the Freezing Tolerances of the Marine Snails Littorina littorea (L.) and Nassarius obsoletus (Say)

Abstract

Littorina littorea, which were found in the intertidal zone throughout the year, displayed a seasonal change in freezing tolerance that was greatest during the colder winter months, was dependent on salinity acclimation, and was independent of temperature acclimation and photoperiod. Nassarius obsoletus, which avoided the freezing stress by migrating to subtidal areas during the winter months, displayed a seasonal change in freezing tolerance that was greatest during the warmer summer months and was dependent on both temperature and salinity acclimation. Acclimation to lower salinities reduced the freezing tolerances of L. littorea and N. obsoletus. The temperature-acclimation-dependent differences in the freezing tolerance of N. obsoletus and the seasonal changes in the freezing tolerance of L. littorea also decreased following acclimation to lower salinities. When collected during the winter months, L. littorea possessed a greater freezing tolerance than N. obsoletus under all of the conditions examined, but the freezing tolerance of L. littorea was lower than that of N. obsoletus when collected during the summer and acclimated to warm temperatures and high salinities. Continuous temperature recordings of intact snails exposed to their LD50 temperatures demonstrated that the seasonal and interspecific decreases in lethal freezing temperatures were not due to a lowering of the temperatures at which tissue ice formation occurred. Calorimetric determinations of the amounts of tissue ice formed at LD50 temperatures showed that the seasonal and interspecific decreases in lethal freezing temperatures were due to increased tolerances to greater quantities of tissue ice.