Seasonal freezing adaptations of the mid-intertidal gastropod Nucella lima from southeast Alaska

Nucella lima from the mid-intertidal zone of Bridget and Sunshine Cove, Alaska is exposed to multiple freezing emersion events during the winter. The average duration of low tide when the air temperature fell below 0 °C increased from 2.91 to 6.78 h between the lower limit and upper limit of the intertidal range of N. lima. Air temperatures below freezing were observed between October 20, 2007 and April 20, 2008. Snails cease feeding and move into crevices, under boulders or into the sediment at the base of rocks in the winter which potentially minimizes their exposure to freezing events. Egg capsules were also observed in the snail habitat between September 27, 2007 and March 12, 2008. Snails supercool below the freezing point of seawater which delays freezing during tidal cycle related emersion. The supercooling point of snail tissues does not vary seasonally. Air temperatures below the maximum supercooling temperature of snails (-4.94 °C) occurred multiple times in December 2007 and January and February 2008. The freeze tolerance of N. lima varies seasonally and is always below the supercooling point indicating that N. lima physiologically tolerates freezing. It is likely that the seasonal synthesis of cellular compatible osmolytes is responsible for the seasonal variation in freeze tolerance: Quantitatively important compatible osmolytes which are found in higher concentration in the winter versus the summer in foot tissue of snails are total free amino acids, taurine (119 mol. Kg wet-1), and glycine (43 mol. Kg wet-1).