Winter quiescence and spring awakening of the Eastern oyster Crassostrea virginica at its northernmost distribution limit

To test the hypothesis that oysters, Crassostrea virginica, from the northernmost part of the species range in the Gulf of St. Lawrence (48A degrees N) open their valves at lower temperatures than those reported for more southern oysters, Hall element sensors were used to monitor their gaping behaviour. These observations were made in a flow-through system and the temperature, salinity and relative fluorescence of unfiltered seawater were monitored. Photoperiod was controlled (15 h dark:9 h light) and light levels were measured but not closely controlled. Gaping behaviour was followed from February to June 2010 (113 days) and from April to May 2011 (34 days) and was classified into three successive phases: quiescent, awakening and active. Although valves were either closed or slightly open during the quiescent phase (maximum gape angle = 0.49A degrees, SE = 0.04), they abruptly opened to maximum angles of about 5.88A degrees (SE = 0.29) during the awakening phase. Moreover, there was noticeable synchrony amongst individuals, since approximately one-half of the monitored population awoke within a 6.6-h period in both study years. Correlative analyses identified temperature as a factor influencing valve movement, and oysters awakened when temperatures were 0.2-4.0 A degrees C (mean = 2.2, SE = 0.2). Oysters exerted their maximal gape angle as soon as temperatures reached 2.8-6.6 A degrees C (mean = 4.8, SE = 0.2). During the active phase, valves remained open 68.6 % (2010) and 79.7 % (2011) of the time. An unexpected result was the observation of a diurnal rhythm in valve openness whereby the openness was greatest near the end of the afternoon and least in the early morning.