Hypoxia tolerance associated with activity reduction is a key adaptation for Laternula elliptica seasonal energetics

Seasonal dormancy is a widespread mechanism for reducing energy expenditure during periods of low energy availability. Seasonal variation in activity and the cost of pumping water through the siphons were investigated to estimate the importance of activity regulation to the seasonal energy budget of the Antarctic clam, Laternula elliptica. In the laboratory, a metabolic rate of 26.35 mu mol O-2 h(-1) was estimated for a 50-mm shell length L. elliptica pumping water at -0.4 degrees C. In the field, the proportion of time siphons were visible at the sediment surface varied seasonally (32% visible in June/July compared to 86% in December/January). L. elliptica were actively pumping for a minimum of 19% of each 24-h period during winter (August) compared to a summer maximum when animals were actively pumping for 73% of the time (February). This resulted in a 3.7-fold seasonal difference in the calculated energy consumption of a 50-mm L. elliptica (19.2 mu mol O-2 h(-1) in February versus 5.0 mu mol O-2 h(-1) in August), which closely matches the 3.0-fold seasonal variation in metabolic rate found previously. Seasonal variation in activity could therefore be responsible for much of the seasonal difference in energy consumption of L. elliptica. Inter-annual variation in timing of the seasonal activity maxima (January 2004 and March 1999) was correlated with variation in the timing of the summer plankton bloom in Ryder Bay. In the laboratory, periods of extended siphon closure (133 +/- 114 min, mean +/- SD) were accompanied by long periods of heart arrhythmia (167 +/- 135 min), during which time blood oxygen levels dropped to values close to zero. Heart arrhythmia is most likely part of a hypo-metabolic adaptation to reduce energy costs during extended periods of siphon closure. Physiological and behavioural dormancy, with associated hypoxia tolerance, appear to be key mechanisms controlling the seasonal energy budget of L. elliptica.