Feast and famine in Antarctica: seasonal physiology in the limpet Nacella concinna

The few studies of Antarctic marine invertebrates that have previously examined seasonal changes in physiology have concentrated on species that cease feeding for extended periods in the austral winter. In contrast, the limpet Nacella concinna (Strebel, 1908) feeds throughout the year. To compare the differing physiological responses demonstrated by a continually feeding species and species that cease feeding during winter we measured seasonal changes in N. concinna faecal egestion, oxygen consumption, O:N ratios, ammonia, urea and primary amine excretion at 4 points in the year. Tissue ash-free dry mass decreased by 47% and faecal egestion 10-fold from summer to winter. Metabolic rates decreased significantly in winter (December, 0.61 +/- 0.04 mumol O-2 h(-1); October, 0.38 +/- 0.04 mumol O-2 h(-1)), but to a much lesser degree than other Antarctic marine invertebrates. Ammonia and urea excretion rates showed significant seasonal variations, while no clear pattern was evident in primary amine excretion. Ammonia excretion decreased during late summer and early winter before increasing in spring (minimum, February, 66.23 +/- 5.72 nmol h(-1); maximum, December, 170.83 +/- 9.50 nmol h(-1)). Urea excretion was low in summer (December, 9.88 +/- 1.60 nmol h(-1)) and maximal (July, 26.70 +/- 4.05 nmol h(-1)) in winter. Urea and primary amine excretion combined accounted for between 10 and 38% of total nitrogen excretion. O:N ratios ranged between 7 and 20 and demonstrated that although protein was the dominant respiratory substrate throughout the year, in late summer and early winter lipids and carbohydrates were also utilised for metabolism. N. concinna has a low degree of metabolic seasonality due to the maintenance of high winter metabolic rates. Although feeding during winter provides some energy, it is insufficient to offset the substantial utilisation of tissue energy stores to balance the metabolic budget.