The seasonal energetics of the Antarctic bivalve Laternula elliptica (Kingand Broderip) at Rothera Point, Adelaide Island

Seasonal changes in feeding activity, metabolic rate and major tissue energ y concentrations of the infaunal bivalve Laternula elliptica were monitored over a 12 degrees month period in relation to phytoplankton standing stock . Chlorophyll a concentration displayed the prolonged winter minima and int ense annual summer peak characteristic of polar environments. Divers made r egular observations on the feeding activity of L. elliptica throughout the year, and siphons were completely withdrawn below the sediment surface at t he start of the study in early September 1998. Siphons reappeared as chloro phyll concentrations rose at the end of September and these concentrations continued until late May 1999 when siphons were again withdrawn. There were , therefore, around 4 months in the austral winter when L. elliptica did no t feed. Oxygen consumption rates were highly seasonal, varying from 4.3 mu mol h(-1) late in the winter (August 1999) to 12.9 mu mol h(-1) in summer ( March 1999) for a 50 degrees mm shell height individual, indicating a summe r increase in oxygen consumption over winter levels of Hx3.0. Nitrogen was excreted as 90% ammonium and 10% urea, and excretion rates were also highly seasonal. The O:N ratio did not vary significantly with size in winter (Se ptember, August), but declined with shell length in summer (November, Janua ry and March), indicating that larger bivalves used a higher proportion of protein to fuel metabolism than younger animals when feeding in summer. O:N ratios in August 1999 ranged from 3 to 16, indicating a predominantly prot ein-based metabolism, ANCOVA showed that regressions of dry tissue masses o f major non-re productive body components (musculature, ctenidia and digest ive tissue) against shell height did not change between late winter and lat e summer. ANCOVA also showed that the regression slope for gonad mass again st shell height was greater in late summer than late winter (slope F = 7.37 , P = 0.009), and gonad mass was greater in all animals by late summer. Alt hough tissue masses remained constant, energy content of non-reproductive t issues (especially musculature) declined from summer to winter. Muscle tiss ue also showed the greatest reduction in protein content, which was consist ent with O:N estimates of primarily protein-based metabolism, especially in winter. Reduction in whole body energy content (5.84 kJ) agreed closely wi th total power utilisation during winter estimated from oxygen consumption (5.78 kJ). The data also showed that in contrast to temperate bivalve speci es, L. elliptica survives the prolonged polar winter with little reduction in body mass.