Oxygen consumption ((M) over dot o(2)), heartbeat rate and form, and circulating hemolymph oxygen content were measured in relation to temperature in the large Antarctic infaunal bivalve Laternula elliptica. After elevations in temperature from 0degrees to 3degrees, 6degrees, and then 9degreesC, (M) over dot o(2) and heartbeat rate rose to new levels, whereas. maximum circulating hemolymph oxygen content fell. At 0degreesC, (M) over dot o(2) was 19.6 mumol O-2 h(-1) for a standard animal of 2-g tissue. ash-free dry mass, which equates to a 8.95-g tissue dry-mass or 58.4-g tissue wet-mass animal. Elevation of metabolism following temperature change had acute Q(10) values between 4.1 and 5, whereas acclimated figures declined from 3.4 (between 0degrees and 3degreesC) to 2.2 (3degrees-6degreesC) and 1.9 (6degrees-9degreesC). Heartbeat rate showed no acclimation following temperature elevations, with Q(10) values of 3.9, 3.2, and 4.3, respectively. Circulating hemolymph oxygen content declined from 0degrees to 3degrees and 6degreesC but stayed at a constant Po-2 (73-78 mmHg) and constant proportion (similar to50%) of the oxygen content of the ambient water. At 9degreesC, (M) over dot o(2) and heartbeat rate both peaked at values 3.3 times those. measured at 0degreesC, which may indicate aerobic scope in this species. After these peaks, both measures declined rapidly over the ensuing 5 d to the lowest measured in the study, and the bivalves began to die. Hemolymph oxygen content fell dramatically at 9degreesC to values between 2% and 12% of ambient water O-2 content and had a maximum Po-2 of around 20 mmHg. These data indicate an experimental upper lethal temperature of 9degreesC and a critical temperature, where a long-term switch to anaerobic metabolism probably occurs, of around 6degreesC for L. elliptica. Concurrent measures of mitochondrial function in the same species had indicated strong thermal sensitivity in proton leakage costs, and our data support the hypothesis that as temperature rises, mitochondrial maintenance costs rapidly outstrip oxygen supply mechanisms in cold stenothermal marine species.