Heat production rates of the benthic amphipod Gammarus oceanicus from the brackish waters of the Gulf of Gdansk were measured by means of direct calorimetry at their usual ambient ("habitat") salinity (7parts per thousand) and 10 degreesC. Animals exhibited locomotor activity during the measurements, so that the total metabolic rate was the sum of both resting and active metabolism. The mean specific metabolic rate amounted to 1.57 +/- 10.61 mW g(-1) wet weight (ww) (n = 73, average wet weight 67.2 +/- 34.0 mg). Smallest animals with a mean wet weight of 24.5 +/- 3.4 mg exhibited the highest specific metabolic rates with 2.24 +/- 0.57 mW g(-1) ww (n = 10), whereas the largest ones with wet weights of 117.7 +/- 19.4 mg showed the lowest values with 1.10 +/-0.32 mW g(-1) ww (n = 15). There was a significant negative correlation (R-2 = 0.49, P < 0.05) between the specific metabolic rate of G. oceanicus and its wet weight. The metabolic rate of females was higher by 29% (P < 0.05) than that of males of the same length due to the differences in wet weight. (C) 2004 Elsevier B.V. All rights reserved.