A twin heat conduction calorimeter is presented that was specially developed as a modular design to allow for easy adaptation to changing experimental conditions. The instrument was built at the Institute of Oceanography, University of Gdansk, Poland for metabolic investigations and locomotor monitoring of aquatic organisms under different environmental conditions (e.g., salinity, temperature, oxygen tension, and pollutants) as well as physiological ones (age, sex, pregnancy, and adaptation to changing external parameters). This calorimeter has flatter and broader 24-ml vessels (height 50 mm and diameter 28 mm) than usual instruments to allow larger aquatic invertebrates to swim around during the experiment. The sensitivity is 99.1 mu V mW(-1), and the time constant is 217 s with 15 ml water and 146 s without water in the vessels. Construction details are presented in this paper and the first results are given for three aquatic crustaceans of different taxonomic groups: the isopod Idotea chelipes, the amphipod Gammarus tigrinus and the decapod Rhithropanopeus harrisii. (C) 2007 Elsevier B.V. All rights reserved.