Combined effects of temperature and salinity on the metabolic rate of <i>Heterocypris salina</i> (Ostracoda, Cyprididae).

Temperature and salinity are considered critical factors affecting the metabolism of freshwater crustaceans. However, experimental research aimed at assessing their interacting effects on oxygen uptake (as a proxy for metabolic rate) has been mainly focused on marine decapods. Here, for the first time, we have experimentally quantified the respiratory responses of a cypridoidean ostracod, Heterocypris salina (Brady 1868), to different combinations of temperature (10, 15, 20, 25, and 30 °C) and salinity (0.5, 5, 10, and 20 PSU). The study species has a Holarctic distribution and high tolerance to a wide range of environmental conditions. As expected, we found significant positive effects of temperature on the ostracod oxygen consumption rate (49% increase per 5 °C on average). Isolated effects of salinity on oxygen uptake were weak. However, we found a strong, negative interaction between salinity and temperature at their upper range (30 °C and 20 PSU), with a 50% mean decline in oxygen uptake. The maximum oxygen consumption rate recorded was 4.15 μL O2 h−1 mg−1 (at 30 °C and 10 PSU), and the lowest was 0.19 μL O2 h−1 mg−1 (at 30 °C and 20 PSU). Our results support the role of salinity in limiting the metabolic responses of freshwater ostracods to high temperatures