Oxygen uptake in two temporary pond ostracods (Crustacea: Ostracoda) in response to different temperatures and salt contents: testing the Metabolic Cold Adaptation hypothesis

Temporary ponds experience wide variation in their physical and chemical conditions, including changes in temperature and salinity, two critical factors strongly affecting aquatic life metabolism. Although ostracods are common in these habitats, little is known on the combined effects of temperature and salt content on their metabolic rates. Here, we compared the oxygen consumption rates of two cosmopolitan ostracods common in temporary ponds: Eucypris virens, which mostly grows dense populations during winter-spring in temperate climates, and Heterocypris incongruens, mostly considered a spring-summer inhabitant in the same habitats. We performed respirometry experiments combining five different temperatures (5-25 °C) and six salinities (0.1-10 PSU), and quantified their combined effects on mass-specific oxygen consumption rates of both species. According to the Metabolic Cold Adaptation (MCA) hypothesis, we expected differences in metabolic rate associated with their different phenologies, with the winter species showing a higher rate at the same temperature. We observed significant effects of temperature and salinity with no interaction between them on the oxygen consumption rates of both species, and confirmed that the winter species had higher energetic demands at all the temperatures tested, as predicted by MCA. Furthermore, significant interaction effects between species and salinity were consistent with their different salinity tolerances. These results provide support for MCA in ostracods and call for further experimental work with a wider range of conditions to better evaluate variation in ostracod metabolic responses at more extreme values of salinity and temperature.