Interactive effects of temperature and salinity on metabolism and activity of the copepod Tigriopus californicus.

In natural environments two or more abiotic parameters often vary simultaneously, and interactions between covarying parameters frequently result in unpredictable, non-additive biological responses. To better understand the mechanisms and consequences of interactions between multiple stressors it is important to study their effects on not only fitness (survival and reproduction) but also performance and intermediary physiological processes. The splashpool copepod Tigriopus californicus tolerates extremely variable abiotic conditions and exhibits a non-additive, antagonistic interaction resulting in higher survival when simultaneously exposed to high salinity and acute heat stress. Here, we investigated T. californicus' response in activity and oxygen consumption under simultaneous manipulation of salinity and temperature to identify if this interaction also arises in these sublethal measures of performance. Oxygen consumption and activity rates decreased with increasing assay salinity. Oxygen consumption also sharply increased in response to acute transfer to lower salinities, an effect that was absent upon transfer to higher salinities. Elevated temperature led to reduced rates of activity overall, resulting in no discernible impact of increased temperature on routine metabolic rates. This suggests that swimming activity has a non-negligible effect on copepod's metabolic rates and must be accounted for in metabolic studies. Temperature also interacted with assay salinity to affect activity and with acclimation salinity to affect routine metabolic rates upon acute salinity transfer, implying that the sublethal impacts of these co-varying factors are also not predictable from experiments that study them in isolation.