Thermal tolerance and thermal preference of the copepod Tigriopus californicus are insensitive to dissolved oxygen levels

Abstract

Shifting climate patterns may impose novel combinations of abiotic conditions on animals, yet understanding of the present-day interactive effects of multiple stressors remains under-developed. We tested the theory of oxygen capacity limited thermal tolerance (OCLTT) and preference in the copepod Tigriopus californicus, a species that inhabits splashpools along rocky coastlines where diel fluctuations of temperature and dissolved oxygen (DO) are substantial. To examine thermal tolerance, egg-mass bearing females were exposed to a 5.5h heat ramp to peak temperatures of 34 - 38°C crossed with each of four oxygen levels: 22, 30, 100 and 250% saturation. The survival of females (four days post-exposure) decreased at higher temperatures but was independent of DO. We quantified behavioral preference of females who were exposed to one of 7 combinations of gradients of temperatures (11-37°C) and oxygen saturation (17 - 206%). Overall, females avoided extremely high temperatures regardless of the DO levels. This pattern was more pronounced when low DO levels coincided with high temperatures. Interestingly, when there was no thermal gradient, the distribution of females shifted toward high DO levels, especially at the constant high temperature. These results question the generality of prevailing theories of OCLTT and raise the possibility of microhabitat selection being used even within tiny splashpool environments to avoid physiologically stressful conditions. Our results emphasize the need to explicitly test interactive effects of multiple factors that may covary in current and future environments.