Heat Wave Intensity Drives Sublethal Reproductive Costs in a Tidepool Copepod

Physiological stress may induce sublethal effects on fitness by limiting energy availability and shifting energy allocation, which can incur reproductive costs. Sublethal reproductive costs may affect vital rates, linking stress events such as heat waves to population demography. Here, we test the hypothesis that heat wave intensity and consecutive days of exposure to heat wave temperatures impact survival and individual reproductive success. We subjected groups of the marine harpacticoid copepod, Tigriopus californicus, to 6 heat wave regimes that differed in maximum exposure temperature, 26°C or 32°C, and number of consecutive exposure days (1, 2, or 7), and predicted that survival and reproductive costs would increase with heat wave intensity and duration. We measured individual survival and offspring production during the heat waves and for 2 weeks following the last day of each experimental heat wave. Despite similar survivorship between the 2 maximum temperature treatments, sublethal effects of heat wave intensity were observed. Consistent with our predictions, individuals that experienced the higher maximum temperature 32°C heat waves produced fewer offspring overall than those that experienced the 26°C heat wave. Furthermore, the number of naupliar larvae (nauplii) per clutch was lower in the 32°C group for egg clutches produced immediately after the final day of exposure. Our results are consistent with the hypothesis that increasing thermal stress can lead to sublethal costs, even with no discernible effects on mortality. Heat waves may not always have lethal effects on individuals, especially for individuals that are adapted to routine exposures to high temperatures, such as those occupying the high intertidal. Costs, however, associated with stress and/or reduced performance due to non-linearities, can affect short-term demographic rates. The effect of these short-term sublethal perturbations is needed to fully understand the potential for population rescue and evolution in the face of rapid climate change.