Sublethal reproductive costs associated with experimental heat waves in the copepod, Tigriopus californicus

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 six 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
two weeks following the last day of each experimental heat wave. Despite
similar survivorship between the two 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.