High Heat Tolerance Is Negatively Correlated with Heat Tolerance Plasticity in Nudibranch Mollusks

Abstract

Rapid ocean warming may alter habitat suitability and population fitness for marine ectotherms. Susceptibility to thermal perturbations will depend in part on plasticity of a species' upper thermal limits of performance (CT_max). However, we currently lack data regarding CT_max plasticity for several major marine taxa, including nudibranch mollusks, thus limiting predictive responses to habitat warming for these species. In order to determine relative sensitivity to future warming, we investigated heat tolerance limits (CT_max), heat tolerance plasticity (acclimation response ratio), thermal safety margins, temperature sensitivity of metabolism, and metabolic cost of heat shock in nine species of nudibranchs collected across a thermal gradient along the northeastern Pacific coast of California and held at ambient and elevated temperature for thermal acclimation. Heat tolerance differed significantly among species, ranging from $25.4°\pm 0.5°C$ to $32.2°\pm 1.8°C$ ( $\overline{x}\pm \mathrm{SD}$ ), but did not vary with collection site within species. Thermal plasticity was generally high ( $0.52\pm 0.06$ , $\overline{x}\pm \mathrm{SE}$ ) and was strongly negatively correlated with CT_max in accordance with the trade-off hypothesis of thermal adaptation. Metabolic costs of thermal challenge were low, with no significant alteration in respiration rate of any species 1 h after exposure to acute heat shock. Thermal safety margins, calculated against maximum habitat temperatures, were negative for nearly all species examined ( $-8.5°\pm 5.3°C$ , $\overline{x}\pm \mathrm{CI}$ [confidence interval]). From these data, we conclude that warm adaptation in intertidal nudibranchs constrains plastic responses to acute thermal challenge and that southern warm-adapted species are likely most vulnerable to future warming.

Keywords: acclimation; gastropod; thermal physiology; thermal sensitivity; trade-off hypothesis.