Traits, behavior, and seasonal physiological shifts mediate thermal stress in intertidal mollusks

Abstract

A key challenge in ecology is predicting how animals respond to environmental change. Ectotherms comprise most animal diversity on Earth and are especially vulnerable to temperature change, because their basic physiological functions are strongly influenced by environmental conditions. Most ectotherms experience substantial fluctuations in temperature and water availability in their habitats, and the physiological consequences of this variability are well-established. However, understanding how physiological responses scale to ecological patterns remains a major challenge. In this dissertation, I use two intertidal mollusk systems (the bubble snail Haminoea vesicula and the California mussel Mytilus californianus) to examine (1) how environmental fluctuations affect physiological processes and (2) how those physiological processes scale to ecological patterns. In Chapter 1, I investigate how well-aligned the thermal sensitivities of development, growth, and survival are to habitat temperatures in embryos of H. vesicula. By integrating laboratory-derived thermal performance curves with environmental temperatures, I show that typical habitat temperatures fell far below temperatures that maximize growth and development (Topts), resulting in suboptimal performance most of the time. However, simulations of embryo performance suggest that elevated Topt allowed embryos to survive seasonally and tidally driven heat spikes. In Chapter 2, I examine whether H. vesicula reproductive timing is shaped by physiological constraints on parents or embryos. The thermal sensitivity of aerobic metabolism shifts seasonally in adults, buffering severe thermal stress. By contrast, embryos face lethal temperatures. Accordingly, egg-laying occurred in synchrony with semilunar tidal cycles, declining when embryos were predicted to experience lethal thermal conditions during development. These results suggest a feedforward mechanism; parents exploited predictable environmental variation to reduce thermal risk for their offspring. In Chapter 3, I investigate selection on body size in a population of M. californianus following an historic aerial heatwave. Selection favored opposite extremes of the body-size distribution, with small mussels favored on the high shore, and large mussels favored on the low shore. Dynamic heat-budget simulations revealed that body size and microhabitat interacted to determine access to cooling behaviors, driving the direction of selection. Together, these studies demonstrate that organismal traits, behavior, and seasonal physiological shifts interact with environmental variability to shape thermal risk and ecological outcomes in marine intertidal mollusks.