Physiological responses to rising temperatures in the Intertidal [electronic resource] : a case study of a Goby and a Mussel
Keywords: Biology.

The intertidal is a harsh environment, existing at the interface of terrestrial and marine habitats. The rise and fall of the tides forces inhabitants to survive a constant flux of environmental stressors including desiccation, temperature extremes, intense solar radiation, and predation from terrestrial and marine predators. Ectotherms, whose body temperatures fluctuate with the temperature of their surroundings, have adapted to survival in this relentless environment. This dissertation evaluates two intertidal species that have mastered the art of intertidal living in a fluctuating thermal environment. In the goby fish, Gillichthys mirabilis, I evaluate global changes in gene expression of gill tissues following acclimation to a wide range of temperature (9°C to 28°C). I compare and contrast strategies for maintaining homeostasis in warm versus cold-acclimated fish, with a particular emphasis on assessing the costs of living at warmer temperatures. None of the acclimated fish expressed genes characteristic of a stress response, reflecting an acclimation process that has largely remedied the effects of acute thermal stress and established a new steady-state condition. However, increased expression of genes involved in protein biosynthesis and membrane-localized transport rise with acclimation temperature, implicating an overall increase in the costs of living at higher temperatures. Higher energetic costs sustained during warm acclimation imply that, despite this species' extreme eurythermy, a trade-off may exist whereby less energy is available for other processes (notably growth and reproduction). Next, I examine how acclimation affects the mechanisms by which these fish respond to acute thermal stress, as might be experienced on a 'hot day' at low tide. I also sought to identify gene expression changes that would define relative severity of sublethal thermal stress (e.g., mild versus extreme, near lethal stress). I found that for each ~10°C increase in acclimation temperature, the global threshold induction temperature for 'stress response' genes shifted upwards by ~2°C. In some cases, warm acclimation appeared to obviate the need for a specific stress gene's induction until the most extreme temperatures were reached. I identified several markers whose up-regulation in gill tissue may be useful in the context of identifying severity of thermal stress in the field (mild stress: HSP70, moderate stress: HSP60, HSP40, and HSPA9, extreme stress: CDKN1B and LONRF1). Elucidation of the effects of acclimation on induction thresholds for the different components of the cellular stress response provides important insights into the fundamental mechanisms of adaptation to highly variable thermal environments. In the California ribbed mussel, Mytilus californianus, I examined populations along a latitudinal range to assess whether all mussels had the same capacity to respond to thermal stress. This species is a dominant intertidal species that ranges from Alaska to Baja California along the west coast of North America, resulting in populations living at widely different temperatures. Interestingly, multiple studies have concluded that this species is genetically homogenous. I found that individuals acclimated to common conditions had significant differences in whole organism thermal tolerance. I investigated several physiological processes that might explain this pattern, including critical thermal maximum (Hcrit) of cardiac function, metabolic capacity (as indexed by tissue activities of two indicator enzymes involved in energy metabolism) and kinetic properties and thermal stability of cytosolic malate dehydrogenase. Although significant interpopulation variation was found in these physiological characteristics, these processes did not fully explain the pattern we found in whole organism thermal tolerance. Nevertheless, we provide evidence that genetic structure or selective post-settlement mortality may occur in this species.