UC Santa Barbara

Salt marshes are dynamic systems characterized by rapid and intense fluctuations in temperature. Because of this, the ectotherms residing in these environments must possess the ability to tolerate thermally variable environments. Mounting these plastic thermal responses requires energy and nutrients, which are acquired through diet. As the nutritional landscape shifts within these highly variable environments, it is critical we examine the modulating effect of diet on the capacity of ectotherms to respond to thermal variability. Using the California Killifish (F. parvipinnis), we tested the influence of fluctuating temperatures and diet quality on cardiac thermal performance through three specific hypotheses: (1) static acclimation temperature will yield a narrower thermal breadth when compared to fluctuating acclimation temperatures, (2) broad diets will yield higher cardiac thermal tolerance compared to a single-item diet and (3) upper thermal limits will vary when tested at the peak vs trough of a fluctuating acclimation cycle. F. parvipinnis were acclimated for 3 weeks at one of five ecologically relevant temperature regimes (11°C, 11-20°C, 20°C, 20-29°C, 29°C) while providing either a single-item diet or broad diet. Cardiac thermal tolerance was measured using an Arrhenius Breakpoint Temperature test. Both maximum heart rate and cardiac upper thermal limits increased with increasing acclimation temperature. Surprisingly, diet quality had a greater influence on cardiac thermal performance at colder fluctuating acclimations when compared to warmer fluctuating acclimations. Lastly, F. parvipinnis were found to exhibit rapid diel plasticity by altering thermal limits throughout a diurnal phase. This work suggests that both diet and fluctuating acclimations influence species’ upper thermal limits and outlines a clear need to incorporate more ecologically relevant temperature acclimations in lab-based research.