Variation in thermal tolerance plasticity and the costs of heat exposure in the estuarine sea hare, Phyllaplysia taylori

Climate change is increasing average temperatures and the frequency and intensity of thermal extremes in coastal marine environments. Organisms in coastal marine habitats are accustomed to environmental fluctuations and possess physiological plasticity that may be advantageous in response to increased occurrence of extremes. To examine whether such plasticity is locally adapted to environmental conditions, we investigated the relationship between genetic diversity and thermal tolerance plasticity in 11 populations of the direct-developing intertidal sea hare, Phyllaplysia taylori, on the western coast of the United States. Using whole-organism metrics of muscle function and metabolic rate and a ddRADseq genomic approach, we were unable to identify correlations between heat tolerance and underlying genetics on a population or individual level. P. taylori from all locations consistently exhibited critical thermal maxima (CT_max) above habitat temperatures (CT_max ranged from 24 to 35°C, average = 30.1 ± 0.2°C; average habitat temperature across habitats ranged from 12 to 20°C, average = 17 ± 2.7°C). We found little evidence for genetic distinctions between populations and high overall genetic diversity, suggesting more gene flow across long distances than was expected from their direct development life history strategy. The breadth of acclimation capacity we observed (11°C) was substantially wider than that reported for other poikilothermic taxa in the literature and did not follow a latitudinal cline. Our findings suggest that high plasticity of thermal tolerance exists across all populations and genetic panmixia is occurring despite life history limitations; thus, heat tolerance traits may not be under positive selection in P. taylori.