Divergence in thermal performance contributes to ecotype maintenance in an intertidal snail: evidence from in-situ transplants

Physiological adaptation across environmental gradients can contribute to ecological speciation by limiting performance outside locally optimal habitats. Intertidal systems provide strong natural thermal gradients, yet the extent to which thermal physiology contributes to divergence across shore height remains poorly resolved. We investigated cardiac thermal performance in two ecotypes of the marine snail Littorina saxatilis occupying different shore heights along the Galician coast (NW Spain): a wave-adapted ecotype on the lower and mid- shore and a crab-resistant ecotype on the mid- and upper shore. Using infrared photoplethysmography, we quantified heart rate responses in both a reciprocal field transplant experiment and laboratory thermal ramping trials. In the field, the Wave ecotype exhibited significantly higher heart rates than Crab ecotype snails under native mid-shore conditions and after 1 day of exposure to upper-shore conditions. However, after 4 days of exposure to the upper shore, Wave ecotype snails showed a marked reduction in cardiac activity, whereas Crab ecotype populations maintained stable heart rates across transplant locations and durations. In laboratory ramping experiments, Crab ecotypes displayed lower baseline cardiac activity and greater thermal insensitivity across the rising phase of the thermal response curve, while the Wave ecotype exhibited higher cardiac performance and an earlier decline in heartrate at high temperatures. Together, these results demonstrate pronounced ecotype divergence in cardiac thermal physiology and suggest that chronic exposure to upper-shore conditions compromises cardiac performance in the Wave ecotype. Such physiological differences likely contribute to vertical zonation and the evolution of barriers to gene flow between these ecotypes.