Thermal responses in the Hawaiian volcano shrimp (Halocaridina rubra): active vs. passive plasticity across genetic lineages.

Animals may respond to threats from climate change through plastic physiological changes (i.e., acclimation), adaptive evolution (i.e., genetic change), or moving to new habitats that match their climatic niche. Organisms from volcanic habitats are underexplored but may serve as models for physiological and evolutionary responses to warming temperatures. Here, we examined how different genetic lineages of the Hawaiian anchialine shrimp Halocaridina rubra respond to temperature, including animals from noticeably warm habitats created during eruptions of Kilauea in 2018. We find that thermal limits are elevated in animals from newer, warm habitats, but decrease to match those of animals from older, cooler habitats after being maintained at room temperature. Laboratory experiments further suggest thermal limits are mainly shaped by acclimation. In contrast, metabolic rates show almost no acclimation responses to temperature, with rates largely explained by test, not acclimation temperatures. There was little difference in thermal acclimation of metabolic rates among animals from different genetic lineages. However, metabolic rates at room temperature were different among shrimps from different genetic lineages, suggesting genetic variation for aerobic metabolism could be a target of selection during climate change. We find that shrimp from newer, warm habitats can live at or near their critical thermal maximum, suggesting some anchialine species may be able to deal with increased temperatures from climate change by being pre-adapted to colonize warm habitats associated with volcanism. We also highlight the potential impacts of climate change on anchialine habitats and suitable experimental designs for categorizing and quantifying thermal acclimation of biological rates.