Heat tolerance and acclimation capacity in unrelated subterranean arthropods living under common and stable thermal conditions

Cave-dwelling ectotherms, which have evolved for millions of years under stable thermal conditions, could be expected to have adjusted their physiological limits to the narrow range of temperatures they experience and be highly vulnerable to global warming. However, the few existing studies on thermal physiology in subterranean invertebrates point that they have lost some of the thermoregulatory mechanisms common in surface species, but there is a lack of evolutionary adjustment to ambient temperature. The question remains whether this surprising homogeneous thermal niche is common for the whole subterranean biodiversity or just a peculiarity of the species tested. In order to test for the generality of such previous findings, we studied basal heat tolerance and thermal plasticity in four species of distant arthropod groups (Coleoptera, Diplopoda and Collembola) with different evolutionary histories but under similar selection pressures, as they have been exposed to the same constant environmental conditions for a long time (inhabiting the same cave). Adult specimens were exposed at different temperatures for one week to determine upper lethal temperatures. Then, surviving individuals from each treatment were exposed to a fixed stressful temperature to determine acclimation capacity. Upper lethal temperatures were similar to those reported for other subterranean species (between 20-25°C), except from that of the diplopod (28°C), widely exceeding the cave temperature (13-14°C). Acclimation responses differed from a positive acclimation response in two of the species to no acclimation capacity or detrimental acclimation effects in the others. Our results show that thermal niche features are not as homogeneous as expected across the subterranean fauna and unrelated to ambient conditions. We show that some species, likely those that colonized subterranean environments more recently, still retain thermoregulation capacity to face temperature changes. Thus, subterranean species, even those living under similar climatic conditions, might be very differently affected by global warming.