How Does Local Temperature Shape Thermal Tolerance? A Test Using Congeneric Snails on Tropical Rocky Shores

Plasticity in individual thermal physiology is dependent on a species’ evolutionary history as well as local environments that influence acclimatization capacity. To test how physiology and acclimatization capacity vary with species and local thermal conditions, we investigated the thermal performance of two rocky shore snails in the genus Nerita. Thermal environments experienced by the snails and their physiological performances (using heart rate) were quantified for two tropical locations (Hong Kong and Singapore; two sites per species in each location) where one or both species were present. Nerita undata, which has a tropical evolutionary origin, was present in both locations, while Nerita yoldii, which has a more subtropical-temperate lineage, was present in Hong Kong but not Singapore. Thermal tolerance of N.undata increased with the maximum environmental temperature in Hong Kong but not Singapore, where sites were hotter and snails had higher but less variable thermal tolerances (~50 °C). Nerita yoldii inhabited hotter sites in Hong Kong than N. undata but, similar to N. undata in Singapore, showed limited plasticity to environmental temperatures. Such extended tolerances but limited plasticity reflect a trade-off that constrains acclimatization capacity, where extreme thermal stress may elicit high energy expenditure but restrict energy gain through feeding when the rock becomes hot and dry. The ability to exhibit physiological plasticity is therefore limited on thermally harsh rocky shores, supporting the hypothesis that tropical ectotherms live close to their thermal limits and are limited in their ability to acclimatize further to survive episodes of extreme thermal stress.