Thermal and hypoxia tolerances of sympatric snappers are linked to partitioning of coral-reef habitats

Zonation on coral reefs is ubiquitous, with differences in species assemblages among habitats linked to a variety of abiotic and biotic factors. We examined whether differences in the physiology of 2 subtropical meso-predatory snappers, Lutjanus carponotatus and L. adetii , reflect their distributions across a gradient of coral-reef habitats (’flats’ and ’slopes’), which have substantial differences in abiotic conditions, at Heron Island, Australia. First, using >225 h of video surveys, we confirmed that L. carponotatus is commonly found on reef flats but L. adetii is absent. We then examined species responses to environmentally relevant changes in temperature and dissolved O 2 by exposing individuals of both species to 2 temperatures (20 or 30°C) for 4 d before measuring minimum and maximum O 2 consumption (ṀO 2min and ṀO 2max ), critical O 2 levels (O 2crit ), and critical thermal maximum and minimum (CT max and CT min ). L. adetii was more temperature sensitive than L. carponotatus , with ṀO 2min of L. adetii increasing ~1.8-fold between 20 and 30°C (ṀO 2min of L. carponotatus did not significantly differ). Additionally, L. adetii had a ~3.8°C narrower thermal tolerance zone (both lower CT max and higher CT min ) and a reduced hypoxia tolerance (~30% higher O 2crit ) than L. carponotatus at both temperatures. As such, physiological differences between L. carponotatus and L. adetii can, at least partially, explain their distribution across coral reef habitat zones. Given that coral reefs are experiencing rapid changes in environmental conditions, improving our understanding of how physiology influences habitat use is critically important for predicting impacts on reef fish assemblages.