Aerobic response to thermal stress across ontogeny and habitats in a teleost fish

Near-future climate change projections predict an increase in sea surface temperature that is expected to have significant and rapid effects on marine ectotherms, potentially affecting a number of critical life processes. Some habitats also undergo more thermal variability than others, and the inhabitants therefore must be more tolerant to acute periods of extreme temperatures. Mitigation of these outcomes may occur through acclimation, plasticity or adaptation, although the rate and extent of a species' ability to adjust to warmer temperatures is largely unknown, specifically as it pertains to effects on various performance metrics in fishes that inhabit multiple habitats throughout ontogenetic stages. Here, the thermal tolerance and aerobic performance of schoolmaster snapper (Lutjanus apodus Walbaum, 1792) collected from two different habitats were experimentally assessed under different warming scenarios (temperature treatments = 30, 33, 35, 36°C) to assess vulnerability to an imminently changing thermal habitat. Larger subadult and adult fish collected from a 12 m deep coral reef exhibited a lower critical thermal maximum (CTmax ) compared to smaller juvenile fish collected from a 1 m deep mangrove creek. However, the CTmax of the creek-sampled fish was only 2°C above the maximum water temperature measured in the habitat from which they were collected, compared to a CTmax that was 8°C higher in the reef-sampled fish, resulting in a wider thermal safety margin at the reef site. A generalized linear model showed a marginally significant effect of temperature treatment on resting metabolic rate (RMR), but there were no effects of any of the tested factors on maximum metabolic rate or absolute aerobic scope. Post hoc tests revealed that RMR was significantly higher for creek-collected fish at the 36°C treatment and significantly higher for reef-collected fish at 35°C. Swimming performance [measured by critical swimming speed] was significantly lower at the highest temperature treatment for creek-collected fish and trended down with each successive increase in temperature treatment for reef-collected fish. These results show that metabolic rate and swimming performance responses to thermal challenges are somewhat consistent across collection habitats, and this species may be susceptible to unique types of thermal risk depending on its habitat. We show the importance of intraspecific studies that couple habitat profiles and performance metrics to better understand possible outcomes under thermal stress.