Too hot to handle? Using movement to alleviate effects of elevated temperatures in a benthic elasmobranch, Hemiscyllium ocellatum

Abstract

Tropical coral reef flats can be 3–4 °C warmer than surrounding deeper reef slopes, and some experience daily temperature fluctuations of up to 12 °C, which will be exacerbated as global temperatures continue to rise. Epaulette sharks (Hemiscyllium ocellatum), predominantly found on reef flats, may have evolved behavioural and/or physiological strategies to mitigate the effects of these dramatic temperature fluctuations. Here, juvenile sharks were acclimated, for at least 6 weeks, to average summer temperatures (28 °C) or predicted end-of-century summer temperatures (32 °C) to investigate the effects of elevated temperatures on growth, survival, and the use of movement to thermoregulate. In addition, sharks experience seasonal temperature changes; therefore, the upper critical thermal limits were determined for adult, wild sharks during both summer and winter months. We found that regardless of acclimation temperature, juveniles maintained the same food consumption rates (~ 5% body mass every other day), but for those living at 32 °C, this resulted in significantly decreased growth rates (body mass and total length). During winter months, maximum habitat temperatures (~ 24 °C) are far below adult sharks’ critical thermal limits (35.92 ± 0.21 °C). During summer months, maximum habitat temperatures (~ 35 °C) are closer to adult critical thermal limits (38.85 ± 0.31 °C). When estimating thermoregulatory behaviour of juvenile sharks maintained at 28 °C, those sharks examined in winter exhibited no thermoregulatory behaviour, while those examined in summer actively sought to control their thermal exposure, preferring 30.7 ± 1.04 °C (day) and 28.54 ± 0.75 °C (night). Furthermore, after acclimation to predicted end-of-century conditions, these same sharks behaviourally sought out 32.94 ± 0.46 °C (day) and 30.74 ± 0.68 °C (night); despite the cost of decreased growth and/or survival. Sharks maintained in control conditions had a mortality rate of 33% during the initial 90-day period of exposure, while mortality was 100% in those sharks exposed to elevated conditions. Ultimately, as ocean temperatures continue to rise, the distribution and abundance patterns for epaulette sharks and many other coral reef species are likely to change if trade-offs associated with acclimation outweigh the benefits of moving to more favourable habitats.