Too hot to handle? Using movement to alleviate effects of elevated temperatures in a benthic elasmobranch, Hemiscyllium ocellatum
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.
We want to thank SeaWorld, Gold Coast for donating sharks and D. Kraver and M. Johnson for rearing embryos. Additionally, thanks are due to the staff of the Marine and Aquatic Research Facilities Unit (MARFU) at James Cook University for help with infrastructure and logistical support, as well as the Heron Island staff for their support in the field. This work was supported by an Australian Research Council (ARC) Super Science Fellowship, ARC Early Career Discovery Award, and ARC Centre of Excellence for Coral Reef Studies research allocation to J. L. R. In addition, this work was funded in part by a Griffith University (Gold Coast) Climate Change Response Group research grant to G. R. and J. L. R.
Compliance with ethical standards
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. Collection permit under the James Cook University accreditation and Marine Park permit, #G14/36697, from the Great Barrier Reef Marine Park Authority, was obtained.
All animal care and experimental protocols used in this study were approved by James Cook University Animal Ethics Committee regulations (permit: A2089) and conducted according to the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes and the Queensland Animal Care and Protection Act 2001. This article does not contain any studies with human participants performed by any of the authors.
Conflict of interest
No conflict interests exist. C. Gervais declares that he has no conflict of interest. T. Nay declares that she has no conflict of interest. G. Renshaw declares that she has no conflict of interest. J. Johansen declares that he has no conflict of interest. J. Steffensen declares that he has no conflict of interest. J. Rummer declares that she has no conflict of interest.
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