Regulate or tolerate: Thermal strategy of a coral reef flat resident, the epaulette shark, Hemiscyllium ocellatum

Abstract

Highly variable thermal environments, such as coral reef flats, are challenging for marine ectotherms and are thought to invoke the use of behavioural strategies to avoid extreme temperatures and seek out thermal environments close to their preferred temperatures. Common to coral reef flats, the epaulette shark (Hemiscyllium ocellatum) possesses physiological adaptations to hypoxic and hypercapnic conditions, such as those experienced on reef flats, but little is known regarding the thermal strategies used by these sharks. We investigated whether H. ocellatum uses behavioural thermoregulation (i.e., movement to occupy thermally favourable microhabitats) or tolerates the broad range of temperatures experienced on the reef flat. Using an automated shuttlebox system, we determined the preferred temperature of H. ocellatum under controlled laboratory conditions and then compared this preferred temperature to 6 months of in situ environmental and body temperatures of individual H. ocellatum across the Heron Island reef flat. The preferred temperature of H. ocellatum under controlled conditions was 20.7 ± 1.5 C, but the body temperatures of individual H. ocellatum on the Heron Island reef flat mirrored environmental temperatures regardless of season or month. Despite substantial temporal variation in temperature on the Heron Island reef flat (15-34 C during 2017), there was a lack of spatial variation in temperature across the reef flat between sites or microhabitats. This limited spatial variation in temperature creates a low-quality thermal habitat limiting the ability of H. ocellatum to behaviourally thermoregulate. Behavioural thermoregulation is assumed in many shark species, but it appears that H. ocellatum may utilize other physiological strategies to cope with extreme temperature fluctuations on coral reef flats. While H. ocellatum appears to be able to tolerate acute exposure to temperatures well outside of their preferred temperature, it is unclear how this, and other, species will cope as temperatures continue to rise and approach their critical thermal limits. Understanding how species will respond to continued warming and the strategies they may use will be key to predicting future populations and assemblages.

Elasmobranch Responses to Experimental Warming, Acidification, and Oxygen Loss—A Meta-Analysis

“…First, one crucial research target, which should provide important insights for integration with different approaches is the identification of the condition thresholds that induce relocation. Although some efforts have been made in this direction (e.g., Gervais et al, 2018;Nay et al, 2021), most experiments rely on a set-up that precludes animal behavioral regulation. This is important across multiple spatial and temporal scales, from transient shifts in the use of specific coastal habitats in response to extreme events to the general poleward movement of species.…”

Swimming performance of sharks and rays under climate change

“…Sharks and rays are known to exploit the thermal heterogeneity in their environment by selecting different temperatures throughout the day, a behavior known as thermotaxis (Fangue and Bennett 2003; Wallman and Bennett 2006;DiGirolamo et al 2012;Speed et al 2012). Elasmobranchs may select specific temperatures to enhance physiological processes, from swimming performance to digestion to reproduction (Wallman and Bennett 2006;Di Santo and Bennett 2011a;Papastamatiou et al 2015), while others do not seem to adjust to changes in the environment by thermoregulation (Nay et al 2021). One example of thermoregulation is the trade-off between optimal foraging and thermal habitats (Sims et al 2006;Sims 2010), where sharks and rays hunt in warmer waters, to maximize muscle performance, and rest in cool waters, to slow down the passage of food across the absorptive surfaces and increase nutrient uptake, thus reducing the daily energy costs (Sims et al 2006;Sims 2010) and increasing digestive efficiency (Di Santo and Bennett 2011a, b).…”

The devastating duo: bycatch and climate change hit the Atlantic wolffish Anarhichas lupus

“…The catastrophic effect of climate change on marine life has been recognised by scientists internationally and is supported by a burgeoning peer reviewed evidence base (Wepener, 2020). In 2021, the Journal of Fish Biology published its largest number of climate change related manuscripts, topics including reef fish assemblages (Eisele et al, 2021), thermal strategies of epaulette shark (Hemiscyllium ocellatum) (Nay et al, 2021) and growth rate of Japanese Spanish mackerel (Scomberomorus niphonius) (Jin et al, 2021), to name a few.In September 2020, growing concern over the impact of climate change on aquatic environments culminated in the 'World Aquatic Societies Statement On Climate Change'. A staggering 111 aquatic scientific societies, representing more than 80,000 scientists, issued the statement, with a plea to mitigate against further degradation caused by man-made climate change (Bonar, 2021).…”

“…The catastrophic effect of climate change on marine life has been recognised by scientists internationally and is supported by a burgeoning peer reviewed evidence base (Wepener, 2020). In 2021, the Journal of Fish Biology published its largest number of climate change related manuscripts, topics including reef fish assemblages (Eisele et al, 2021), thermal strategies of epaulette shark (Hemiscyllium ocellatum) (Nay et al, 2021) and growth rate of Japanese Spanish mackerel (Scomberomorus niphonius) (Jin et al, 2021), to name a few.…”