The upper thermal limit of epaulette sharks (Hemiscyllium ocellatum) is conserved across three life history stages, sex and body size

Abstract

Owing to climate change, most notably the increasing frequency of marine heatwaves and long-term ocean warming, better elucidating the upper thermal limits of marine fishes is important for predicting the future of species and populations. The critical thermal maximum (CTmax), or the highest temperature a species can tolerate, is a physiological metric that is used to establish upper thermal limits. Among marine organisms, this metric is commonly assessed in bony fishes but less so in other taxonomic groups, such as elasmobranchs (subclass of sharks, rays and skates), where only thermal acclimation effects on CTmax have been assessed. Herein, we tested whether three life history stages, sex and body size affected CTmax in a tropical elasmobranch, the epaulette shark (Hemiscyllium ocellatum), collected from the reef flats surrounding Heron Island, Australia. Overall, we found no difference in CTmax between life history stages, sexes or across a range of body sizes. Findings from this research suggest that the energetically costly processes (i.e. growth, maturation and reproduction) associated with the life history stages occupying these tropical reef flats do not change overall acute thermal tolerance. However, it is important to note that neither embryos developing in ovo, neonates, nor females actively encapsulating egg cases were observed in or collected from the reef flats. Overall, our findings provide the first evidence in an elasmobranch that upper thermal tolerance is not impacted by life history stage or size. This information will help to improve our understanding of how anthropogenic climate change may (or may not) disproportionally affect particular life stages and, as such, where additional conservation and management actions may be required.

Physiological responses to acute warming at the agitation temperature in a temperate shark

“…Thermal tolerance mechanisms in elasmobranch fishes are poorly understood. To date, eight studies have quantified CT max in seven species of elasmobranch fishes: (i) four tropical species: the epaulette shark (Hemiscyllium ocellatum; Gervais et al, 2018;Wheeler et al, 2022), ribbontail stingray (Taeniura lymma; Dabruzzi et al, 2013), blacktip reef shark (Carcharhinus melanopterus; Bouyoucos et al, 2020Bouyoucos et al, , 2021 and sicklefin lemon shark (Negaprion acutidens; Bouyoucos et al, 2021); and (ii) three temperate species: the Atlantic stingray (Hypanus sabinus; Fangue and Bennett, 2003), Port Jackson shark (Heterodontus portusjacksoni; Gervais et al, 2021) and chain catshark (Scyliorhinus rotifer; Lupton and Bennett, 2023). These studies predominantly tested for effects of temperature acclimation on CT max without testing underlying mechanisms.…”

Nonlethally assessing elasmobranch ontogenetic shifts in energetics

“…In addition to the captive epaulette sharks, data were also collected from wild sharks during catch and release sampling. Epaulette sharks were sampled at Balgal Beach, QLD ( n = 12; −19.021387, 146.418124) from May to June in 2021 and 2022 and on the Heron Island reef flat, QLD ( n = 60; −23.444622, 151.914889) from October to November 2021 (Wheeler et al ., 2022). Sharks were caught and measured in the same manner as described above.…”

“…In addition to the captive epaulette sharks, data were also collected from wild sharks during catch and release sampling. October to November 2021 (Wheeler et al, 2022). Sharks were caught and measured in the same manner as described above.…”

The ecological relevance of critical thermal maxima methodology for fishes

“…A meta-analysis by Bell et al (2009) found that several behavioural traits that were repeatable often differed among age classes, across sexes (also reported in O' Donnell et al 2020) and between field and laboratory studies. Many factors can elicit plasticity (variation) in CT max estimates, including differences in life stages (e.g., Illing et al, 2020;Recsetar et al, 2012), diet (Isaza et al, 2019), water quality (e.g., Ern et al, 2016;Potts, 2020;Sardella et al, 2008), habitat (Rodgers et al, 2019), reproductive stage (Auer et al, 2021;Dahlke et al 2020;Johnson, 1976;Wheeler et al, 2022) and social status (Gilmour & Bard, 2022;LeBlanc et al, 2011). Chronic stress in fish can also impair responses to thermal stress (e.g., Gilmour & Bard, 2022;Claireaux et al, 2013;LeBlanc et al, 2011).…”