Simulated heatwave and fishing stressors alter corticosteroid and energy balance in neonate blacktip reef sharks, Carcharhinus melanopterus

The increasing frequency and duration of marine heatwaves attributed to climate change threatens coastal elasmobranchs and may exacerbate existing anthropogenic stressors. While the elasmobranch stress response has been well studied, the role of the unique corticosteroid-1 alpha-hydroxycorticosterone (1 alpha-OHB)-in energy balance is not understood. Therefore, 1 alpha-OHB's utility as a stress biomarker in elasmobranch conservation physiology is equivocal. Here, we analyse the roles of corticosteroids, 1 alpha-OHB and corticosterone, and metabolites, glucose and 3-hydroxybutyrate (3-HB), in response to stress in a protected tropical shark species, the blacktip reef shark (Carcharhinus melanopterus). Wild-caught neonates were exposed to ambient (27 degrees C) or heatwave conditions (29 degrees C) and subsequently a simulated fishing stressor (1 min air exposure). Blood samples were taken prior to temperature exposure, prior to air exposure, and 30 min, 1 h, 24 h, and 48 h post-air exposure at treatment temperatures. Plasma 1 alpha-OHB was elevated for 48 h in 27 degrees C-exposed sharks but declined over time in 29 degrees C-exposed sharks. Plasma 1 alpha-OHB was not correlated with either metabolite. Plasma glucose was higher and plasma 3-HB was lower in 29 degrees C-exposed sharks. In a separate experiment, blood samples were collected from both neonate and adult sharks immediately following capture and again 5 min later, and analysed for corticosteroids and metabolites. Plasma 1 alpha-OHB increased in neonates within 5 min, but neonates displayed lower plasma 1 alpha-OHB and higher glucose concentrations than adults. We conclude that 1 alpha-OHB does not serve as a classic glucocorticoid role in C. melanopterus under these stressors. Furthermore, we show for the first time, ontogenetic differences in plasma 1 alpha-OHB. Ultimately, our findings provide insights into hormonal control of energy mobilization during stress in C. melanopterus, particularly during simulated heatwave conditions, which seem to alter both endocrine and energy mobilization. Further work is needed to determine the utility of 1 alpha-OHB as a biomarker for the mobilization of energy during a stress event in elasmobranchs.

Automated summary

Our research analyzed the stress response of the protected tropical shark species, the blacktip reef shark, under different temperature and fishing stress conditions, showing that 1α-OHB did not play a typical glucocorticoid role under these stressors. Furthermore, we demonstrated for the first time ontogenetic differences in plasma 1α-OHB levels in sharks of different developmental stages.