Thermal tolerance and metabolic scope of Sphoeroides annulatus juveniles, a fish with aquaculture potential

Abstract

Ocean warming, driven by current anthropogenic climate change, affects the physiological responses of marine ectotherms, including Sphoeroides annulatus (bullseye puffer fish), a fish with aquaculture potential. This subtropical-tropical fish inhabits shallow estuarine and coastal environments along the eastern Pacific and is exposed to extreme temperature fluctuations and marine heatwaves, which may threaten its distribution and cultivation. This study aimed to determine thermal tolerance (CT_max and CT_min), thermal polygon area, acclimation capacity via the acclimation response ratio (ARR), thermal safety margins (TSM), and plasticity of the thermal metabolic scope (TMS) of S. annulatus juveniles. Prior to experiments, juveniles (N = 300) were acclimated to five temperatures (20, 23, 26, 29, and 32 °C ± 1 °C) for 30 days. CT_max ranged from 37.8 to 41.4 °C and CT_min from 12.0 to 18.7 °C. The thermal polygon area (293.0°C²) indicated moderately eurythermal behavior, consistent with the species’ broad distribution. ARR values ranged from 0.2 to 0.4 for CT_max and 0.1 to 0.9 for CT_min, suggesting greater plasticity to cold than to warm conditions. Present and future TSM estimates suggest that S. annulatus populations in tropical zones may be more vulnerable to ocean warming than those in subtropical regions. The highest TMS values—an indicator of aerobic capacity—were observed in juveniles acclimated at 23–26 °C, indicating that this temperature range may represent a thermally favorable condition for maximizing aerobic performance. This study provides insight into the physiological limits of S. annulatus under ocean warming conditions and helps define optimal conditions to improve juvenile growth in aquaculture systems.