Thermal physiology of tropical rock lobster (Panulirus ornatus); defining physiological constraints to high temperature tolerance

This study investigated the effect of chronic temperature acclimation (24, 26, 28, 30, and 32 °C) on growth, aerobic metabolism, feeding, and gut evacuation rate of early juvenile spiny lobster, Panulirus ornatus (initial weight = 0.48 ± 0. 03 g) to determine thermal optima for growth and identify physiological constraints at high temperatures for this emerging aquaculture species. Groups of 7 juvenile lobsters were communally cultured (n = 4) for 45 days post-acclimation and solely fed on a formulated feed. Lobster specific growth rate (SGR), apparent feed intake (AFI), and specific feed consumption (SFC) displayed unimodal response to temperature peaking at 28.0, 27.9, and 26.2 °C, respectively. Similarly, survival had a significant quadratic relationship with temperature; however, it displayed an inverse relationship to SGR, AFI, and SFC where the lowest survival was found at 29.1 °C. Continuous exponential increases in lobster standard, routine and maximum metabolic rates, and aerobic scope were observed up to a maximum non-lethal temperature of 32 °C. Gastrointestinal evacuation rate (GIER) (>96%) increased minimally between 24 and 30 °C from 2.5 h to 2.9 h and substantially rose to 5.1 h at 32 °C. While the feed conversion ratio improved with increasing temperature, the temperature optimum for metabolic feeding efficiency closely matched specific growth rate and apparent feed intake. These findings show that optimum temperature for growth performance of juvenile P. ornatus does not align with maximum aerobic scope, rather growth was limited by prolonged GIER (up to 5.1 h) at high temperatures 32 °C which constrained food consumption. These findings contribute to the growing body of literature debating the universality of aerobic scope to define physiological thermal boundaries of aquatic ectotherms suggesting thermal performance of lobsters under sub-lethal temperatures to be regulated by feed intake and driven by digestive processes such as GIER. Future research should focus on lobster feed formulation and its influence on digestive processes such as GIER, foregut evacuation, and digestibility which may influence the thermal tolerance of spiny lobsters in culture.