Developing isotopic proxies to reconstruct the metabolic rates and thermal histories of octopus.

Understanding an animal's metabolic rate and thermal history is pivotal for ecological research. Recent studies have proposed the use of stable carbon and oxygen isotopes (δC and δO) in biogenic carbonates as proxies of metabolic rate and experienced temperature, respectively, to overcome the challenges of directly measuring these data in the field. Our study represents the first experimental investigation to develop δC and δO proxies in octopus. Octopus berrima hatchlings were raised in captivity, at varying water temperatures, for up to 110 days. O. berrima statoliths were then subsequently analysed for δC and δO values. The proportion of metabolically derived carbon, or respired carbon (C), increased as the octopus grew (slope = 0.076, R = 0.72), suggesting an influence of somatic growth rate and body mass on δC values. Additionally, we identified an inverse correlation between δO values and environmental temperature (slope = -0.163, R = 0.91), which was subsequently used to develop a thermal reconstruction model. Our experiment aids in interpreting stable isotopic values in statoliths and their application as temperature and metabolic proxies in wild-caught octopus. Such proxies will increase our monitoring capabilities of these ecologically and commercially significant cephalopods and contribute to their conservation and effective management.