Interplay between hormonal and morphological changes throughout a critical period of larval rearing in the orbicular batfish

•The Orbicular batfish is a major emerging species in Polynesian aquaculture.•Metamorphosis is a critical transitionary stage in the aquaculture of P. orbicularis.•Metamorphosis occurs around 10–12 days after hatching in this species.•Metamorphic climax coincides with a peak in the thyroid hormone (T3).•These findings may benefit the Pacific Island aquaculture industry. Advancement and diversification of the aquaculture industry is reliant on the development of captive breeding and rearing protocols for novel fish species. Orbicular batfish (Platax orbicularis), a major emerging species in Polynesian aquaculture, live in brackish and marine waters around coral reefs, and are highly prized by Pacific Island communities for their high-quality meat. The present study describes the larval growth of P. orbicularis from hatching until 16 days after hatching (DAH) using meristics and thyroid hormone (T3) quantification. Our study highlighted that metamorphosis of P. orbicularis is critical to their production in aquaculture, as found for other species. Levels of the thyroid hormone T3 in P. orbicularis reached a peak at 12 DAH (i.e. hormonal metamorphosis). This peak occurred in concert with important morphological changes and increased mortality and growth between 9 and 12 DAH, clearly illustrating the sensitivity of this fish during metamorphosis. Overall, our study sheds light on the metamorphosis and larval development of a novel aquaculture species, and the interplay between hormonal and morphological changes throughout a critical period of its rearing. These findings may promote the development of protocols for mass-scale production of orbicular batfish in captivity, which may be particularly beneficial to the aquaculture industries of many Pacific Island countries.