Thyroid axis dynamics during metamorphic development of hatchery-reared grey mullet larvae

The flathead grey mullet (Mugil cephalus) is a widely distributed euryhaline teleost of ecological and economic importance. During early development, mullets undergo a prolonged larval phase culminating in metamorphosis, a critical developmental transition that is associated with high mortality and pronounced variability in growth trajectories that limit its commercial production. To better understand the regulation of metamorphic development in M. cephalus, we characterized the ontogeny of the hypothalamic-pituitary-thyroid axis, the key regulator of metamorphosis in teleosts. Immunostaining revealed that thyroid follicles begin producing T3 and T4 at the initiation of external feeding, with follicle number and area increasing significantly from metamorphosis onset onward. Whole-body thyroid hormone measurements showed that T4 peaks at metamorphosis onset and T3 peaks at its climax. Gene expression analysis indicated stage-specific regulation across the axis with early expression peaks of hypothalamic (trh, crhb) and pituitary (tshba, galr1a, crhr1, crhr2) genes at yolk sac depletion, followed by increased expression of thyroidal genes (tg, dio2) and thyroid hormone receptors (thrb, thrab) during metamorphic development. Together, these findings describe a coordinated endocrine program in which thyroid follicle maturation, hormone production, receptor availability, and axis regulation are temporally aligned to ensure timely metamorphosis. Beyond their relevance for overcoming larval rearing bottlenecks in aquaculture, these results establish a reference framework for thyroid axis function in M. cephalus, offering broader insights into endocrine control of metamorphic development in teleosts.