Thyroid hormone levels and lipid mobilization in weatherfish Misgurnus fossilis during early development.

The weatherfish Misgurnus fossilis is a freshwater species that adapts to hypoxic conditions during its developmental stage through the presence of external filamentous gills, which serve as primary respiratory organs. Our experimental study aimed to investigate biochemical changes occurring in weatherfish during its development, from ovulated oocytes to larvae with external gills. The concentrations of thyroid hormones were low and relatively stable up to the larval stage at hatching, provided by maternal reserves. However, levels of free triiodothyronine and total thyroxine increased during the larval stage when external gills were present, likely reflecting preparation for their reduction. This indicates that the function of the endogenous self-synthesis thyroid axis begins in weatherfish at a developmental stage preceding larval metamorphosis. Our results indicate that the free fraction of triiodothyronine is more important during fish metamorphosis than its total content, which generally exhibits limited biological activity within the organism. The increase of thyroid levels coincides with an increase in lipid levels; the rise in cholesterol and triglycerides suggests an elevated energy demand essential for metamorphosis. Consequently, thyroid hormone synthesis coupled with lipid mobilization is likely a key predictor of imminent metamorphosis in fish.