[Metamorphoses in fishes and their endocrine regulations]

The most emblematic metamorphoses in the animal kingdom remain those of the caterpillar into a butterfly and the tadpole into a frog. However, some other vertebrates also present, at one or more steps of their biological cycle, drastic changes in their morphology, physiology and behavior, allowing them to adapt to a new environment (habitat) and way of life, and thus considered as metamorphoses. This is the case within fish, for some representatives of teleosts (the largest group among vertebrates) and of cyclostomes (the most ancient group among vertebrates). Thus, a larval (or primary) metamorphosis takes place, as for amphibians, in pleuronectiformes (e.g. flatfish), elopomorphs (e.g. eels) and also in cyclostomes (e.g. lamprey). This larval metamorphosis is controlled, in all cases, by thyroid hormones. However, this regulation is remarkably opposite, stimulatory in teleosts as in amphibians, and inhibitory in the lamprey. These opposing regulations, as well as the presence of metamorphoses only in certain groups or even certain species, suggest that the recruitment of thyroid hormones for the control of metamorphoses would have occurred repeatedly and independently during the evolution of vertebrates. The neuroendocrine control of the production of thyroid hormones during larval metamorphosis is ensured in amphibians by the stimulatory control of pituitary thyrotropin, itself under the stimulatory control of hypothalamic corticotropin-releasing hormone instead of the thyrotropin-releasing hormone classically involved in the thyrotropic control of metabolism. A similar neuroendocrine control would also occur for teleost larval metamorphosis, but investigations are yet limited. Data are still lacking concerning the neuroendocrine control of the production of thyroid hormone which exerts an inhibitory control on lamprey metamorphosis. In some amphidromous migratory fishes, a so-called secondary metamorphosis, because it occurs at the juvenile stage, allows the passage from the freshwater (river) to the seawater (ocean). These are silvering in eels and smoltification in salmons. Salmon smoltification is a transition between two phases of juvenile body growth, under the positive control of thyroid hormones, as the larval metamorphosis of amphibians and teleosts. In contrast, eel silvering marks the end of the feeding and body growth phase as well as the initiation of the reproductive phase, and is controlled by the gonadotropic axis with sex steroids. The additional involvement of other hormones, such as cortisol for larval and secondary metamorphosis and growth hormone for smoltification, has also been demonstrated in the control of vertebrate metamorphoses. Overall, the larval (primary) and secondary metamorphoses observed in various vertebrates, and the recruitment of thyroid hormones and different neuroendocrine axes for their triggering, have contributed to the evolution of complex life cycles adapted to diverse habitats.