Involvement of thyroid hormones in the control of larval metamorphosis in Sicyopterus lagocephalus (Teleostei: Gobioidei) at the time of river recruitment.

After oceanic migration, post-larvae of the amphidromous Sicyopterus lagocephalus recruit to rivers in Reunion Island. As they enter the river mouth, post-larvae undergo many morphological, physiological and behavioural changes. These drastic changes, which allow them to change feeding regime and to colonise the juvenile and adult freshwater habitat, are defined as metamorphosis. The endocrine control of these changes has never been investigated in Gobioid fish. Here, we investigated whether thyroid hormones (TH) influence metamorphosis in recruiting S.lagocephalus. An analytical study was first performed on a cohort of 2400 fish caught at post-larval stage 1 and maintained for 37 days after capture in a flume tank (fluvarium), which replicates as closely as possible the natural conditions. Biometrical parameters (total and standard lengths, corner of mouth angle, body mass and condition factor) and whole-body thyroxine (T(4)) and triiodothyronine (T(3)) contents were measured on fish, sampled at regular intervals during these 37 days (192 fish). TH levels, measured by radioimmunoassays, were highest when morphological changes, such as the change in the position of the mouth, were most important. An experimental approach was then used to test the effect of the hormonal treatment (T(4) or thiourea, TU, a TH inhibitor) on biometrical parameters of 576 post-larvae. The change in the position of the mouth was significantly accelerated in the T(4)-treated post-larvae, while it was significantly delayed in the TU-treated post-larvae, compared to controls. Our study suggests that S.lagocephalus post-larva undergoes a true metamorphic event under the control of thyroid hormones at the time of its recruitment into the river.