Looking for environmental and endocrine factors inducing the transformation of Sicyopterus lagocephalus (Pallas 1770) (Teleostei: Gobiidae: Sicydiinae) freshwater prolarvae into marine larvae

During their ontogenetic development, many species of fishes undergo drastic changes which may be defined as true metamorphosis when they are induced by thyroid hormones. Sicyopterus lagocephalus is an amphidromous goby that has to change biome twice during its life cycle. The first biome shift occurs few hours after hatching in the river. The aim of the present study is to determine what external/environmental and internal/endocrine factors induce the transformation of freshwater prolarvae into marine larvae. First, we experimentally determined that the minimum salinity threshold inducing the transformation of all the prolarvae into marine larvae is 1.5, whereas a salinity of 0.2 induces the transformation of a few prolarvae. Similarly, an 18-h immersion in seawater before the return to freshwater is enough to induce the transformation of all the prolarvae into marine larvae, even though an immersion for 3 h can induce the transformation of a few prolarvae. Furthermore, we demonstrated that a simulated increase in the osmotic pressure of freshwater does not trigger the transformation of the prolarvae into marine larvae. Our study also reveals that among the various constituent salts of seawater, it is primarily NaCl and K+ that induce the transformation of the prolarvae, leading to the assumption that the membrane protein Na+/K+-ATPase is activated simultaneously with the prolarval transformation. Finally, we showed that thyroid hormones and cortisol are not involved in the prolarval transformation, leading to the conclusion that this first transformation in the life cycle of S. lagocephalus cannot be considered as a true metamorphosis.