Silvering: Metamorphosis or Puberty?

Abstract

Eels have a complex migratory life cycle with the occurrence of two metamorphoses (for reviews see: Sinha and Jones 1975; Tesch 1977; Haro 2003). They represent a typical larval (first) metamorphosis, the leptocephalus larva being transformed into the glass eel. After this drastic transformation, the growth phase starts in the continental waters and glass eels become “yellow” eels. After many years in freshwater, the yellow eels transform into “silver” eels which stop growing and start their downstream migration towards the ocean and the area of reproduction. To allow this transition from sedentary life in freshwater to migrant life in seawater, eels undergo their second metamorphosis, known as silvering. Silvering not only preadapts the eel to deep-sea conditions (S补rt 2003), but also prepares for sexual maturation, which will only be completed during the oceanic migration towards the Sargasso Sea (Tesch 1982, 1989; Dufour and Fontaine 1985; Fontaine 1985; Dufour 1994). At the silver stage, eels are blocked at a prepubertal stage (Dufour et al. 2003) but chronic administration of carp or salmon pituitary extract (gonadotropic treatment) is able to induce sexual maturation (Fontaine et al. 1964; Fontaine 1975). The lack of sexual maturation at the silver stage is due to a deficient production of pituitary gonadotropins (GtHs) (Dufour et al. 1983). Stimulation of synthesis and release of pituitary gonadotropin in the silver eel can be induced by combined treatment with a GnRH-agonist and a dopamineantagonist, indicating that a double neuroendocrine blockade was responsible for the deficient pituitary gonadotropic function: a lack of endogenous stimulation by GnRH due to a deficient production of GnRH and a strong dopaminergic inhibition of GnRH action (Dufour et al. 1988, 1991; Vidal et al. 2004). At the yellow stage, in contrast, gonadotropic treatment has no significant effect on both gonad weight (Leloup-Hatey et al. 1989) and body color (Lopez and Fontaine 1990).