Silvering in the eel： Changes in morphology, body fat content and gonadal development.

Abstract

The commonly used criterion of maturing degree of the eel is skin color, with immature adults as ''yellow eel'' and maturing adults as ''silver eel''. The process of metamorphosis in eel is thus named ''silvering''. In eels, the common characters of silvering process are accompanied by morphological (e.g. color and thickness of integument, eye size, body length and weight, size of pectoral fins) and physiological (e.g. age at maturity, digestive tract, visual pigments, composition and function of skeletal muscle, swim bladder structure, locomotory behavior, density of chloride cells in the gill) modifications, together with the development of gonads. These modifications are beforehand preparation for the future spawning migration in the environment of deep sea. The knowledge about the phenomena occurring in eel silvering comes mainly from studies in temperate eel species, namely Japanese, American, and European eels, and this review is mainly concerned on the morphological and gonadal changes during the silvering of eels.