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, thickness of 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 deep sea environment.