Relation of visual changes to the onset of sexual maturation in the European eel Anguilla anguilla (L.)

Eye size was correlated with body length and gonad development in 112 untreated and 33 hormone injected female Anguilla anguilla . A length‐related measure of eye size was developed as an index of sexual maturity. Eels of eye index of 6.5 or less were classed as sexually immature adults , those of < 6.5 as sexually Maturing adults . Structural changes in the eye during maturation were examined. Total numbers of rods increased markedly while density of photoreceptors remained approximately constant. Cone density decreased with degenerative changes occurring. Changes in the eye were found to be essentially complete early in the maturation process.