Histological changes in the gonad, skin, intestine and olfactory epithelium of artificially‐matured male American eels, Anguilla rostrata (LeSueur)

Migratory (silver) male American eels were injected weekly for 5 weeks with human chorionic gonadotropin (hCG), salmon pituitary extract, hydrocortisone 21‐hemisuccinate (cortisol), or saline to determine the effects of induced maturation on the histology of their gonads, skin, intestines and olfactory epithelia. Treatment with hCG induced full sexual maturity, salmon pituitary evoked only limited spermatogenesis, and neither hydrocortisone nor saline had any effect on the gonad. Eels injected with hCG and salmon pituitary experienced no changes in skin morphology, but the epidermal thickness of saline‐ and hydrocortisone‐treated fish decreased. Intestinal morphology did not change in any of the treatment groups. Both the presumed sensory and non‐sensory portions of the olfactory epithelium of hCG‐ and pituitary‐treated males exhibited a decrease in thickness, structural degeneration, and non‐significant reductions in mucous cell density. Some of these changes in maturing male eels are similar to those previously observed in maturing females but others are different; it appears likely that maturing Anguilla are sexually dimorphic and that these changes are adaptive and not artifacts of the hormone treatments.