Ontogeny of ACTH and MSH cells in Japanese flounder (Paralichthys olivaceus) in relation to albinism

Japanese flounder larvae were reared for 43 days in glass aquariums with an d without sand in the bottom to study the influence of the substrate on the pigmentation pattern, pituitary growth and ACTH and MSH production. Simila r results were obtained in terms of survival, growth and ocular side pigmen tation in both groups. However, a clear difference was observed in blind si de pigmentation and only 28% of the fish cultured with sand developed this abnormality, contrasting with 58% of those reared without sand. A clear inc rease in the pituitary volume was observed during larval development, being maximal at post metamorphosis. No differences in the pituitary volume coul d be found among pigmented, albino and ambicoloured juveniles, although tho se reared with sand had a larger pituitary than those reared without sand. ACTH and MSH producing cells were detected by immunohistochemistry. The pro portion of the pituitary occupied by ACTH or MSH cells (ratio) was highest at early larval stages, and decreased with development to a minimum value a t the climax of metamorphosis. No significant difference in the ACTH ratio was found between the two groups, although juveniles reared without sand sh owed a slightly higher ratio, probably reflecting the stress experienced by these fish. The MSH ratio was significantly higher in fish reared with san d. No significant differences were found in the ACTH and MSH ratios among p igmented, albino and ambicoloured fish. Fish produced with a sandy bottom s howed a lower degree of hyperpigmentation and possessed larger pituitaries that had higher MSH ratios than those reared without sand. (C) 2001 Elsevie r Science B.V. All rights reserved.