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 and without sand in the bottom to study the influence of the substrate on the pigmentation pattern, pituitary growth and ACTH and MSH production. Similar results were obtained in terms of survival, growth and ocular side pigmentation in both groups. However, a clear difference was observed in blind side pigmentation and only 28% of the fish cultured with sand developed this abnormality, contrasting with 58% of those reared without sand. A clear increase in the pituitary volume was observed during larval development, being maximal at post metamorphosis. No differences in the pituitary volume could be found among pigmented, albino and ambicoloured juveniles, although those reared with sand had a larger pituitary than those reared without sand. ACTH and MSH producing cells were detected by immunohistochemistry. The proportion of the pituitary occupied by ACTH or MSH cells?ratio.was highest at early larval stages, and decreased with development to a minimum value at the climax of metamorphosis. No significant difference in the ACTH ratio was found between the two groups, although juveniles reared without sand showed a slightly higherratio, probably reflecting the stress experienced by these fish. The MSH ratio was significantly higher in fish reared with sand. No significant differences were found in the ACTH and MSH ratios among pigmented, albino and ambicoloured fish. Fish produced with a sandy bottom showed a lower degree of hyperpigmentation and possessed larger pituitaries that had higher MSH ratios than those reared without sand