BOLKER, J.A.; VAN CISE, A.; HAKALA, T.F.; Univ. of New Hampshire, Durham: Pigmentation development in summer flounder

Flounders offer a remarkable opportunity to study the developmental patterning and differentiation of pigment cells. As in other ectotherms, overall pigmentation is based on the morphology and distribution of several types of chromatophores, of which melanophores are the most important. Although flounder embryos and early larvae start out with symmetrical pigmentation, body coloration (like other aspects of morphology) becomes profoundly asymmetrical at metamorphosis, which therefore represents a second, distinct phase of pigmentation patterning. We have applied two complementary techniques to document skin and pigmentation development in summer flounder (Paralichthys dentatus): paraffin histology, which shows both general skin structure and the specific location of melanophores, and a biochemical (DOPA) assay that reveals melanophores at early stages of their differentiation. Newly hatched larvae lack DOPA-reactive cells, but scattered cells on both ocular and blind sides show a positive DOPA reaction by late premetamorphosis. Following metamorphosis, DOPA treatment fails to detect early-stage melanophores; however, a layer of melanin is present between the dermal and epidermal layers of ocular-side skin. This melanic layer is completely absent on the blind side of normal individuals. In juveniles, dermal melanophores gradually replace epidermal melanin as the primary basis of dark pigmentation on the ocular side. Malpigmented juveniles display the same skin structure and melanin distribution as normal individuals at the histological level; however, albino fish show typical blind-side morphology in ocular-side skin, and ambicolored fish have ocular-type skin and pigmentation on both sides of the body. Supported by UNH-AES Hatch Grant 399 and USDA NRI 99-35208-8586 to JAB.