The dorsal muscles of the red sea bream Pagrus major, when reared in an outdoor tank, be-come marked with numerous black lines. The darkish appearance of these tissues has been known to be markedly improved upon rearing in dark conditions for three months. In this study, we examined (1) cytological properties of the black lines, (2) their changes under dark adapta-tion and (3) possible implication of acid phosphatase in paling or depigmentation processes using transmission electron microscopy and its associated cytochemistry. The results indicate that (1) the black lines distributed along the blood vessels in muscles of the fish reared in an unshaded out-door tank are composed of a large number of fully differentiated melanophores, each stretching melanosome-laden cytoplasmic processes; (2) the melanophores in the muscles of dark-adapted fish appear as a collapsed mass of melanosomes, indicating that they are in an aggregated state; (3) no apparent difference is observed in acid phosphatase activity between light- and dark-adapted fish.
The similar population density of melanophores in the periphery of the blood vessels between the light- and dark-adapted fish strongly suggests that paling of the muscle tissues induced by dark adaptation results from melanosome aggregation in these cells, and not from a marked decrease of melanophores.
