Mechanisms of metamorphosis in flatfishes - structural and functional analyses

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The most typical metamorphosis occurs in flatfishes, associated with eye migration and asymmetrical body transformation. The structural and functional changes occurring during flatfish metamorphosis were examined mainly in the Japanese flounder (hirame).Marked elevations in tissue thyroid hormone levels were observed at the climax phase of metamorphosis in all seven species examined. Cortisol levels also elevated just prior to thyroid hormone surge in a few species, but it didn't exhibit marked elevation in other species and/or different conditions. During metamorphosis of hirame prominent developmental changes were observed in the digestive system: differentiation of functional stomach with gastric gland and pyloric caeca and marked increase in pepsin-like enzyme activity. These changes were significantly accelerated by exogenous thyroid hormone. Small lymphocytes differentiated during metamorphosis, this suggesting transfer of the body defense mechanism from larval to adult types. Thymus involution occurred at the climax phase related to cortisol surge in hirame. This suggests an intimate relationship between endocrine and immune systems.Dorso-ventral differentiation of melanization in the epidermis of hirame occurs during metamorphosis, at which melanoblast differentiation was significantly promoted in the ocular side while it was degrade in the blind side. This process seemed to be controlled be melanization promoting factors in the ocular and the inhibiting factors in the blind sides.