Thyroxine as a Mediator of Metamorphosis of Atlantic Halibut, Hippoglossus Hippoglossus

The response of morphological, histological and endocrinological development to exogenous 1-thyroxine (T₄) and to water depth during metamorphosis in Atlantic halibut, Hippoglossus hippoglossus, was investigated. Exogenous T₄ was given in daily doses of 0.1, 0.05 ppm or a control treatment to halibut larvae at 550 daydegrees (posthatch, premetamorphic) for 14 days. Water depths of 40 cm, 10 cm or 1.5 cm were used to rear halibut larvae from 590 daydegrees for 21 days. Halibut larvae given exogenous T₄ at 0.1 ppm had accelerated eye migration relative to MH in fish given 0.05 ppm and in control fish. Pigmentation was correlated with dosage after 14 days. The volume of thyroid tissue was expressed in a dose-dependent manner and exhibited a size-dependency within each treatment. However, the follicles were atypical with reduced colloid, increased lumen and low epithelial cells even in the control group. The results indicate that T₄ is a mediator in halibut metamorphosis. In the water depth experiment, only cortisol levels of larvae reared in 1.5 cm water were significantly affected after 21 days, but this was not correlated with metamorphic rate. Hormone profiles, morphological changes and size suggest the existence of a window of opportunity for metamorphosis in halibut extending from about 16 mm and tapering off about 21 mm SL. The pooled hormone profiles indicate the commencement of a hormonal cascade similar to that of other flatfishes during metamorphosis. The results indicate that growth, neural and skeletal transformation, and pigmentation are biochemically separate processes in the metamorphosis of Atlantic halibut.