Thyroid hormones in development of coloration of two species of Neotropical cichlids.

Neotropical cichlids demonstrate an enormous diversity of pigment patterns, a morphological trait that plays an important role in the adaptation and speciation. It has been suggested that alterations of the activity of the thyroid axis, one of the main endocrine axes regulating fish ontogeny, are involved in the development and diversification of pigment patterns in the Neotropical cichlids. To test this hypothesis, we assessed the thyroid hormone developmental dynamics and pigment patterning, and experimentally induced hyper- and hypothyroidism at different developmental stages in convict cichlid, Amatitlania nigrofasciata, and blue-eye cichlid, Cryptoheros spilurus. We revealed that both species display a similar type of coloration development and similar reactions on changes of the thyroid hormone level, but species-specific differences in hormonal dynamics and thyroid hormone-responsiveness. These findings indicate that thyroid hormone is a necessary but not sufficient signal to induce the transition from larval to juvenile coloration, and is a component of a complex, concerted endocrine cascade that drives skin development.