The Influence of Thyroid Hormone Deficiency during Pigment Pattern Transformation on the Development of Adult Coloration in the Firemouth Cichlid Thorichthys meeki (Cichlidae)

Thyroid hormones are involved in the regulation of many ontogenetic processes and may act as a link between the changing environment and phenotypic development in cichlid fishes. To date, the potential for reversible changes in pigment pattern development of cichlids caused by thyroid hormone deficiency during metamorphic transformations remains poorly understood. In this work, using Thorichthys meeki as an example, it was demonstrated that a decrease in thyroid hormone levels, resulting from the addition of thiourea (final concentration in water 0.03%), beginning during the late larval period, led to changes in the timing and rate of development of pigment pattern elements. A pronounced retardation in pigment patterning led to the emergence of an alternative coloration phenotype in juveniles, characterized by gray-yellow shades due to changes in the iridophore and melanophore populations on the body, and xanthophore oblique lines on the dorsal fin. After the cessation of the influence on the thyroid axis, compensatory changes occurred during subsequent development, including the replacement of chromatophores in the color elements, which ultimately led to the formation of a typical adult pigment pattern. The obtained data indicate an important role of thyroid hormones in reversible plasticity during sensitive periods of post-embryonic development in cichlids.