Abstract: We examined the distribution characteristics of melanin in different tissues and organs of Triplophysa stenura and T. orientalis by using histological method. The results showed that melanin was distributed in the head skin, dorsal skin, lateral skin, peritoneal kidney layer, spinal cord cavity wall, peritoneal wall layer, pericardial cavity wall, neurocranial cavity wall and eyes of both Triplophysa species. Melanin was not found in the abdomen skin, liver chorion, gonad capsule and spleen capsule. Melanin was distributed in the dermis layer and subcutaneous layer in the skin, and in the endothelial layer or wall layer in other tissues and organs. Melanin was mainly distributed on the back, with sparse and symmetrical distribution on both sides of the body. Melanin was more abundant and densely distributed on the dorsal and lateral skin with stripes than areas without stripes. Melanin in the no stripes skin was only partially aggregated or formed intermittently distributed melanin patches. The distribution of melanin in different tissues and organs of the same Triplophysa species was different, and the percentage of distribution area and thickness of melanin layer were significantly different. However, the distribution characteristics of melanin in the same tissues and organs were similar in two Triplophysa species. The distribution of melanin in the tissues and organs of both species were related to the intensity of ultraviolet radiation they received, which was an adaptation to the intense ultraviolet radiation environment of the plateau.

Key words: ultraviolet radiation, Triplophysa stenura, Triplophysa orientalis, histology, melanin