Melanin deposition in the gut of the monogenean Macrogyrodactylus polypteri Malmberg 1957.

Macrogyrodactylus polypteri, a skin parasite of the African freshwater fish Polypterus senegalus, has a black pigmented gut. Unusually, the gut is striped; 4 pigmented transverse bands become apparent within 2 h of birth and intensify in colour as the parasite ages. Ultrastructurally, the gut is a syncytium, similar to that of other gyrodactylids. Within the pigmented bands, electron-dense ovoid (1.0 micron x 0.25 micron) granules accumulate in feeding vacuoles in the gut syncytium, and the same granules are abundant in the gut lumen. These granules have the histochemical characteristics of melanosomes, and were also found in the epidermis of the host fish. It is concluded, therefore, that the black coloration is due to melanin derived from host epithelial cells in the diet, and that this parasite, in common with most other monopisthocotylean monogeneans, is an epithelial browser. The function of the gut banding is unknown, but may create a disruptive colour pattern, more difficult to see against the pigmented scales of the host than a uniformly dark gut.