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. Ultrastructur ally, the gut is a syncytium, similar to that of other gyrodactylids. Within the pigmented bands, electron-dense ovoid (1.0 pm x 0.25 mu m) granules accumulate in feeding vacuoles in the gut syncytium, and the same granules are abundant in the gut lumen. These granules have the h istochemical characteristics of melanosomes, and were also found in th e epidermis of the host fish. It is concluded, therefore, that the bla ck coloration is due to melanin derived from host epithelial cells in the diet, and that this parasite, in common with most other monopistho cotylean monogeneans, is an epithelial browser. The function of the gu t banding is unknown, but may create a disruptive colour pattern, more difficult to see against the pigmented scales of the host than a unif ormly dark gut. (C) 1997 Australian Society for Parasitology. Publishe d by Elsevier Science Ltd.