Black and soft sferical structures (4-5 mm in diameter) were observed at the caudal fin-base of swordtail (Xiphophorus helleri)-platy (Xiphophorus maculatus) hybrids. Examinations using light and electron microscopes revealed that there were abundant melanine pigments in these regions. It was observed that melanocytes were euchromatic and contained one nucleous. The meianocytes were degranulated in some regions and had iytic and membraneous structures in their cytoplasm. It is believed that the structures which had not developed metastasis is a harmless pigment cell tumour. Observations of the fish with tumour for six months showed that this anomaly did not cause mortality and therefore it might be carried by heredity. © TÜBİTAK
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