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TWO TYPES OF MELANOMAS IN A NEW EXPERIMENTAL SYSTEM OF PLATYFISHSWORDTAIL HYBRIDS

A new experimental system for investigating platyfish‐swordtail hybrid melanomas was developed using ornamental platyfish (Xiphophorus maculatus) and swordtails (Xiphophorus helleri). The incidence of melanomas in the hybrids was studied in relation to individual age factors. Based on the age of the onset of the melanomas, two types of the melanomas were discerned and were designated as the fry and the adult melanomas, respectively. The fry melanomas were characterized by early onset, wide‐ranged lesions, an invasive nature owing to densely pigmented cells, and a tendency toward regression. The adult melanomas were characterized by occurrence in the later stage of life, a small number of primary sites, and rapid, expanding, and invasive growth due to sparsely pigmented cells.