Lipoma in a farmed northern bluefin tuna, Thunnus thynnus (L.)

An adult male northern bluefin tuna from an ongrowing site was sent to the laboratory for histopathological examination. The mass was located in the dorsal musculature over the vertebral column. It was oval shaped, firm in consistence and measured 35 X 22 cm. The tumour appeared well delimited by the surrounding tissues. The cut surface showed a lobulated appearance and had a pale colour. Microscopically, the tumour was constituted mainly of well differentiated adipose cells with large fat vacuoles and flat nuclei pushed to the periphery of the cells. The fat cells were supported by a thin stroma which condensed to form thick septa giving a lobulated feature to the neoplasm. Those septa of connective tissue were continuous with a thin capsule externally delimiting the tumour. On the basis of the gross and histological findings the tumour was diagnosed as lipoma. Lipoma, fibrolipoma and liposarcoma have rarely been reported in fish and, when present, they showed the same gross and histological patterns observed in higher animals. Eight tumours have been described in the genus Thunnus by different authors cited by Munday et al. (2003); although lipoma seem to be the most frequent neoplasia, generally they were subcutaneous masses. In our opinion, a dysraphic status of bone tissue must be considered as a possible predisposing factor also in fish when lipomas arise close to the bone.