Lymphoma in the Japanese medaka Oryzias latipes

Six adult Japanese medaka Oryzias latipes were diagnosed with lymphoma, over a 6 mo period, from a control population of broodstock fish. Tumors were equally split among males and females, and presented as either grossly visible cranial masses, unilateral exophthalmos, or with no gross lesions. Tumors were classified as renal, thymic, or disseminated based on histologic distribution and the extent of neoplastic infiltrate. Metastatic lesions were found in all 6 fish and occurred via both direct extension and vascular invasion. The etiology of the tumors has not been determined.