Spontaneous thyroidal tumors in the Swordtail Xiphophorus montezumae.

Early studies of the thyroid tumors of trout (3—5)have demonstrated the relationship between iodine supply and hyperplastic growth of thyroid cells. Experimental work with fishes provided much of the background for the present under standing of the etiology of human goiters. In con trast with encapsulated mammalian goiters, how ever, hyperplastic thyroids in fishes are remark able for their invasive reaction to surrounding tis sues. This apparently invasive property of unen capsulated fish thyroidal cells and follicles led earlier workers to consider the glandular tumors as “carcinomas―(1) . However, later work showed that the regression of thyroidal tumors in trout could be induced by adding iodine to the water in which the fish were maintained (3). Spontaneous thyroidal tumors have been re ported in at least eighteen species of fresh-water and marine fishes. Like trout, all of them have been observed under cultivation in aquaria or in fish hatcheries (6). While the goiters in fresh-water forms are often caused by a low iodine supply in the water or food, this explanation hardly seems applicable for salt-water species which live in a relatively iodine-rich environment and which feed upon such iodine-rich foods as ocean shrimp and clams (6). The morphologies of thyroidal tumors in fishes are all quite similar. In this report we propose to describe a upon taneous thyroidal tumor of high incidence in a species of Mexican fresh-water fish, the swordtail, Xiphophorus montezumae. Our knowledge of the environmental conditions under which these tu mors develop makes it possible to suggest certain generalizations concerning their natural history