Epidermal Papillomas in the Flathead Sole, Hippoglossoides Elassodon, With Notes on the Occurrence of Similar Neoplasms in Other Pleuronectids2

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Abstract
Numerous fish of many species were collected in the waters of the San Juan Islands, Washington, from July, 1963, to July, 1964. Surface tumors were observed only in the Pleuronectidae. In the flathead sole, Hippoglossoides elassodon, there were 281 tumor-bearing fish in a total collection of 4,364 fish (6.4% tumor incidence). Isolated instances of similar neoplasms in 3 other pleuronectids were reported. In the flathead sole, the tumors originated in the 1st year of life as small, 1 to 2 mm, angiomatous proliferations of dermal connective tissue capped by a layer of hyperplastic epidermis. These lesions were given a descriptive name: angioepithelial nodules. Some angioepithelial nodules developed into typical epidermal papillomas near the end of the 1st year of life. Simultaneously, the dermal angiomatous component usually regressed. The epidermal papillomas spread to cover extensive portions of the fish, including body, fins, eyes, and gills. The number of tumors (both angioepithelial nodules and epidermal papillomas) per fish was highest in the 1st year of life (1–23), less in the 2d year (1–5), and least in the 3d year (1–2). No tumors were observed in flathead sole older than 4 years. Angioepithelial nodules were never seen on older fish with large epidermal papillomas. No new tumors of either kind ever appeared de novo on numerous normal and tumor-bearing fish of all age groups kept alive in the laboratory for 1 to 8 months. Although tumors occurred on any portion of the outer surface, there was a significant tendency for them to be located on the pigmented (right) side. The high frequency of natural occurrence of epidermal papillomas in the flathead sole is a striking feature of the data.

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