Three Types of Skin Papillomas of Flatfishes and Their Causes

Three structurally different skin papillomas occur in flatfishes. Apparently, different causative agents are responsible . (1) Flat 'fish pox'-like tumours with a low connective tissue content occur in the Atlantic dab Limanda limanda. The high rate of occurrence in the German Bight (up to 6.6 "h) might be related to the dumping of wastes consisting of sulphuric acid and titanium compounds. (2) Collagen-rich papillomas on the tip of the lower jaw appear on both Atlantic and Pacific flatfishes kept in aquaria over long periods of time. These develop as a result of mechanical stimulation of the tissue as the fishes rub against aquarium walls. (3) Complex papillomas, composed in a characteristic manner of so-called X-cells and envelope cells between them, are common in flatfishes of the Pacific region. The structure and the distribution of these complex papillomas indicate that a virus is probably involved. There also exists some relationship between tumour occurrence rate and degree of coastal water pollution.