The recent literature concerning the structure of teleost epidermis is reviewed; particular attention is given to salmonids and epidermal variability and plasticity. A preliminary survey of hatchery-reared and wild 0+ brown trout Salmo trutta L. suggested that quantitative and qualitative variation in epidermal structure are related inter alia to development and disease. A subsequent, temporal investigation of disease-free brown trout revealed a decline in the density of superficial mucous cells and a change in their histochemistry early in development; these findings are discussed in relation to habitat and the functional role of epidermal mucous secretion. Infestations by the ectoparasite Ichtyobodo necator were shown to cause a significant depletion of epidermal mucous cells and an increase in the abundance of a previously undescribed acidophilic cell in 0+ brown trout; these pathogenic effects were found to be reversible by therapeutic formalin administration.

The sequential histopathology of costiasis in cultured and wild juvenile brown trout is described. Ichtyobodo infestation stimulated an initial hypersecretion of mucus; as infestation intensified Malpighian cell proliferation occurred, mucous cells disappeared and the density of acidophilic cells continued to increase. In the chronic phase of the disease there was a cycle of epidermal hyperplasia and but by autumn, as infestation became less severe, progressive re-development of a normal epidermal structure took place in surviving fish. Analogous changes in cell composition were observed in fish subjected to formalin prophylaxis, and possible applications to fish farm management are suggested. The contents of a presumed secretory acidophilic cell in brown trout, which was also shown to occur in other salmonid taxa, were found to be granular and proteinaceous. Finally, the possibility that there are non-specific epidermal responses to surface active irritation in teleosts is considered in relation to effects of irritants on epithelial mucosae in higher vertebrates.