Changes in the concentration and histochemistry of epidermal mucous cells during the alevin and fry stages of the Brown trout Salmo trutta

Changes in the concentration and histochemistry of epidermal mucous cells have been shown to be similar in both natural and hatchery‐reared Brown trout during the early stages of development. Superficial mucous (goblet) cell concentration is greatest immediately post‐hatching (≈ 1500 cells/mm 2 ) and remains high in the alevin epidermis during the period of yolk‐sac resorption. Once the yolk sac has been resorbed and the free‐swimming fry have left the spawning redds, the epidermal mucous cell concentration falls to ≈700 cells/mm 2 . During the alevin‐fry transition there is also a progressive decrease in the relative proportion of mucous cells containing sulphated (or a mixture of sulphated and carboxylated) acid mucopolysaccharides and an increase in the proportion of cells containing exclusively carboxylated acid mucopolysaccharides. These findings are discussed in relation to habitat and to the functional roles of epidermal mucous secretion in fishes. It is suggested that the high potential for production of mucus in the alevin stages is related to the need for protection against abrasion during the period of residence in the gravel of the spawning redds.