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Skin on the gill and non-gill surface of fish produces mucus which may form a discrete layer between tissue and water. Goblet cells produce the bulk of mucus, but other cell types are implicated.

The status of mucus layers in healthy unstressed fish, particularly on the gills, is uncertain. Neither the thickness nor the concentration of macromolecules in the mucus layer has been determined with accuracy. It is clear, however, that stress and disease often lead to the release of mucus and the build-up of thick layers of mucus. Mucus is thought to have many roles, but few are well supported by experimental analysis.

Mucus has specific interactions with some externally applied chemicals, but for others it acts only as an unstirred layer. It may contribute to epithelial impermeability in some circumstances. There is much interest in techniques to modify the properties of mucus or to remove it, particularly to enhance penetration of …