Operculum of peppered loach, Lepidocephalichthys guntea (Hamilton, 1822) (Cobitidae, Cypriniformes): a scanning electron microscopic and histochemical investigation

Lepidocephalichthys guntea frequently darts and burrows in mud or sand, and spends most of the time buried in the soft bottom of water bodies. The epidermis covering its outer surface (OE) and the epithelium lining the inner surface of the operculum (EISO) differ noticeably in their surface architecture seen under SEM, and in their glycoprotein secretions analysed histochemically in whole mounts. This indicates their important physiological activity as mediators for extrinsic factors to which these are exposed. Micro-ridges arranged compactly may impart firm consistency to the free surfaces of the epithelial cells in the OE as an adaptation to mechanical stress during burrowing. In the EISO, which is not prone to abrasion during burrowing, in contrast, widely spaced micro-ridges suggest less rigid surfaces. Mucous cells are of two types, A and B, in the OE, and only one, type C, in the EISO. Most mucous cells in the OE are type A, and elaborate mainly sulphated glycoprotein, which is associated with an increase in the viscosity of mucus providing protection against possible mechanical damage during burrowing and against pathogens. In contrast, glycoproteins with oxidisable vicinal diols and with carboxyl groups elaborated by type C mucous cells in the EISO are considered to lower viscosity of the mucus. Mucus with lower viscosity is washed away more easily by water currents, preventing accumulation on the surface that may obstruct or disturb the smooth flow of respiratory current across the branchial chamber. Presence of prominent taste buds in the OE is considered as an adaptation to locate food with increased efficiency. Presence of a large number of taste buds in the EISO, is regarded as an adaptation to detect other chemicals that could enter the buccal cavity during respiration. Possible functional significance of glycoprotein secretions in the taste buds are discussed.