Morphology of adhesive organ of the snow trout Schizothorax richardsonii (Gray, 1832).

This paper describes the morphology and fine structure of adhesive organ (AO) of the snow trout Schizothorax richardsonii (Gray, 1832). The species lives in the sub-Himalayan streams and rivers, where they adhere to submerged rocks and stones with its AO located beneath the mouth opening. The AO consists of a crescent callus part and a modified labial fold, bearing numerous tubercles. In both parts, the epidermis is exceptionally thick (150-170 µm) and contains keratinocytes rich in bundles of tonofilaments; elsewhere, the epidermis contains single, disperse filaments, mucous cells and other usual cells seen in teleosts. Unlike epidermis elsewhere, the outer 4-5 row cells of the AO epidermis undergo keratinisation and are cytokeratin positive. The individual keratinised cell layers, which are degenerated electron-dense sheets, lack nuclei and cytoplasmic organelles, but possess filaments embedded in a dense cytoplasmic matrix and few, large mucous-like granules. Beneath the keratinised layers, keratinocytes are compactly aligned into 20-25 rows. They possess abundant thick bundles of clumped tonofilaments over the entire cytoplasm and peripherally, small mucous granules. Towards the basal epidermis, the cells are elongated and show major cytoplasmic organelles, few tonofilament bundles, but no mucous granules. It is evident that the keratinocytes of the mid-epidermis are differentiated for the production of tonofilaments. The outer keratinised part of the AO epidermis provides structural support and protection (from damage) during adhesion. It appears that the callus part acts by creating vacuum first, while the tuberculated part of the AO epidermis assists in adhesion to the rocky substratum. [ABSTRACT FROM AUTHOR]