Surface Ultrastructure of the Oromandibular Region of Three Stone Loaches (Teleostei: Nemacheilidae) from Northeast India

Stone loaches of the family Nemacheilidae are bottom-dwelling inhabitants of freshwater shallow streams of hilly areas and fast-flowing stretches of large rivers. The present study was undertaken to study and understand the nature, types and pattern of distribution of epidermal gustatory features, specifically taste buds and unculi, on the oromandibular region of three nemacheilid fishes from the Brahmaputra River basin, namely Mustura dikrongensis, ‘Nemacheilus’ corica and Paracanthocobitis mackenziei. Scanning electron micrographs revealed similarity and variability in the presence (or absence) of taste buds (TBs) on the lips, barbels and rostral cap of these three fishes, which can be put under three categories — Type I, Type II and Type III TBs. Type I TBs were observed only in ‘N.’ corica, but absent in M. dikrongensis and P. mackenziei. Similarly, Type III TBs were observed in M. dikrongensis and P. mackenziei, whereas absent in ‘N.’ corica. Additionally, unculi and keratinzed cells are present on the oromandibular epithelium including the breathing valve. These epidermal modifications of the mouth and associated structures often exemplify evolutionary adaptations of teleost, which can pave a new direction towards future comparative morpho-taxonomy of loaches of northeast India.