Ciliary function in the olfactory organs of sharks and rays

The sensory channels in the olfactory organs of modern sharks and rays (neoselachians) are lined with fields of non-sensory cilia. Using evidence from 31 species of neoselachian, this review attempts to identify the fluid - water or mucus - propelled by these cilia. The apparently common structure of the olfactory epithelium in these 31 diverse species implies that the cilia perform the same function and that this function is therefore independent of neoselachian phylogeny, lifestyle, gross nasal anatomy and the principal means by which water is circulated through the single major olfactory channel. On balance, the evidence suggests that the cilia propel mucus. This evidence includes short cilia, the presence (or absence) of water currents in the nasal region, mucociliary interactions, poor ciliary alignment, rare or absent interciliary connections, deep ciliary anchorages and the observation of cilia performing effective strokes. The short cilia are particularly telling. Goblet cells within and on the edge of the sensory epithelium are likely to be the source of any mucus layer. If the cilia of the neoselachian olfactory epithelium do propel mucus, there are three crucial implications. First, given that ciliary function is potentially independent of neoselachian phylogeny, it is possible that cilia in all neoselachians propel mucus. Second, a large part of the neoselachian olfactory organ must be actively involved in protecting the sensory epithelium from damage. Third, flow of water through the sensory channels must be driven by flow of water through the major olfactory channel.