Functional nasal morphology of chimaerid fishes.

Holocephalans (chimaeras) are a group of marine fishes comprising three families: the Callorhinchidae (callorhinchid fishes), the Rhinochimaeridae (rhinochimaerid fishes) and the Chimaeridae (chimaerid fishes). We have used X-ray microcomputed tomography and magnetic resonance imaging to characterise in detail the nasal anatomy of three species of chimaerid fishes: Chimaera monstrosa, C. phantasma and Hydrolagus colliei. We have shown that the nasal chamber of these three species is linked to the external environment by an incurrent channel and to the oral cavity by an excurrent channel via an oral groove. A protrusion of variable morphology is present on the medial wall of the incurrent channel in all three species, but is absent in members of the two other holocephalan families that we inspected. A third nasal channel, the lateral channel, functionally connects the incurrent nostril to the oral cavity, by-passing the nasal chamber. From anatomical reconstructions, we have proposed a model for the circulation of water, and therefore the transport of odorant, in the chimaerid nasal region. In this model, water could flow through the nasal region via the nasal chamber or the lateral channel. In either case, the direction of flow could be reversed. Circulation through the entire nasal region is likely to be driven primarily by the respiratory pump. We have identified several anatomical features that may segregate, distribute, facilitate and regulate flow in the nasal region and have considered the consequences of flow reversal. The non-sensory cilia lining the olfactory sensory channels appear to be mucus-propelling, suggesting that these cilia have a common protective role in cartilaginous fishes (sharks, rays and chimaeras). The nasal region of chimaerid fishes shows at least two adaptations to a benthic lifestyle, and suggests good olfactory sensitivity, with secondary folding enhancing the hypothetical flat sensory surface area by up to 70%.