Getting Nosy: olfactory rosette morphology and lamellar microstructure of two chondrichthyan species.

To smell, fish rely on passive water flow into their olfactory chambers and through their olfactory rosettes to detect chemical signals in their aquatic environment. The olfactory rosette is made up of secondarily folded tissues called olfactory lamellae. The olfactory morphology of cartilaginous fishes varies widely in both rosette gross morphology and lamellar microstructure. Previous research has shown differences in lamellar sensory morphology depending on the position along the rosette in hammerheads (family Sphyrnidae). Here, we investigate if this pattern continues in members of two other chondrichthyan families: Squalidae and Chimaeridae. Using contrast- enhanced microCT and scanning electron microscopy, we investigated patterns in lamellar morphology based on lamellar position along the olfactory rosette in Pacific spiny dogfish (Squalus suckleyi) and spotted ratfish (Hydrolagus colliei). We describe the gross olfactory rosette anatomy and lamellar microstructure of both species. We also put forth a new method, combining 3D morphological microCT data with 2D SEM microstructure data to better approximate lamellar sensory surface area. We found that in both species, lamellae in the center of the rosette were larger with more secondary folds. However, we found no significant differences in lamellar sensory surface area among lamellar positions. Previously, differences in lamellar sensory morphology have been tied to the internal fluid dynamics of the olfactory chamber. It is possible that the internal flow dynamics of these species are like other chondrichthyan models, where water flow patterns differ in the lateral versus the medial part of the organ, and the consistent distribution of sensory tissue does not correspond to this flow. Alternatively, the olfactory morphology of these species may result in uniform flow patterns throughout the olfactory chamber, correlating with the consistent distribution of sensory tissue throughout the organ. This study emphasizes that further investigations into chondrichthyan fluid dynamics is paramount to any future studies on the correlations between distribution of sensory tissues and water flow.