CHI*, K.-J.; KAJIURA, S.M.; SUMMERS, A.P.; Duke U.; FAU; UCI: A running nose: Biomechanics of peripheral olfactory organs of the spiny dogfish and hammerhead sharks

The mechanics of odor perception in fishes is poorly understood. The nasal rosette is morphologically diverse raising the possibility of variable flow regimes, and in actively swimming sharks we suppose ventilation of the nasal capsule must be passive. We used spiny dogfish as a model system to examine the incurrent mechanism through nares placed perpendicularly to the body axis, as well as the internal flow regimes and odor-encounter patterns under controlled ambient flow. Coomassie blue was used to visualize flow and stain the rosettes. We confirmed that flow is induced passively through nasal capsule at a nonzero ambient flow. At a maximal ambient flow at 1/4 BL/sec, Reynolds numbers (Re) suggest a laminar internal flow: Re<1 between lamellae and Re<800 at the raphe. Odor transport is not solely a diffusive event as indicated by a Peclet number ~26 between lamellae. Convection is presumably mediated by ciliary activity. Dye released from the same upstream location relative to the incurrent naris yielded consistent regional staining patterns. A separate experiment revealed that the area of the incurrent naris where dye entered the rosette determined where it would exit the excurrent naris. These results suggest that the path taken by the odor molecules may be determined by how they enter the naris and where they are within the incurrent water column. This is of particular importance in hammerhead sharks, a group with diverse morphology in their incurrent naris. Our previous study found that the incurrent naris width varies more than 5 fold among all hammerheads and two out-group species. The prominent prenarial groove characteristic of most hammerhead species could be �mapped� onto the rosette in such a way that the shark forms a somatotopic map of where odor particles intersect the head.