How water flow interacts with the rhinophores in Tritonia tetraquetra

Abstract

We were interested in learning how olfactory cues that guide navigation in the gastropod nudibranch Tritonia tetraquetra, formerly known as Tritonia diomedea are received by the rhinophores in turbulent flow. We measured boundary layer thickness at various speeds, and characterized water flow turbulence caused by their rhinophore. This was done using Particle Image Velocimetry (PIV) around a fixed (dead) rhinophore on a clay model slug. We predicted with increasing flow speed, boundary layer thickness would become thinner. Results from this experiment were deemed inconclusive. We also characterized cilia-generated flow on the clavus of three species of nudibranch (Tritonia diomedea, Tritonia festiva, Armina californica). Dye flow patterns show cilia-driven currents from near the distal region of the rhinophore to the proximal region of the rhinophore. In Tritonia, the current originated at the distal tip and flowed proximal into the folds of the clavus, then spread outward towards the circumference of the base of the clavus. In Armina, the flow was unidirectionally proximal along the infolds of the rhinophore. The cilia on the rhinophore may help the animal respond faster to odor changes by removing the boundary layer (i.e. sniffing). Scanning electron micrographs showed that the tuft at the distal end of the rhinophore (a.k.a. clavus) had large patches of dense cilia on its vertical inner folds, but lacked dense cilia on the parts of clavus that are more exposed around the circumference.