Meeting Abstract
Chemosensation is a key component of navigation and communication for aquatic invertebrates. The posterior tentacles of nudibranchs are called rhinophores and are their primary olfactory organs. We videoed and measured active water currents driven by cilia around the clavus of rhinophores using dyes and neutrally-buoyant glass beads to observe speed and patterns of flow. The speed of particle flow toward the rhinophore averaged between 0.1 and 1.0 mm/s across five species, and particles were apparently pulled in viscous laminar flow toward the rhinophore from up to 3-5 mm away. For the lamellate rhinophores found in dorid species, fluid is split into medial and lateral lamellae at the midline of each rhinophore and moved anterior to posterior through the lamellae. These rhinophores can rotate around their vertical axis to pull in water from the left or right. In other dendronotid species, fluid is pulled downward into the cup-shaped clavus of the vertically-oriented rhinophore and released in all directions at the base of the clavus before the stalk. In a burrowing arminid fluid moves distally to proximally parallel to the ridges of the conical rhinophore. Scanning electron microscopy showed densely-ciliated areas on the unexposed surfaces of the rhinophores which facilitates fluid movement through the leaflets of the clavus. Exposed surfaces had small patches of presumably-sensory cilia as found on all skin. We hypothesize that these currents minimize the boundary layer thickness and thus decrease the response latency of olfactory receptors to changes in odor density, and also increase the volume of water sampled per time. Some species show little or no current flow and a comparative study will help us determine the adaptive function of sniffing.
