Fluid dynamics and microscale chemical movement in the chemosensory appendages of the lobster, homarus Americanus

Every chemosensory structure has a boundary layer surrounding it through which chemical signals must pass before contacting receptor cells. Fluid motion in this boundary layer is slow and odor movement is mainly by diffusion. The boundary layer structure depends upon external fluid velocities and the morphology of the appendage. High-speed (10-200 Hz) electrochemical recordings from microchemical electrodes were used to quantify chemical transport in the microscale environment of three morphologically different chemosensory appendages of the lobster, Homarus Americanus: lateral antennule, medial antennule and walking legs. Controlled pulses of the odor tracer (dopamine) were delivered to the three appendages at three different flow speeds (0, 3, 6 cm/s). The amplitudes of the pulses increased with increasing flow speed, indicating that boundary layer thickness decreased with increasing flow speed. Larger pulse amplitudes were measured in the walking legs than in the lateral or medial antennules at all flow speeds. In addition, larger amplitudes were recorded in the medial antennule than the lateral antennule. Changes in pulse amplitude with increasing flow speed were larger than changes in pulse duration. These results demonstrate that pulse amplitude is affected more than pulse duration by boundary layer thickness and that the morphology of the receptor strucure helps determine the structure of signals arriving at receptor cells. This may explain why animals have adopted sampling strategies that reduce boundary layer thickness.