A new type of water vibration receptor on the crayfish antenna

Summary

• 1. The flagellum of the second antenna of the crayfishAstacus leptodactylus has two main types of sensory hairs, normal erect hairs and procumbent hairs, which bridge the junction between adjacent segments of the flagellum (Tautz et al. 1981). The upright hairs respond to particle motion or touch, and the procumbent hairs to bending of the flagellar articulation that they span.

• 2. Their mode of excitation suggests that the procumbent hairs might be able to detect and perhaps also locate nearby moving objects by the underwater disturbances they create. To test this idea we have developed a mathematical model that predicts the stimulation of the procumbent hairs produced by sources of different types, strengths, frequencies and axes of vibration. The model has been tested by measuring underwater antenna motion in response to a dipole source (a vibrating sphere) using laser Doppler vibrometry. The model appears to be adequate up to ∼ 30 Hz.

• 3. We have also used a second model, based on the work of Fletcher (1978), to predict the stimulation of the upright hairs. Comparison of results from the two models suggests that the procumbent hairs generally have a larger “receptive field”, particularly beyond the tips of the antennae. The procumbent hairs may also be less affected by random background stimulation than upright hairs and would not be influenced by eddies or turbulence created by water flowing across the antennae.

• 4. Based on our model of antenna bending and on previous neurophysiological work, the angular response threshold of procumbent hair sensilla may be <0.0003 ° at their best frequency. Thus the procumbent hair sensilla should be capable of detecting fish large enough to be potential predators at least 20 cm away.

• 5. It is possible that a crayfish, by analyzing the pattern of stimulation produced by a source of underwater disturbances, might also be able to determine its location, restricting its position to within a certain Probable Location Region (PLR). The size and location of PLRs is affected only slightly by the source strength and frequency of vibration, but differences in source type and axis of vibration increase the size of the PLR of an unknown source. A crayfish might also determine the approximate direction to a source by moving its antennae and monitoring the change in procumbent hair stimulation.