Mechanoreceptors in calanoid copepods: designed for high sensitivity.

The mechanoreceptors of the first antennae of Pleuromamma xiphias, a mesopelagic calanoid copepod, are critical for the detection of potential threats. These receptors exceed the physiological performance of other crustacean mechanoreceptors in sensitivity to water velocities as well as in frequency response. A study of these receptors was initiated to elucidate structure-function relationships. Morphologically, the receptors resemble the arthropod scolopidial organs by the presence of a scolopale tube. However, the rigidity of the copepod receptors greatly exceeds those described for crustaceans and other arthropods. The scolopale tube completely encloses the distal dendrites and it is firmly anchored to the cuticle. Microtubules are organized in register and arise from microtubule subfibers associated with crescent-shaped rods which extend from the basal body region to the setal socket. The distal dendrites are filled with a large number of cross-linked microtubules. Termination of the distal dendrites inside the lumen of the setae is gradual with a firm anchoring to the cuticle. A likely mechanism for mechanotransduction would involve a linkage between individual microtubules and mechano-gated channels in the dendritic membrane. The rigidity probably contributes to the high frequency sensitivity, and termination of the dendrite inside the seta increases the overall sensitivity of these receptors.