Statocysts in Crabs: Short-Term Control of Locomotion and Long-Term Monitoring of Hydrostatic Pressure

Abstract

Crabs show well-coordinated locomotion. They have proprioceptors similar to those of lobsters, but they differ in terms of their balancing systems and their condensed nervous system, which allows rapid interganglionic conduction. Typically they exhibit dynamically stable locomotion with a highly developed semicircular canal system that codes angular acceleration in each of three orthogonal planes (horizontal and vertical at 45 degrees and 135 degrees to the pitching plane). Left and right interneurons each code one direction of angular acceleration, carrying information between the brain and the thoracic ganglia. Cell A codes head-up vertical plane angular accelerations. Cell B codes rotations in the horizontal plane. Interneurons C and D code headdown vertical plane information, carrying it ipsilaterally and contralaterally respectively. These interneurons have a central role in locomotion. They are activated and have their responsiveness to angular acceleration enhanced before and during locomotion. Such simple activation pathways point to how an angular-acceleration-controlled robot (CRABOT) could be constructed. Hydrostatic pressure information carried by the thread hairs, which also sense angular acceleration, is filtered out from direct pathways onto the interneurons, but spectral analysis shows that it still has an influence via central pathways. Long-term recordings from equilibrium interneurons in free-walking crabs taken from the wild into constant conditions show tidally changing frequencies