Semicircular Canal Morphology and Function in Crabs

Abstract

All crabs have a well-developed balancing organ or statocyst which has long been considered analogous to the vertebrate vestibular system. The foundations for our understanding of the statocyst were laid by Hensen (8) and others, and further developed by Dijkgraaf (1). However, it was 1972 before two independent lines of research indicated the closeness of the analogy between the crab statocyst and the vertebrate semicircular canal system. Sandeman and Okajima (11) in Canberra demonstrated that the statocyst of the Australian mud crab Scylla consisted of two toroids at right angles. They also showed by introducing dye into the canals that during angular accelerations, fluid was displaced by inertial forces differentially in the separate canals, and they were able to investigate the responses of the various receptor hairs to fluid displacement. At the same time, I discovered a set of large interneurons each with input from one statocyst in the green shore crab Carcinus which illustrated the orthogonal arrangement of the statocyst. Each in terneuron responded optimally to rotation in one direction about either the vertical axis or one of the two horizontal axes midway between pitch and roll axes (2,3). Using these interneurons or the sensory cells innervating the thread hairs and free hook hairs to monitor the working of the statocyst, it has been possible to investigate many properties of the statocyst and its associated central pathways in Scylla(4,7,9,10,12), in Carcinus(6; un- published observations; Campbell, unpublished observations), and to a lesser extent in other crabs.