Meeting Abstract

91.2  Friday, Jan. 7  Getting around when you’re round: Quantitative analysis of the locomotion of the thick-spined brittlestar (Ophiocoma echinata). ASTLEY, H.C.; Brown University henry_astley@brown.edu

The gaits of bilaterally symmetrical animals have been extensively studied and show remarkable convergence, even between phyla which have evolved limbs independently (e.g. tetrapods & arthropods). However, little data exists on the locomotion of brittle stars (Order Ophiurida), a group of pentaradially symmetrical echinoderms who use five multi-jointed arms to move along the seafloor. It is uncertain whether the ring-shaped nervous system lacking a distinct anterior is capable of generating rhythmic coordinated movements of multiple limbs. This study tested whether brittle stars possess distinct gaits with strong inter-limb coordination as seen in other phyla or instead move each arm independently according to local sensory feedback. Thirteen individuals were recorded moving across a sand substrate and each arm tip and the central disk were digitized for 58 cycles of movement. All individuals became functionally bilateral, moving along the axis of a non-locomotory central arm via synchronous motions of pairs of limbs on the right and left. The majority (90%) of cycles used a “rowing” gait, consisting of synchronous movement of the anterior arm pair adjacent to a leading central arm, while on rare occasions (10%) all four limbs were synchronously used for propulsion while the central arm trailed behind. In each cycle, the central disk would come to a complete stop, at which time the locomotory arms would move through swing phase. Propulsive limbs showed minor changes in length (< 20%), but moved through large angular excursions (> 40°). While some kinematic events were highly variable, others were closely coupled (+- 10% phase lag between rowing arms), supporting the hypothesis that brittle stars employ distinct and coordinated gaits.