BELANGER, JH; DEWELL, RB; MILLET, L; VIDAL-GADEA, AG; Louisiana State University, Baton Rouge; Louisiana State University, Baton Rouge; Centenary College of Louisiana; Louisiana State University, Baton Rouge: Comparative studies of walking in crabs: From neuroanatomy to behavior

Spider crabs (Libinia emarginata) and shore crabs (Carcinus maenas) are two brachyuran crabs with different walking strategies and environmental niches. L. emarginata walks preferentially forwards and is strictly aquatic. C. maenas preferentially walks sideways and is amphibious. Furthermore, C. maenas is capable of locomotion on land with limb kinematics that closely resemble those seen while it walks underwater. Libinia does not appear to be capable of walking on land, or when carrying an added load of more than 25% of its body weight. Are there differences in physiology and/or morphology that reflect these differences? We are using a combined approach of neuroanatomy, neuromuscular physiology, and EMG combined with kinematics in freely-behaving animals, to answer this question. While the neuromuscular anatomy of the distal limb segments is similar in the two species, there is a more anterior placement of Libinia�s fused thoracic ganglia, and a reduction in proximal leg musculature, compared to Carcinus. The latter difference may reflect the reduced ability of spider crabs to walk when laden. Furthermore, in isolated preparations, limb muscles in Libinia show much lower rates of facilitation than do similar muscles in Carcinus. Both species show similar alterations in limb kinematics as a response to increased load. In addition to increasing the duration and within-burst frequency of power stroke muscles, Carcinus responds to added load by altering the timing of muscle activity relative to limb kinematics. We are currently investigating whether Libinia show similar changes in the timing of muscle activation.