Spider Crab

Crustaceans have been successfully employed to study legged locomotion for decades. Most studies have focused on either forwards-walking macrurans, or sideways-walking brachyurans. Libinia emarginata is a Majoid crab (Brachyura) and as such belongs to the earliest group to have evolved the crab form from homoloid ancestors. Unlike most brachyurans, Libinia walks forwards 80% of the time. We employed standard anatomical techniques and motion analysis to compare the skeleton, stance, and the range of motion of the legs of Libinia to the sideways-walking green shore crab (Carcinus maenas), and to the forwards-walking crayfish (Procambarus clarkii). We found animals tended to have greater ranges of motion for joints articulating in the preferred direction of locomotion. Leg segments proximal to such joints were comparatively longer. Thorax elongation, leg length and placement at rest also reflected walking preference. Comparative studies of walking in Libinia and other brachyurans may shed light on the neuroethology of legged locomotion, and on the anatomical and physiological changes necessary for sideways-walking in crustaceans.

Decapod crustaceans have been the focus of neuroethological studies for decades. With few exceptions, however, their musculature remains scarcely described. We study the neuroethology of legged locomotion in the portly spider crab, Libinia emarginata (Brachyura, Majoidea), which preferentially walks forward. Majoid crabs are thought to be among the first to have adopted the crab form (carcinification) from lobster-like ancestors, making them interesting subjects for comparative and phylogenetic studies. The radial arrangement of the legs around the thorax, coupled with its unidirectional walking modality makes L. emarginata a good candidate for the presence of anterior and posterior limb specializations. Here we describe the complete muscular anatomy of all the pereopods of L. emarginata and compare our findings with other decapods described in the literature. The number of proximal muscle bundles differs between the anterior and posterior pereopods of L. emarginata. We describe an intersegmental bundle of the flexor muscle similar to the one present in distantly related, forward walking macruran species. The behavioral repertoire, amenability to experimental investigations, and phylogenetic position make spider crabs useful species for the study of the neural control of legged locomotion. To our knowledge, this is the first instance of a complete description and comparison of the musculature in all the locomotor appendages of one species.