The Structural Morphology of Walking Mechanisms in Eumalacostracan Crustaceans

Nearly all eumalacostracan orders possess members that are capable of walking by means of their pereopods. This study analyses the skeletomuscular mechanics that make walking possible. It includes at least one member of each order, except for the Stomatopoda: Isopoda (four suborders), Tanaidacea (two suborders), Cumacea, Spelaeogriphacea, Amphipoda (two suborders), Mysidacea (two suborders), Theromosbaenacea, Decapoda and Syncarida. In each case an attempt was made to find primitive or generalized forms. The movement of living animals was observed wherever possible. Because the condition of the endopodal podomeres tends to be relatively stereotyped, primary attention was focused on the base of the limb, that is, body, coxa and basis. All taxa have some features in common. Most importantly, the orientation of the podomeres is such that the entire limb lies in a plane (the `limb plane'), whose integrity is preserved in normal locomotion. However, the limb plane can bend to compensate for bottom irregularities, and for the benefit of grooming, feeding, swimming or burrowing. In walking, `extensible strut' motions (Gray 1944) result from extension and flexion within the limb plane, and `rowing' motions are accomplished at the limb base. Rowing motions are complex, involving tilting and rotation of the limb plane and require special adaptations. Two major patterns exist. In eucarids and syncarids, the coxa forms a gimbal, where the dicondylic body-coxa articulation allows promotion/remotion, and the dicondylic coxa-basis articulation allows abduction/adduction. In most peracarids, body-coxa articulation is either immobilized or capable of limited abduction/adduction, and coxa-basis articulation is monocondylic and can perform a complete suite of motions. Exceptions of varying degrees exist within tanaids and amphipods, and the mysidaceans show an intermediate morphology. I suggest that the percaridan condition is apomorphic and that the evolution of the midventral thoracic marsupium was the driving force for change. Because the oostegites are coxal outgrowths, coxal promotion/remotion as a part of normal walking would disrupt the marsupium. As a result, the coxa lost this function, and coxa-basis articulation evolved to a morphology that would allow promotion/remotion as well as abduction/adduction.