Why "swimming crabs" are able to swim - The importance of the axial skeleton: A comparison between the "swimming crab" Liocarcinus depurator and two other brachyuran crabs (Cancer pagurus, Carcinus maenas) using µCT and 3D-reconstruction.

ABSTRACT

Most brachyuran crabs use their pereiopods as walking legs, but there are also a number of species, in which the last (5th) pair of pereiopods (P5) are specialized to permit a unique mode of swimming. One of these P5-swimming crabs is Liocarcinus depurator, commonly found on European shores. We present 3-dimensional µCT-based reconstructions of the axial skeletons and 5th pereiopods (P5), including the intrinsic and extrinsic musculature of the P5, of L. depurator and of two other non-swimming brachyuran crabs, Cancer pagurus and Carcinus maenas. In Liocarcinus, we also present a reconstruction of the 4th pereiopod (P4) together with its intrinsic musculature. We further use 3-dimensional prints of the reconstructed P5 coxa and basi-ischium of L. depurator as well as thoracal parts near the thoracal-coxal arthrodial cavity to build a model which can simulate the effects of muscular activity based on muscle insertions in our 3D reconstruction and under various simulated tensile vectors. This enables us to test muscular functions that had previously been speculated upon. Reconstructed morphological structures are compared to find differences that may explain why of the three species, only L. depurator is able to swim. Significant differences between Liocarcinus and the non-swimmers Cancer and Carcinus were found in the shape of the axial skeleton, external P5 morphology and the dimensions and origin sites of the extrinsic P5 musculature, but not in the intrinsic musculature of the P5. Inclination angle measurements of P1 thoracal-coxal articulation axes against P5 axes showed that in Cancer and Carcinus, angles in the longitudinal plane were smaller than in the lateral one, whereas in Liocarcinus, they were greater. Inclination angles in the longitudinal plane were also much greater in Liocarcinus than in Cancer and Carcinus. 3D print muscular activity simulation showed that muscles inserting at the basi-ischium, which are often referred to as "levator" or "depressor" muscles, may actually also function as promotors or remotors, depending on the tensile vector within which the muscle is acting.