Foot extension and retraction in the clam Calyptogena okutanii without any Keber's valve: An inflatable fastener bag model

In order to investigate the foot manipulation of a clam without a Keber's valve, Calyptogena okutanii was examined by light microscopy, magnetic resonance imaging and computed tomography. The foot chamber was divided into two compartments by a dense muscle, fastener zone (FZ) in which a pedal artery and sinuses in the mid-sagittal plane in between muscles running in the anterior-posterior oblique direction. The distal part of the foot chamber (inflatable fastener bag: IFB) had a loose superficial muscle layer. The proximal part of the foot chamber (visceral reservoir: VR) was covered by a dense superficial muscle layer. The outlet of the VR was connected with the hinge-ligament duct, consisting of the hinge ligament, a pair of shells and the pericardium. Based on these anatomical structures, the foot extension starts from contraction of muscles in the FZ, so that flow in the FZ is stopped. Then, the superficial muscles of the foot contract, and the pressure of the IFB increases so that the foot can extend. The foot retraction starts from the relaxation of muscles in the FZ so that the hemolymph returns to the VR. The hinge-ligament duct allows a constant return flow from the foot chamber to the gills and the heart. The heart rate and the flow in the FZ, which decreased and increased during the foot extension and retraction, respectively, supported this model. In conclusion, the FZ of the Calyptogena okutanii could play an alternative role similar to that of the Keber's valve in the Anodonta.