Narrowing, twisting, and undulating: Complicated movement in shark spiral intestine inferred using ultrasound.

Shark intestine presents a complicated three-dimensional morphology, characterized by the development of a coiled internal septum. A basic question regarding the intestine is its movement. This lack of knowledge has prevented the testing of the hypothesis on its functional morphology. The present study, to our knowledge, for the first time, visualized the intestinal movement of three captive sharks using an "underwater ultrasound" system. The results indicated that the movement of the shark intestine involved strong twisting. We suspect that this motion is the mechanism that tightens the coiling of the internal septum, enhancing compression of the intestinal lumen. Our data also revealed the presence of active undulatory movement of the internal septum, of which the undulatory wave propagated in the opposite (anal-to-oral) direction. We hypothesize that this motion decreases the flow rate of the digesta and increases absorptive time. These observations indicate that the kinematics of the shark spiral intestine are more complicated than expected based on morphology, and the fluid flow in the intestine is likely highly regulated by intestinal muscular activity.