Meeting Abstract
Elasmobranchs, the sharks, skates and rays, have an expansion of the intestine that contains a convoluted internal structure called the ‘spiral valve’, and thus, this intestinal region is called the spiral intestine (SI). It has been supposed, with little basis, that the SI in sharks slows digesta transit rate. We experimentally tested the flow rate using water. The flow rate is slowed by 3.5x through the SI as compared to the proximal intestines (PI) in Squalus suckleyi, Sphyrna tiburo, Sphyrna lewini, and Carcharhinus limbatus; four species with radically different intestinal morphology. We found resistance is higher in the SI compared to the PI. In the SI, resistance is higher in the reverse direction, which helps maintain anteroposterior directional flow. We measured the contraction rate of the intestinal smooth muscle of S. suckleyi, and found that an average of 48 contractions is necessary for complete transit of a medium viscosity (20 Poise) substance. Finally, we provide the first 3D images of spiral intestines for S. suckleyi, Dasyatis say, Rhinoptera bonasus, and Sphyrna tiburo. These 3D reconstructions from CT scans of lyophylized spiral intestines provide a modern replacement for the heavily cited and revised Parker (1885) illustration. This investigation provides a new way of quantifying intestine volume, surface area created by the intestinal folds, and visualizing how flow may occur through the various spiral structures. It also provides a mechanism for the quantification of the functional morphology of the SI and opens the door to examining the function of the gastrointestinal tract of fishes and sharks.
