What valve-like structure in a shark's intestine increases nutrient absorption area? The spiral valve increases a shark's intestinal surface area for nutrient absorption.
A spiral valve is a corkscrew-shaped structure inside the lower intestine, specifically a modification of the ileum. Instead of requiring a very long intestinal tube, the shark folds a large absorptive surface into a comparatively short section. The coiled design also slows the movement of digested food, giving nutrients more time to pass into the body.
This arrangement helps explain why sharks can have short intestines compared with many mammals without sacrificing digestive efficiency. After material passes through the spiral valve, it enters the rectum and eventually leaves through the cloaca. Sharks also possess a rectal gland, which helps remove excess salt from the body.
The spiral valve is not unique to sharks. Similar structures occur in rays and skates, sturgeons, paddlefish, bichirs, gars, and lungfishes. It is also easy to confuse with the pyloric caeca, finger-like pouches near the stomach or intestine in some fishes. Those structures aid digestion and absorption, but the corkscrew-shaped intestinal feature described here is the spiral valve.