Sharks, skates and rays are known for their unique intestines, which are spiral shaped and located behind the stomach. This unusual spiral intestine naturally slows digestion and expands the intestine's surface area to improve nutrient absorption. The inside of the spiral intestine can vary greatly in shape, but had previously only been studied through two-dimensional illustrations, losing the complexity of the structure's three-dimensional shape and preventing researchers from fully understanding the role that these designs may play in digestion. Samantha Leigh from California State University Dominguez Hills, USA, and colleagues from the University of Washington, Biomark Inc., and the University of California Irvine, USA, wanted to compare the three-dimensional (3D) shape and function of the spiral intestines from 22 shark families to better understand whether corkscrew shaped intestines could predict a shark's diet type and how food moves through these unconventional intestines.
After obtaining museum specimens of species ranging from the small and rare combtooth dogfish (Centroscyllium nigrum) to the large and charismatic smooth hammerhead (Sphyrna zygaena), the team removed the sharks’ spiral intestines and CT scanned them at Friday Harbor Laboratories, USA, and the University of California Irvine. Analysing the structures of the intestines in 3D, the team realised that the digestive tracts came in four distinct forms: column-shaped, funnel-shaped toward the end, funnel-shaped toward the beginning and scroll-shaped. The team then selected four sharks, each representing one of the four intestine designs, and pushed three different solutions, ranging in viscosity from water to 25% glycerol, through the intestines to measure their flow rates. Lastly, the team injected a hormone that stimulates muscles to contract into the intestines of five dogfish sharks, to measure how long it takes for food to move through these spiral intestines as the animals digest dinner.
The researchers found that some shapes, such as the funnel-shaped spiral intestines, have a slower flow rate than others, suggesting that sharks with these shapes likely have slower digestion rates. This may also increase the water and nutrient absorption in the spiral intestine, so that sharks can harvest the most energy from their food. Though spiral intestines vary in shape, these shapes did not predict what the sharks chose to eat. In addition, when the team analysed how fluids flow through the pulsating tissue, they realised that spiral intestines work like a Tesla valve, which only allows fluid to move in one direction without the benefit of additional moving components, such as flap valves. In short, spiral intestines prevent food from moving backward, in the wrong direction. The team also realised that the pulsing contractions that propel food through these intestines can be concentrated in just a few locations and could help to mix and move food better as it progresses through the digestive system.
Most incredibly, this corkscrew intestine design evolved in a shark ancestor around 450 million years ago, so this structure is older than land animals! Yet, despite its success at sticking around for so long, it has only evolved in a few groups of fishes and occurs most in the cartilaginous fishes (such as the sharks, skates and rays). The main benefit of the curly design is that it naturally prevents blockages and food from accidentally flowing backward, maintaining a slow progress that is ideal for digestion. While the four different spiral intestine shapes may not be directly related to the diets that these animals consume, they do affect how fast food moves through. Future studies might uncover why these different shapes evolved in the first place and what role the difference in food flow speed may have on a shark's digestion.
