Many years ago, I was snorkeling in warm Pacific waters when I noticed what looked like a spreading ink stain in the water before me. It was faint at first, a barely perceptible sliver of black against the vast backdrop of blue. But it grew and darkened, until it finally resolved into the unmistakable shape of a manta ray. It was several meters wide, from one gracefully beating wing tip to the next. Its head was topped by those distinctive, devilish, paddle-shaped fins. And its meter-wide mouth was, of course, agape.
Agape is pretty much the default setting for a manta ray. As it swims, water is constantly sucked into its cavernous mouth and out through its gills, which filter out the tiny bits of plankton that compose the fish’s diet. The manta ray is effectively a living, swimming, car-sized colander that continuously strains the oceans for food.
But colanders eventually clog. Food gets trapped in the holes, and they must then be cleaned. Mantas, on the other hand, never seem to do that. “These things can go for up to 10 minutes without closing their mouths,” says Misty Paig-Tran from California State University at Fullerton. “They don’t cough or clear anything. They don’t have a tongue.” So how do they avoid getting clogged?
When Paig-Tran first started studying mantas a decade ago, she tried to understand what they were doing by swimming with them and pointing cameras into their mouth. That didn’t work. “Mantas are highly maneuverable and incredibly intelligent,” she says. “They have the biggest brain-to-body ratio of any fish, and they don’t like you to bother them.” Undeterred, she went to three separate natural-history museums and examined the mantas that had been preserved in their collections. Specifically, she and her colleagues wanted to understand the specialized body parts that allow these fish to filter food.
Stare headlong into a manta’s mouth, and you’ll see five successive pairs of white, V-shaped rods—think of putting your thumbs and index fingers together to form a diamond. These are called gill arches. Now think of each arch as a double-sided comb, with teeth protruding both forward and backward. These are called gill rakers. Now imagine that each tooth is lined with small flaps, which overlap like rows of toppled dominos.
Based on this anatomy, Paig-Tran thought she understood how manta filtration works, and why it never clogs. As water enters the animal’s mouth, she hypothesized, it flows over the gill rakers before exiting the gills. Some of the water is diverted into the narrow gaps between the dominos, and plankton gets trapped in those gaps. But thanks to the angle of the rakers, the same incoming currents also dislodge the trapped plankton, sweeping them into the manta’s throat. Technically, this is called cross-flow filtration. More simply, it’s a sieve that unclogs itself.
But this explanation fell apart when Paig-Tran’s team, including Raj Divi, a master’s student, created a 3-D-printed model of a gill raker. When they placed it in a water tunnel that had been loaded with tiny shrimp eggs, they realized that it could filter particles that were smaller than the gaps between the dominos—a feat that should have been impossible.
The team found out what actually happens by adding dye to its water tunnel, tracking the paths of the colored currents, and observing the movements of food particles within those currents. They noticed that when water hits the leading edge of each domino, it forms a swirling vortex, which you can see in the video below.
When food particles encounter these vortices, they accelerate. This boost of speed prevents them from sliding down the gaps between the dominos. Instead, they repeatedly bounce across the tops, skimming from one to the next like the bouncing balls on sing-along lyrics. This phenomenon, which Paig-Tran calls “ricochet separation,” means that food gets concentrated in the thin layer of water that stays within the manta’s mouth, and gets swept into its throat. All the other water gets funneled through the rakers and out of its body.
This perfectly explains why mantas never need to unclog. They’ve evolved a self-cleaning filtration system that never clogs in the first place. While they swim at high speeds, mouth agape, their gill rakers can automatically extract solid food from liquid currents—and clean themselves in the process.
No one animal is known to feed in this exact way, but Eva Kanso from the University of Southern California notes that the Hawaiian bobtail squid uses a similar mechanism to select beneficial microbes. Tiny beating hairs on its surface create a flow field that selectively filters particles of bacterial size. “It is quite amazing how nature converged on such interesting solutions, which work either actively (in the squid) or passively (in the manta ray) to generate flows to [an animal’s] advantage,” Kanso says.
“Understanding these mechanisms can one day lead to bio-inspired technologies and novel devices for filtration,” adds Kakani Katija from the Monterey Bay Aquarium Research Institute, who studies how marine creatures manipulate fluid dynamics to feed and move.
Indeed, Paig-Tran has now filed a provisional patent to develop filters that are based on manta gills. Together with colleagues in Toledo, she is hoping to build a system that can remove harmful algae from fresh water. She also wants to shrink the filters so they can sieve microscopic bits of plastic from water. “That would be ideal for cleaning wastewater” she says.
The tragedy is that the very same gill rakers that inspired this work are also dooming the mantas. Historically, these fish were not targeted by humans: Their flesh isn’t tasty, and they’re difficult to catch. But in recent years, several fisheries have started killing mantas specifically for their gill rakers, to meet a sudden demand from Asian markets. The rakers, when ground into a powder and swallowed, are said to cleanse the body. There’s no evidence for this, but that hasn’t stopped people from killing thousands of these fish every year.
In 2011, both species of manta ray were classified as “vulnerable” to extinction. Since then, they’ve gained some protection under the Convention on International Trade in Endangered Species of Wild Fauna and Flora, the Endangered Species Act, and other pieces of legislation. But these animals are slow to reproduce, and it will take time for depleted populations to recover.
And to add insult to injury, gill rakers aren’t even part of traditional medicine. “The demand has arisen due to product marketing by retailers, who have falsely ‘revived’ a remedy that never existed in the traditional literature,” according to the Manta Trust, a nonprofit. It’s enough to set one’s mouth agape.