Manta rays filter water in ways never seen before

(Inside Science) – Manta rays flying over oceans can capture prey using filters that repel food rather than trap it. This new mechanism, unlike any previously known filtering technique, could lead to new ways of cleaning water from microscopic pollutants that pose risks to people and the environment.

Mantas are the largest rays in the world – a recorded specimen had a wingspan of 9.1 meters (nearly 30 feet). Despite their size, mantas are harmless to humans, feeding instead on plankton and microscopic crustaceans in open water. They swim with their mouths open, catching food while simultaneously expelling seawater from their slots.

Many other animals, such as oysters and humpback whales, feed by filtering food out of the water. Oysters rely on sticky mucus from their gills to appear on food, while humpback whales swallow bunches of fish and then use jaw structures called baleen to sift the seawater.

Previous research has suggested that mantas also sift their food, using structures known as branchiards. However, when the author of the study, Misty Paig-Tran, marine biologist and biomechanist at the University of California, Fullerton, watched mantas feed, she discovered that they only closed their mouths and swallowed all the few minutes. clear their gills of food. "Which, at least for me, did not seem to be sieving, since you would expect the filter to clog very quickly," she said.

Analyzing how living mantas live is tricky because "mantas are animals that swim very fast, and it's hard to film them in their mouths," said Paig-Tran. Instead, she and her colleagues experimented with 3D printed models of the giant manta giant's filtering device (Manta Birostris) they built from specimens borrowed from museum collections.

Scientists placed their models in tanks filled with eggs of blue shrimp and brine that were dripping at a rate that the researchers felt the gills of the manta rays would meet depending on the swimming speed and the anatomical characteristics of the fish . They then used video cameras and digital fluid dynamics software to monitor the movement of dye and eggs.

The gills of the manta rays consist of long parallel networks of leaf-shaped filter lobes. "The anatomy of a manta filter is unlike any other I have ever seen before," Paig-Tran said.

Scientists found that when water leaked on these filter lobes, large vortices appeared in the pores of the lobes. The brine cooks eggs, mimicking the behavior of any plankton suspended in the water, then basically "ricocheted" out of the filtering device, backing towards the location where the manta ray's throat is located .

"Although filter fish make up about 25% of the world's fish catch, we know very little about how these fish get their own food," said biologist Laurie Sanderson at the College of William & Mary in Williamsburg, Va. . In this study. "To explore the" black box "of manta ray mouths, this research has applied multiple cutting-edge techniques."

This new mechanism, called ricochet separation, is unlike any other filtration system previously described. "This is an excellent example of developing animal performance with surprising innovations, taking advantage of the physical principles that engineers have not even developed yet," said Jeff Shimeta, marine ecologist at the Royal Melbourne Institute. of Technology in Australia. participate in this research.

The fact that the separation of ricochets is not fouling could have major industrial applications, said the researchers. Filtering technologies, from screen filters in many air conditioners to water filters attached to many faucets, typically rely on sticky elements or sieves, but are often clogged, making them expensive and time-consuming to clean.

"The filter mechanism in the mantas is exciting because the particles bounce off the filter, so the filter stays clean and maintains high flow rates," said study co-author James Strother, a theoretical biologist of the Oregon State University in Corvallis.

Scientists are currently studying whether the separation of ricochets could extract microscopic plastic pieces from wastewater. When such "microplastics" enter the food chain, they can be potentially dangerous for wildlife and humans. "Filtering microplastics is a major technical challenge, and the high flow rates and sealant resistance we've seen in Manta ray filters could be very beneficial," Strother said.

In addition, these results could help conserve mantas and their relatives in the mobulas. "Knowing what an animal eats helps us to protect the areas where they are likely to feed," Paig-Tran said.

Additional studies may reveal whether the filtering mechanism of the manta ray plays a role in determining the prey they capture, Shimeta explained. "These results also suggest that artisans from other fish species should be examined more closely for surprising filtration mechanisms."

Overall, these results underscore that "funding for basic research is important, and eleven years ago, when I began this research, I could not predict that it would lead to a new, non-clogging filter. "said Paig-Tran. "You never know when nature's observations will turn into a usable application for humans."

In the future, scientists also hope to see the separation of ricochets occur in real mantas. "Believe me, I've tried it," Paig-Tran said. "I think I've found a good way to get this data in a live manta without harassing them, so hopefully that will happen."

Paig-Tran and Strother, along with senior author of the Raj Divi study, detailed their findings online Sept. 26 in the journal Scientists progress.