Manta rays filter tiny plankton from the water column 鈥� even those smaller than their filter pores. A unique filtration systems makes solid particles ricochet back into the ray鈥檚 mouth while draining out seawater.
Filter-feeders use different strategies to trap food. Most fish have gills that work as a sieve, letting water through and retaining food particles larger than the sieve pores. Sea sponges, on the other hand, secrete sticky mucus to capture food.
It was long thought that manta rays used sieve filtration, but then gut analysis found large amounts of plankton that should have passed through the pores in their gills. This suggested the rays must have a different mechanism to separate food.
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To find out how manta ray filters work, Misty Paig-Tran at the California State University, and her colleagues, 3D printed a segment of ray fish gill and let a stream of water carrying solid particles of various sizes run through it at a speed matching the natural ocean flow. Although the 3D printed filter had a pore size of 340 micrometres, it managed to capture nearly all particles larger than 150 micrometres.
Domino tiles
鈥淭his is like catching rice grains in a pasta filter, which doesn鈥檛 happen except by chance,鈥� Paig-Tran says.
Using simulation, the team found manta rays have a unique mechanism to filter solid particles out of water. The manta ray filter looks like a row of slanted domino tiles, and it alters the trajectories of solids and liquids differently. When water hits the filter, the course of its flow turns sharply and it passes through the gaps between the tiles. But solid particles floating in the water are not able to make such a sharp turn. Instead, they ricochet back off the filter and into the fish鈥檚 mouth. The researchers call this unique filtration system 鈥榬icochet separation鈥�.
Such a filtration mechanism is not only effective for capturing tiny plankton, but also resistant to clogging around the pores. Paig-Tran suggests that filtration systems inspired by the manta ray may be useful in wastewater treatment and in helping remove microplastic pollution聽from water.
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