manta rays look strange when they eat: they open their huge mouths and swim in the sea. These scary-looking large cartilaginous fish actually live on filter food, and the sea flows into their big mouths, which are filtered by Gill rakes, leaving behind small plankton, and the rest of the water flows away from the Gill cleft.

(manta rays filter food. (original video: Oscar Olsson)

for human beings, filtering is also a very common operation, and the simplest idea is to use a filter to block particles larger than its pore diameter. But doing so is always accompanied by a worry: the particles are easy to block the strainer and have to be replaced or cleaned frequently.

but manta rays that filter sea water every day do not seem to be bothered by blockage, and they have nothing to do with cleaning the filter. Why aren't manta rays' gills blocked by food particles?

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in order to understand this problem, some researchers have carried out experiments with 3D printing models and computer simulations. They chose the manta ray (Manta birostris) as the reference prototype, imitated its anatomical structure, and speculated the flow rate during filtration by swimming speed. These big food filtering devices are made up of rows of parallel structures that look a bit like sloping dominoes.

(filter feeding structure of manta rays. Photo Source: Misty Paig-Tran)

experiments show that these filter structures work very differently from the filter screen. The filter screen should only hold particles larger than the diameter of the opening, which can easily get stuck in the opening, while the filter structure of manta rays can leave smaller particles than the opening, and the small particles will not accumulate at the opening of the crevice. They will be "bounced off" here and continue to move along the surface of the filter structure (see diagram below). As a result, the sea water flows through the cracks, while the food particles move all the way into the digestive tract of manta rays.

(original video source: James Strother)

what prevents small particles from entering the gap in the filter structure? When the flow encounters a filter structure, it creates eddies at the opening (see image below), which the researchers say is the key to preventing food particles from entering the crevice.

(see the blue whirlpool at the opening of the gap? (original video: Rachel Gutman)

using manta rays, it may also be possible to design industrial filters that are less clogged, such as plastic particles in waste water.

original paper: http://advances.sciencemag.org/content/4/9/eaat9533

related report: https://www.theatlantic.com/science/archive/2018/09/how-manta-rays-gill-rakers-filter-water-without-clogging/571324/