Filter feeders are essential to the aquatic ecosystem and can be found in a variety of species, ranging from minuscule crustaceans and certain types of coral and krill to various molluscs, barnacles, and even massive basking sharks and baleen whales.
Recent findings by MIT engineers have revealed that one filter feeder has evolved to sift food in ways that could revolutionize the design of industrial water filters.
In a study published this week, researchers delve into the fascinating filter-feeding mechanism of the mobula ray, a family of aquatic rays that includes two manta species and seven devil rays. Mobula rays capture food by swimming with their mouths wide open through areas rich in plankton, filtering the plankton particles into their gullet as water flows into their mouths and exits through their gills. Both sides of the mobula ray’s mouth are lined with parallel, comb-like structures known as plates that draw water into the ray’s gills.
The MIT researchers discovered that the size of these plates may enable incoming plankton to bounce across the plates and move deeper into the ray’s cavity instead of being expelled through the gills. Additionally, as the ray feeds, its gills absorb oxygen from the water that flows out, allowing the ray to breathe simultaneously.
“We show that the mobula ray has evolved the geometry of these plates to be the perfect size to balance feeding and breathing,” says study author Anette “Peko” Hosoi, the Pappalardo Professor of Mechanical Engineering at MIT.
The engineers created a simple water filter inspired by the unique plankton-filtering abilities of the mobula ray. Through detailed studies of water flow in filters equipped with 3D-printed plate-like structures, they obtained critical experimental results. The team used the findings from these tests to create a design blueprint that they believe can assist designers in enhancing industrial cross-flow filters, which share a similar structure to that of the mobula ray.
“We want to expand the design space of traditional cross-flow filtration with new knowledge from the manta ray,” says lead author and MIT postdoc Xinyu Mao. “People can choose a parameter regime of the mobula ray so they could potentially improve overall filter performance.“
In the new study, the team created a simple filter modeled after the mobula ray. The design of the filter is what engineers call a “leaky channel,” essentially a tube with strategically placed openings along its sides. The channel comprises two flat, transparent acrylic plates that are securely glued at the edges, creating a small gap between them for fluid flow.
At one end of this channel, the researchers placed 3D-printed components that mimic the grooved structures found on the floor of the mobula ray’s mouth. The researchers then introduced water into the channel at different flow rates, along with colored dye, to enhance the visualization of the flow. They discovered an intriguing behavior: at lower pumping rates, the fluid flowed smoothly and effortlessly slipped through the grooves into a reservoir.
However, when the researchers increased the pumping speed, the faster-moving fluid failed to pass through, instead swirling at the entrance of each groove, forming a vortex, akin to a small cluster of hair tangled within the teeth of a comb.
“This vortex is not blocking water, but it is blocking particles,” Hosoi explains. “Whereas in a slower flow, particles go through the filter with the water, at higher flow rates, particles try to get through the filter but are blocked by this vortex and are shot down the channel instead. The vortex is helpful because it prevents particles from flowing out.”
The team concluded that vortices play a crucial role in the filter-feeding mechanism of mobula rays. As the ray swims at an optimal speed, water entering its mouth forms vortices between the grooved plates. These vortices successfully obstruct plankton particles, including those smaller than the gaps between the plates. The particles then bounce across the plates and continue deeper into the ray’s cavity while the remaining water flows freely between the plates and exits through the gills.
The researchers utilized the findings from their experiments, along with measurements of the filtering structures in mobula rays, to create a design for cross-flow filtration.
“We have provided practical guidance on how to actually filter as the mobula ray does,” Mao offers.
“You want to design a filter such that you’re in the regime where you generate vortices,” Hosoi says. “Our guidelines tell you: If you want your plant to pump at a certain rate, then your filter has to have a particular pore diameter and spacing to generate vortices that will filter out particles of this size. The mobula ray is giving us a really nice rule of thumb for rational design.”
Journal reference:
- Xinyu Mao, Irmgard Bischofberger, and Anette E. Hosoi. Permeability–selectivity trade-off for a universal leaky channel inspired by mobula filters. PNAS, 2024; DOI: 10.1073/pnas.2410018121
