Quirks & Quarks

Manta ray dining habits could be the secret to filtering pollution from water

The giant rays have an innovative filter feeding technique different from any of our technologies

The giant rays have an innovative filter feeding technique different from any of our technologies

Manta ray's unique feeding mechanism could inspire a revolution in filtering technology for things like water pollution that we desperately need. (The Associated Press)
Listen7:49

Some of the ocean's largest inhabitants eat some of the smallest creatures to achieve their formidable size.

Many whales, sharks and rays are filter feeders that consume huge amounts of almost invisible zooplankton — tiny crustaceans and fish larvae .

They do so by ingesting vast quantities of water and separating out the tasty little creatures from it.

Dr. Jim Strother and his colleagues became interested in the unique feeding system of the Manta ray.  These giant, bat-like swimmers can have wingspans of up to six meters.

Dr. Strother is an assistant professor of integrative biology in the Oregon State University's College of Science.

He thinks the Manta ray's unique feeding system could inspire a revolution in filtering technology for things like water pollution that we desperately need.

Manta ray's feeding mechanism

Manta rays feed by opening up their wide mouths to let in water.

The filter system then flushes out the water through the gill slits on the underside of their body, while food particles get directed away from the slits, into the Manta ray's mouth for consumption.

According to Strother, 100 liters of water pass their body every second, and they consume from 20 to 30 kilograms of zooplanktons every day.

The manta ray filtering apparatus effectively separates plankton from seawater. (A) Manta ray during feeding behavior (B) Gill raker (left) and tracing of filter lobes (right) (C and D) Fluid pathlines visualized using dye injection for filter lobes in wing (E and F) Trajectories of solid particles passing over filter apparatus and vertical velocity of particles. (From research paper)

The manta ray's filtering apparatus is made up of cascading sheets of gill rakers — slanted protrusions attached to the gill arch of the animal near their gill slits.  

When water passes over the rakers, it creates a pattern of swirling eddies which capture food particles and ricochets them from one raker to the next. The food thus passes over the filters, while the water travels downward between the rakers and exits through the gill slits.

This allows Manta rays to retain food particles much smaller than their gill slits. Strother found, though, that the ray's swimming speed is key.  For the filtering to work effectively, water has to be flowing in at a high speed to create the right pattern of eddies.

No more clogging

The Manta ray's food filtration system is unlike any man-made filter system we have at the moment.

It's much more sophisticated than a sieve filter, for example, which uses a mesh to separates items by size.

It can operate at a high flow rate and capture plankton much smaller than the filter.

Manta ray's unique filtering system can be used to improve wastewater treatment to prevent microplastics pollution in the ocean. (AFP/Getty Images)

It's resistant to clogging because particles don't get trapped or accumulate on the filter. Manta rays never stop to clean themselves because their gills are kept pristine, said Strother.

His team has started exploring how to adapt the animal's unique filtering mechanism for industrial applications.  

One of them, he points out, would be to improve wastewater treatment to prevent microplastic pollution entering the ocean.

"These animals have been traversing the oceans for 30 million years. There's plenty we can learn from them."

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