This article is from
Creation 41(4):26–27, October 2019

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Manta ray might inspire new filter design


lp2studio © 123RF.commanta-ray
Figure 1. Calculated trajectories of fluid (blue) and solid particles (center of mass, orange; diameter, 350 µm; neutrally buoyant) as they pass over the filtering apparatus. The outline of a representative particle (red) shows the size of the particles relative to the filtering apparatus.

We have already written about the manta ray’s swimming agility, which designers of underwater robots would like to copy.1 Now their filter-feeding system turns out to be a revolutionary design that could inspire man-made micro-filters.

The Manta birostris is 7 m (23 ft) wide. Yet despite their huge size, mantas feed on some of the smallest animals, such as krill which are small shrimp-like creatures. They herd them into a ball, and then engulf them in their metre-wide mouth. Water is forced through their gills, and their prey is filtered out and swallowed. But how do they cope with the usual problem of filters—getting clogged by the very things they are filtering out?

Misty Paig-Tran from California State University at Fullerton studied the gill lining to find out. This is covered with gill arches like V-shaped rods—each arch is like a double-sided comb with small ‘teeth’ projecting both forward and backwards. These teeth are called ‘gill rakers’ (see image below). Paig-Tran found that the gill rakers could filter particles smaller than the gaps between them, which should not be possible.

© Steven Lundebergmanta-ray-filtracion
In many parts of the world, manta rays are being overfished—mostly for their gill rakers (image above), which are dried and sold as ‘medicinal products’

The solution to this problem is that these rakers generate vortices (eddies) of liquid. The particles in the liquid are sped up so much that they bounce along the tops of the arches (Figure 1). So they are concentrated in the thin layer of water on the surface of the gill lining, and are easily swallowed. And because they don’t really enter the filter, they never clog it.2

Dr Paig-Tran calls this previously unknown filtration method “ricochet separation”.3 She proposes that engineers will be able to borrow this design for filtering out harmful algae from freshwater. Further, by miniaturizing the design, it could remove microscopic plastic particles from water. “That would be ideal for cleaning wastewater”, she says.3

This newly discovered filtering technique is another example of an ingenious engineering design that human engineers had never dreamed of. So all they can do is copy the Master Designer—a rapidly expanding field called biomimetics.4

Manta misfortune

Manta rays are being hunted for the Asian market for the very structures highlighted here. The gills are ground into a powder which is claimed to ‘detoxify’ the body and even help with fertility. This hasn’t the slightest evidence, and is not even part of ‘Traditional Chinese Medicine’. Unfortunately, mantas are nonetheless being killed by the thousands, and are classified as “vulnerable” to extinction on the Red List of the International Union for Conservation of Nature (IUCN).*

  • *Reef manta ray, georgiaaquarium.org, accessed 17 Dec 2018
Posted on homepage: 26 October 2020

References and notes

  1. Catchpoole, D., Manta motion marvel, Creation 38(3):56, 2016; creation.com/manta-motion. Return to text.
  2. Divi, R.V., Strother, J.A., and Paig-Tran, E.W.M., Manta rays feed using ricochet separation, a novel nonclogging filtration mechanism, Science Advances 4(9):eaat9533, 26 Sep 2018. Return to text.
  3. Yong, E., Why your vacuum clogs but a manta ray doesn’t: The self-cleaning structures in the animal’s mouth could inspire new designs for human-made filters, theatlantic.com, 26 Sep 2018. Return to text.
  4. See creation.com/biomimetics. Return to text.

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