[OANNES Foro] The Marvelous Filters in the Manta Ray's Mouth

[Mario Cabrejos]

The car-sized, kite-shaped fishes don't have to clear their throats because
of their unique method of filtration that could be applied to preventing
plastic pollution in the seas.

The Marvelous Filters in the Manta Ray's Mouth

By  <https://www.nytimes.com/by/veronique-greenwood> Veronique Greenwood

Sept. 26, 2018

https://www.nytimes.com/2018/09/26/science/manta-ray-filters.html?rref=colle
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It all began with a simple question: Why don't manta rays clear their
throats?

The car-size, kite-shaped fishes filter their plankton food from seawater,
but they don't pause, close their mouths and snort clogs from their filters
nearly as often as you would expect, according to Misty Paig-Tran, a marine
biologist and a professor at California State University, Fullerton. If
their filters work like sieves, then they must get clogged over time, like
all similar systems, from vacuum cleaners to your water-filter pitcher.

But Dr. Paig-Tran and her colleagues' latest research,
<http://advances.sciencemag.org/content/4/9/eaat9533> published Wednesday in
Science Advances, shows that the manta ray is using a previously unknown
method of filtration that causes particles to glide over its straining
system, rather than go through it. It doesn't need to clear its filters much
because they're rarely clogged.

To understand how the manta's filters work, imagine a series of tiny angled
slats lined up in its mouth. When seawater rushes over these structures,
according to experiments by Raj Divi, a student in Dr. Paig-Tran's lab, it
forms whirlpools between each pair of slats. These vortices don't suck
particles down. Instead they push up, keeping the fragments of plankton and
other seaborne particles from falling into the crevices.

 

As a result the particles ricochet off the slats, growing concentrated in
the mouth while the water drains away. They never actually get in the
filter, according to both lab experiments washing colored dye and particles
over plastic versions of the structures, and mathematical models of what's
going on. They are bounced out before they get the chance, and then are
swallowed by the ray.

Video

This is not the first time that researchers have found that filter feeders
are probably not simply sieving.
<https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0150106>
According to one study looking at this question in whales, water may rush
across the front and back of the giant mammals' baleens at different speeds,
causing a pressure difference that allows plankton to build up on the
interior of the mouth. But other marine creatures do seem to be sieving,
such as the basking shark, which sometimes closes its mouth and clears clogs
in its filters with a kind of cough.

Understanding how the manta ray is feeding and what it is eating may assist
conservation efforts. "This is a protected animal that is being harvested
like crazy. And we don't even have a good handle on what they're feeding
on," Dr. Paig-Tran said, noting that this could aid in understanding what
organisms they depend on and whether they are ingesting plastic particles
floating in the ocean.

 

It could also lead to filters for human use, relying on this method. In the
past, machines that use clever engineering to avoid the buildup of
obstructions while filtering have met with striking success: some vacuum
cleaners, for instance, separate dust from the air using centrifugal force,
rather than a filter that grows caked with grit.

Dr. Paig-Tran hopes that this discovery will aid in the battle against
microplastics in the ocean.

Tiny plastic pieces may wind up in wastewater, she said, "but they don't get
treated in wastewater treatment because they're not equipped for that size
particle." Manta rays, in contrast, are good at filtering at such a scale.

Imagine a clog-resistant filter modeled on the manta's mouth and placed in a
treatment plant to catch plastic fragments before they are released into the
environment. To Dr. Paig-Tran, that seems like an intriguing potential use
for the system she and her colleagues have described.

 



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