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Some of the largest animals in the ocean feed on the tiniest little critters.
And if you’ve ever wondered why they don’t just scarf down a big fish and move on with
their lives, well, their strategy actually isn’t as impractical as it might seem.
That’s because, in an ecosystem, only 10% of the energy from prey gets passed up the
food chain to its predator.
So it’s more efficient for a giant creature to sustain itself on, say, krill, than on
fish much higher on the food chain.
It would have to eat a whole lot more fish to get the same amount of fuel.
And it takes a lot of fuel to sustain a giant creature.
So, many of them rely on a technique called filtering to separate out tiny, edible critters
from the water as they swim.
But not all filter feeders were created equal!
Large ocean dwellers have some pretty unique ways of fueling up.
Even among the giants of the sea, blue whales are truly giant.
Adults can be nearly as long as three school buses and weigh 150 tonnes.
That makes them the largest animal to have ever lived on Earth.
Instead of having teeth, these whales have long plates of what’s called baleen hanging
from the top of their mouths.
It’s made of keratin, the same stuff your hair and nails are made of, so it’s both
strong and flexible.
As blue whales feed, small organisms get stuck in the baleen, and the whales lick them off
with their tongues.
Blue whales will eat whatever tiny critters get stuck, but they’ve become specialists
in hunting for one particular type of food: krill.
That’s because these whales need a lot of energy, and krill make for a high-energy meal.
But the whales need a lot of them.
So they’ve evolved a special feeding strategy known as lunge feeding.
This involves diving down deep into the ocean to pick up speed and then turning around and
swimming up super fast.
Then, just before they reach a patch of krill, they open their mouths wide and lunge straight
into it.
With each lunge, the whale engulfs the krill plus a massive volume of water—like, sometimes
more than its own weight in water.
This can give them a very bizarre shape, sort of like a half-blown-up balloon, but not for
long.
All of the water gets pushed back out through the baleen, almost as fast as it came in,
leaving only krill behind.
Lunge feeding uses a lot of energy, because if you’re a whale, plunging through water
with your mouth open creates a huge amount of drag.
But the rewards are enormous.
A whale can engulf a tonne—literally, one tonne—of krill in just a few hours of lunge
feeding, providing it with enough energy for the entire day.
But not all filter feeders, or even all baleen whales, eat the same way.
Gray whales are also baleen whales, and they need a lot of fuel too.
They’re smaller than blue whales, but they’re still about as long as a school bus, and they
weigh in at about 40 tonnes.
But they’ve evolved a completely different way of eating than their gigantic cousins,
probably because of what they eat.
Gray whales eat tiny crustaceans known as amphipods, which also make for a high-energy
diet, like krill.
But these tiny critters don’t swim around in the water—they live in burrows on the
seafloor.
So to feed, gray whales have to suck them out of their burrows.
They do this by slowly swimming on their sides above the seafloor, using the side of their
mouth to suction up the mud and water where the amphipods live.
Then they push the mud back out with their tongues, leaving just the amphipods behind,
stuck on their baleen plates.
You can often tell where a gray whale’s been feeding, because it’ll leave large
pits behind on the seafloor.
But although gray whales prefer to feed by vacuuming up the seafloor, they’re also
pretty flexible.
They’ve even learned how to chase after herring when they’re around—which is not
a small feat when you have plates instead of teeth!
And that’s probably part of the reason these whales are such survivors.
They’ve been around for millions of years and survived many changes in Earth’s climate
and sea levels.
And even though they were hunted extensively in the 1900s, their numbers are on the rise
again.
As famous as whales are for their filter feeding, they’re not the only ones that eat this
way—many large sharks are filter feeders too.
The biggest of them is the whale shark.
In fact, it’s the biggest fish in the ocean, and these giants can be even larger than the
gray whale—growing over 18 meters long.
Unlike the other filter feeders we’ve talked about, whale sharks have teeth, but they don’t
play a big role in feeding.
Instead, whale sharks have three methods of filter feeding, and they switch between them
depending on how much food is in the water around them and how much energy they’re
willing to use.
Sometimes they just slowly swim through the water with their mouths open, passively allowing
the food to filter through their mouths.
This way of feeding uses the least amount of energy and might be used while searching
for higher-energy food.
But once they sniff out a patch of plankton or something that they want to feast on, they
can also hang vertically in the water, with their tails down, and suction water into their
mouths like a vacuum.
Researchers think this method might be used for slurping up small patches of high-energy
prey.
But in some cases, whale sharks combine these two methods in a move known as ram feeding,
where they swim while suctioning.
That uses a lot of energy, but it maximizes the amount of food they’re consuming at
one time.
Once they’ve gulped up their food, the water gets pushed through a collection of filtering
pads and out through their gills.
Scientists aren’t exactly sure how the flow of food and water interacts with the filtering
pads, but one hypothesis is that the food particles travel more or less parallel to
the filters as the water filters out.
These particles collect in a ball at the back of their throats, and once the whale shark
has enough, it swallows.
Another filter-feeding shark isn't as striking in size, but it does have a gigantic mouth…
which it’s named for.
The megamouth shark is about as long as a canoe, and its mouth is over a meter across.
Researchers think megamouth may have been the first filter-feeding shark to evolve this
ability, because it belongs to the most primitive lineage of sharks.
But we don’t know exactly how this shark eats because it’s really rare, and we’ve
never observed it eating in the wild.
In fact, there have only even been 69 confirmed sightings since its discovery in the 1970s.
But its unique mouth offers some hints, so researchers have some theories.
One clue is the fact that these sharks have a distinctive white strip of tissue on their
upper lip.
It’s made of denticles, the same tooth-like material that shark skin is made from, and
is coated with something called guanine.
These are crystals that are pretty good at reflecting light, so scientists believe this
strip is reflective.
And that shiny strip may help them lure prey into their giant mouth.
But in addition to luring fish toward it, the megamouth also goes after its prey, which
include plankton and small fish.
Researchers believe that it swims into a cloud of prey with its mouth wide open.
And they think it can even stick its jaws out beyond its lips, which helps create suction
to pull the prey in.
When the shark closes its giant mouth, it gulps up food and water all at once.
Then, it quickly expels a bunch of water out through its gills—pushing out over 500 liters
at once!
This is where filters come in: Its gills have spongy, finger-like protrusions called gill-rakers
that help keep food in as the water rushes out.
Unlike most filter feeders, which tend to feed close to the surface, the megamouth shark
follows its prey as they make their daily migration between the surface and the ocean’s
deeper waters.
This is a little unusual for a filter feeder, but this odd habit may help explain why we
rarely encounter megamouth in the wild.
Finally, in addition to filter-feeding whales and sharks, there are manta rays—enormous
creatures with wingspans up to seven meters.
Most rays are hunters, but manta rays have evolved a more peaceful, filter-feeding lifestyle.
And they even look graceful while they eat!
They take in tiny organisms through their open mouths as they glide through the water,
but they don’t just swim in a straight line.
Instead, you can catch them doing somersaults to stay in a single spot that's packed with
krill, or following each other in a circle to create a cyclone and trap their food.
And while this might sound more peaceful than the whale shark slurping up clouds of plankton,
this is considered a type of ram feeding too.
Manta rays also use a pair of smaller fins near their mouths, known as cephalic lobes,
to funnel food particles into their mouths.
They have plates on their gills that help keep food in as they expel the water—but
surprisingly, these plates never seem to clog like other filter feeders.
The authors of a 2018 paper found that the reason seems to be that manta rays filter
their food in a completely different way:
In most filter feeders, the filter acts like a physical obstacle that keeps food from passing
through.
But that’s not what happens with mantas.
They have rows of plates as filters, and they’re spaced in such a way that, as seawater flows
over them, little whirlpools form between them.
That pushes food particles up and keeps them from sticking to the plates.
Then, these particles end up ricocheting off these slats, concentrating in the ray’s
mouth as water drains away out the gills.
And since they never actually pass through the slats, they can’t get stuck.
Which is great for the manta ray—and it may also be useful for us.
This discovery may help researchers develop new methods of filtration, especially when
it comes to microplastics, which can easily clog many commonly used filters.
When you think about it, it’s pretty incredible that all of these ocean-going giants have
evolved so many creative ways to strain tiny creatures out of seawater.
One thing is for certain though—these giants may all be eating small animals, but thanks
to their massive body sizes and appetites, their meals are anything but tiny.
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