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Not all animals have a mouth, or even need one to eat! These different feeding strategies can teach us a lot about our ancestors and how they went from not needing a mouth at all to only eating with one.

Hosted by: Hank Green

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Sources:

Trichoplax adhaerens
https://doi.org/10.1093/icb/icm015
https://doi.org/10.1371/journal.pone.0136098
https://doi.org/10.1038/nature07191
https://doi.org/10.1016/j.cub.2014.05.046
Bryozoans
https://doi.org/10.1007/s12526-019-00948-w
https://animaldiversity.org/accounts/Bryozoa/
https://doi.org/10.1666/13-029
https://doi.org/10.1111/pala.12189
Hydra vulgaris
https://doi.org/10.1016/j.bpj.2016.01.008
https://doi.org/10.1002/bies.200800183
Paracatenula
https://doi.org/10.1371/journal.pone.0034709
https://doi.org/10.1073/pnas.1105347108
https://doi.org/10.1073/pnas.1818995116
Hagfish
https://doi.org/10.1098/rspb.2010.2784
https://doi.org/10.1038/srep00131
https://animaldiversity.org/accounts/Eptatretus_stoutii/

Images:

https://commons.wikimedia.org/wiki/File:Trichoplax_adhaerens_photograph.png
https://commons.wikimedia.org/wiki/File:Trichoplax.jpg
https://commons.wikimedia.org/wiki/File:Trichoplax_revised_anatomy.png
https://commons.wikimedia.org/wiki/File:Plumatella_repens_from_Haeckel_Bryozoa_drawing_Commons.jpg
https://commons.wikimedia.org/wiki/File:Freshwater_Bryozoan234.JPG
https://en.wikipedia.org/wiki/File:Polyps_of_Cnidaria_colony.jpg
https://oceanservice.noaa.gov/education/tutorial_corals/coral01_intro.html
https://www.inaturalist.org/observations/28006959
https://www.inaturalist.org/observations/33610065
https://www.eurekalert.org/multimedia/pub/110296.php?from=320887
https://www.eurekalert.org/pub_releases/2016-03/cp-itm030216.php
https://commons.wikimedia.org/wiki/File:FMIB_50097_Hydra_vulgaris.jpeg
https://www.eurekalert.org/multimedia/pub/197645.php?from=426129
https://vimeo.com/290672261
https://www.flickr.com/photos/dirtsailor2003/5562913211
https://en.wikipedia.org/wiki/File:Eptatretus_stoutii_1.jpg
[♪ INTRO].

Humans, along with most other animals, cannot make their own food. I mean, we can whip up a sandwich if we're feeling peckish.

But, we have to consume proteins, sugars, and other essential molecules made by something else. And we do that with the help of our very useful body part: our mouths. But not all animals have a mouth, and even some that have one are able to eat without it.

In fact, it's likely that the very first animals didn't have mouths at all, at least not like we do. So understanding exactly how and why animals accomplish mouthless eating can teach us a lot about evolution! The first animal on our list is a tiny member of the phylum Placozoa.

It does not have a common name, apparently it is not common enough, but its scientific name is Trichoplax adhaerens, which literally means ‘hairy sticky plate'. We should just call it the hairy sticky plate. And it is the simplest animal we know of.

It's smaller than some single-celled organisms, and it has no, well, anything. No mouth, no gut, no nerves, no brain, etcetera. It's the size of a sesame seed and its body is just three layers of cells.

Though, those layers consist of at least fifty thousand cells which fulfill a few different jobs. Since it doesn't have a mouth, when this creature wants to eat, it relies on hair-like cell projections called cilia to help it move on top of a piece of algae. Then, certain cells on its underside secrete chemicals to digest the algae, while others absorb the sugars and other nutrients released by the meal.

Basically, its entire underside is an external stomach. So, it doesn't need a mouth! And it provides scientists with some clues as to how mouths might have evolved in the first place.

Researchers think that the very first animals probably looked a lot like these simple, hairy plates. And if you start with a flat thing with a “stomach” on the bottom, and then curl it up and connect the sides a bit—voila! You've got something with a stomach and a mouth!

Many members of the phylum Bryozoa kinda look like giant water boogers. But they are actually animals. And, with few exceptions, they are actually colonies of animals.

These colonies are sometimes composed of millions of individuals. And all of them have mouths... but only some of them use them. Bryozoans are filter feeders, kinda like sponges or corals.

In fact, people sometimes confuse them with corals because each individual looks a bit like a coral polyp. That's thanks to a structure called a lophophore: a crown of ciliated tentacles that can be extended and retracted. In the middle of this structure is the individual's mouth.

Still, only some members of the colony actually eat. They're called autozooids. And essentially, their job is to collect food particles from the water with their lophophore.

They also do the hard work of digesting the food. The other individuals in the colony are collectively known as heterozooids. They're responsible for pretty much everything else the colony does, like incubating eggs and defending the colony against threats.

But, and here's the weird part, even though they have mouths, they cannot collect or consume food themselves. So instead, they get all of the nutrients they need from the autozooids through a series of connective pores. This collective feeding strategy appears to have paid off, as bryozoans have been around for almost five hundred million years!

And though their bodies might seem simple, evolutionarily, they are a big jump up from our friend the hairy sticky plate. See, each individual has both a mouth and an anus. And buttholes were a pretty big development.

So by studying these animals, scientists can gain clues as to how more complex digestive systems evolved. So far, we've seen that you can eat without a mouth if you use an external stomach or are hooked up to a buddy that can feed you. But there is another way to eat if you are a hungry, mouthless animal.

You can just rip open a hole in yourself every time you need one. And yeah, that is a real phenomenon that occurs in an animal called Hydra vulgaris. These freshwater relatives of corals are tiny, like in the submillimeter to millimeter range.

And while their evolutionary cousins have mouths, they don't. At least, not permanently. They're essentially very simplified anemones.

Just a column of flesh with a ring of tentacles at one end that sticks to a rock or other hard surface with the other end. The animal does have a stomach in the middle, but it's totally sealed off from the outside world. When an unsuspecting morsel touches this hydra's tentacles, it releases hooked barbs that grab on tight.

These barbs also contain a toxin that paralyzes the hydra's victim, you know, just in case. Then, the hydra retracts its tentacles, pulling its dinner back to where its mouth will appear. For that to happen, electric signals must trigger contractions in the inner layer of its cells.

Those contractions change the shape of those cells, creating an opening. When it's done shoving its meal in its belly, the hydra closes the hole, but that's not even the end of this. A little while later, after it digests the delicious meal, it rips itself open once more so it can spit out any indigestible bits.

And this mouth opening closes up completely each time, leaving no sign it ever existed. Scientists are still trying to figure out exactly why they do this, as it would teach us a lot about the tradeoffs of having, you know, a permanent mouth. In the meantime, they're investigating the details of how these hydra seal things up all neat and tidy every time, as that may help us learn more about wound healing in general.

The marine flatworm Paracatenula does things a little differently from everyone else I've talked about so far. These little guys live buried in the warm, shallow coastal sediments that you might find near coral reefs and mangrove forests. And, unlike other flatworms, they have no mouth or gut.

Never fear though; chemosynthetic bacteria called Riegeria make sure they want for nothing. Chemosynthetic simply means these bacteria can turn chemicals into food. So, they perform a photosynthesis-like process, but the energy produced comes from chemicals instead of light.

And their relationship with the worms is mutually beneficial. The bacteria get a home inside an animal, and in return, they produce food and help manage the waste products. These flatworms are far from the only species to feed themselves with the help of bacterial partners.

But generally, animals that rely on bacterial feeders have to acquire their food-making pals somehow. Like, they need to ingest the bacteria or soak them up through their skin when they're young. The bacteria that reside inside of Paracatenula are different.

They live inside the worm's cells. They act like food-producing organelles which serve up nutrients in the form of little droplets the worm can access as needed. So, they're passed to each new worm when those cells divide, much like organelles like mitochondria are.

And that's not the only way these bacteria stand out. Usually, mutualistic bacteria are just a small fraction of the overall animal. But the Riegeria in a worm's cells account for as much as half of its total body weight!

What's even stranger is their genomes are super tiny, even when compared with other chemosynthetic bacteria. And yet they're able to produce a wide range of nutrients for the worms, including fats, proteins, sugars, and vitamins. This group of bacteria are also the oldest known group that forms these kinds of symbiotic relationships with animals.

So they've been passed from worm to worm for more than five hundred million years. Talk about a family heirloom! So studying these creatures can provide us with unique insights into how symbiotic relationships are formed and maintained throughout evolutionary history.

Last but certainly not least, we have the only vertebrate on this list: the hagfish. Now, this fish may seem like it doesn't really belong on the list because it has a mouth that it does eat with, but bear with me for just a minute. Hagfish are eel-like fish famous for, or perhaps infamous for, their remarkable slime producing abilities.

They're scavengers that live off the dead and dying animals on the bottom of the ocean. So, essentially, they are the vultures of the sea. Just slimier.

And their skin is lined with sensory organs similar to taste buds which help them find food in the dark, muddy environments they live in. That skin doesn't just point the way, either. It also consumes food.

Hagfish are the only known vertebrate that can eat something without opening its mouth. When they find a nice, rotting carcass, like a dead whale that's sunk down from above, they start their feast by using their mouth and tooth-like structures to take bites. Then, they bury themselves deep into the tasty goodness.

And while they're hangin' out inside their meal, their skin and gills can absorb nutrients from the carcass directly. Hagfish likely evolved this trait because they sometimes go months without food. So, when a meal does appear, they benefit most by maximizing their intake.

Now, this is especially interesting because it suggests that all vertebrates may have originally done this kind of skin-eating. Switching from feeding through the skin to feeding through a gut was an important step in the evolution of non-bony life. And hagfish are thought to be the oldest living connection to the first vertebrate animal.

So this skin-feeding might imply a switch in feeding style was important in our lineage, too. And if this is true, then it is especially interesting that they are the only vertebrate who still feeds through their skin. Evolutionary biologists think understanding how and why these fish eat this way could help explain how and why the rest of us vertebrates became mouth-eaters.

In the end, whether an animal absorbs nutrients through their skin, partners with microbes, or simply digests their food outside of them, mouthless eating clearly works. And learning about these different feeding strategies can teach us a lot about our ancestors and how they went from not needing a mouth at all to only eating with one. And honestly after learning about hagfish and hydras,.

I'm, like, a little disappointed that I have to put food through this, like, weird, permanent hole in my face. Thanks for watching this episode of SciShow! And especially, thank you to all of you who support the show, including our patrons on Patreon.

I'm biased, of course, but I think we have the most awesome community of people supporting us here at SciShow. If you are one of our Patrons, thank you so much, we really couldn't do it without you. So thanks for being such a big part of it!

And if you want to learn more about joining this sensational, science-loving community, go on over to Patreon.com/SciShow [♪ OUTRO].