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Go to Brilliant.org/SciShow to learn how you can take your STEM skills to the next level and get 20% off an annual premium subscription! [ ♪ INTRO ] Humans have a bunch of permanent fixtures like our ears, mouths, and so on. But some organisms like to renovate those as they go on living.

Like sealing their mouths and ripping them apart when they need a bigger one, which is part of the day-to-day for a tiny microorganism called hydra. Take hydra’s mouths, for example. Imagine if you could stretch your mouth to devour that cheeseburger all in one bite.

That’s pretty much what the hydra does. See, the mouth of this tiny, freshwater organism isn’t a /permanent/ opening. Instead, its mouth hole is sealed over by a thin sheet of cells packed closely together.

These cells are joined by what are called septate junctions, little links that look like ladders and hold cells together. But when it’s time to eat, hydra will literally rip this layer of cells apart. Muscles around the edges of the mouth hole, called myonemes, contract and pull the covering open, eventually ripping up those septate junctions.

It’s similar to how the muscles of the human iris contract to open the pupil… minus the tearing part. It might sound gruesome, but it means the hydra’s mouth can open wider than its body, so it can eat food that’s much larger than itself. Then when it’s time to close it back up again, the layer of cells thickens and wrinkles.

Scientists aren’t exactly sure why hydra has such a dramatic way of eating. But they think it might be because having a large, permanent mouth would put too much strain on the hydra’s small body. In any case, by understanding how the hydra rips and regrows its mouth, researchers can understand more about morphogenesis or how tissues get their shape.

That could help them understand processes in other animals, like how the mammalian heart starts out as just a tube and rearranges itself to become four-chambered. But it’s not just the hydra’s mouth that grows when it eats. Its whole body can be a drastically different size depending on its diet.

It’s not like us humans, who store excess food as fat to use as an energy source later on. Hydras put their extra energy directly toward growing more cells. They also use it to grow buds, baby-hydra that later drop off to become new individuals.

In an experiment from 1977, hydras were put on different feeding regimes for two weeks. Those that were fed more, grew more cells and buds than those that were fed less. And the ones who were starved shrunk in size.

What’s interesting about the experiment is that the rate the hydra grew was independent of how much the hydra were fed. Which means hydras change their bodies to keep the amount of food /per cell/ the same. And that might just make the hydra the ultimate example of phenotypic flexibility, or changing up your look based on the environment.

As in, when food’s good and available, make the most of it! But when it’s not, adjust your body’s requirements. And because the process of budding is all about growing cells really fast, it can be potentially used in the future as a model for metastasis or understanding how cancer spreads.

Now, with all this growing and shrinking, other parts of the hydra’s body need to change too. Like its brain, which is spread across its entire body in the form of a net. Hydras are constantly losing nerve cells as they get sloughed away from near the outside of their bodies, or lost to those buds.

That also means they need to constantly make new nerve cells. The tricky thing is, hydras have around seven different types of nerve cells, so they need a way of making sure they have the right balance of each. Hydras do that by starting off with a blank canvas, known as a stem cell, which then gets marked to become a nerve cell..

But it’s not until the cell settles in either the body or the extremities that it changes into its final form. Scientists are still figuring out the finer details of how that all happens. If they can, it would help them understand not only how hydra regrows their heads, but also maybe how to grow nerve cells in people who have brain injuries or degenerative brain diseases.

So when it comes to the phrase “you are what you eat,” hydras definitely take the cake. Big thanks to James Weiss for letting us use his footage! James is the Master of Microscopes on the YouTube channel Journey to the Microcosmos where you can see more of his beautiful footage and find out even more about Hydras.

Big thanks to James Weiss for letting us use his footage! James is the Master of Microscopes on the YouTube channel Journey to the Microcosmos where you can see more of his beautiful footage and find out even more about Hydras. And another big thank you to today’s sponsor, Brilliant!

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It includes some fun and challenging exercises to help you stretch your analytic muscles. If you’re interested, you can sign up at Brilliant.org/SciShow will get 20% off the annual Premium subscription. And last but not least, thank you for watching this episode of SciShow! [ OUTRO ♪ ]