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There are some weird animals out there, but few environments have produced stranger creatures than the deep ocean!

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Original Episodes:
This Worm's Gut Has No Way In or Out

Meet The Black Swallower: Nature's Top Competitive Eater

The Terrifying Fish with Transparent Teeth

These Animals Don’t Need Oxygen?!

The Shapeshifting Deep Sea Jellyfish... With a "Pet"


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[Scishow theme music] Let's be real.

There are some weird animals out there, but few environments have produced stranger creatures than the deep ocean. Evolving in a dark, cold, high-pressure environment like the bottom of the sea will do some bizarre things to your biology.


So it's no surprise that we've talked about a lot of these creatures over the years, and now, it's time for a highlight reel. First up, we've got a worm that makes absolutely no sense to our human understanding of things because its gut has no way in or out.


How does it get food? How does it produce waste? And why is it like this? I'll pass the baton to Hank.

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There are plenty of creatures out there in the world with only one opening to handle the business of both taking in food and getting rid of the leftovers. Jellyfish, for example, get along fine with a mouth that is also an anus. But there's at least one animal out there that doesn't have a gut opening at all. The giant tube worm Riftia pachyptila lives over a kilometre deep in the ocean along ocean ridges, where hydrothermal vents are common and spew boiling hot chemical-laden water into the freezing cold deep sea.

These chemicals include stuff like hydrogen sulfide, which isn't all that great for most animals, but these bizarre-looking worms are adapted to live in this hostile habitat without so much as a mouth to make things easier. Riftia almost looks like a lipstick, with their vivid red plumes and tube-like outer casing. Within, they have a structure called a trophosome, which is like a gut but with no way in or out.

They were first discovered in 1977 when the submarine ALVIN accidentally landed on a cluster of them while it was investigating hydrothermal vents near the Galapagos. Red blood gushed up around the sub.

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The researchers on board later discovered that the tube worm's plume has blood vessels full of hemoglobin.

In us mammals, hemoglobin is mainly responsible for transporting oxygen. In Riftia, the tube worm's plume acts like a gill.

Its hemoglobin helps pull that hydrogen sulfide from the vent water and move it into the trophosome. Inside the trophosome are millions of symbiotic bacteria, accounting for up to half of the body weight of the worm. The bacteria are able to convert the toxic vent water chemicals into a food source for the worm, through a process called chemosynthesis.

Much like plants use photosynthesis to produce food using sunlight, these bacteria are getting their food through chemical reactions that use hydrogen sulfide to produce energy. And the inside of a worm, it turns out, is a much better habitat for them than then the open vents, or so the hypothesis goes. Not only have the bacteria turned this hostile environment into an advantage, the worms have capitalized right along with them.

And they don't need a mouth or anus- they just get fed by their bacterial partners, which produce enough food to keep everyone happy. It's like having your kitchen in your body.

 NewSection (3:13)

Okay, so that's how the food gets in, but how does it get out? Well, the waste produced from digesting this food can be transported back out via the worms' bloodstream. There's no need for either a mouth or an anus. But this makes you want to ask another question: if there's no way in or out, how do those bacteria get in there?

Researchers asked this question back in 2006 and found that it's weird. The bacteria enter through the tube worm's skin when it's still a larva. So basically, this is a bacterial infection.

Hydrothermal vents are an unpredictable place to call home. Thanks to the constant tectonic activity happening along ocean ridges, they may be there one day and gone the next. Once the vents stop venting, the tube worms die because their bacteria's food source gets cut off.

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The distance between them can be several miles, which is a long swim when you're a little worm.

So researchers aren't sure how tube worm larvae get from place to place without a food source. Hypotheses range from whale falls to shipwrecks, which could supply enough of the chemicals the bacteria need to stay alive.

The discovery of these weird worms and their unique way of eating has led to researchers finding chemosynthetic communities in ecosystems around the world, from elsewhere in the ocean to Yellowstone National Park. Who knew a giant, mouthless, buttless worm could completely redefine the way we thought about how life works on our planet?

 NewSection (4:42)

Okay, so that generally makes sense, but that worm is by no means the only weird eater in the ocean. There's also a fish called the black swallower, and if you thought those hot dog-eating contests were impressive, they have nothing on this thing. Here's one from Olivia.

 NewSection (4:59)

Compared to other creepy deep-water critters like gulper eels or anglerfish, Chiasmodon niger looks pretty unassuming, kind of like an ugly anchovy. It grows to about 25 centimeters long and is found all around the world at about 700 to 2,800 meters deep in what scientists call the twilight and midnight zones of the ocean because there is so little light. And while Chiasmodon is not, you know, a beautiful fish, with its rows of long, needlelike teeth and a mostly scaleless body, it's still pretty normal-looking, as long as you catch it before it eats. Its common name is the black swallower because it can eat meals several times larger than itself and make competitive eaters worldwide jealous of its stretchy stomach.

In 2007, a fisherman off the Cayman Islands found a dead nineteen centimeter-long black swallower floating on the surface of the ocean. Inside its guts was an eighty-six centimeter-long snake mackerel. That's four times as long as the swallower.

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Other reports suggest they can eat meals twelve times their mass.

If humans could eat that much in one sitting, the world record for competitive hot dog eating would be closer to 8,500 hot dogs rather than just over 70.

Swallowers manage this incredible feat of gastronomy thanks to their large mouths and very elastic stomachs, which can expand so far that the skin becomes thin enough to see through. Fish in the swallower family also have pelvic fins that aren't fused, much like the lower jawbones in a snake, so they can stretch their chest to make room when their belly expands. Ichthyologists think that when swallowers hunt, they first seize their victim by the tail and then bit by bit, scoop the jaws up like a boa constrictor until the luckless prey is completely engulfed. And it makes sense that they've evolved the ability to eat pretty much anything they can get their big mouths around.

Down in the deep ocean, where food can be pretty scarce, having that kind of flexibility would be a great adaptation for survival. But no one knows yet exactly how they pull it off. When snakes eat a big meal, they have to ramp up their metabolism and even large organs, like their liver and heart, to fully digest their food.

It's likely these little fish have evolved similar ways to cope with such large meals, but their big appetites can sometimes get the better of them. Digestive juices can only work so fast, and if a meal is just too big, like a nearly ninety centimeter-long snake mackerel, the prey can start to rot and decompose inside the stomach. Like all rotting food, a decomposing meal releases gases, and these can inflate the swallower's stomach like a balloon, lifting the creature to the surface of the ocean and killing it.

That's what happened to the one from the Cayman Islands, and it's actually how we first discovered this species back in the 1860s. In fact, for over a century, their habit of fatally overeating was the main way scientists collected specimens, other than a few that were dragged up by deep-water fishing.

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