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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.

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. 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.

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. 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?

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.

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. 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. Now, thanks to better deep-sea vehicles, we're able to spy on them in their natural habitat.

In 2017, for instance, a NOAA rover spotted one in the Gulf of Mexico with a bit more of a dignified profile, and unsurprisingly, a very full belly.

You know, when they aren't totally stuffed, they're actually kinda cute. Not this next fish, though; it's got a terrifying maw, not that you'd notice in the water because to blend in in the deep, this thing has evolved transparent teeth. Here's another one from Hank.

Meet Aristostomias Scintillans, a species of fish that lives in the deep dark waters hundreds of meters beneath the surface of the Pacific. It's occasionally called the "shiny loosejaw" because sometimes, when you name a thing, you just call 'em like you see 'em. The animal is part of the dragonfish family, and like most of its relatives, it's a creepy looking thing with a disproportionately huge jaw, long pointy teeth, and a bioluminescent barbel for attracting prey. But of all of the dragonfish, which are wonderful, this one is special.

It's tiny, usually only about fifteen centimeters long, but it's also a fierce hunter with an amazing adaptation. To catch prey, the fish has fangs that are transparent. Most animals' teeth are, and this probably won't be surprising, not transparent.

Vertebrate teeth are typically hard, calcified structures in the mouth and although they're not quite the same, they tend to look pretty similar to bone. They have an inner layer of what's called dentin, and an outer, hard layer called enamel, and they look opaque because they reflect, absorb, or scatter most light. This dragonfish has a much more unique situation because its tooth composition is very different from what we see elsewhere.

Both the enamel and the dentin are made of unique mineral matrices containing nano-scale crystal rods- basically, very tiny crystals. They're even smaller than the wavelengths of light that normally hit them, which means they scatter barely any light. Instead, the light can go right through their teeth.

Additionally, these teeth don't have dentin tubules-these are little channels that run through our teeth and they are a major place where light gets scattered. So not having them helps this dragon fish keep its mouth crystal clear.

This is all super cool, but there is still a pretty big question here, what's the point of having clear teeth? It seems like they're working really hard for this, so why?  Well that comes down to the dragon fish's hunting style. These things swim around with their mouths open and when some unsuspecting prey gets too close, their mouths snap shut like traps.

So the point of this fish's transparent teeth is to make its maw virtually undetectable to prey by not reflecting any of the dim light in the deep ocean. Combined with the dark colour of its body, this gives this species a unique level of stealth as it hunts.

This fish is very cool in it's own right but there is actually something beyond just the wow factor here because researchers are hoping to use this animal as inspiration for advancing material science. Engineers aim to copy the nanostructure of these teeth to make transparent ceramics, which would be used for super strong armored windows, laser housings and other tech. This is just one example of biomimicry, a field of engineering that aims to adapt the awesomeness of nature into useful technology.

Biomimicry has lead to better bullet trains based on bird beak shapes, more sustainable building ventilation based on termite mounds and much much more. So I guess like, never be afraid to look a gift dragon fish in the mouth.

Now not all weird things in the ocean are strange because of their eating habits. Other animals are just exist to defy our understanding of what life is capable of. Like these next animals, they don't even need oxygen to survive. Here's Olivia with more.

I don't know if you've noticed but animals kind of need oxygen. That's because animals generally get their energy from cellular structures called mitochondria, and those processes require oxygen to work.

So if somebody stole all of the Earth's oxygen, things would end pretty quickly around here. Except as it turns out, there are at least some animals that would be perfectly fine because in 2010, scientists published a paper announcing that they'd found three species of them that straight up don't need oxygen.

Now to be clear, not all life needs oxygen. There are plenty of single celled microbes that are anaerobic, meaning they can survive just fine without the stuff. Instead of oxygen, these organisms can use other molecules like sulfate or nitrate. But for years, scientists thought a system like that wouldn't work for animals since their complex, multicellular bodies have higher energy requirements. Instead, they thought animals needed the more efficient energy production that takes place in the mitochondria. 

And then came that 2010 paper, This discovery happened in the L'atalante Basin, 3000m below the surface of the Mediterrian sea. L'atalante Basin is a deep hyper-saline anoxic basic, meaning its super salty and completely devoid of oxygen. It's the kind of place you wouldn't expect to find animals. And indeed, when a research team visited three times between 1998 and 2008, that's generally what they saw. They did find a lot of single celled organisms living in the basin, but most of animals they saw were dead; the result of a so-called "rain of cadavers" from oxygenated waters above.

Most of the animals, but not all of them because the team also found an unusually high abundance of tiny sediment dwelling animals called loriciferans, and they were seemingly very alive. Loriciferans are pretty weird creatures to begin with. Their heads are covered in spines and their bodies are typically encased in a vase-like shell called a lorika. But finding them in an oxygen-free basin was a whole new level of weird.

The researchers observed that the loriciferans were still taking up nutrients and that some had recently molted. Some even had developing offspring inside them. So these animals apparently spend their lives buried in this sediment with no oxygen, not only surviving but thriving. Part of incredible survival might be down to their size.

At less than one millimeter, loriciferans have pretty low energy needs. But they also seems to have some pretty unique adaptations. For one thing, they don't have mitochondria. Instead, they have cellular structures that look a lot like hydrogenosomes- these are organelles that some microbes use to produce energy, and they use hydrogen ions in place of oxygen.

Alongside these structures, the researchers also noticed shapes that might be microbes living inside the loriciferan cells. That's intriguing because some anaerobic single-celled organisms also have symbiotic microbes that live alongside their hydrogenosomes.

All in all, it looks like these loriciferans have developed similar cellular structures to anaerobic microbes for living in the same way. Although it's not clear how they did this.

One option is that loriciferans retained these adaptations from an earlier ancestor more similar to anaerobic microbes. But it's also possible that their ancestors swiped genes from microbial neighbors, allowing them to use the same cellular tricks for survival.

Ofcourse, this is an extraordinary claim and some researchers have doubts. For example, a study published in 2015 looked in the same basin and was unable to find independent evidence of living loriciferans. The researchers of the original study are still confident in their results, but it may take more conformation to convince everyone. 

If these results do hold up though, it could change how we understand the requirements of complex life. It would have implications for the diversity of animal life in the world today for scientists interested in how life got started in an oxygen-deficient early Earth and maybe even for scientists looking for life elsewhere in the solar system. Ultimately, life is so adaptable and endlessly diverse that we wouldn't be shocked if there were more surprises to be found.

Finally, to close out this whole saga, let's wind down with a deep sea jelly-fish we still don't entirely understand. This creature is weird in its own right but it also has a quirk no-one can explain. It seems to keep some kind of pet. Here's one last episode from Stephen.

Imagine you're cruising the sea in a submarine and your lights illuminate what looks like a large plastic bag floating through the water. Well, a lot of trash ends up in the ocean, so this doesn't seem like a complete surprise until the bag begins to change shape, and then you realize, it's an animal!

Meet Deepstaria. This weird looking creature has been mistaken for a lot of things, including a whale placenta and a sea monster as well as a garbage bag. And less often, it's recognized for what it is, a jellyfish. But it's a jellyfish that always seems to have a friend in tow, and we have no idea why.

There are two species of deepstaria jelly, and they don't look like jellyfish. They lack the long stinking tentacles that are a hallmark for many jellies. Instead all you see is their bell, which is very thin, fragile and extremely oversized in comparison to other jellies, measuring over half a meter wide. 

To swim they actually ripple these huge delicate bells which is pretty unusual and may also contribute to their shape shifting reputation. But that's not where the weirdness ends.

Deepstaria jelly belong to the Ulmaridae Family, one of the most ancient lineages of jellyfish. If you're familiar with the moon jelly, they are also a member of this family. Ulmaridae jelly are cousins to another order of jelly called Rhizostomeae, and some of these aren't any more familiar looking either.

Some Rhizostomeae have ditched tentacles all together. Rather than tentacles, these jellies eat and catch their prey using oral arms- specialized appendages that help them catch food and move it to their mouths.

And it turns out we also see oral arms in deep sea species of Ulmaridae, then instead of tentacles, they dot their bellies with stinging cells instead. This may be a more useful strategy than tentacles for catching hard-to-find prey in the deep sea and might help to explain why Deepstaria is missing them.

Researchers believe that Deepstaria jellies are ambush predators, lying in wait with their bodies spread out wide until some unfortunate creature swims into them; then they close their bell around the prey and cinch themselves up tight like a trash bag. 

And nothing escapes the inside of a Deepstaria jelly alive, except one creature. Almost every Deepstaria jelly encountered in the deep sea has a little friend living inside of it, the giant isopod Anuropus. Picture your typical garden pill bug and then supersize it, making it several centimeters long. 

And scientists aren't quite sure who benefits from this, and how. Specifically, they don't know if the jelly gets anything from this nor do they know whether the isopod is doing any harm, or if it's just along for the ride.

They do know that these isopods are great at finding large jellies like Deepstaria and then settling in for the long haul. But there's still a lot left to learn about these weird deep sea critters, like how do they meet up to reproduce? Why are they so hard to find? And why are they so appealing to those isopods?

But now that you know about these awesome jellies, you'll never mistake one for a trash bag, at least once you spot it's little hitch-hiker.

Thanks for watching this episode of Sci Show. If you enjoy these episodes, we've got plenty of ocean content where this came from, including a few episodes made in partnership with the folks over at Monterey Bay Aquarium.

If you want to watch one after this, you might try our extremely cute episode about animals that adopt.