[♩INTRO].

Imagine you're cruising the ocean in a submarine and your lights illuminate what looks like a large plastic bag floating through the water. Well, lot of trash ends up in the ocean, so this doesn't seem as a complete surprise until the bag begins to change shape, and 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 jellies, and… they don't look like jellyfish.

They lack the long stinging tentacles that are a hallmark of 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 jellies belong to the Ulmaridae family, one of the most ancient lineages of jellyfish. If you're familiar with the moon jelly, they're also a member of this family.

Ulmaridae jellies are cousins with another order of jellies, the Rhizostomeae. And some of these aren't any more familiar-looking, either. Some Rhizostomae have ditched tentacles altogether.

Rather than tentacles, these jellies catch and eat 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 bells 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 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 jellyfish 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 they can get several centimeters long.

Scientists aren't quite sure who benefits from this relationship 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 is 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 you know about these awesome jellies, you'll never mistake one for a trash bag. At least, once you spot its little hitchhiker.

Thanks for watching this episode of SciShow, and thanks especially to the isopod to our jellyfish, this month's President of Space, Matthew Brant! Your continued support makes this channel possible, so thank you. If you'd like to help support SciShow, and maybe become President of Space yourself, check out patreon.com/scishow. [♩OUTRO].