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I don't think you're ready for this jelly. Sure, it looks kind of unassuming, what with it's strange habit of hanging out, wrong-way up on the seafloor.

But it can sting you without actually touching you. That's all thanks to the stinging snot rockets it launches into the water above. Yes, I said stinging snot rockets.

I told you you weren't ready. Cassiopea xamachana, better known as the upside-down jelly, spends its life in still, coastal waters, nestled in near the roots of mangrove trees. It uses its bell like a suction cup to anchor itself to the bottom.

So in a way, it acts more like its distant cousin the sea anemone than a proper jelly. It does this because its tentacles contain symbiotic algae called zooxanthellae which use light and carbon dioxide to make food for the jelly. And in return for providing these yums, the algae get a cozy home with one heck of a security system.

Upside-down jellies are still jellies, after all, and they have potent stinging cells called nematocysts. In addition to warding off threats, these stingers allow the jellies to add some variety to their diet, in the form of small critters like brine shrimp that swim about in the waters above them. The jellies don't saunter up to grab their meals, though.

They send stingers to them. For decades, people swimming in areas with these jellies have reported inexplicable patches of “stinging water”, and scientists have long noted clouds of mucus hovering above the jellies, but they hadn't taken a close look at what was going on. Not until 2016 anyway, when aquarists with the Smithsonian Museum of Natural History collected some of the mucus that upside-down jellies in their care released during feedings and stuck it under a microscope.

To their surprise, they saw tiny specks racing around in it. And when they added brine shrimp, the specks kept bumping into them like mini go-karts. Lethal go karts, that is, because the shrimp were killed on contact.

Upon closer examination, the team discovered these specks had an outer layer composed of stinging cells and wiggling, hair-like projections called cilia which help them move around in the mucus. And inside were living zooxanthellae—basically, a solar-powered energy pack! They dubbed these swimming structures cassiosomes, and the team believes they're used as a long-range weapon.

Basically, the jellies can ooze a cloud of cassiosome-filled mucus up to 20 centimeters high into the water above them. That, presumably, kills lots of tasty little morsels. Then, they can then slowly suck the dead into their mouths.

The researchers also discovered that the cassisomes themselves can survive up to 10 days, which could explain those mysterious patches of stinging water. If some of this mucus gets churned up into the water column say, by the kicks of a snorkeler's fins then there could be little stinging snot rockets zooming around long after the actual jelly has moved on. And, it turns out, it's not just this one species.

The team discovered similar cassiosomes in four of its relatives including ones that don't chill on the bottom. They believe cassiosomes evolved in this lineage of jellies, the Rhizostomeae, to give them a bit of extra firepower. And it's clearly served them well, as they're the most diverse order of true jellies.

Is it me, or do jellies just get more amazing the more we learn about them?! Thanks for watching SciShow! If you enjoyed learning about these weirdly wonderful cnidarians, you'll probably enjoy our episode on their cousins, hydras, and how they can live forever.

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