The Jelly With Stinging Snot

Unlike other jelly species, upside-down jellies spend most of their adult lives pulsing on the sea floor in shallow water. Facing upside-down allows these jellies to expose the symbiotic algae that live inside their tissues—called zooxanthellae—to the sun so they can photosynthesize energy from the sun into food. But perhaps the most interesting quality of these jellies is—believe it or not—their mucus.

Aquarists at the National Aquarium, in collaboration with other researchers, have recently discovered that upside-down jelly mucus contains clusters of stinging cells, which are armed with harpoon-like structures called nematocysts. A description of these mobile, microscopic balls of snot, which also house zooxanthellae in their centers, is currently under peer review for publication in a scientific journal.

The stinging balls of snot—which researchers have named "cassiosomes" based on the genus of upside-down jellies (Cassiopea)—are capable of killing brine shrimp. Even though upside-down jellies get a large portion of their food through photosynthesis, they also receive nutrients by trapping plankton with the cassiosomes in their mucus. This means that larval fish or plankton that come into contact with the mucus can get stung without even touching the jelly!

According to the recent findings, upside-down jellies also release cassiosome-filled mucus as a defense mechanism when disturbed, creating the phenomenon that experts call "stinging water." The sting from upside-down jelly mucus is not considered dangerous to humans, but toxins released from their stinging cells typically cause an itch or mild irritation.