The lion's mane jellyfish, Cyanea capillata, is found in the cooler waters of the Arctic, northern European Atlantic, and North American Atlantic and Pacific Oceans. A similar jellyfish (possibly the same species) is found off the coasts of Australia, New Zealand, and the Southern Ocean surrounding Antarctica. The jellyfish has a red and yellow umbrella-shaped bell, and hanging below the bell is a mass of long, hair-like tentacles that is likened to a lion's mane. The size of C capillata varies tremendously with the largest animals found in areas of higher latitude. The Arctic C capillata can reach a bell diameter over 2 meters (7 1/2 feet) and trail tentacles 35 meters (120 feet) long, while bell sizes of just 50 cm (20 inches) are more common in lower latitudes. The stinging cells (nematocysts) are found on the tentacles and upper surface of the bell.

Cyanea capillata venom is capable of producing dermonecrosis. In addition, at least in animal studies, the venom is cardiotoxic and musculotoxic but does not cause hemolysis.