This colorful jelly has a very toxic sting, but reports of human fatalities are few. It's considered a giant jelly—its bell can reach about eight feet (2.4 m) in diameter and its tentacles can grow to more than 100 feet (30.5 m) long. That's longer than a 90-foot (27.4 m) blue whale (the largest mammal on Earth) but smaller than the 130 feet (39.6 m) of a giant siphonophore (a jelly relative). The largest specimens are found in Arctic waters.
Leatherbacks are unique among sea turtles in that their primary food is jellyfish! Leatherback populations have plummeted in recent years and many are highly endangered. Help the sea turtles and jellyfish, along with countless other species, by tossing trash into recycle bins or trashcans; don't let it blow or float into the jellies' habitat.
A Cyanea sea jelly was the murder weapon in a Sherlock Holmes mystery called "The Adventure of the Lion's Mane." Although the lion's mane's sting can be potentially fatal, most swimmers who encounter this gentle beast survive to tell the story. And some fishes, such as the southern harvestfish and gulf butterfish, feed on it. These fishes, resistant to the toxin of the lion's mane, are often found nibbling on its gelatinous bell.