Scientists debate whether blooms are becoming more common globally due to human activities or whether people are simply noticing them more as humans increasingly engage with the sea. But many agree that in certain areas—especially in enclosed waters near harbors—large jellyfish blooms are becoming more frequent, and these blooms tend to be dominated by a certain species: the moon jelly.
“If any species has increased, moon jellies definitely have,” says Jennifer Purcell, a marine scientist at Western Washington University in Bellingham. A moon jelly (Aurelia aurita) looks like a delicate, transparent UFO with four petal-like gonads on the top of its bell and a trail of short, thin tentacles.
The venom of moon jellies isn’t very powerful, so their stings aren’t a danger to people. But moon jellies are relatively large, reaching up to almost 16 inches wide, and their unpredictable blooms can be massive beyond imagination. One bloom in Japan’s Uwakai Sea in 2000 contained an estimated 583 million jellyfish along 62 miles of coastline, concentrated in an area of less than 1.5 square miles…
Moon jellyfish (Moon jelly, Common jellyfish, Saucer jelly)
Aurelia aurita (Semaeostomeae - Ulmaridae), the Moon jellyfish, often lives in large groups in the sea. You can easily identify them by their four moons’ in the middle. These are the reproductive organs. Males have white and females have pink moons’.
Moon jellyfish have short tentacles along the edge of the bell and four short arms situated around the mouth for catching food. The tentacles of the moon jellyfish are poisonous for small marine animals but people are not affected by the toxin since it does not penetrate the skin.
Aurelia aurita is a cosmopolitan species, found near the coast, in mostly warm and tropical waters.