Simple body, complex tasks
Instead of a brain, a jelly has a nerve net. A simple system of nerves and muscles lets a jelly pulse its bell to swim up or down as well as drift with the current. Instead of blood to carry oxygen, its thin skin absorbs it from the surrounding ocean water. The mouth does triple duty: food goes in, waste goes out, and eggs or sperm pass through. The most substantial part of a moon jelly's anatomy are horseshoe-shaped structures in the bell—its reproductive organs.
Passive, but well-armed, predators
A short fringe of hairlike tentacles rim the moon jelly's bell, and frilly oral arms dangle from the bell's center. Both are loaded with venomous stinging cells. When krill, larvae, or other tiny zooplankton bump into the jelly's appendages, the stinging cells fire dartlike nematocysts into the prey. Stunned prey is easier—and safer—for a fragile jelly to eat. Like floating flypaper, the mucus-coated bell also catches drifting prey. All food moves conveyorlike from the stinging tentacles to the oral arms, up into the bell and to stomach pouches located below the reproductive organs.
Moons on the menu
Moon jellies are the primary prey for many other large jellies. They are also eaten by tuna, sunfish, some sharks, seabirds and all seven species of sea turtles, especially the huge leatherbacks. Despite jellies' high water content, the gelatinous mesoglea, which gives the bell its structure, is a source of protein.
Purple Stripe Nettle
Japanese Sea Nettle