The Moon Jellyfish (Aurelia aurita) is one of the most widespread jellyfish species, often encountered in coastal waters and harbors worldwide. Moon Jellyfish do possess tentacles, but they are dramatically different from the long, trailing stinging structures associated with more dangerous species like the Sea Wasp. Their anatomy is structured to capture very small prey and minimize the chance of causing harm to larger animals. This species utilizes specialized structures for sensing, feeding, and movement.
The Marginal Tentacles
The Moon Jellyfish’s tentacles are a fine, hair-like fringe lining the perimeter of the bell, known as the margin. These delicate structures are short, typically 1 to 5 centimeters long, and are densely packed with stinging cells called nematocysts. Their primary function is the detection and capture of microscopic prey, such as zooplankton and small crustaceans.
Once prey brushes against the fringe, the nematocysts fire barbs that immobilize the organisms. The food is then trapped in a layer of mucus coating the lower surface of the bell, which is covered in beating cilia. These cilia sweep the mucus and captured plankton toward the four central feeding canals on the underside of the bell.
Interspersed among this marginal fringe are eight specialized sensory structures called rhopalia. Each rhopalium acts as a sensory center, aiding in navigation and orientation. They contain statocysts, which sense gravity to maintain balance, and ocelli, which are simple eyespots that detect light intensity.
The Purpose of the Oral Arms
Hanging beneath the bell are four frilly structures correctly identified as the oral arms. These arms are significantly larger and more prominent than the marginal tentacles, appearing as four elaborate, curtain-like projections surrounding the central mouth. The four translucent, horseshoe-shaped gonads are often visible through the top of the bell, adding to their striking appearance.
The oral arms serve two primary purposes, beginning with the final stage of the feeding process. After the marginal fringe and cilia collect food into the feeding canals, the oral arms receive the plankton-laden mucus. They transport this mucus to the central mouth opening, acting as funnels to ensure the captured food reaches the gastric pouches for digestion.
The oral arms also play a direct role in reproduction. Males release sperm into the water, which is drawn into the female’s mouth to fertilize eggs in the gastric pouches. The female broods the developing larvae (planulae) within specialized folds on her oral arms until they are released into the water.
Moon Jellyfish and Human Contact
The Moon Jellyfish is one of the least threatening species to humans, despite the common concern about painful stings. The nematocysts on its tentacles are designed to penetrate the thin exoskeletons of planktonic prey. These stinging cells are typically too small and weak to effectively pierce the thicker outer layer of human skin.
Contact usually results in a sensation ranging from a mild tingling or itch to no feeling at all. Any reaction is limited to a minor, temporary skin irritation or slight rash. This mild effect stands in sharp contrast to the highly toxic and potentially life-threatening venom delivered by other species, such as the Box Jellyfish, whose stings can cause severe pain and systemic reactions. The low potency means most encounters are harmless.