Moon Jellies (Aurelia aurita) are a common and easily recognizable type of jellyfish found across the world’s oceans. These marine invertebrates are frequently encountered in coastal waters, estuaries, and harbors, captivating observers with their distinctive, almost translucent appearance.
Key Characteristics
Moon jellies possess a delicate physical structure. Their disc-shaped, transparent bell typically ranges from 5 to 40 centimeters in diameter. Visible through the bell are four horseshoe-shaped gonads, often resembling a four-leaf clover. These reproductive organs can exhibit various colors, including mauve, violet, reddish, pink, or yellowish.
Composed of about 95% water, moon jellies lack complex organs like a brain, heart, or respiratory system. Instead, they rely on a simple nerve net for coordinating movements and responses. Rhopalia, their sensory structures, help them perceive their position in the water. Short, fine tentacles fringe the edge of their bell, distinguishing them from many other jellyfish species with long, trailing tentacles.
Oceanic Life and Diet
Moon jellies inhabit temperate and tropical waters globally, thriving in coastal areas, estuaries, and harbors. They tolerate a wide range of water temperatures (6°C to 31°C) and low salinity levels (as low as 0.6% salt content). As planktivores, their diet primarily consists of plankton, including small crustaceans like copepods, larval fish, and other tiny marine organisms.
Their short, fine tentacles capture small prey, which is then moved to a mucous layer on the bell’s edge. Specialized oral arms transport the food to the mouth, located on the underside of the bell. While capable of rhythmic bell contractions for movement, moon jellies are not strong swimmers. They largely rely on ocean currents for passive dispersion, often drifting with the flow and sometimes washing up on beaches after strong tides or storms.
The Moon Jelly Life Cycle
The life cycle of a moon jelly involves an alternation of generations, moving between a sessile (attached) polyp stage and a free-swimming medusa stage. Adult medusae, the familiar bell-shaped jellyfish, reproduce sexually by releasing eggs and sperm into the water. Fertilization occurs externally, and the fertilized eggs develop into small, ciliated larvae known as planulae.
These planula larvae are free-swimming for about 10 days before settling and attaching to a hard surface on the seafloor. Once attached, they transform into a small, sedentary polyp stage called a scyphistoma. Under suitable environmental conditions, these polyps undergo strobilation, where they segment and bud off small, immature, free-swimming ephyra. These ephyra then grow and mature into adult moon jellies, completing the cycle.
Moon Jellies and People
Moon jellies are recognized for their generally mild interaction with humans. Their sting is typically very gentle, often causing only a slight itching or stinging sensation, if any, and is not considered dangerous. The stinging cells, called nematocysts, located on their short tentacles, are not potent enough to penetrate human skin effectively for most individuals. While rare, some sensitive individuals might experience a more noticeable, temporary irritation.
These jellies are a common sight in public aquariums worldwide due to their graceful movements, relatively easy care, and docile nature. Their presence provides educational opportunities, allowing people to observe and learn about marine life. Moon jellies also play a role in marine ecosystems, serving as both predators of zooplankton and prey for larger animals like sea turtles, sunfish, and certain birds, contributing to the balance of the food web.