The Moon Jellyfish is easily recognized by its translucent body. Its widespread presence makes it one of the most common and familiar jellyfish species globally, often observed floating near coastlines. This animal, which has no brain or heart, represents a simple yet ancient form of life that has thrived in marine environments. The jellyfish stage, known as the medusa, appears as a shimmering, bell-shaped form that drifts through the water column. Species within the genus Aurelia are found across a vast range of temperate and tropical seas.
Defining Physical Characteristics
The species most commonly referred to as the Moon Jellyfish is Aurelia aurita, a cnidarian known for its umbrella-like bell. This disc-shaped body typically measures between 10 and 40 centimeters in diameter in its adult medusa stage. The bell is composed of over 90 percent water, allowing its internal structures to be clearly visible.
The most distinctive feature is the presence of four horseshoe-shaped structures located centrally on the top of the bell. These structures are the gonads, which are often colored pink, violet, or yellowish due to the plankton in the digestive system or the sex of the individual. The gonads are positioned around the stomach and connect to the four oral arms that hang beneath the bell.
A fringe of hundreds of short, fine tentacles lines the margin of the bell. These tentacles are equipped with stinging cells, or nematocysts, but they are generally too small to cause significant harm to humans. The four frilly oral arms extend from the mouth beneath the bell, serving to capture food and transport it to the central mouth.
Global Habitat and Lifestyle
Moon Jellies inhabit coastal and shelf waters across the Atlantic, Pacific, and Indian Oceans. They thrive in temperate and tropical zones, often preferring the surface waters of estuaries, bays, and harbors. Their ability to tolerate a wide range of temperatures and salinities contributes to their success in these variable near-shore environments.
The Moon Jellyfish is not a strong swimmer, largely depending on ocean currents for transport, which sometimes results in large aggregations, or “blooms,” near shorelines. They achieve limited movement and maintain their position in the water column by rhythmically pulsing their bell. This muscular contraction pushes water out, allowing the jellyfish to direct its movement slightly.
These animals are passive, opportunistic carnivores whose diet consists mainly of zooplankton, small crustaceans, and other tiny invertebrates. They capture prey using a layer of mucus that coats the underside of their bell. The short marginal tentacles sting and stun small organisms, which then become trapped in this sticky mucous layer. Tiny hair-like structures, called cilia, then sweep the trapped food particles toward the bell’s edge, where the oral arms collect the food and move it into the mouth.
Understanding the Complex Life Cycle
The life cycle of the Moon Jellyfish involves an alternation of sexual and asexual generations. The cycle begins with the adult medusa, which is either male or female, reproducing sexually by releasing sperm and eggs into the water. Following external fertilization, the resulting zygote develops into a tiny, ciliated, free-swimming larva known as a planula.
The planula drifts for a short period before settling onto a hard substrate, such as a rock or shell, on the seafloor. The larva attaches itself and undergoes metamorphosis to become a sessile polyp, a stage called a scyphistoma. This scyphistoma is a small, stalk-like organism that feeds with its own ring of tiny tentacles. The polyp can also reproduce asexually by budding, creating new polyps that form colonies.
When environmental conditions are favorable, the scyphistoma enters a process called strobilation. During strobilation, the polyp body begins to divide horizontally, forming a stack of disk-like segments, known as a strobila. Each segment then detaches sequentially to become a tiny, star-shaped, free-swimming juvenile jellyfish called an ephyra. The ephyra feeds on small plankton as it grows, gradually developing the classic bell shape and increasing in size to become the mature medusa, completing the cycle.
Interaction with Ecosystems and Humans
In marine ecosystems, the Moon Jellyfish acts as both a predator and a source of food. They are consumers of zooplankton, helping to regulate the populations of these small organisms in the water column. Conversely, they are a food source for several specialized marine predators, including leatherback sea turtles and the ocean sunfish (Mola mola).
The most common human interaction involves the Moon Jelly’s sting, which is a concern for swimmers. Their tentacles possess nematocysts, or stinging cells, but the sting is mild in most cases. The small size of the nematocysts means they are unable to penetrate the thick skin of humans. A sting from a Moon Jelly is described as a mild itch, slight rash, or a barely perceptible sensation.
The effects are rarely painful or long-lasting, making the species harmless to beachgoers. The mucus secreted by the jellyfish, which contains trace amounts of enzymatic compounds, may cause skin irritation even without direct contact with the tentacles. Despite their mild nature, the sheer number of individuals in a large bloom can occasionally lead to numerous minor stings for those swimming in the area.