Jellyfish (Moon)

Overview

The moon jellyfish (Aurelia aurita) is a cosmopolitan scyphozoan jellyfish characterized by its transparent to slightly milky white bell, which typically measures 5-40 cm in diameter. Its common name derives from its circular, moon-like appearance and the translucent quality of its bell. The most distinctive feature is the presence of four horseshoe-shaped reproductive organs (gonads) that are visible through the bell, appearing as purple, pink, or pale coloration depending on diet and maturity. This species inhabits temperate and tropical coastal waters across all major oceans, thriving in harbors, bays, and estuaries where nutrient levels are high. Moon jellyfish are planktonic organisms that drift with ocean currents, though they can pulse their bell to provide some directional movement. They are highly adaptable and can tolerate a wide range of salinities and temperatures, which contributes to their global distribution and frequent population blooms. Aurelia aurita plays an important role in marine food webs as both predator and prey. They feed primarily on microscopic zooplankton and are themselves consumed by sea turtles, ocean sunfish, and some seabirds. Moon jellyfish are often studied in marine biology due to their availability and relatively simple body plan, making them valuable for research into cnidarian biology, marine ecology, and the impacts of climate change on ocean ecosystems.

How to identify

Moon jellyfish have a translucent, saucer-shaped bell ranging from 5-40 cm in diameter with a smooth, dome-like appearance. Four horseshoe-shaped gonads arranged in a cloverleaf pattern are visible through the bell, typically appearing pink, purple, or violet. The bell margin has numerous short, fine tentacles and eight sensory organs (rhopalia). Four oral arms hang from the center underside, appearing frilly and trailing. The body is nearly transparent to milky white, sometimes with a slight blue or pink tint depending on diet.

Habitat

Coastal waters, harbors, bays, estuaries, and nearshore marine environments. Prefers calm, shallow waters with temperatures between 9-19°C but can tolerate a wide range from -6 to 31°C. Common in waters with high nutrient content and often found in large aggregations or blooms during favorable conditions.

Distribution

Cosmopolitan distribution found in temperate and tropical oceans worldwide. Present in the Atlantic, Pacific, and Indian Oceans, including European coastal waters, North American coastlines, Mediterranean Sea, East Asian waters, and Australian shores. One of the most widely distributed jellyfish species globally.

Diet

Carnivorous, feeding primarily on microscopic zooplankton including copepods, fish eggs, larvae, small ciliates, and other planktonic organisms. Prey is captured by mucus on the bell surface and oral arms, then transported to the mouth via ciliary action. Can also absorb dissolved organic matter directly through the epidermis.

Behavior

Drifts with ocean currents as part of the plankton community but capable of weak swimming through rhythmic pulsations of the bell. Often forms large aggregations or blooms when conditions are favorable. Exhibits daily vertical migration in some populations, moving toward the surface during day and deeper at night. Can survive in low-oxygen conditions better than many marine species.

Reproduction & life cycle

Complex life cycle alternating between sexual and asexual phases. Adult medusae reproduce sexually, releasing sperm and eggs into the water column where fertilization occurs. Fertilized eggs develop into free-swimming planula larvae that settle on hard substrate and metamorphose into polyps (scyphistomae). Polyps reproduce asexually by budding and can also produce young medusae (ephyrae) through a process called strobilation. Ephyrae mature into adult medusae over several weeks to months.

When to look

Present year-round in many regions, but blooms are most common in spring and summer months when water temperatures rise and plankton abundance increases. Timing varies by geographic location and local oceanographic conditions.

Cultural significance

Moon jellyfish have been kept in public aquariums worldwide due to their graceful appearance and are popular subjects for marine biology education. In some Asian cuisines, jellyfish including Aurelia species are processed and consumed, though A. aurita is less commercially valuable than other species. Used extensively in scientific research for studying cnidarian biology, regeneration, and as bioindicators of marine environmental change. Their blooms are sometimes viewed negatively when they interfere with fishing, tourism, or power plant operations.

Similar species

• Aurelia labiata (Brown-banded moon jelly)
• Aurelia coerulea (Asian moon jelly)
• Chrysaora species (Sea nettles)

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