Moon Jellyfish | Marine Species

Moon jellyfish（圖片來源：Luc Viatour，採用 CC BY-SA 3.0 授權）

The moon jellyfish (Aurelia aurita), commonly known as the moon jelly, is one of the most familiar jellyfish species and belongs to the class Scyphozoa. Its body is transparent and delicate in appearance, with four horseshoe-shaped gonads clearly visible in the center of the bell, which serve as a key identifying feature. Adult individuals typically reach a bell diameter of approximately 25 to 40 cm. Although they are capable of slow propulsion through rhythmic pulsation of the bell, they generally drift passively with ocean currents.

In the wild, moon jellyfish are widely distributed across the world's three major oceans and are commonly found in temperate to tropical coastal waters. They are particularly abundant in estuaries, harbors, and sheltered bays. The species possesses strong environmental tolerance: it can survive in brackish water with salinity as low as 6‰ and can withstand a broad temperature range from 6 to 31°C. During summer, when dissolved oxygen levels in the water may drop sharply, many fish reduce their activity or even die from hypoxia. Moon jellyfish, however, can continue feeding efficiently under low-oxygen conditions. This physiological resilience allows them to form large seasonal blooms that further compress the ecological space available to other marine organisms.

Their diet is omnivorous but predominantly carnivorous. They mainly feed on zooplankton, including crustacean larvae, mollusk larvae, polychaete larvae, fish eggs, protozoans, and algae. Prey is captured using nematocysts located on the tentacles and is then transported toward the gastric cavity by ciliary motion and mucus for digestion. Although the sting of Aurelia aurita has minimal effect on humans—most people experience only mild irritation or no sensation at all—it remains an effective predatory tool for capturing small organisms.

The life cycle of the moon jellyfish includes several distinct morphological stages. Fertilized eggs develop into planktonic planula larvae, which eventually settle onto a substrate and transform into a sessile polyp stage known as a scyphistoma. The scyphistoma can reproduce asexually through budding and may undergo transverse fission known as strobilation, producing juvenile forms called ephyrae. These ephyrae later develop into free-swimming adult medusae. In natural environments, the lifespan of moon jellyfish usually does not exceed one year, although individuals maintained in stable aquarium conditions may live longer.

Some studies suggest that moon jellyfish may exhibit a phenomenon resembling rejuvenation under certain conditions, in which mature individuals can revert to earlier developmental stages. This characteristic has drawn scientific interest in studies of development and evolutionary biology.

Circular aquarium tank（圖片來源：Edilberto "Pay" Aponte of Jellyfish Art.com，採用 CC BY-SA 4.0 授權）

Moon jellyfish（圖片來源：Hans Hillewaert，採用 CC BY-SA 4.0 授權）

Group of moon jellyfish（圖片來源：Johannes Maximilian，採用 GNU Free Documentation License 1.2 授權）

Housing Requirements in Captivity

Moon jellyfish are relatively suitable for beginner jellyfish keepers, yet their bodies remain extremely fragile. For this reason, specialized aquarium designs are required in captivity. The most ideal system is a circular Kreisel tank, a type of aquarium designed to generate a continuous circular flow that keeps jellyfish suspended in the water column. This design prevents them from colliding with hard surfaces or being trapped in corners by water currents.

Jellyfish are not strong swimmers, so the water flow must be gentle and uniform. The circulation should be strong enough to keep food suspended in the water while avoiding excessive turbulence that might damage the animals.

Regarding water conditions, moon jellyfish prefer temperatures between 16 and 19°C. Temperatures that are too high may cause abnormal behavior known as balling, in which the jellyfish contract into tight clumps. Cooling systems are therefore often required during summer, while heating may be necessary in winter. Salinity should be maintained between 30‰ and 35‰. Marine salts with higher calcium content—such as coral salt mixes—are recommended, as they help maintain stable pulsation rhythms. Water changes should be carried out slowly to avoid sudden fluctuations in temperature or salinity that could stress the animals.

Moon jellyfish do not require strong lighting and can survive well even in low-light environments. Nevertheless, many aquarists use colored LED lighting to enhance their display value, as the transparent bodies of jellyfish can create a striking visual effect under illumination. Stronger lighting, however, can accelerate algae growth, requiring additional maintenance. For daily care, it is advisable to siphon out leftover food and waste from the bottom of the tank and periodically clean the tank walls to maintain good water quality.

When transferring jellyfish between containers, they must remain fully submerged in water at all times. They should never be lifted directly with a net, as this can easily cause the delicate body to rupture. The safest method is to scoop them together with seawater using a container filled with water.

Feeding Conditions

In captive environments, the most common and safest food source is Artemia nauplii, especially newly hatched brine shrimp. Their small size and slow swimming behavior make them easy prey for moon jellyfish. Feeding is generally recommended two to three times per day. Aquarists can monitor feeding success by observing whether the gastric pouches contain visible orange-colored food masses. When the four gastric cavities show clear coloration, the feeding amount is typically sufficient.

Uneaten food that settles on the bottom of the tank can decay and degrade water quality. For this reason, leftover food should be removed promptly after feeding. To maintain water cleanliness, some keepers temporarily move the jellyfish to a smaller feeding container, allow them to feed there, and then return them to the main tank. Although slightly inconvenient, this method significantly reduces contamination of the primary aquarium.

In addition to live food, commercially prepared powdered feeds designed for jellyfish are also available, such as Medusa-G. These feeds must first be dissolved in seawater before being introduced into the aquarium. Because powdered feeds tend to settle quickly, they are often better used in small feeding containers rather than in the main tank. For aquarists with limited time, large batches of Artemia nauplii may be hatched in advance, frozen in portions, and thawed daily for feeding.

Breeding Conditions

In nature, moon jellyfish reproduce through a distinctive life cycle that alternates between sexual and asexual stages. Sexually mature males release sperm into the water column. The sperm are carried by water currents into the gastric cavity of females, where internal fertilization occurs. Fertilized eggs develop into planula larvae, which eventually settle and transform into scyphistoma polyps.

These polyps can reproduce asexually and, under suitable environmental conditions, undergo strobilation to produce ephyrae. The ephyrae then grow into new medusae. Because this life cycle includes both sexual and asexual reproduction, populations can expand rapidly when environmental conditions are favorable.

In captivity, reproduction is usually not the primary goal of jellyfish husbandry, though it may still occur. If polyps appear in the aquarium, aquarists should consider preparing a separate rearing tank. Since polyp growth and strobilation can increase the number of individuals, excess polyps may need to be removed if the main aquarium cannot support the growing population. For most hobbyists, maintaining stable water quality and environmental conditions is far more important than deliberately attempting to induce breeding.

Preventing Invasive Spread

Moon jellyfish occur naturally in oceans around the world, and they can also appear in waters surrounding Taiwan. However, individuals raised in captivity may differ from wild populations, and releasing them into the ocean could disrupt natural ecosystems. Moreover, aquarium-raised animals may carry pathogens that could spread to wild populations and cause ecological harm.

For these reasons, aquarists must follow the principle of never abandoning or releasing captive organisms into the wild. If it is no longer possible to continue keeping the jellyfish, the responsible approach is to transfer them to another suitable keeper or consult nearby marine aquarium stores to see whether they are willing to accept them, rather than releasing them into natural marine environments.

Author: Shui-Ye You