Moonjelly

Aurelia aurita

ORDER

Semaeostomeae

FAMILY

Ulmaridae

TAXONOMY

Aurelia aurita Linnaeus, 1758, Baltic Sea.

OTHER COMMON NAMES None known.

PHYSICAL CHARACTERISTICS

The diameter of the medusa swimming bell may reach 20 in (50 cm). The eight lobes of the bell are marked by shallow indentations, each with a rhopalium. The bell is translucent, usually with a pink tinge. The gonads resemble a pink four-leaf clover, as seen inside the semitransparent bell. Hundreds of short, fine tentacles hang in a single circle from the bell margin. The oral arms extend only to about the edge of the bell and may have bright reddish orange larvae brooded in pockets at the edges. Polyps are white and about 0.1-0.2 in (2-4 mm) long, with a single ring of tentacles. They often occur in large aggregations. Developing ephyrae are orange.

DISTRIBUTION

The moon jelly is reported from all oceans, from tropical to temperate waters between 70°N and 55°S latitude. They are common in coastal European, North American, and Japanese waters. They also are reported from some locations in Asia, Australia, Pacific Islands, South America, and Africa. This species may be endemic to Europe and introduced elsewhere. It closely resembles other species in the genus. Recent molecular studies indicate perhaps six species in the genus that may be easily confused.

HABITAT

This species occurs in a wide range of conditions, in waters with near-freezing winter temperatures, to 90°F (32°C), and with salinity levels of 14-38 ppt. Medusae generally are present only in warm months in temperate locations, but they occur throughout the year in tropical and some temperate locations. They often are found in estuaries, fjords, and bays, usually at the surface to 100 ft (30 m). Large populations may form in semi-enclosed bays with restricted tidal exchange. The polyps are found at depths above 65 ft (20 m) on the undersides of hard structures.

BEHAVIOR

The moon jelly is remarkable in forming large aggregations of medusae. Aggregations may be a mile or more (2 km) in length and can contain millions of individuals. These groups may be seen from low-flying airplanes and detected by "fish finders" on fishing boats. The aggregations are formed because of the tendency of the medusae to swim either up or down against directional water flow. They also are reported to swim horizontally by orienting to a specific compass direction in sunlight, causing them to gather in certain locations.

FEEDING ECOLOGY AND DIET

The fine tentacles of the medusae catch mainly small crustacean zooplankton, such as copepods and cladocerans. They also feed on fish and mollusk larvae, small hydromedusae, and even microzooplankton, such as ciliates. They may catch food in the mucus on the outer surface of the swimming bell.

REPRODUCTIVE BIOLOGY

The life cycle is typical of semaeostome scyphomedusae, having both a medusa (sexual) stage and a polyp (asexual) stage. Moon jelly aggregations are believed to increase fertilization success by bringing females and males into proximity. The males release sperm strands into the water, which are taken up by the females during feeding. The fertilized eggs form larvae that are brooded in pockets on the oral arms. The larvae attach to hard surfaces and become polyps, which then bud medusae.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

Aggregations sometimes have clogged the seawater intakes of power plants in Asia. The medusae also may reduce commercial herring populations by feeding on larvae in Kiel Bight, Germany. Their sting is not painful to humans. They are raised easily for aquarium exhibits and sometimes are kept in special home aquariums. ♦

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