Moon Jelly

They loom suddenly from the darkness of their underwater realm.  Mysterious.  Alien.  Glowing.  Sensual.  These strange creatures are jellyfish.  Parents and children alike will be fascinated by these floating aliens – not fish at all, really, but breathtakingly beautiful and ancient sea creatures who survive with no brain, no eyes, no ears, no heart, and a body that is almost entirely composed of water.

The moon jelly, or Aurelia aurita, is probably the most common and widely recognized type of jelly. They can be found in the Atlantic, Arctic and Pacific Oceans near the coasts. The animal ranges in size from 5cm to 40cm across. The moon jelly is easily recognizable by its four violet or pink crescent shaped gonads on the underside and at the center of its translucent bell or umbrella. Also, its bell is thicker toward the middle, thinning toward the edge. Like many other species of jellyfish, the moon jelly’s ability to move by itself is limited, so it is subjected to the water currents of the ocean.

The moon jelly has two main stages in its life cycle – the polyp stage (asexual reproduction) and the medusa stage (sexual reproduction).  A mature polyp reproduces asexually, known as budding, forming an entire colony of polyps. Polyps specializing in reproduction produce ephyra (small medusae) by budding. The medusae swim off and mature. They then reproduce sexually. From the egg and the sperm of two medusae, a zygote is formed. The zygote develops into a planula (larva), which leaves the adult medusae, finds a shaded surface, and attaches itself to it. The planula eventually develops into a new polyp, and the life cycle of the moon jelly starts again.

The diameter of the medusa swimming bell may reach 20 in. The eight lobes of the bell are marked by shallow indentations. 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. long, with a single ring of tentacles. They often occur in large aggregations.

Their behavior depends on a number of external conditions, in particular, the food supply. Moon jellies swim by pulsations of the bell, functioning primarily to keep the animal at the surface of the water rather than to make progress through the water. They swim horizontally, keeping the bell near the surface at all times. This allows the tentacles to be spread over the largest possible area in order to catch food more easily. The coronal muscle allows the animal to pulsate in order to move. The moon jelly has rhopalial (neuron) centers, which allow it to control the pulsations. As the oxygen rate in the water goes down, so too does the respiratory rate of the jellyfish.

The moon jelly is carnivorous and feeds on zooplankton. 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 catch food in the mucus on the outer surface of the swimming bell, which is then directed by currents eventually into the stomach.

The moon jelly is remarkable in forming large aggregations of medusae. Aggregations may be a mile or more 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.

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, and recent molecular studies indicate perhaps six species in the genus that may be easily confused. They are prevalent in both inshore seas and oceans. Their habitat includes the costal waters of all zones, and they occur in huge numbers. They are known to live in brackish waters with as low a salt content as 0.6%. Decreased salinity in the water diminishes the bell curvature, and vice versa. An optimum temperature for the animals is 9-19°C.

Their sting is not painful to humans, and moon jellyfish are plentiful and are not listed as endangered.