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
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
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
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.