Common moon jelly

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Onglets

Description

Distinguishing features

Common moon jellies are easy to recognize by their translucent, disk-shaped body, called a bell, edged with about 1,200 short tentacles. Inside the bell, you can usually see four horseshoe-shaped gonads, the reproductive organs.

These marine animals move by rhythmically pulsing their bell, but they still rely heavily on currents.

The mouth sits at the centre of the underside of the disk. It is actually a single opening that leads to the gastric cavity and serves as both mouth and anus. Four oral arms (manubria) surround it. Common moon jellies have no brain, no heart, and no skeleton, and their bodies are more than 95% water.

Reproduction

The life cycle of the moon jelly (Aurelia aurita) has two main stages: a polyp stage, attached to the seafloor, and an adult stage called a medusa.

Adult jellies are sexual. In the fall, males, recognizable by their pink to purple gonads, release sperm into the water. Females capture it to fertilize their eggs, which are still held in their pale, whitish gonads. The embryos then develop in a specialized pouch on the female’s oral arms, which shelters them until they hatch.

Hatching produces a tiny larva called a planula, only about 1 to 2 mm long. It swims briefly before attaching to the seafloor, beginning the second phase: the polyp. The polyp looks like a small sea anemone, anchored at one end, with a mouth surrounded by tentacles at the other to capture food.

Common moon jellies can remain in the polyp stage for several years and may form small colonies that carpet the seafloor.

When conditions become favourable, usually toward the end of winter, the polyp begins a segmentation process and turns into a strobila. Through strobilation, it divides into a stack of sections and releases tiny young jellies called ephyrae. This asexual stage produces large numbers of genetically identical individuals.

The young jellies grow steadily and reach adult size in about three months.

Diet

Plankton is the staple of the diet of both adult jelly and polyps alike. Young jelly also capture fish larvae.

The predation technique of cnidaria—from jelly to anemones to coral—is essentially the same across species. Their tentacles are covered in nematocysts—tiny cells that, at the slightest contact, release a stinger to inject a paralyzing agent. The prey is then at the mercy of the jelly (or the anemone), to be swallowed at the predator’s leisure.

The common moon jelly’s abundant mucus is usually enough to trap and subdue its tiny prey. Its tentacles are also so fine and light that they often don’t trigger a prey animal’s escape response.

Predators

Its main predators include sea turtles (especially leatherback turtles), certain fish such as tuna and salmon, and seabirds. Humans rarely eat common moon jellies, but they are sometimes used in Asian cuisine. Because they are made mostly of water, they have little nutritional value.

Habitat

Common moon jellies are found in temperate and tropical coastal waters, often in shallow areas. They prefer calm seas and can also survive in slightly brackish water, such as in estuaries, bays, and fjords. They occur in nearly all of the world’s oceans and are a common species in the Gulf of St. Lawrence.

Ecology, behaviour

Moon jellies play an important role in marine food webs by helping regulate zooplankton populations. During a mass increase in numbers (a bloom), they can disrupt ecosystems and human activities such as fishing and power generation.

A very current problem is the increasingly frequent jellyfish blooms in our oceans. Many factors may be at play in this phenomenon, but it always involves an imbalance in the marine ecosystem.

Overfishing is one possible cause. The disappearance of major fish populations leaves an ecological niche empty. In the absence of competition, jelly proliferate, eating the plankton and small forage fish that these fish used to feed on. Furthermore, the disappearance of certain predators of jelly, such as sea turtles and tuna, simply amplifies the phenomenon.

In turn, increased numbers of jelly have an impact on commercial fish stocks, as they feed on their hatchlings.

Global warming, ocean acidification and anoxia (depletion of dissolved oxygen) may also be major drivers of jellyfish blooms, since jellies tend to be better able to withstand these difficult conditions.

French name

Méduse lune

Scientific name

Aurelia aurita

Phylum

Cnidaria (coelenterata)

Class

Scyphozoa

Order

Semaeostomeae

Family

Ulmaridae

Size

5 to 40 cm in diameter

Life span

Adults (free-floating form) live three to six months, while polyps (tiny form attached to the seabed) can live for a few years.

Status

Non-endangered species—sometimes tends to be invasive.

At the Biodôme

Keeping jelly in captivity presents a number of challenges, due to the animal’s fragility and lack of mobility. A jelly is a gelatinous creature consisting mostly of water. Any contact with an object or the walls of the aquarium could cause injury.

Furthermore, as these animals have a limited ability to move, they can neither see nor avoid obstacles. This is why they must be kept in special tanks called kreisel aquariums. In a kreisel aquarium, a circular current is created along the walls, which keeps the jelly suspended and forms a protective liquid barrier between these animals and the side of the tank.

At the Biodôme, common moon jellies are fed mainly brine shrimp (Artemia), small planktonic animals.

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