Moon jelly

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Onglets

Description

Distinguishing features

The moon jelly, also known as the moon jellyfish, looks like a gelatinous saucer surrounded by very short tentacles (about 1,200). Four horseshoe-shaped gonads (or, more rarely, three to seven) are visible through its translucent body. Its mouth is located in the center of the subumbrella. This opening, which is connected to the gastric cavity, is surrounded by four oral arms (manubria) and serves as both mouth and anus.

The moon jelly’s body is 98% water.

Reproduction

The life cycle of the moon jelly is split into two main phases: The medusa stage (free-floating adult form) and the polyp stage (miniature form attached to the seabed).

Medusae (i.e. the adults) may be male or female. In the fall, the males release spermatozoa into the water, which the females capture to fertilize the ova in their gonads. The eggs are incubated in a special pouch on the female’s oral arms.

Small (1-2 cm) planula larvae then form, which make their way to the seabed to begin the second phase of the medusa’s life cycle: the polyp. The polyp, which resembles a tiny hydra, is attached to the seabed at one end, while the free end has an opening surrounded by tentacles for feeding. When the conditions are right (generally towards the end of winter), the polyp divides to form minuscule medusae that look like snowflakes (ephyrae). After three months’ growth, these will constitute the next generation of adult jelly.

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.

Due to the small size of the Aurelia aurita’s prey, neither the penetration force of its nematocysts nor the potency of its venom needs to be very powerful. This is why humans do not generally feel a moon jellyfish sting. The large amounts of mucus produced by the moon jelly are generally enough to entrap and hold its prey. Another peculiarity: its tentacles are so light and fine that they do not trigger its prey’s flight reflex.

Predators

As moon jellies are 98% water, they do not constitute a very tempting source of food and therefore have few predators. However, sea turtles, tuna and moonfish are known to eat them.

Habitat

Moon jelly can be found in every ocean on Earth, always near the coast and generally in warm to temperate waters. It is also present in cold waters, but not in polar waters. This species is very common in the Gulf of St. Lawrence.

Ecology, behaviour

A very current problem is the increasingly frequent proliferation of jelly populations 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. The situation can therefore quickly spiral out of control.

Global warming, ocean acidification and anoxia (depletion of dissolved oxygen) may also be significant factors in the proliferation of jelly, as they are better able to withstand these difficult conditions.

French name

Méduse lune, Aurélie, Méduse commune

Scientific name

Aurelia aurita

Phylum

Cnidaria (coelenterata)

Class

Scyphozoa

Order

Semaeostomeae

Family

Ulmaridae

Size

5-40 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.

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