Moon jellyfish are a species of jellyfish known by the scientific name Aurelia aurita. They are found mostly in warm and tropical waters near sea-coasts. They are also called moon jelly, saucer jelly, common sea jelly, and even violet moon jellyfish. These jellyfish can survive temperatures as low as -6 °C and as high as 31 °C. They live in both inshore seas and oceans, as well as in brackish waters with salt content as low as 0.6%. Although local to Virginia's waters, they are found in oceans across the globe. In the Pacific Ocean, they range from Alaska to Southern California, and in the Atlantic Ocean, they range from the Arctic to Mexico.
The scientific classification is as follows:
- Domain, Eukarya
- Kingdom, Animalia
- Phylum, Cnidaria
- Class, Scyphozoa
- Order, Semaeostomeae
- Family, Ulmaridae
- Genus, Aurelia
- Species, Aurelia aurita
Moon Jellyfish Are Not Fish
Fish are vertebrates belonging to phylum Chordata, while moon jellyfish are invertebrates belonging to phylum Cnidaria. Phylum Cnidaria includes corals, sea anemones, sea whips, sea pens, hydras, the Portuguese man-of-war, and sea fan corals, along with moon jellyfish. Members of phylum Cnidaria are multicellular, diploblastic (meaning they have two cell layers), and radially symmetric. They all feature special microscopic cells called nematocysts, or sting cells, which let them stun and capture prey.
They Resemble Floating Mushrooms
Moon jellyfish look like floating mushrooms. They have transparent, milky-white, or light purple bell-shaped bodies. From the bell, numerous long thread-like arms called tentacles branch out while floating underwater, like the branches of a weeping willow. The bell-shaped upper body can be as large as 18 inches, giving the animal the appearance of a flying saucer. That is why they are also often called saucer jelly. The bell is divided into eight parts or canals, which branch further and connect to the digestive system. These jellyfish have only one opening, it serves as both the mouth and the anus.
These Sea Jellies Are 95% Water
Moon jellyfish do not have a brain, heart, blood, head, eyes, or ears. They are 95% water and are basically a floating mouth and digestive system. They have a complex set of nerves that respond to stimuli, but they cannot think. They have no lungs, gills, or any kind of breathing system. Instead, they receive oxygen simply by diffusion through a thin membrane. Their large bell-shaped body gives them a big surface area for this process, which is all they need.
Two Main Stages of Life
A moon jellyfish goes through two stages of body plan during its lifetime. The first is called the polyp stage and the second is the medusa stage. The polyp stage has two features: a mouth that opens upwards, and an attachment that anchors it to a surface. The medusa stage brings the familiar bell-shaped body, with the mouth opening downwards. In this stage, the jellyfish is no longer attached and becomes free-swimming. In some species, the polyp stage does not exist at all.
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They Depend on Ocean Currents for Locomotion
In their larval stages, moon jellyfish use cilia for movement. The polyps use their gastrovascular cavity as a hydrostatic skeleton, and also use a few muscle-like cells for contraction and extension. They creep along surfaces using these muscle cells. In the medusa stage, they move differently, using muscle cells called the coronal muscle, found inside the bell. This coronal muscle lets the animal pulsate by contracting, which drives it through the water. The impulses sent to the coronal muscles come from the sub-umbrellar nerve net.
When the muscle cells contract in a rhythmic fashion, movement by jet propulsion occurs. The jellyfish controls this propulsion with the help of rhopalia centers. This swimming is not really meant to travel from place to place, but to keep the jellyfish near the surface. They swim horizontally so that the bell stays close to the surface, which lets them spread their tentacles over a large area to catch the most prey. Even so, these creatures are carried from place to place solely by the movement of ocean currents, they are true drifters of the sea. You can learn more about how jellyfish hunt and feed in our dedicated guide.
Sting Cells
The net of tentacles on the lower part of the jellyfish is covered with scores of poisonous sting cells called cnidocytes or nematocysts, which can inject poison. The moment the tentacles brush against a prey, the numerous sting cells explode, launching barbed stingers and poison into the prey's body. These sting cells are also a defense mechanism, protecting the jellyfish from predators. Nematocysts also play an important role in food capture.
Frequently Asked Questions
What is the scientific name of the moon jellyfish?
Aurelia aurita
What percentage of a moon jellyfish's body is water?
95%
How big can a moon jellyfish get?
The bell can be as large as 18 inches across
Do moon jellyfish have a brain?
No, they have no brain, heart, blood, head, eyes, or ears
Are moon jellyfish stings dangerous?
Most moon jellyfish stings are harmless; the bell cannot sting at all
What do moon jellyfish eat?
Zooplankton, mollusks, rotifers, fish larvae, crustaceans, and other small plankton
Moon Jellyfish Are Carnivorous
These invertebrates are carnivores that feed on zooplankton. Their diet includes mollusks, rotifers, nematodes, diatoms, fish larvae, tunicate larvae, crustaceans, protozoans, and several other small planktons. When tentacles brush against prey, the sting cells make it inactive. The inactive prey collects mainly on the surface of the jellyfish, where it gets caught in mucus. Then, by flagellar action, food is passed through the mouth and along the eight canals inside the jellyfish's body. These canals connect to the stomach via the ring canal, where digestive enzymes break the food down.
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Gonads Are a Distinctive Feature
Moon jellyfish have 4 gonads situated at the bottom of the stomach. They reach sexual maturity mostly in spring and summer. Reproduction is sexual, with distinct male and female individuals, and the brightly colored gonads are visible through the transparent bell body. When a moon jellyfish is viewed from above, the gonads appear as 4 horseshoe-shaped parts, a key identifying feature of this species. Animals observed during August or September show distinct pink arms, caused by the many larvae developing at the edge of the bell-shaped upper body.
Moon jellyfish are commonly washed onto the shores of Long Beach Island beaches during summer high tides and strong winds. By the time they reach the beach, their tentacles are ruptured and worn out. The bell of the moon jellyfish cannot sting, and if you do get stung by the tentacles, most of them are harmless. The stung area will hurt, but applying some household vinegar or urine has proven quite effective in treating the sting.
Fun fact for today!Moon jellyfish are not fish at all, they belong to phylum Cnidaria, the same group as corals and sea anemones!