Moon jellyfish, scientifically known as Aurelia aurita, are translucent marine invertebrates found across the globe. Often called common jellyfish or saucer jellies due to their distinctive appearance, they are a frequent sight in coastal areas. Their delicate form has captured the interest of researchers and the public.
Distinctive Features
Moon jellyfish have a translucent, saucer-shaped bell, typically 10 to 35 centimeters (4 to 14 inches) in diameter, though some reach 40 centimeters (16 inches). The bell is often colorless but can show subtle hues of blue, pink, or purple, sometimes influenced by diet. Four horseshoe-shaped gonads, responsible for reproduction, are visible through the bell and can appear in various colors such as mauve, violet, reddish, pink, or yellowish.
Along the bell’s fringe are numerous short, fine tentacles, 1 to 5 centimeters (0.4 to 2 inches) long. These tentacles contain stinging cells (nematocysts) used for capturing prey. Four frilly oral arms hang from the bell’s underside, longer than the marginal tentacles, aiding in feeding. Moon jellyfish lack a brain, heart, or complex circulatory and respiratory systems, relying on a simple nerve net for sensing and thin skin for oxygen absorption.
Where They Live
Moon jellyfish inhabit coastal waters across the Atlantic, Pacific, and Indian Oceans, making them a cosmopolitan species. While primarily pelagic, they are frequently observed in shallower coastal areas, estuaries, and harbors. They prefer temperate and tropical waters, thriving between 9 and 19 degrees Celsius (48 to 66 degrees Fahrenheit), though they tolerate a wider range from 6 to 31 degrees Celsius (43 to 88 degrees Fahrenheit). They can also survive in waters with low salinity, as low as 0.6% salt content.
Moon jellyfish often form large aggregations, called “blooms,” consisting of hundreds or thousands of individuals. These blooms are influenced by water temperature, prey availability, and ocean currents. Their presence in large numbers can impact local marine ecosystems and human activities.
What They Eat
Moon jellyfish are carnivorous filter feeders, primarily consuming zooplankton. Their diet includes a variety of small marine organisms such as planktonic crustaceans, copepods, mollusks, fish eggs, worms, protozoans, and diatoms. Not strong swimmers, they largely drift with currents, using feeding mechanisms to capture food.
Prey particles become entangled in a sticky mucus layer covering their bell and short tentacles. Cilia then move the trapped food along the bell’s surface towards the four oral arms. These arms transport food to the central mouth, located on the bell’s underside. From there, food enters stomach pouches for digestion.
Life Cycle
The moon jellyfish life cycle involves both sexual and asexual reproduction, transitioning between two forms: the free-swimming medusa and the sessile polyp. Male medusae release sperm, which female medusae take in to fertilize eggs internally. Fertilized eggs develop into small, ciliated larvae known as planulae, brooded by the female, often on her oral arms.
Planulae are free-swimming briefly before settling onto a hard surface, such as rocks, shells, or artificial structures. Upon attachment, the planula transforms into a small, sedentary polyp resembling a tiny sea anemone. Under favorable conditions, the polyp undergoes asexual reproduction via strobilation, dividing horizontally into multiple disc-shaped segments. Each segment detaches as a tiny, immature ephyra, which grows into the adult medusa stage, completing the life cycle.
Are They Dangerous?
Moon jellyfish are generally not considered dangerous to humans; their sting is very mild. Their tentacles contain nematocysts (stinging cells), but these are small and not potent enough to significantly harm human skin. A sting is often barely noticeable, like a minor prickle, or may result in only slight, temporary irritation or a mild rash.
If a mild sting occurs, rinsing the affected area with seawater is recommended, as fresh water can reactivate stinging cells. Applying a paste of baking soda and seawater can also alleviate persistent irritation. It is not advisable to use urine, as this is a common myth and can worsen the discomfort. While individual stings are weak, large aggregations, or “blooms,” might still cause mild skin irritation due to the sheer number of stinging cells.