Moon Jellyfish

Aurelia aurita, often called the common or moon jellyfish, belongs to the Ulmaridae family and is widely recognizable by its translucent, saucer-like body. Typically measuring between 25 to 40 centimeters (10 to 16 inches) in diameter, these jellyfish display four distinct horseshoe-shaped gonads visible through the top of their bell-shaped bodies. They rely on tentacles to catch their food, which includes medusae, plankton, and molluscs, transporting it into their bodies for digestion. Unlike many jellyfish, moon jellies have short, delicate tentacles that are not dangerous to humans, as their sting is ineffective.

Aurelia aurita is found across a vast geographical range, inhabiting the North, Black, Baltic, and Caspian Seas, as well as regions in the Northeast Atlantic, near Greenland, the northeastern 🇺🇸 United States, Canada, the Northwest Pacific, and South America. They thrive in coastal areas and estuaries with ocean temperatures spanning from 6 to 31 degrees Celsius (43 to 88 degrees Fahrenheit), favoring conditions between 9 and 19 degrees Celsius (48 to 66 degrees Fahrenheit). These jellyfish are highly adaptable, tolerating salt concentrations as low as 6 parts per thousand.

In summer months, especially July and August, Aurelia aurita populations surge in oxygen-poor waters, where others struggle. When dissolved oxygen levels drop below 2.0 mg per liter, moon jellyfish remain unaffected, often forming large groups that can drastically consume mesozooplankton biomass, as observed in places like the Seto Inland Sea. While predatory fish find low oxygen levels challenging, reducing competition for food, moon jellyfish maintain effective feeding and growth, gaining an advantage in these conditions.

These jellyfish feed primarily on plankton, including small organisms such as molluscs, crustaceans, tunicate larvae, and fish eggs, along with gelatinous zooplankton like hydromedusae and ctenophores. Both adult jellyfish and their young use specialized stinging cells, called nematocysts, for capturing prey and defense. Their digestive process involves trapping food in mucus, followed by the action of digestive enzymes.

Aurelia aurita has a unique anatomy; it lacks complex respiratory systems and depends on diffusing oxygen from water through its thin membrane. With no respiratory, excretory, or circulatory systems, its structure includes a transparent bell, membrane margins, and tentacles at the base. Inside, there are four gonads and components for food distribution, like the manubrium and radial canals. Water flow driven by cilia enhances oxygen diffusion, necessary for cellular respiration.

During its lifespan, Aurelia aurita undergoes several stages. After hatching as microscopic larvae called planulae, these organisms settle and transform into polyps, eventually budding off into juvenile medusae. This jellyfish exhibits an intriguing ability to revert to previous life stages, similar to the "immortal jellyfish" (Turritopsis dohrnii), allowing them to return to youth under certain conditions. This incredible feature underscores their resilience and adaptability in diverse marine environments.