The moon jellyfish is almost entirely translucent and can be recognised by four bright pinkish/purple horseshoe-shaped circles, the gonads i.e., the reproductive system, seen through the top of the bell. The jellyfish has short thread-like tentacles around the edge of the bell attached to the bottom of a marginal membrane. It has four fringed oral arms that are longer than its tentacles.

Behaviour

The moon jellyfish is not a strong swimmer, mainly using its short tentacles to stay near the water surface, or pulsing the bell to push itself slowly forwards. It lives quite a solitary life, but wind and water currents often group the jellyfish together.

Moon jellyfish are meat eaters. They eat molluscs and plankton, but also fish eggs and shrimps. They have a large stomach and will eat whenever they find something floating by. The tentacles have powerful toxins that immobilize the prey which is passed via cilia into its oral cavity for digestion. It relies on digestive enzymes in the stomach lining to break the prey down.

The jellyfish life cycle has a sexual and asexual component. Each adult (called a medusa) is either male or female. Jellyfish release sperm and eggs into open water. Fertilized eggs develop and grow for a few days before attaching to the sea floor and growing into polyps. The polyp resembles an upside down medusa. Polyps asexually bud off clones that develop into mature medusae.

Size

• Average diameter: 25 – 40 cm
• Average lifespan: 6 months to one year (as an adult)

Status

Common; not threatened or endangered

Distribution

The moon jellyfish is common along the Atlantic coast of Europe. They frequent coastal and epipelagic areas (top layer of the ocean) and can survive the lower salinity of estuaries and bays. They can withstand temperatures as low as -6°C and as high as 31°C, though the optimum temperature is between 9°C and 19°C. Moon jellyfish are the most common jellyfish found in UK seas, especially the sheltered waters off the west coast of Scotland.

When to see

January to December; most spectacular swarms are seen in late summer when the jellyfish are at their largest.

Facts

• Moon jellyfish species live their lives without brains, ears, heart, lungs, blood or eyes. They are basically made up of three main elements: 95% water, a mouth, and a digestive system.
• Moon jellyfish do sting, but their venom is mild and considered harmless to humans. Any clinging tentacles may be rinsed off with salt water and the venom deactivated with heat, vinegar, or baking soda.
• There are several known predators of the moon jellyfish. These include sea turtles that rely on them for food during their annual migration. Jellyfish have very little nutritional value, so predators that specialize on them must eat hundreds in order to maintain their required energy levels.