Invertebrates
Moon Jellyfish
Aurelia aurita · also called moon jelly, common jellyfish, saucer jelly
The moon jellyfish is the jelly everyone has seen — the pale, saucer-shaped drifter pulsing gently in a harbour, washed up limp on a beach, or lit up in an aquarium tank. It has a sting too weak to feel, no brain to speak of, and a body that is almost entirely water, and yet it is one of the most successful animals in the coastal ocean.
Overview
Moon jellyfish live in coastal and shelf seas nearly worldwide, from the tropics to near the poles, and are especially common in sheltered, nutrient-rich water — harbours, estuaries, fjords. They drift near the surface, pulsing the flat, translucent bell (usually 10–35 cm), and feed by trapping plankton and small animals on a sheet of sticky mucus over the bell, then sweeping it to the mouth. The four horseshoe-shaped gonads show through the top like a four-leaf clover.
Almost harmless
Its stinging cells are small and its venom mild, so most people feel nothing touching one — which is why aquariums allow it. It can irritate thin skin, lips, or eyes, so it should still not be handled.
No brain
The moon jelly has no brain, no blood, no bones. A loose nerve net, a nerve ring, and eight simple sensory structures (light spots and gravity sensors) are enough to coordinate swimming, right the animal if it flips, sense day and night, and move food to the mouth.
Shrink and regrow
Starved, an adult can reabsorb its own tissue and shrink, then grow again when food returns, and it can regenerate lost arms and bell sections.
Blooms
The visible jelly is half the life cycle. A bottom-living polyp buds off stacks of baby jellies when conditions suit; a bloom forms when many polyps release at once. Human pressures — warming, nutrient pollution, overfishing of jelly predators, and artificial surfaces for polyps — may be making blooms worse in some regions.
Status
The moon jellyfish has not been assessed for conservation and is abundant worldwide — one of the few animals that may benefit from a degraded ocean.
Moon Jellyfish: common questions
Can a moon jellyfish sting you?
Technically yes, practically usually not. Like all jellyfish it is covered in stinging cells (nematocysts), but in the moon jelly they are small, the barbs are short, and the venom is weak, so they generally cannot penetrate the thick skin of a human palm, arm, or torso — which is why aquariums let visitors touch them. The venom can still cause a mild rash or prickling on thin or sensitive skin, and it will sting if it contacts the lips, the inside of the wrist, or especially the eyes, so you should still avoid handling one and wash your hands afterwards. Compared with a box jellyfish or a Portuguese man o' war, the moon jelly is essentially harmless.
How does a jellyfish live without a brain?
The moon jellyfish has no brain, no central nervous system, and no head. Instead it has a diffuse "nerve net" — a mesh of nerve cells spread through the bell — plus a ring of nerves around the rim and eight sensory structures (rhopalia) that contain simple light-sensitive spots and gravity-sensing statocysts. This is enough to coordinate the rhythmic pulsing of the bell for swimming, to right the animal if it tips over, to sense day and night, and to move captured food to the mouth. It does not need to make decisions in the way an animal with a brain does; it drifts with the current, pulses to stay near the surface, and passively filters whatever food touches its mucus-covered bell.
Why do you see so many moon jellyfish sometimes and none other times?
Because the visible adult jelly is only half the life cycle, and the timing is controlled by the hidden half. Moon jellies spend part of their life as a tiny polyp fixed to a hard surface on the sea floor — a pier piling, a rock, a shell. When water temperature, day length, and food are right, each polyp buds off a stack of miniature jellyfish (ephyrae), one after another like plates coming off a pile, which then grow into the familiar medusae over weeks. A large "bloom" appears when many polyps release at once into favourable conditions; when the adults die off in late summer or autumn, the jellies vanish, but the polyps stay put, ready to produce the next generation.
Are jellyfish blooms getting worse, and is that our fault?
In some places, apparently yes, though the global picture is debated and data are patchy. Several human pressures can favour jellyfish like the moon jelly — warmer water speeds up polyp budding, nutrient pollution from farming and sewage boosts the plankton jellies eat, overfishing removes the small fish that both compete with jellies for that plankton and eat young jellies, and artificial structures such as docks, oil platforms, and aquaculture cages give polyps vast new surfaces to attach to. Where these factors combine, moon jelly blooms can clog fishing nets, block the cooling-water intakes of power plants, and disrupt fish farms. It is one of the clearer examples of an animal that may benefit from a degraded ocean.