A species
Moon jellyfish
Aurelia aurita
No brain, no heart, almost all water – yet a survivor for ages.
You and the moon jellyfish last shared an ancestor about 700 million years ago (on 1 November, if life's history were one year).
Imagine an animal with no brain, no heart, no bones and a body that is about 95% water. That is the moon jellyfish. Jellyfish as a group have drifted in the oceans for more than 500 million years, long before the dinosaurs. Surprisingly, its cousins include corals such as the staghorn coral. Both catch food and defend themselves with tiny stinging cells.
The adult bell can grow to about 40 cm across, as wide as a large dinner plate. It looks like a clear saucer, milky white with a blue-grey disc in the middle. Four glowing pinkish shapes show through the top like a four-leaf clover. Underneath sits a square mouth with four frilly, curtain-like arms. Around the rim hangs a fringe of short, fine tentacles that give only a very mild sting.
It drifts near the surface in shallow bays, harbours and river mouths in warm and mild coastal seas. It pulses its body to move, rather than swimming strongly. Tiny stinging cells on its tentacles and arms catch tiny drifting animals such as small shrimp, which are then swept towards its mouth.
A master of shrinking
When food runs short, it can shrink to about one-tenth of its size to save energy, then grow back when food returns.
From sea flower to jelly
It starts life as a young swimmer that settles on the seafloor as a polyp that buds off baby jellyfish which grow into adults. Without helpful microbes, this budding stops and it can hardly make young.
Food for turtles
All seven kinds of sea turtle eat it, along with ocean sunfish, tuna and other fish. Though it has almost no calories, it carries fatty acids that help them grow and breed, richest in fully grown breeding adults.