Invertebrates
Atolla Jellyfish
Atolla wyvillei · also called atolla jellyfish, alarm jellyfish, coronate medusa
The atolla jellyfish drifts through the cold darkness half a kilometre or more below the surface, deep red and, in that lightless water, effectively invisible. But grab it, and it turns into a spectacle: a rapid pinwheel of blue light spinning around the rim of its bell — a signal that is not a warning to its attacker but a call for a bigger one.
Overview
The atolla jellyfish lives in the deep open ocean worldwide, mostly between about 500 and 5,000 m. The bell runs from a few centimetres to about 15 cm across, deep red to maroon, with a ring of marginal lappets and one tentacle far longer than the rest trailing below.
Hidden in red
Its maroon colour is effectively black in the deep sea, because red light does not reach those depths, so the animal reflects nothing and cannot be seen. The red gut wall also hides the glow of any bioluminescent prey it has eaten.
The burglar alarm
When something seizes it, the atolla jellyfish fires a rapid series of blue flashes that seem to rotate around the bell like a spinning wheel, visible over a long distance. The best explanation is that this attracts a larger predator, which then attacks the jelly’s attacker — a call for help rather than a threat.
Filming the giant squid
A scientist built an electronic lure mimicking this spinning display, reasoning it would summon big predators. In 2012, that lure drew a giant squid in front of a deep-sea camera for the first time.
Status
The atolla jellyfish has not been assessed. It is one of the most common animals of the deep midwater and is at no conservation risk.
Atolla Jellyfish: common questions
Why does the atolla jellyfish light up when it is attacked?
The flashing, spinning light show is a defence, but an indirect one. Rather than trying to startle or poison its attacker, the atolla jellyfish appears to be advertising the attack itself. A bright, moving light in the pitch-dark deep sea is highly conspicuous and draws the attention of larger predators over a wide area. If one of those larger predators comes to investigate and attacks the animal that is attacking the jelly, the jelly benefits, either because the attacker is eaten or because it releases its grip and flees. The behaviour has been nicknamed the burglar alarm, because it works like a shop alarm that summons the police rather than fighting the thief directly.
How can something be bright red in the deep sea and still be hidden?
It comes down to what light is available. Sunlight is filtered out with depth, and red wavelengths disappear first, so below a few hundred metres there is essentially no red light in the water, only faint blue. An object can only reflect colours that are shining on it. A deep-red animal in blue-only water has nothing red to reflect and nothing much else either, so it appears black and effectively vanishes. Many deep-sea animals are red, black, or transparent for exactly this reason, and red pigment in the gut wall also stops the light of any glowing prey the animal has eaten from shining out through its body.
What is the atolla jellyfish's role in filming the giant squid?
The deep-sea biologist Edith Widder reasoned that if the atolla jellyfish's light display really does summon large predators, then a device mimicking that display could be used to attract them to a camera. She built an electronic lure, sometimes called the e-jelly, that reproduces the spinning blue pinwheel. Deployed on a quiet, low-light camera platform in 2012, first off Japan and then in the Gulf of Mexico, the lure drew a giant squid into view on more than one occasion, producing the first video ever taken of the animal alive in the deep sea.
Is the atolla jellyfish dangerous to people?
In practical terms, no. It lives hundreds to thousands of metres down in the open ocean and is almost never encountered by swimmers or divers, who do not go anywhere near its depths. Like other jellyfish it has stinging cells on its tentacles for catching prey, and handling one carelessly at the surface, for example after it has been brought up in a net, could cause a mild sting, but it is not considered a hazardous species and there is no context in which an ordinary person would meet one in the water.