Pyrosome

A pyrosome is a glowing, free-floating colony of tunicates that looks like a jellyfish but is nothing of the kind: hundreds to thousands of tiny animals joined into one firm, hollow tube. Its name comes from the Greek for fire and body, and Pyrosoma atlanticum can flash a brilliant blue-green.

It drifts in the open ocean in temperate seas across the world, usually between 50°N and 50°S, mostly in deep water by day and rising towards the surface each evening. It is most plentiful below about 250m.

Appearance

Look for a rigid cylinder, up to 60cm long and 4 to 6cm wide, in pale pink, yellowish or bluish. One end is narrower and closed, and the other is open with a strong diaphragm around it. The jelly-like skin is dimpled with blunt knobs that point backwards, and each knob belongs to one of the zooids, the tiny individuals that make up the colony, none more than 8.5mm long.

Behaviour

Colonies make a huge daily commute, climbing in the evening and sinking again around dawn. A large one may cover 760m in a day, and even a colony only a few millimetres long can manage 90m. Each zooid has a pair of light organs beside its inlet siphon, and when stimulated these switch on and off to give a rhythmic flashing.

Feeding

Rhythmic beating of tiny hairs pulls water in through the zooids' gill slits and on into the middle of the tube, and plankton and other particles stick to mucus filters on the way. They are very effective at this. A study in the Indian Ocean compared several plankton grazers and found that pyrosomes took a higher share of particles above 10 micrometres than the rest, which suggests they rely on eating a great deal, not on saving energy.

Life cycle

A colony grows by budding new rings of zooids around its edge, gradually lengthening the tube.

Threats

Predators include bony fish such as the spiky oreo, big-eyed cardinalfish and pelagic butterfish, as well as dolphins and whales, among them the sperm whale and giant beaked whale.