Is That One Animal or Thousands? The Answer Is Genuinely Unclear

The body is built from many separate units, each of which develops from the same fertilised egg by budding and each of which is, developmentally, an individual animal of the same kind as a small jellyfish or a polyp. Those units are specialised for different jobs and are physically joined into a single functioning structure, sharing a common digestive space so that food caught by one feeds all. No unit can live independently and none performs more than its own function. Whether the result is one animal or a colony of many is a question the group forces, and biologists use both descriptions depending on what they are discussing.

Each kind of unit does one job and nothing else:

• •A gas-filled float, in the species that hang at the surface
• •Swimming bells, contracting in coordination to drive the colony
• •Feeding units, with a mouth and a stomach and no tentacle of their own
• •Long tentacle-bearing units that catch prey
• •Protective units forming a shield over the rest
• •Reproductive units producing eggs or sperm

The confusion is universal and the difference matters. A jellyfish is a single animal with one body, whereas this is a colony of specialised units with no single body plan. The float is a gas-filled sac the colony inflates and can partly deflate to submerge briefly, and it carries a crest that acts as a sail, so the animal does not swim at all and goes where the wind takes it, which is why they arrive on beaches in enormous numbers when the wind sets that way. Individuals come in left-handed and right-handed forms that sail in opposite directions relative to the wind, which spreads the population rather than driving all of them ashore together.

The tentacles carry enormous numbers of stinging capsules, each of which fires a coiled thread under pressure when triggered, penetrating skin and delivering venom in well under a millisecond. Contact with a man o' war produces immediate severe pain, welts along the line of contact and, less often, systemic effects. Detached tentacles and animals stranded on a beach remain capable of firing for a long period, which is why beached ones are still dangerous. Rinsing with vinegar prevents unfired capsules discharging in several species, while fresh water triggers them, which is the reason the standard advice is specific about what to use.

The surface-floating species are a small minority and the rest live in open water at depth, where the group reaches remarkable proportions. A specimen recorded off Western Australia in 2020 was estimated at over forty metres long, arranged as a feeding spiral, which would make it among the longest animals ever measured by any definition. These deep species hunt by hanging a curtain of tentacles and waiting, some of them producing light to lure prey, and one genus uses a lure that mimics a small fish with striking accuracy. They are extremely fragile and disintegrate in nets, which is why they were poorly known until remotely operated vehicles allowed them to be filmed intact.

Many units budded from one egg, each developmentally an individual and each specialised for floating, swimming, feeding, catching, defending or reproducing, are joined into one structure sharing a digestive space, and none can live alone. The man o' war is one of these rather than a jellyfish, and it sails rather than swims, with left and right-handed forms going opposite ways. Deep-water species exceed forty metres.