Floating on the currents of the vast open ocean, a long, translucent string looms into view.

Drifting atop the waves on a gas-filled sac, the infamous Portuguese man-o-war trails its deadly tendrils 100 feet long through the water below. With its paralysing tentacles, and gelatinous body, it is often mistaken as a jellyfish. But although it appears to be a single organism, the Portuguese man of war is in fact a colony. It is a siphonophore, not one creature but thousands of different microscopic animals joined together into one super-organism. Siphonophores are rare and peculiar creatures. They are cnidarians, related to corals and anemones.

But what are the creatures that make up these vast colonies? They’re known as zooids, small multicellular organisms that filter feed by catching plankton in the water. Small, thin tentacles dangling beneath each feeding zooid help them to increase their surface area, allowing them to catch more prey.

Individual zooids are tiny, often only a millimetre long. But when they come together, they can be found in immense sizes, with some able to grow to lengths of 130 feet (40 m). That makes them longer than the blue whale, which is often considered Earth's largest animal.

Each zooid in a colony is a clone, but different groups of these clones take care of different functions. The man-of-war is made up of four separate sections called polyps, containing different varieties of zooids each specialised to fulfil a specific purpose in the siphonophore.

The uppermost polyp is its buoyant bladder, known as a pneumatophore. The tentacles are the second polyp, covered in venom-filled nematocysts that can paralyse and kill small creatures. Muscles in its tentacles draw prey up towards a polyp containing digestive organisms, known as gastrozooids, that provide the whole colony with nutrients. This is a vital process in siphonophores, as unlike in most colonial animals, there is an immense degree of specialisation between the zooids. Those that are adapted to one function, lack the structures associated with other functions. Nectophores that propel the colony through the water, can’t eat, and feeding polyps cannot swim.

They are each dependant on the others to do what it cannot do itself, demonstrating the astounding role of symbiosis in allowing these colonies to survive. Furthermore, unlike other colonial animals like ants and penguins, the specialised zooids of a siphonophore are organised in a precise pattern that remains the same from colony to colony, only differing between different species. This shows that despite their simple appearance, siphonophores are incredibly complex organisms, but in an entirely unique way. Whereas most animals consist of specialised cells that are arranged into tissues, organs and systems, siphonophores are made up of specialised zooids, precisely arranged. Understanding how evolution shaped these colonies into such complex organisms, teaches us a great deal about how evolution was able to lead to complex multicellular organisms like ourselves.

All the zooids in a single colony are descended from a single fertilised egg. The egg develops into a protozooid, which is a polyp that gives rise to all the other zooids of the colony through budding.

There is an astounding diversity to be seen among the different varieties of these siphonophores. All siphonophores are predators, using some incredibly efficient methods to capture crustaceans and small fish.

Some, like the Erenna sirena, lure their prey by wiggling glowing bioluminescent lures, much like anglerfish do, and thus they get their name from deadly mythological Sirens, who lure sea-farers towards them with their beauty.

But while siphonophores are long, thing pelagic colonies that drift through the open ocean, there are other kinds of colony organisms to be found in the depths too.

This translucent sock-like structure is a pyrosome. Like siphonophores, it is a colonial tunicate made up of thousands of individual zooids, but their difference lies in their appearance. Their blue-green bioluminescence is much brighter than siphonophores, and they often resemble hollow tubes where they catch and digest their prey in a deadly trap. There is very little specialisation between zooids within pyrosomes. The clones here are identical to one another, but together in a structure of this size, they are an intimidating force. The name pryosome describes their luminescent quality, and stems from the Greek words for fire, “pyro,” and body, or “soma.”

There is another way in which zooids are known to arrange themselves as colonies. This is a Bryozoa, a filter feeding colony that often encrusts surfaces, grows branching structures, or, in freshwater species, can form a gelatinous blob. Unlike siphonophores and pyrosomes, Bryozoa produce a hard calcium carbonate skeleton due to the exoskeletons secreted by individual zooids. This hard material means that Bryozoa are able to be fossilised, and their ancestors show up in the fossil record, dating back 500 million years. In fact, they may be the most abundant fossil on earth. It is likely Bryozoa existed long before this time, but with entirely soft-bodies like siphonophores.