Does the Spiral Siphonophore Reign as the Longest Animal in the World?

The creatures of the open ocean are often flashy, in both an abstract and literal sense, thanks to the flickering luminosity of many marine organisms. But maybe more fantastic than glittering squid and glowing, frilled fish are a group of animals that resemble nothing more than globs of gelatin or tangles of twine.

Drifting through the mysterious depths of the water, siphonophores reach surprising lengths, and rule as some of the longest animals in the world.

The siphonophores are an order of organisms in the Cnidaria phylum that look like lumps or spirals of string. Found in all oceans and all ocean depths, these animals are abundant and adaptive. And though these organisms seem like loners as they float through the open ocean, a single siphonophore is actually composed of thousands of tiny individuals — called zooids — that are chained together to form functional colonies.

All of the zooids in a specific siphonophore colony share the same genes and genetically identical individuals, though they are morphologically specialized to serve separate functions. Some protect the colony, and some propel the colony through the ocean. Others catch or process prey or keep the colony afloat. In fact, these zooids are so fine-tuned for their specific tasks that they are often unable to survive on their own.

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This siphonophore group contains creatures that grow to lengths of over 150 feet, making them some of the longest animals alive today.

Scientists say that a single zooid (formed from a single fertilized zygote) starts the development of a siphonophore colony through the process of budding. In this process, new zooids are developed from small, detached segments, or buds, that shoot off the bodies of older zooids.

And because all zooids in a specific siphonophore colony are cloned from older zooids in the colony, they are all made to match — though mutations can change their genetic makeup, specializing their form and function.

Linked together, these zooids act as a single, lethal individual. While the zooids in charge of movement maneuver the siphonophore into the perfect position for finding food, the zooids in charge of predation are armed with fluorescent tentacles that twitch and flash to attract prey.

These predatory zooids are also capable of stinging and capturing the creatures that they attract, fueling the colony with a diverse diet of crustaceans and fish.

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Scientists have identified over 175 species of siphonophores, which come in a variety of shapes and sizes that are suited for a variety of oceanic zones. That said, the majority of siphonophores stick to the open ocean, with longer siphonophores drifting in deeper waters than shorter siphonophores, since powerful currents close to the surface can shred siphonophore colonies apart.

If they manage to stay together, these longer siphonophores can reach remarkable lengths. In fact, several members of the Praya and Apolemia genuses are among the longest animals in the world, including the giant siphonophore, the woolly siphonophore, and a spiraling specimen of siphonophore from the Apolemia grouping.

Giant siphonophores arrange themselves in long stem structures. These structures are composed of chains of several sorts of zooids, including transparent, tentacled zooids that are specialized for snatching food, and they are attached to globular, gelatinous floats filled with gas. These floats allow the colonies to drift at their desired depth, and attached to these floats are pairs of pulsing swimming bells that propel the colonies through the sea.

• Size: Length up to 130 feet long

• Diet: Small crustaceans, small fish and larvae

• Depth: Open ocean (between about 2,300 and 3,300 feet below the surface)

• Year of Discovery: 1827

The woolly siphonophores arrange themselves in much the same structure as the giant siphonophores, with both species sharing a similar float and a similar stem. But woolly siphonophores tend to be a bit shorter, and their tentacled fringe tends to be a bit fuller.

• Size: Length up to 50 feet

• Diet: Small crustaceans and larvae

• Depth: Open ocean (between about 0 and 13,000 feet below the surface)

• Year of Discovery: 2013

In 2020, a team of researchers from the Schmidt Ocean Institute spotted a startlingly long, siphonophore of an unknown species in the genus Apolemia off the coast of Western Australia. Capturing footage of the creature with a underwater robot, the team announced that the siphonophore's spiraling stem was approximately 150 feet in length, making it the longest siphonophore — and seemingly longest animal — to be discovered and described.

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Of course, the siphonophores aren't the only large creatures in the ocean, and they aren't the only large colonial creatures, either. Corals are colonial animals, formed from thousands of tiny zooids, and so, too, are pyrosomes — cylindrical or cone-shaped colonies up to 60 feet in length.

But unlike the siphonophores and their zooids, pryosomes are always arranged in similar, structures, and their zooids always perform the same, shared functions, sucking water in and spewing water out to filter food and push the colony forward. As such, pryosomes represent a simple sort of colony, unlike the siphonophores, which take "coloniality to an unparalleled extreme," according to a Current Biology article by biologist Casey Dunn.

Ultimately, it is not only the length of the siphonophores that makes them so special, but the complexity and connectivity of their lives. "Siphonophores challenge us to think about what we mean when we call something an individual," writes Dunn, who specializes in siphonophores. "Is a single zooid or an entire colony the siphonophore 'individual?'"

The answer, it seems, is complicated and long, just like the siphonophores themselves.

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