The siphonophore is a deep-sea marine invertebrate known for its incredible length, challenging the definition of a single organism. These animals are typically transparent and gelatinous, drifting through the ocean’s water column. Found in every ocean basin, they are among the longest and most fragile creatures in the world, thriving in cold, dark waters far below the surface. Their unique biology places them among the most unusual life forms on the planet.
The Colonial Structure
A siphonophore is not one animal but a highly organized, intricate colony of smaller, genetically identical units called zooids. These zooids develop asexually from a single fertilized egg, remaining permanently attached to a long, hollow stem. The colony functions as a single body, with each zooid specializing in a different life-sustaining task.
The zooids are structurally unique, resembling either polyps or medusae from other Cnidarians, such as jellyfish. Locomotion is handled by bell-shaped zooids called nectophores, which propel the entire organism through the water using coordinated jet propulsion. Other specialized units include gastrozooids, responsible for feeding and digestion, each equipped with a single tentacle to capture prey.
To maintain buoyancy and orientation, many siphonophores possess a pneumatophore, a gas-filled float located at the anterior end of the colony. This complex arrangement means that no single zooid can survive independently. The entire colony must work together for the whole organism to live, making the siphonophore a “superorganism.”
Measuring the Longest Specimens
Measuring a siphonophore’s length is complicated by its colonial nature and its extremely fragile, gelatinous body. Unlike a single organism, its total length includes its main stem and its vast, trailing curtain of feeding tentacles. The extreme depths and movements of these animals make accurate measurement difficult, often relying on estimates from remotely operated vehicles (ROVs).
One of the longest species is the giant siphonophore, Praya dubia, which forms a long, rope-like chain reaching up to 40 meters (130 feet). This length is greater than that of the largest blue whales. The species is found in the midwater zone, where it drifts like a delicate, transparent ribbon.
A record-breaking specimen belonging to the genus Apolemia was discovered in 2020 off the coast of Western Australia. The colony was observed in a giant, spiral-shaped feeding posture, with the outer ring measuring approximately 47 meters (154 feet) across. Scientists estimated its total length could reach up to 119 meters (390 feet) if fully stretched, making it one of the longest free-living life forms ever documented.
Locomotion and Predation
Siphonophores are active predators in the deep ocean, utilizing a synchronized movement system to hunt. The nectophores contract in a pulsed, coordinated fashion to push water out, generating the jet propulsion that moves the entire colony. Smaller nectophores are often concentrated toward the front, helping to steer and turn the long organism.
Movement is often intermittent, allowing the animal to pause and set its enormous, trailing net. The long, slender tentacles, which extend for many meters behind the main body, function as a massive fishing net. These tentacles are lined with specialized stinging cells called nematocysts.
When prey, such as copepods, small crustaceans, or fish, brush against the tentacles, the nematocysts fire microscopic, venom-injecting harpoons. This paralyzes the victim, which is then reeled in to be consumed by the feeding zooids along the stem. Some siphonophores, like the species Erenna, can also use bioluminescence as a lure, flashing red light to attract or confuse their prey in the dark depths before ensnaring them in their deadly net.