The Longest Ocean Creature Is Actually Not A Jellyfish

When you think of the ocean’s largest inhabitants, your mind might instantly leap to images of mighty blue whales or colossal great white sharks. Others might picture the sweeping tentacles of the lion’s mane jellyfish, often deemed one of the longest creatures on Earth.

However, the true titleholder for the longest ocean animal might surprise you—and it certainly isn’t a jellyfish. Meet the siphonophore: a mysterious, deep-sea colonial organism that can stretch across unbelievable distances, challenging our very definition of what a single "animal" is.

What Is A Siphonophore?

Blue whales, with a length, measured up to 110 feet (33.5 m), are the heaviest creatures ever known and among the ocean's longest. The lion’s mane jellyfish, which can stretch its tentacles beyond 120 feet, sometimes rivals that whale length. Yet, siphonophores, surpass both creatures and can measure over 130 feet (40 m.)

Siphonophores are not single animals in the traditional sense; they’re colonial organisms. Each siphonophore is composed of numerous individual units, known as zooids. Although related to jellyfish and corals, siphonophores differ because each zooid within the colony performs specialized tasks—feeding, reproduction, or defense—yet cannot survive independently. Together, these zooids form a larger, integrated "super-organism" drifting through the ocean.

Often mistaken for jellyfish at first glance, siphonophores are delicate, sometimes transparent, and can appear ribbon-like as they move. Their bodies can include a variety of shapes and colors depending on the species. In many instances, they showcase glowing or bioluminescent patches, creating a mesmerizing display in the deep sea. When fully extended, they can become astonishingly long reaching maximum lengths of 130 feet (40 m), sometimes forming spiral or looped shapes that help them capture prey.

You’re unlikely to spot a siphonophore on a casual scuba dive. They typically inhabit open ocean waters, sometimes at great depths where sunlight barely reaches—or doesn’t penetrate at all (at depths between 5,200 to 7,500 feet or 1,600 to 2,300 metres.) Deep-sea submersibles and remote-operated vehicles (ROVs) have provided most of our knowledge about these creatures. Footage from these expeditions reveals how gracefully they float, suspended in the water column.

The Biology Of Siphonophores

One of the most fascinating aspects of siphonophores is the division of labor among their zooids. Each colony contains specialized units:

• Feeding zooids with tentacles that capture prey.
• Reproductive zooids that release eggs and sperm.
• Defensive zooids armed with stinging cells for protection.

Despite being genetically identical, these units have unique forms and roles, functioning as a single integrated lifeform.

Siphonophores are predatory, using long, stinging tentacles laden with nematocysts to ensnare small fish, plankton, and other organisms. Think of them as a drifting net, slowly sweeping through the water column, entrapping any unwary creature that brushes against their filaments. After capture, the prey is passed along the colony’s shared gastrovascular cavity, ensuring that nutrients are distributed among all zooids.

Reproduction in siphonophores is a colony-wide affair. The reproductive zooids release eggs and sperm into the water, where fertilization occurs. This results in larvae that eventually develop into new siphonophore colonies. Interestingly, each new colony is essentially a clone of the parent, maintaining the same genetic code while forming its own specialized zooids over time.

Living in vast stretches of open ocean, siphonophores must capitalize on any feeding opportunity. Their extreme length acts like a biological trawling net, extending their reach to capture prey scattered throughout the water column. This design is an elegant solution to the scarce and unpredictable distribution of food resources in deeper waters.

The colony’s flexible, chain-like architecture also allows siphonophores to contract or expand depending on circumstances, much like an accordion. If food is plentiful, they can extend and form a larger net. During less bountiful times, they may contract to conserve energy. This structural adaptability enhances their survival in the challenging mid-water environment, far from the sea floor and the sunlight above.