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Siphonophores (from Ancient Greek σίφων (siphōn), meaning "tube" and -φόρος (-phóros), meaning "bearing") are cnidarian animals of the hydrozoan order Siphonophorae.

According to the World Register of Marine Species, the order contains 194 species described thus far. Siphonophores are highly polymorphic and complex organisms, which blur the line between individual organisms ("regular" animals, built from organs and cells) and colonial organisms (like e.g. corals, where each polyp can in principle live on its own). Each specimen is composed of medusoid and polypoid zooids that are morphologically and functionally specialized. Zooids are multicellular units that develop from a single fertilized egg and combine to create functional colonies able to reproduce, digest, float, maintain body positioning, and use jet propulsion to move. Most colonies are long, thin, transparent floaters living in the pelagic zone. Like other hydrozoans, some siphonophores emit light to attract and attack prey. While many sea animals produce blue and green bioluminescence, a siphonophore, Erenna sirena, was only the second life form found to produce a red light (the first being the scaleless dragonfish Chirostomias pliopterus).

Zooids are the multicellular units that build the colonies of a siphonophore. Siphonophores are unique in that the pro-bud initiates the production of diverse zooids with specific functions. The functions and organizations of the zooids in colonies vary widely among the different species, however the majority of colonies are bilaterally arranged with dorsal and ventral sides to the central stem (the stem is the vertical branch in the center of the colony to which the zooids attach).

In general, siphonophore colonies have a modular body plan, with many different zooids making up the overall structure. This adaptation is unique to zooids living in siphonophore colonies.

However, bracts are not present in all species of siphonophore.

Further observations of the siphonophore species Nanomia bijuga indicate that the pneumatophore feature potentially also functions to sense pressure changes and regulate chemotaxis in some species.

A siphonophore is a complex aggregate colony made up of many nectophores, which are clonal individuals that form by budding and are genetically identical.

Generally, the diets of strong swimming siphonophores consist of smaller prey, and the diets of weak swimming siphonophores consist of larger prey. Like other Cnidaria, many siphonophore species exhibit nematocyst stinging capsules on branches of their tentacles called tentilla. There are four types of nematocysts in siphonophore tentilla: heteronemes, haplonemes, desmonemes, and rhopalonemes. Due to the scarcity of food in the deep sea environment, a majority of siphonophore species function in a sit-and-wait tactic for food. Nanomia, among other siphonophore species, can move both forward and backwards to escape a stimulus from the opposite side, using their nectophores for propulsion.

Monoecious species contain male and female gonophores in a single zooid colony, whereas dioecious species harbor male and female gonophores separately in different colonies of zooids. Some siphonophore species within the Calycophorae clade release eudoxids, which are zooid clusters, instead of reproduction through gonophore organs.

Bioluminescent lures are found in many different species of siphonophores, and are used for a variety of reasons.

The discovery rate of siphonophore species was slow in the 18th century, as only four additional species were found. He introduced 46 "new species"; however, his work was heavily critiqued because some of the species that he identified were eventually found not to be siphonophores. On April 6, 2020, the Schmidt Ocean Institute announced the discovery of a giant Apolemia siphonophore in submarine canyons near Ningaloo Coast, measuring 15 m (49 ft) diameter with a ring approximately 47 m (154 ft) long, possibly the largest siphonophore, and longest animal, ever recorded. Another notable recent discovery occurred in early 2025, when a team led by researchers at Yale University, the University of New South Wales, and Griffith University determined that the Portuguese man o'war is actually at least four distinct species.

Quick Facts

• While many sea animals produce blue and green bioluminescence, a siphonophore, Erenna sirena, was only the second life form found to produce a red light (the first being the scaleless dragonfish Chirostomias pliopterus).
• Like other hydrozoans, some siphonophores emit light to attract and attack prey.
• Zooids are multicellular units that develop from a single fertilized egg and combine to create functional colonies able to reproduce, digest, float, maintain body positioning, and use jet propulsion to move.
• Siphonophores are highly polymorphic and complex organisms, which blur the line between individual organisms ("regular" animals, built from organs and cells) and colonial organisms (like e.g. corals, where each polyp can in principle live on its own).
• According to the World Register of Marine Species, the order contains 194 species described thus far.

Source material: Wikipedia - "Siphonophore". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.