Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine animals belonging to the phylum Cnidaria. Although a siphonophore appears to be a single organism, each specimen is actually a colony composed of many individual animals. Most colonies are long, thin, transparent pelagic floaters. Some siphonophores superficially resemble jellyfish. The best known species is the dangerous Portuguese Man o’ War (Physalia physalis). With a body length of 40–50 m (130–160 ft), another species of siphonophore, Praya dubia, is one of the longest animals in the world.
Siphonophores are of special scientific interest because they are composed of medusoid and polypoid zooids that are morphologically and functionally specialized. Each zooid is an individual, but their integration with each other is so strong that the colony attains the character of one large organism. Indeed, most of the zooids are so specialized that they lack the ability to survive on their own. Siphonophorae thus exist at the boundary between colonial and complex multicellular organisms. Also, because multicellular organisms have cells which, like zooids, are specialized and interdependent, siphonophores may provide clues regarding their evolution.
Like other hydrozoans, certain siphonophores can emit light. A siphonophore of the genus Erenna has been discovered at a depth of around 1,600 meters (5,200 ft) off the coast of Monterey, California. The individuals from these colonies are strung together like a feather boa. They prey on small animals using stinging cells. Among the stinging cells are stalks with red glowing ends. The tips twitch back and forth creating a twinkling effect. It is theorized that twinkling red light attracts small fish that have been found eaten by these siphonophores. While many sea animals produce blue and green bioluminescence, this siphonophore was only the second lifeform found to produce a red light (the first being the scaleless dragonfish Chirostomias pliopterus).
Ernst Haeckel described a number of siphonophores, and several plates from his Kunstformen der Natur (1904) depict members of the taxon: