|Portuguese Man o' War, Physalia physalis
Siphonophora Eschscholtz, 1829
Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates 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 especially scientifically interesting 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).
Due to their highly specialized colonies, siphonophores have long misled scientists. They were for a long time believed to be a highly distinct group, but now are known to have evolved from simpler colonial hydrozoans similar to Anthomedusae or Leptomedusae. Consequently, they are now united with these in a subclass Leptolinae.
The Siphonophorae have long fascinated scientists and layfolk alike, due to their dramatic appearance as well as the large size and dangerous sting of several species. Compared to their relatives, their systematics are relatively straightforward:
- Family Abylidae
- Family Clausophyidae
- Family Diphyidae
- Family Hippopodiidae
- Family Prayidae
- Family Sphaeronectidae
- Family Agalmatidae
- Family Apolemiidae
- Family Athorybiidae
- Family Erennidae
- Family Forskaliidae
- Family Physophoridae
- Family Pyrostephidae
- Family Rhodaliidae
The genus Stepanyantsia is of unclear affiliations; it might belong in the Agalmatidae.
Haeckel's siphonophores 
- Dunn (2005)
- Haddock et al. (2005)
- MarineSpecies.org (2008)
- Dunn, Casey (2005): Siphonophores. Retrieved 2008-JUL-08.
- Haddock, S.H.; Dunn, C.W.; Pugh, P.R. & Schnitzler, C.E. (2005): Bioluminescent and red-fluorescent lures in a deep-sea siphonophore. Science 309(5732): 263. PMID 16002609 doi:10.1126/science.1110441 (HTML abstract)
- Mapstone, Gillian M. (2009): Siphonophora (Cnidaria, Hydrozoa) of Canadian Pacific waters NRC Research Press, Ottawa ISBN 978-0-660-19843-9
- MarineSpecies.org (2008): Anthomedusae. Retrieved 2008-JUL-08.
- PinkTentacle.com (2008): Siphonophore: Deep-sea superorganism (video). Retrieved 2009-MAY-23.
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