Now what are siphonophores? They are Hydrozoans from the Phylum Cnidaria, which include the Anthozoa (sea anemones, corals), Cubozoa (box jellies), and Scyphozoa (jellyfish). You probably already know one, the Portuguese man-of-war, but what you might not know is that they are not strictly ‘single organisms’. Siphonophores are of great interest to scientists because of this and they are better thought of as a collection of different type of units, called zooids, every one genetically identical to the other. Each zooid is highly specialised for a specific function, such as gastrozooids which are responsible for feeding, the gonozooids which perform reproduction tasks, and others may be used for buoyancy and propulsion. Although each of these units is just part of a colony they are so highly specialised that they cannot survive by themselves.
By Bernard DUPONT from FRANCE (Siphonophore) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
Siphonophores can be found all over the oceans with some floating on the surface and others anchored to the sea floor. Their length can vary widely also, from centimetres up to 50 metres (164ft) such as the Praya dubia (the largest blue whale recorded was 33m, or 108ft long). Siphonophores can be defined into three categories, physonects, cystonects and calycophorans, which normally contain a float, nectosome, and a stem. The float provides buoyancy to the colony and the nectosome is covered in medusae which give propulsion, the stem has all the other specialist zooids attached to it.
By US dept of Commerce, NOAA
The most famous species of the siphonophores is the Portuguese man-of-war. They are normally found drifting in the open oceans of the tropical and sub-tropical regions. They consist of a float and three units; dactylozooid for defence, gonozooid for reproduction, and gastrozooids for feeding. Interestingly they do not posses any means of propulsion, relying on ocean currents and the sail of their float to move them across the oceans. The tentacles of this species can range from 10m (33ft) to 50m (164ft). The man-of-war tends to prey on small marine organisms and plankton and despite the potency of their venom they do have some predators, such as the Loggerhead turtle. There are some animals that have developed immunity to the man-of-war’s venom. The Blanket octopus has been recorded wearing their tentacles, possibly using the venom contained within to offer defence against predators. The man-of-war fish also appears to show a partial immunity to the venom as it chooses to make its home among the tentacles.
The other species of note from this order is the Praya dubia. As previously mentioned, it is one of the longest organisms in the ocean, and can be found in the Gulf of Mexico and the Atlantic. Not much is known about them as they live in the water column which makes them hard to find. Adding to this problem is they have a hydrostatic skeleton, so the pressure change of brining them to the surface to study will cause them to burst. Check out these videos of siphonophores in the wild.