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Pyrosoma is a genus of colonial, pelagic (open-ocean) tunicates. Colony size ranges from less than a centimeter to several meters in length. Each colony forms a transparent tube, closed at one end and open at the other, that is composed of hundreds or even thousands of outward-facing individuals (or zooids). These tiny zooids, each just millimeters long, are joined together by a gelatinous tunic. Water is drawn into each zooid through an oral siphon by beating cilia, creating a feeding current. Plankton are filtered out of the water and the depleted water is then expelled into the interior of the colony and out the posterior opening. This flow of water not only facilitates food acquisition, but also allows the colony to move by graceful jet propulsion, although Pyrosoma are mainly planktonic (passively free-floating).
Pyrosomes are planktonic, which means that their movements are largely controlled by currents, tides and waves in the oceans. On a smaller scale, however, each colony can move itself slowly by the process of jet propulsion, created by the coordinated beating of cilia in the branchial baskets of all the zooids, which also create feeding currents.
A striking feature of Pyrosoma tunicates is their dramatic bioluminescence, which is visible for several meters underwater and appears in waves within the colony as flashing by individual zooids is triggered by flashes from their neighbors. Flashing can also be triggered by physical disturbance. When disturbed, individual zooids protect themselves by closing off their oral (intake) siphons, stopping the beating of their cilia, and emitting a flash of light. Neighbouring zooids detect the flash with their photoreceptors and respond in turn with protective responses and light emission. Protective responses thus spread by photic signalling and propagate from zooid to zooid through the colony (Mackie 1995).
Pyrosomes, genus Pyrosoma, are free-floating colonial tunicates that live usually in the upper layers of the open ocean in warm seas, although some may be found at greater depths.
Most Pyrosoma species are tropical. Unlike most tunicates, which are benthic (bottom-dwelling) and sessile (fixed in one place) as adults, Pyrosoma are pelagic at all life history stages, floating freely in the open ocean, sometimes in enormous numbers. One recent study off the coast of West Africa (Lebrato and Jones 2009) suggests that Pyrosoma tunicates that die and sink quickly to the bottom of the ocean may represent a major food resource for both benthic microbes and larger benthic organisms in the deep sea and should be included in models of large-scale cycling of elements such as carbon.
Pyrosoma are not known to face any special population threats.
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