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).
Conservation and Management
Pyrosoma are not known to face any special population threats.
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).
Pyrosoma are hermaphroditic, with each zooid producing both eggs and sperm. The fertilized egg gives rise to an embryo that develops into four attached zooids which subsequently reproduce asexually by budding off new zooids (Brooks 1906). This budding process is responsible for the growth of the colony. Thus, the life history of Pyrosoma includes both sexual and asexual phases.
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 may migrate hundreds of vertical meters each day. A study of Pyrosoma atlanticum occurring in offshore waters of the Ligurian Sea (Northwestern Mediterranean) in April of 1991 found that daytime depths and amplitudes of the diurnal migration were correlated with colony size. The amplitude of the migration ranged from 90 m for 3-mm-length colonies to 760 m for 51-mm-length colonies, with a mean amplitude of 410 m for the overall population pooled (Andersen and Sardou 1994). In the same study, the results of horizontal hauls at a given depth around sunrise and sunset revealed a marked diurnal symmetry of the migratory cycle relative to noon, and showed that migration of the population was not cohesive. For example, the larger the colonies, the later after sunset they reached the upper layers during their upward migration.