Pyrosomida are colonial pelagic tunicates that belong to the Class Thaliacea. The "body" is a permanent hollow tubular, pink or yellowish pink colony, open at one end and closed at the other — the oldest part of the colony. The wall of the tube is gelatinous or cartilaginous and serves as a common tunic or test for the individual zooids, that are arranged with the oral aperture on the outside and the anal aperture on the inside of the tube. There is a constant inflow of water through the wall of the colony and an outflow from the open end of the tube, resulting in a sort of slow but continuous jet propulsion. Sizes of adult colonies of Pyrosomida vary from a few centimetres to twenty metres or even more. The colonies are transparent, usually colourless but sometimes bluish, yellowish or slightly grey in colour. Each zooid has luminous organs on both sides of the pharyngeal aperture; the brilliant phosphorescence was reason for the name Pyrosoma.
The blastozooids (resulting from asexual propagation) are more or less oval in shape, numerous and independent, embedded in the common test. The blastozooids are essentially similar to a solitary ascidian but the buccal siphon lies opposite from the atrial siphon along the body axis. The muscles are weakly developed and consist of branchial and atrial sphincters, additional circumoral muscles, and a pair of transverse cloacal muscles. The blastozooids are hermaphrodite and blastogenic. They take care of the growth of the colony and the propagation of the species. The zooids are only short living and blastogenic transient stages. In this, the alternation of generations is different from that in the other thaliacean orders Salpida and Doliolida.
The alimentary system comprises a buccal cavity, a branchial sac in a branchial chamber, and posterior to the latter is a stomach and an intestine, that is always curved to the left as it leaves the stomach. The intestine opens in the atrial chamber. Every zooid has the ventral endostyle facing the anterior, closed end of the colony; the endostyle extends almost the whole length of the branchial chamber. Around the pharynx are peripharyngeal bands, which unite behind the dorsal nerve ganglion at a broad angle. There is a number of branchial bars in the walls of the branchial sac, supporting the gill slits (or stigmata); this number is of value in the identification of pyrosomal species.
Asexual reproduction occurs by budding from the stolon; the new zooids settle in the colony. Pyrosomas are hermaphrodite. The oldest zooids, at the apex of the colony, are protrandrous, and the youngest zooids, farmost from the apex, are protogynous. The zooids in between have their testes and ovary simultaneously maturing. The eggs grow into rudimentary larvae, the cythozooid. By asexual budding, the cythozooid produces the first larval zooids of a colony, the ascidiozooids. At this stage, the young colony is set free, after which further development of the colony occurs by budding from the stolon of the zooid (Fraser, 1981).
Pyrosomida are primarly found in all tropical and warm waters of the world. Most of the pyrosomids are living in the epi- or mesopelagical; few are living deeper. Regarding their preferences for warmer waters, pyrosomids are rare in the NE Atlantic waters north of 52°N, from where only two species are known: Pyrosoma atlanticum and Pyrosoma spinosum (Fraser, 1981). It is rather unlikely that P. atlanticum will be encountered in the North Sea, though the species has been recorded for this area (see Ihle, 1927).
There is an affinity of the Pyrosomida with the sessile colonial tunicates of the Class Ascidiacea. There is only one family, the Pyrosomidae, with two subfamilies, the Pyrostremmatinae with the single genus Pyrostremma and two species, and the subfamily Pyrosomatinae with two genera Pyrosomella and Pyrosoma, and six species (Godeaux et al., 1998).