Pyrosomatida
Introduction

Pyrosomatida is an order of holoplanktonic colonial tunicates which are well-known for the production of bioluminescent flashes. According to Metcalf and Hopkins (1919), they are said to be the most brilliantly luminous of all marine organisms. Despite their presence in warm and temperate waters of all oceans, the distribution of species of this order is little-known because of their erratic occurrence in planktonic samples. Only a few species are known, in most cases with sparse records (Van Soest, 1981). Inasmuch as the classification of the Pyrosomatida remained obscure until relatively recently, it is not difficult to understand why the number of specialists on the group still does not exceed the fingers of one hand.

As characteristic for the class Thaliacea, the Pyrosomatida have a reproductive cycle with alternate sexual and asexual generations (e.g., Brien, 1948). The asexual individual or oozooid (called cyathozooid in Pyrosomatida) forms a blastogenetic stolon that produces sexual individuals or blastozooids (or ascidiozooids). The latter remain embedded in a common test, forming a colony that ranges in size from a few centimetres (for example, up to 2.5 by 1.5 cm in Pyrosoma aherniosum, cf. Van Soest, 1981), to over 20 m in length and 1.2 m in diameter (Pyrostremma spinosum, cf. Baker, 1971).

Colonies are tubular, closed at one end and open at the other, with the ascidiozooids distributed along the wall. The oral siphon is directed toward the exterior and the cloacal siphon toward the inner cavity of the tube (longitudinal section, detail of an ascidiozooid). The ventral region of the ascidiozooids is directed toward the closed extreme of the colony. The water, carrying the food particles that are filtered by the zooids, enters the oral siphon, flows through the cloacal siphon toward the interior of the colony, and is finally ejected through the open extremity of the tube, thus producing the retropropulsive movement of the colony.

The ascidiozooids are hermaphroditic, having one testis and one ovary. The ovary produces only a single egg at a time. Eggs are fertilized in situ. Depending on the species, the sexual development is either protandrous, protogynous, or synchronous (Ivanova-Kazas, 1956, 1960, 1961, 1962, 1978; Van Soest, 1979, 1981; Godeaux, 1957, 1990). Two groups of species, treated as subfamilies by Van Soest (1979), can be distinguished on the basis of their embryological development: Pyrostremmatinae and Pyrosomatinae. Pyrosoma atlanticum (Pyrosomatinae), has been the object of studies since the advent of embryology (e.g. Kowalevsky, 1875; Seeliger, 1889; Salensky, 1891, 1892; Korotneff, 1905; Julin, 1912; Neumann, 1913a; Godeaux, 1957; Ivanova-Kazas, 1978). Later, Godeaux (1987) also studied the development of Pyrosomella verticillata, another species of the same subfamily. In both species, the embryo produces a cyathozooid of simplified structure, which in turn produces, by budding, only four primary ascidiozooids (cyathozooid of P. verticillata). In the genus Pyrostremma (Pyrostremmatinae) the cyathozooid has practically all the organs of a normal zooid, but some of them remain histologically undifferentiated (Godeaux, 1990), producing 30-80 primary ascidiozooids (Ivanova-Kazas 1956, 1960, 1978; Van Soest, 1981). In both groups, the ascidiozooids of the primary colony are released from the mother colony, and produce stolons from which secondary ascidiozooids originate by budding. Initially, budding takes place only in the stolons of the primary ascidiozooids. Later on, the stolons of the secondary ascidiozooids begin to proliferate as well, resulting in subsequent growth of the colony. Marking a difference between the two subfamilies, in Pyrosomatinae the secondary ascidiozooids are moved to their definitive location in the colony by special test cells. Conversely, in the Pyrostremmatinae the buds cannot be moved once they separate from the stolon. Finally, these factors determine the particular spatial organization of the ascidiozooids in the colony.

Pyrosoma atlanticum, the commonest species of Pyrosomatida, can form immense swarms (Bary, 1960; Berner, 1967; Goy, 1977; Angel, 1989; Drits et al., 1992). As important consumers of phytoplankton and producers of very large quantities of fecal pellets, they must play an important role in the flux of organic carbon in marine waters. Fecal carbon fluxes per unit biomass appear to be higher in these animals than in salps (Andersen and Sardou, 1994).