Pyrosomatida
Geographic and vertical distribution

The distribution of Pyrosomatida is still very poorly known. In his monograph on the order, Van Soest (1981) recognized only eight species, two of which (Pyrosomella operculata and Pyrosoma ovatum) have been found on only two or three occasions. A third one (Pyrosoma godeauxi) is known only from its original description from a single locality in the Indian Ocean (Van Soest, 1981). In addition, two other species have been recorded only in Indo-Pacific waters (Pyrosomella verticillata and Pyrosomella operculata). Since Pyrosomella verticillata has been found near Cape of Good Hope (Van Soest, 1981), it is likely to occur in the South Atlantic Ocean along with the other five South Atlantic species (Pyrostremma spinosum, Pyrostremma agassizi, Pyrosoma atlanticum, Pyrosoma aherniosum, and Pyrosoma ovatum). Their records are so scarce that all the available information has been compiled in the following maps: Pyrostremma spinosum M, Pyrostremma agassizi M, Pyrosomella verticillata M, Pyrosoma aherniosum M, Pyrosoma atlanticum M, Pyrosoma ovatum M. The present synopsis includes information from Van Soest (1981) and Godeaux (1977) for the African coasts, and a couple unpublished records from the southwestern Atlantic Ocean.

Pyrosoma atlanticum is the most widespread pyrosomatid species, occurring in all oceans between 50°N and 50°S. Several closely related species (or subspecies, forms, hybrids and intermediate groups) have been described (Lesueur, 1813, 1815; Seeliger, 1895; Brooks, 1906; Neumann, 1909a; Metcalf and Hopkins, 1919). Van Soest (1981), however, based on a thorough study of hundreds of colonies of Pyrosoma atlanticum from all oceans concluded that all these "taxa" are merely misidentifications of damaged or poorly preserved materials, or represent extreme morphological variants of the species.

Pyrostremma spinosum and P. agassizi are widely distributed between 40°N and 45°S in all oceans, but do not reach the Mediterranean Sea. The first species is rarer than the second. Samples often include only fragments of colonies, which can reach giant dimensions (see Introduction). Although the Pyrosomatida are not generally considered neritic, P. spinosum has been repeatedly recorded in neritic areas (Griffin, et al., 1970; Baker, 1971).

Pyrosoma aherniosum occurs in tropical waters of all three oceans and in the West Pacific, between 30°N and 40°S.

Pyrosoma ovatum is a very rare species that has been recorded by only two authors: Neumann (1909b, 1913b) from the South Atlantic Ocean, and Van Soest (1981) from the South Atlantic and tropical Indian Oceans.

Data on the abundance of Pyrosomatida are very scarce. Pyrosoma atlanticum is the only species known to form large swarms (see Introduction), but their occurrence seems to be erratic. Berner (1967) reported that in the California Current region, during the 1949-1958 period, large populations of this species occurred only in 1955 and 1956. Among the highest densities reported for the Atlantic Ocean (northwestern African coasts), Goy (1977) observed 100-2000 colonies per 1000 mö3 in the 0-300 m water column, and Drits et al. (1992), in the same area, reported maximum densities of 9500-41000 colonies per 1000 mö3 in the 0-10 m layer at night.

Most of the data published on their vertical distribution refer to Pyrosoma atlanticum (Franqueville, 1971; Goy, 1977; Van Soest, 1981; Roe et al., 1987; Angel, 1989; Andersen et al., 1992; Andersen and Sardou, 1994). Van Soest (1981) also provides some observations on Pyrostremma spinosum and P. agassizi. Available information indicates that these animals undergo extensive diel vertical migrations, probably exceeding 1000 m. Andersen and Sardou (1994) studied a large population of P. atlanticum in offshore waters of the Ligurian Sea (northwestern Mediterranean Sea). Descriptions of vertical distribution and diel migrations in the 0-965 m water column as a function of their size were based on large numbers of colonies. Migration ranged from 90 m (for 3 mm-long colonies) to 760 m (for 51 mm-long colonies), with a mean amplitude of 410 m for the whole population, all sizes pooled. The migratory cycle showed a clear diurnal symmetry relative to noon. The larger the colonies, the later they reached the upper layers after sunset and, symmetrically, the sooner they left it before sunrise.

Although pyrosomes have been reported frequently from near-bottom samples (Trégouboff, 1956; Hurley and McKnight, 1959; Monniot and Monniot, 1966; Griffin et al., 1970; Baker, 1971), this is probably not their normal habitat (Van Soest, 1981).