Salpida
Geographic and vertical distribution

Salps are important members of zooplankton communities in the open ocean, where they are widely, but sparsely, distributed (Van Soest, 1975c; Kashkina, 1978). Most species do not occur in neritic environments, but there are exceptions such as Brooksia rostrata, Cyclosalpa floridana (cf. Meurice et al., 1984), and species of the genus Thalia, especially Thalia democratica (Atkinson et al., 1978; Heron and Benham, 1984; Deibel, 1985b; Paffenhöfer et al., 1991, 1995; Madin, 1995).

The patchy distribution of salps makes them difficult to sample quantitatively; therefore, the sparse data on densities are, in general, hardly comparable. Kashkina (1978) summarized trawl data from the North and South Atlantic (without identifying species), reporting concentrations of salps ranging from 0 to near 2000 individuals per 104 mö3. Population blooms occur sporadically, even in oligotrophic waters (Madin et al., 1996). Obviously, salps are most conspicuous during swarming periods, when they may cover large areas of low- or high-latitude oceans at densities of hundreds per mö3 (Thompson, 1942; Foxton, 1966; Berner, 1967; Brattström, 1972; Wiebe et al., 1979; Le Borgne, 1983; Casareto and Nemoto, 1986; Lancraft et al., 1989; Huntley et al., 1989).

Most of the species are warm-water cosmopolites, but several seem to have a more restricted distribution, as they are restricted to tropical areas of the Indian and Pacific Oceans (Van Soest, 1975b). Among these, Helicosalpa younti, a very rare species, was known only from two solitary zooids (Yount, 1954; Kashkina, 1973) until a recent finding off northern Chile, which led to the description of aggregate zooids, and a complete description of solitary zooids (Esnal et al., in press).

Another species, T. sibogae, was described by Van Soest (1973a) from a very rich sample from the type locality (5°N, 120°E, Celebes Sea) and from a specimen captured in the Philippine Archipelago by the Albatross Expedition (1908). The most recent record also comes from the same geographical area (Esnal et al., 1993; Daponte et al., 1996). Cyclosalpa strongylenteron exhibits an even more restricted distribution, it was found only at two sites in the tropical zone of the Eastern Pacific Ocean (Berner, 1955; Esnal, 1976).

Two species are restricted to the Antarctic Ocean: Ihlea racovitzai, with a circumpolar distribution (Foxton, 1971, Esnal and Daponte, 1990); and Salpa gerlachei, probably limited to the Pacific sector (Foxton, 1961, 1966). Some authors suggest, however, that the latter species may be merely an extreme clinal variant of S. thompsoni, since the forms differ only biometrically and intermediate morphotypes have been found (Caldwell, 1966; Esnal, 1970c; Van Soest, 1974b, 1975b; Casareto and Nemoto, 1987).

The first data on the horizontal and vertical distribution of salps in the South Atlantic Ocean was derived from early expeditions, such as the "Challenger" Expedition (Herdman, 1888), the Plankton Expedition (Traustedt, 1893; Apstein, 1894), the Tiefsee Expedition (Apstein, 1906a), the Deutsche Südpolar Expedition (Apstein, 1906b), and the "Meteor" Expedition (Krüger, 1939). Later studies covered either broad areas (Foxton, 1961, 1966, 1971; Van Soest, 1972, 1973a, b, 1974a, b, 1975a, b; Esnal, 1978, 1981a), or more restricted ones (Van Zyl, 1959; Godeaux, 1962, 1969, 1977; Amor, 1966a, b; Caldwell, 1966; Tavares, 1967; Esnal, 1968, 1970a, b, 1975; Godeaux and Goffinet, 1968; Meurice, 1970, 1974; De Decker, 1973; Esnal and Castro, 1974; Godeaux and Meurice, 1978; Lazarus and Dowler, 1979; Meurice et al., 1984; Daponte et al., 1993).

All the available information has been compiled in Sal-table, which shows the latitudinal distribution of the species recorded from the South Atlantic Ocean to date.

Iasis zonaria, Salpa fusiformis, Thalia democratica, Salpa aspera, and Pegea confoederata are the most widely distributed species, ranging to 50°S. However, only the first three species are frequent, occasionally abundant. The latter two, on the other hand, occur more erratically. Pegea socia, which has obviously been confused in the literature with Pegea confoederata, was recently found in the South Atlantic (Daponte et al. 1993).

Another species group: Weelia cylindrica, Brooksia rostrata, Thalia cicar, Thalia orientalis, Salpa maxima, Salpa younti, Ritteriella retracta, Pegea bicaudata, Ihlea punctata, and Thetys vagina, does not range south of 40°S. Only the first four species can be considered frequent.

Both groups present therefore, a tropical-subtropical distribution pattern.

A third species group: Cyclosalpa bakeri, Cyclosalpa polae, Cyclosalpa affinis, Cyclosalpa danae, Cyclosalpa floridana, Cyclosalpa pinnata, Helicosalpa virgula, Ritteriella amboinensis, and Traustedtia multitentaculata, limited to warm waters, does not extend beyond 20°S. The first two species are the most frequent. Ritteriella amboinensis is included here, although Apstein's (1906a) records from the Gulf of Guinea (included in the distribution chart of Meurice, 1970) are doubtful (Van Soest, 1974b).

The rest of the species are associated with cold waters. Salpa thompsoni is the most frequent and abundant species in Antarctic and subantarctic waters; Ihlea magalhanica occurs from north of the Subtropical Convergence southward to the Antarctic Convergence; and Thalia longicauda is associated with the Malvinas (=Falkland) and Benguela currents.

As to their vertical distribution, salps are most frequent in the upper strata. However, there is evidence indicating that some species perform diel vertical migrations over considerable distances (at least 800 m for Salpa aspera, according to Wiebe et al., 1979).

While diving near Bermuda, Madin et al. (1996) observed salps in the top 30 m during non-swarm conditions, noting that various species migrate vertically in and out of the surface layer at different, non-overlapping, times of day. These patterns are consistent with the hypothesis that vertical migration serves to concentrate the salps near the surface, where spawning occurs (Purcell and Madin, 1991). However, not all the species are migratory; Gibbons (1997a), for example, observed that Thalia democratica did not undergo diel vertical migrations over the Agulhas Bank, South Africa; and Madin et al. (1996) reported that only about half of over 16 species found at Bermuda appear to be migratory. These authors believe that the different migratory patterns reflect behavioral differences acting as isolating mechanisms among species.