Doliolida
Geographic and vertical distribution

The general distribution pattern of doliolids in the South Atlantic Ocean was established by the great expeditions of the end of the last century and the beginning of the present one (Herdman, 1888; Traustedt, 1893; Borgert, 1894; Neumann, 1906, 1913). Garstang (1933), based on materials obtained by the Terra Nova Expedition clarified the systematics of the group, whereas Krüger (1939) reported new information from the Meteor Expedition. Subsequently, the only information available was provided by Van Zyl (1960); Godeaux (1962, 1977), De Decker (1973), Godeaux and Meurice (1978), and Lazarus and Dowler (1979), for the eastern part of the South Atlantic Ocean; and by Tavares (1967) plus some unpublished personal data, for its western part. All this information has been compiled in the following maps:

geographic 1 Doliolina muelleri; Doliolina intermedia; Doliolina sigmoides
geographic 2 Dolioletta gegenbauri; Dolioletta mirabilis; Dolioletta valdiviae
geographic 3 Dolioloides rarum; Doliopsoides meteori; Doliopsis rubescens
geographic 4 Doliolum denticulatum
geographic 5 Doliolum nationalis

Most species are warm-water cosmopolitans, thus tropical zones present the highest specific diversity. Only one species, Doliolina intermedia, is known to have an Antarctic distribution. Neumann (1906, 1913) recorded it in the Antarctic Drift and south of 64°S (as Doliolum resistibile), and Garstang (1933) found it in material from the Terra Nova Expedition at 67°S. This species was also found in the Arctic (Borgert, 1894), and in the Bay of Biscay (Fowler, 1905). Krüger (1939) recorded it (both as D. intermedium and as D. resistibile) from 32°S south in the western Atlantic, always at 400 m or deeper, with the exception of a station off the Argentine coast at about 41°S, where it appeared at depths of 50-100 m. In the eastern South Atlantic Ocean, Krüger (1939) recorded Doliolina intermedia at 15°S at depths between 200-400 m. Godeaux (1977) assigned some nurses with very narrow muscular bands to this species, found off the equatorial coast of western Africa, where they are supposed to occur in deep waters.

Doliolum denticulatum is, no doubt, the most widely distributed species and also the most abundant one. However, its occurrence is irregular, like that of the rest of the species. De Decker (1973) mentioned large swarms off the coasts of South Africa but, in general, data on species densities are not available in the literature. According to Lazarus and Dowler (1979), Doliolum denticulatum proved to be a good indicator of the warm waters of the Agulhas Current, the 15°C isotherm apparently acting as an ecological barrier. In agreement with this observation, Berner and Reid (1961) mentioned that along the Californian coast, D. denticulatum is strongly limited by low temperatures (about 14-15°C). However, Neumann (1913) found this species in the Benguela Current, at 11.8°C. The closely related species Doliolum nationalis is less frequent and also less abundant, showing different ecological preferences. Neumann (1913) did not find it in waters of the Benguela Current. In accordance, Garstang (1933) mentioned that along the western coast of Africa it is abundant in the equatorial zone, but suddenly and completely disappears when the cool waters of the Benguela Current are encountered. On the other hand, D. denticulatum, appearing together with Doliolum nationalis throughout the tropics, bloomed at high densities in the Benguela Current. Godeaux (1973, 1977) reported neritic preferences for Doliolum nationalis and oceanic preferences for D. denticulatum.

Doliolina muelleri is probably the most abundant species after Doliolum denticulatum, also showing a very wide distributional range. Neumann (1913) concluded that its distribution boundary may be determined by the 15°C isotherm; however, Doliolina muelleri var. krohni is the most frequently-occurring form in temperate and cold waters off South America as far south as Tierra del Fuego.

Another very frequent and abundant species is Dolioletta gegenbauri. Its distribution may be similar to that of Doliolina muelleri. Neumann (1906) found it in the Benguela Current, at 12.8°C. Fraser (1955) considered it a cosmopolitan oceanic species, but Tokioka (1960) mentioned a tendency for this species to increase in abundance toward the coasts, which agrees with Paffenhöfer et al. (1991, 1995), who found high densities of this species in coastal areas of the Peninsula of Florida.

The rest of the species have a more erratic occurrence and most of them have never been recorded since the days of the old expeditions. Thus, their distribution ranges are still very poorly known.

The information available on the vertical distribution of doliolids is also very scant. It is generally accepted that they are surface dwellers, preferably in the upper 100 m (e.g. Braconnot, 1970). Using continuous video tape recording from a descending submersible in a coastal area off eastern Florida, Paffenhöfer et al. (1991) reported vertical abundance profiles from the surface to near the sea-floor (30 to 60 m), with a 2 m resolution. The authors observed that concentrations could change dramatically over a range of just 2 m, and recognized two general patterns of vertical distribution: the first one characterized by a peak in abundance at the thermocline, and a second one by a more or less even distribution throughout most of the water column. Doliolids were scarce or absent in the lowermost 10 m and also within the uppermost 10 m of the water column.