Hydromedusae
Geographic distribution

In total, 197 species of hydromedusae have been listed from the South Atlantic, defined as 0° to 60° latitude, coast to coast. Of these, 149 are meropelagic (neritic, epipelagic, or slope species), and 48 are holopelagic. The distribution of species among higher taxa (shown in Hydromedusae table 1 ) is rather different from that observed in other seas, like the Mediterranean and the Indo-Pacific (Bismarck Sea). Comparison of the three areas shows that while the total number of species is very similar, the number of Anthomedusae is always higher than that of Leptomedusae, the number of Narcomedusae is highest in the Mediterranean (10.4%), and the numbers of Limnomedusae and Trachymedusae are proportionally highest in the South Atlantic (7.6 and 14.3%, respectively).

Other comparisons indicate that: (1) 131 (66.5%) of the South Atlantic hydromedusae are also found in the North Atlantic, 11 of them being restricted to the Atlantic; (2) 85 (43.1 %) of the South Atlantic hydromedusae are also found in the Mediterranean, and only 1 (Amphinema rubra) is restricted to these two regions; (3) 125 (63.5%) of the South Atlantic hydromedusae are also found in Indo-Pacific waters, and 16 of them are restricted to these two areas; (4) 68 (34.5 %) are found in Atlantic, Mediterranean and Indo-Pacific waters, and may thus be considered cosmopolitan; (5) 50 (25%) species are found in Subantarctic or Antarctic waters; 41 of these are also present in other sectors; 6 are endemic to the Subantarctic area, 3 are endemic to the Antarctic; and 6 are bipolar; (6) 28 (14.2%) of the total number of species are endemic to the South Atlantic.

These results show that, although there are great similarities between the hydromedusae of the Northern and Southern Atlantic, the South Atlantic shares as many species with the Indo-Pacific fauna. All South Atlantic species found in the Mediterranean are also present in the North Atlantic, in the Indo-Pacific, or in both areas, except for one (Amphinema rubra), which is present only in the first two regions. The South Atlantic fauna contains also a rather large Subantarctic and Antarctic component, many of these species being epipelagic here and dwelling in deeper waters at lower latitudes.

The eastern South Atlantic (West African coasts) hosts 109 species, the Western Atlantic (South American coasts) 141 (135 + 6 Subantarctic endemics). Forty-five (41.2%) of the Eastern Atlantic medusae are not known from the western part, and 77 (54.6%) species of the western fauna have not been found in eastern waters; 65 (33%) of the total number of medusae are common to the two areas, 2 of them (Haliscera alba and Sarsia gracilis) are endemic to the South Atlantic. Among the 45 species found only in the eastern area, 20 (44%) are present in the North Atlantic; 14 (31%) in the Mediterranean; 22 (48%) in the Indo-Pacific; 6 (13%) in the three areas, and 8 (18%) are endemic to this area. Among the 77 species found in the western waters only, 55 (71.4%) also inhabit the North Atlantic; 29 (37.6%) the Mediterranean; 40 (52%) the Indo-Pacific; 16 (20.7 %) are found in the three areas, and 17 (22%) are endemic to the western area. The western South Atlantic medusa fauna seems thus to have more affinities with the North Atlantic fauna, and to include more endemics than the eastern one.

The general endemism of the South Atlantic medusa fauna (14.2%) is lower than, for example, that of the Mediterranean (18%), which is one of the most thoroughly studied areas in the world. The endemic medusa fauna of the South Atlantic certainly contains many false endemics. Among the 8 eastern South Atlantic "endemic" species for instance, 7 have been described only recently (Pagès et al., 1991, 1992), and it is very doubtful that they indeed are restricted to this area. Endemism often reflects either the scarcity and dispersion of observations, many of the species having been reported only once; or is linked to incomplete knowledge of the species distribution. Furthermore, in some cases it may also be attributable to the questionable validity of some species, resulting from inadequate descriptions (e.g., Haliscera alba, Euphysa monotentaculata, Irenium teuscheri, Solmissus atlantica, etc.).

For distributional and ecological information about the Hydromedusae treated in this chapter, open the files "Hydromedusae_Distribution" and "Hydromedusae_Ecologic setting" files in the IdentifyIt module.

In the western South Atlantic the relationships between hydromedusan distribution patterns and local water masses have been investigated by Vannucci (1957b, 1963) and Goy (1979). Navas-Pereira (1981) studied the distribution of the hydromedusae of the continental platform of Rio Grande do Sul (Brazil). Zamponi and Genzano (1994) analysed the diversity, seasonality, and abundance of the hydromedusae from Samborombón Bay (Argentina). They observed a marked seasonality in the distribution, with peak densities in the summer months (about 100 ind. mö-3), and another important maximum in the autumn (>40 ind. mö-3).

Pagès et al. (1994) studied the composition and distribution of macro- and megaplanktonic hydromedusae in the eastern part of the Weddell gyre (55°35.2' to 66°53.8'S) between 0 and 2000 m. Fourteen hydromedusae were identified, their abundance varying from 0.21 to 30 specimens per 104 mö3, the most abundant being Pantachogon haeckeli. They found high diversity at bathypelagic depths, but medusae were most abundant at mesopelagic depths, reaching 46 specimens per 104 mö3 in the 500-1000 m range. Farther south, Pagès and Kurbjeweit (1994) studied the abundance and distribution of mesoplanktonic medusae from the Weddell Sea (Antarctic Atlantic sector, about 60°30' to 71°S), reporting 8 hydromedusae, among which Pantachogon scotti (to 11.671 ind. 1000 mö-3) and Arctapodema ampla (to 960 specimens 1000 mö-3) were the most abundant.

The eastern South Atlantic has been investigated much less than the western part, except for the south-eastern sector, which is one of most productive areas in the world and where the distribution and ecology of hydromedusae have been studied by Pagès (1992), Pagès et al. (1991), and Pagès and Gili (1991, 1992a, b, c).

Navas-Pereira and Vannucci (1994) analysed the ecological preferences of the Subantarctic and Antarctic hydromedusae for different water masses (between 20°W in the Atlantic region and 145°E in the Pacific sector of the Antarctic). They confirmed earlier observations showing that eurybathic, but stenohaline and/or stenothermal, species may often be found at the surface in the Antarctic Ocean and at great depths at equatorial latitudes or even north of the Equator; and that in the Southern Ocean the numbers of both individuals and species of holoplanktonic forms exceed those of meroplanktonic ones. They also confirmed the usefulness of hydromedusae as water mass indicators.

Most literature data confirm that on the average the Subantarctic and Antarctic fauna is rather poor in Leptomedusae (15,7 %) compared to Anthomedusae (23.5%), and that the Laingiomedusae and Limnomedusae seem to be absent from Antarctic waters. As mentioned above, holoplanktonic forms are the most abundant in Subantarctic and Antarctic waters, representing 60.8 % of all species, 22.3% for the Narcomedusae and 38.5% for the Trachymedusae, apparently the most characteristic groups in those areas.