Tintinnoinea
Geographic patterns in the Southwestern Atlantic

Absolute abundances of 40 tintinnid species along a transect of surface oceanic waters in subtropical to Antarctic latitudes of the southwestern Atlantic (30°S to 60°S, along 55°W; Alder et al., unpublished) suggest four biogeographic zones (biogeographic zonation) clearly associated with hydrology, but with seasonally and multi-annually variable limits:

—Subtropical Oceanic (Zone 1, north of 30°S).
These assemblages are strongly dominated (up to 46%) by Eutintinnus medius.

—Transitional Oceanic (Zone 2).
The largest of all four biogeographic zones, encompassing 3 subzones: a northern one extending from 34 to 37°S (Subzone 2A); an intermediate one between 37-43°S (Subzone 2B); and a southern one located between 45 and 47°S (Subzone 2C). Species assemblages of the former two are similar, dominated by the genus Dictyocysta (Dictyocysta mitra, Dictyocysta elegans var. speciosa, Dictyocysta elegans var. lepida), and the species Epiplocylis acuminata, Steenstrupiella steenstrupii, Codonellopsis pusilla and Protorhabdonella curta. Subzone 2B differs from 2A by the presence of Acanthostomella norvegica, a typically Arctic species (Davis, 1985, Boltovskoy et al., 1991), which in surface and subsurface waters occurs only between the Antarctic Convergence (around 56° in this study) and 37°S. Subzone 2C defines the boundary between transitional and subantarctic waters; abundant species in subzones 2A and 2B are scarce here, whereas Protorhabdonella simplex becomes the sole dominant species.

Acanthostomella norvegica (A. norvegica) was never reported (under this name) from tropical-subtropical waters. This species is usually considered bipolar, and several hypotheses were offered to explain its disjunct distribution (see Pierce and Turner, 1993). However, our data suggest a different distribution for this taxon. A. norvegica is present in the Arctic Sea, but was never found in typically Antarctic waters south of the Polar Front (=Antarctic Convergence), not even at layers as deep as 1000 m (Alder, 1995). In subantarctic latitudes, as already mentioned, A. norvegica is frequent within the oceanic domain, but only rarely found in neritic waters (see Souto, 1979). Acanthostomella lata Kofoid and Campbell, 1929, which has been reported from tropical and subtropical waters (Kofoid and Campbell, 1929: Tropical Pacific; Balech, 1968: northern Tropical Atlantic; Souto, 1979: southern Tropical Atlantic), is herein considered a form of Acanthostomella norvegica, differences between the two taxa being extremely minor, involving only total length and the degree of development of aboral horns (both traits without diagnostic value). Their distributional patterns, on the other hand, are different and well-defined: Acanthostomella norvegica, forma lata (A. norvegica 3) is restricted to warm waters, while Acanthostomella norvegica, forma typica (A. norvegica, A. norvegica 2) is a typically cold-temperate organism.

—Subantarctic Oceanic (Zone 3).
Extending from ca. 49 to 55-56°S, it is characterized both by the presence of Acanthostomella norvegica and the dominance of Cymatocylis antarctica. The latter is virtually the only species of Cymatocylis (a typically Antarctic genus) whose distribution follows the Circumpolar Antarctic Current (Alder, 1995), although it is also frequently found in subantarctic waters.

—Antarctic Oceanic (Zone 4).
Typical species of this zone are Codonellopsis gaussi (with its formae typica, globosa and cylindroconica), and Laackmanniella naviculaefera, forma naviculaefera. These are extremely polymorphic taxa whose loricae can be either hyaline or covered to a varying extent with foreign particles (mainly diatoms). The degree of agglutination of the loricae seems not to be affected by distance from the coast, but is inversely related to ice-coverage (Alder, 1995).

Comparison of plankton collections from the Southwestern Atlantic with sediment trap materials from the eastern Tropical Atlantic (unpublished data) indicates that tintinnid species assemblages are roughly similar on both sides of the ocean.