Euphausiacea
Zoogeography

Previous biogeographies of the South Atlantic Ocean using euphausiids alone have been confined to the waters off South Africa (Barange et al., 1992; Gibbons et al., 1995), although biogeographies that have included euphausiids amongst other zooplankton taxa have been constructed for the SW Atlantic (Dadon and Boltovskoy, 1982; Boltovskoy, 1986; Tarling et al., 1995). Basin-wide studies have not been attempted until now, largely because of the poor areal coverage that the region has enjoyed, and the inconsistent manner in which material has been collected. Having said that, preliminary results of one such study by Gibbons (1997) are presented below. It should be noted at the outset that this analysis is based on presence:absence records from a scanty literature and comment is confined to obvious general patterns: specifics are dealt with only for regions with good areal coverage.

There are two broad assemblages of euphausiids in the South Atlantic; one associated with tropical/sub-tropical (to approximately 25°S) and temperate waters (to approximately 50°S), and the other associated with sub-Antarctic (to 65°S) and Antarctic waters. Similarities between the two broad groupings are obviously small (<10%). The temperate assemblages are further split into those with warm or cold affinities around latitude 35°S, although distinctions between the two are often not clear in the region of overlap. Major borders between regions are thus latitudinal and generally reflect the horizontal distribution of the major water masses. "Truly" Antarctic assemblages, i.e. those restricted to neritic areas south of 65ºS, are defined by Euphausia crystallorophias (rather than the more widespread E. superba). East-west differences in assemblages that reflect local circulation (e.g. Benguela and Brazil Currents) are not clear from this analysis. Having said that, it can be seen that tropical, and warm and cold temperate assemblages extend further south in the western South Atlantic than the east. Likewise, cold and warm temperate assemblages, as well as Subantarctic assemblages, extend further north up the eastern than western boundaries to the region. This clearly reflects the influence of the South Atlantic Gyre, although distinct assemblages associated with its core are missing. Schem. map of zoogeographic. To construct the schematic map of zoogeographic provinces, the distribution records of all euphausiid species from the literature were plotted by scoring their presence:absence in each 5° grid square:

Tables 3a, 3b and 3c. Distribution of euphausiid records amongst the zoogeographic provinces identified in

Schem. map of zoogeographic:
Eu table 3a
Eu table 3b
Eu table 3c
Sources tables 3a, 3b, 3c[/textfile][/l]

In order to minimise the effects of differential areal coverage, gaps in the [l][m]Glossary[/m][r]distribution[/r]distribution record of some species were filled in. This interpolation was confined to grid squares adjoining presence records (as Gibbons et al., 1995). To further reduce bias in the data set, rare species (e.g. Euphausia sibogae) or uncommon species with disjunct distributions (e.g. some members of the genus Thysanopoda) were discarded as uninformative; as were grid squares with less than 3 species. Similarity (Bary-Curtis Index) amongst grid squares was then calculated using Plymouth Routines in Multivariate Ecological Research software and the classification analyses plotted. Provinces are delimited at the 55% similarity level.

Assemblages associated with, for example, the Agulhas Current are not immediately distinct from other warm temperate areas and only become demarcated from the remainder at the 55% level of similarity, despite the fact that it has a number of unique species (see tables 1a and 1b below). This absence of distinct assemblages at the level of particular current systems reflects in part the influence of the larger number of species which are shared with other assemblages, and part the non-quantitative way in which the samples were analysed. This is particularly clear in, for example, the case of the Falklands (=Malvinas) Current where assemblages differ only in numerical composition from those of regions further south (Tarling et al., 1995).Thus, while congruence between the zoogeographic provinces identified here and those put forward by previous authors as part of larger, global biogeographies (e.g. van der Spoel and Heyman, 1983) is generally good, it breaks down at the biogeochemical level (e.g. Longhurst, 1995).

Eu table 1a
Eu table 1b
[l][textfile]extended_legend_tables_1a_1.txtExtended legend tables 1a, 1b[/textfile][/l]