Abstract
Species composition, distribution and biogeography of polychaetes collected from the soft bottoms of the Straits of Magellan (South America) in February-March 1991 are reported. In 16 benthic samples, collected with different tools (Charcot and triangular dredges, van Veen grab), a total of 1132 individuals belonging to 119 taxa of polychaetes were collected; only 49 of the taxa found have been determined at species level. Eighteen species were recorded for the first time in the Straits of Magellan. Species richness was relatively high considering both the number of individuals collected, and the taxa known from previous studies on the area (182 species). The biogeographical analysis, conducted only on the 49 taxa classified at the species level, showed the dominance of Magellan-Antarctic-Subantarctic species (M-An-S 50%), followed by Magellan-American (M-Am 14%), Magellan-Antarctic (M-An 12%), Magellan-Subantarctic (M-S 6%), Cosmopolitans (C 10%), and also by a few species with disjunct distribution (D 8%). Values of the Sorensen similarity index among stations were very low (below 0.30) with a few exceptions. Both number of species and of individuals were higher in the Atlantic sector of the Straits, especially at some stations characterized by heterogeneous mixed sediments (gravels and pebbles) and biogenic debris (mollusc thanatocoenosis). The high number of species recorded, coupled with low similarity values among stations, suggests that the soft bottoms of the Straits of Magellan show, at medium scale, a highly diversified mosaic of different biotopes. This can be due to various factors which are expected to vary along the wide geographic area investigated, such as the wide bathymetric range sampled, the type of sampling gears used, and last but not least the occurrence of many different environmental situations along the Straits.
Highlights
The Straits of Magellan (South America) (53°45’S Lat., 71°30’W Long.) represent one of the most interesting Subantarctic areas from both a biogeographical and an ecological point of view
The relative closeness of this area with the Antarctic continent, from which it is separated through the Scotia Arc, and the continuity with the American continent define the nature of biogeographic crossroads of this zone (Guzmán, 1992)
The material analyzed in this study was obtained during the 2nd Italian oceanographic cruise (February-March 1991) on board the R/V “Cariboo” (Guglielmo, 1991). Samples for both hydrological and benthic studies were collected over a large spatial scale from 24 stations distributed along the whole Straits of Magellan, from the Pacific Ocean mouth to the Atlantic Ocean (Fig. 1)
Summary
The Straits of Magellan (South America) (53°45’S Lat., 71°30’W Long.) represent one of the most interesting Subantarctic areas from both a biogeographical and an ecological point of view. The relative closeness of this area with the Antarctic continent, from which it is separated through the Scotia Arc, and the continuity with the American continent define the nature of biogeographic crossroads of this zone (Guzmán, 1992) This is well documented for both the benthic flora (Skottsberg, 1941) and the fauna (Brattström and Johanssen, 1983; Bastida et al, 1992). The hydrological regime of the Straits is a reflection of the complex geomorphological situation and bottom topography that largely influence sediment features at large and medium scale (Brambati et al, 1991) These climatic, environmental and sedimentological features strongly influence the distribution and ecology of benthic organisms, and have great importance for the biogeographic and ecological comparison of bottom fauna with both the Antarctic and other Subantarctic areas
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