Echinoderms are benthic marine invertebrates widely distributed throughout the world ocean. In southern South America, this group was early studied by L. Feuillée at the beginning of 1770 (Larraín 1995); during the following centuries, most information from the Atlantic Ocean was produced on the basis of material collected by HMS Challenger (1873-1876) and RV Discovery (1925-1936) (Mortensen 1936, Fisher 1940). The early work on taxonomy and biology of echinoderms of the Argentine Sea are contributions about echinoids, asteroids and ophiuroids of southern South America (Bernasconi 1947, 1964a,b,c, Bernasconi and D'Agostino 1977). More recently, inventories have been developed in north Patagonian gulfs (Zaixso and Lizarralde 2000) and in the Straits of Magellan (Larraín et al. 1999, Mutschke and Ríos 2006), which provide lists of species of echinoderms collected in particular environments. Other contributions have shown the distribution patterns of the most conspicuous species in large Atlantic shelf areas, between 26 and 38°S (Tommasi et al. 1988a,b), and along the shelf break frontal area, between 36 and 43°S (Escolar 2010).

The assessment of biodiversity in terms of species richness in marine systems is important to understand the ecological patterns of species distribution as well as the functioning of ecosystems, and to manage the use of marine resources and the identification of priorities for conservation (Gray 2001). Anthropogenic impacts and the need for systematic conservation planning have prompted further analyses of the patterns of diversity (Worm et al. 2006).

The current zoogeographic scheme established for the southwestern Atlantic between 34 and 56°S, with the Argentine and Magellan Provinces (Balech 1954), has been confirmed for various groups of invertebrates in recent work on amphipods (López Gappa et al. 2006), hydroids (Genzano et al. 2009) and polychaetes (Bremec et al. 2010a). In the case of echinoderms, a great biodiversity is found in the Argentine Sea. Most echinoderm species are distributed from southern Brazil and Uruguay to the Province of Buenos Aires in Argentina, or belong to the sub-Antarctic fauna that can reach southern Uruguay (Brogger et al. 2013). However, knowledge of the taxonomy, ecology and biogeography of echinoderms on the Argentinean continental shelf is still incomplete.

The aim of this study was to compile and analyse available historical information on echinoderms in the southwestern Atlantic, in order to make a synthesis of present taxonomical knowledge and to identify patterns of geographical distribution. A database with geo-referenced records of echinoderm species that covers the Argentinean and Uruguayan continental shelves was used for the first time to test the validity of the current zoogeographical scheme.

A total of 110 species of echinoderms distributed in 5 classes, 16 orders and 37 families were recorded in the study area (Appendix 1) according to the information available in 36 taxonomic and other published works up to 2005 (Appendix 2). Twenty species were not considered because of inaccurate locations, presence in only one location or only one report. Therefore, a matrix of 152 squares for 90 species was used in the analysis. The geographical coverage of sampling fully covers the study area, but there were areas with higher sampling intensity such as the coast of Uruguay, Buenos Aires, Chubut, Tierra del Fuego and the Malvinas Islands (Fig. 2).

The class Asteroidea presented the highest species richness (47 species) representing over 50% of the total; the species richness in Ophiuroidea, Echinoidea and Holothuroidea was 18, 11 and 14 species, respectively.

We analysed the presence and distribution of 110 species of echinoderms, distributed in 5 classes, 16 orders and 37 families in the southwestern Atlantic. However, our analyses were performed with the most frequent species (N=90), in agreement with Brogger et al. (2013) in a recent contribution. It is interesting to point out that Crinoidea Antedonidae were early reported by Mortensen (1917, 1920) and Bremec et al. (2010b) on the Argentinean slope, but excluded in this study due to their scarcity.

A biased distribution of the sampling effort occurred in the study area, a fact already reported for other groups of benthic invertebrates in the Argentine Sea (López Gappa 2000, López Gappa and Landoni 2005, López Gappa et al. 2006). Although geographic coverage of historical sampling is wide, certain coastal areas have been sampled more intensively than others; this was the case of coastal bottoms of Uruguay, Buenos Aires, Chubut, Tierra del Fuego and the Malvinas Islands, where the number of species found could be a good estimation of species richness.

The latitudinal gradient in species richness is largely documented in both terrestrial and marine environments (Brown and Lomolino 1998). The most clearly observed pattern occurs in the northern hemisphere, with the highest richness in the tropics and decreasing towards the polar regions (Roy et al. 1998, Crame 2000, Hillebrand 2004). In the southern hemisphere, there is no clear evidence of any increase in species richness from Antarctica towards the Equator (Crame 2000, Valdovinos et al. 2003, Barnes and Griffiths 2008). The results of this study indicated that species richness of echinoderms in the southwestern Atlantic increases significantly with latitude (between 34 and 56°S); the highest species richness was observed between 46 and 56°S in the Argentine Sea. A similar pattern was observed for Bryozoa (López Gappa and Lichtschein 1988, López Gappa 2000), Porifera (López Gappa and Landoni 2005) and Amphipoda (López Gappa et al. 2006) in the southwestern Atlantic and for Mollusca (Valdovinos et al. 2003) and Polychaeta (Lancellotti and Vásquez 2000; Hernández et al. 2005) along the southeast Pacific coast. Some authors postulate that the main component that generates this asymmetry of species richness in the latitudinal pattern, in comparison with the northern hemisphere, is the high species richness in the Antarctic Region (Griffiths et al. 2009). In particular, echinoderms are well represented on both sides of the Drake Passage (Arntz et al. 2005). Antarctica is considered a “hot-spot” in terms of echinoderm diversity (O’Loughlin et al. 2011). In addition, the Antarctic Region is the centre of origin and radiation of various taxa; many species that originated in the region have been able to migrate to cold temperate waters surrounding the sub-Antarctic region (Briggs 2006).

Specific composition of echinoderms changed through the studied bathymetric gradient, the most noticeable change being registered at depths greater than 100 m.

The bathymetric distribution patterns of echinoderms have been explained by physical factors (pressure, temperature, dissolved oxygen and sediment quality) and biological factors (mode of larval dispersal, predation and intra-and inter-specific competition) (Sokolova 1972, Gage and Tyler 1982, Ventura and Fernandes 1995), which could be modified through the bathymetric gradient. According to Iken et al. (2010), the echinoderm associations could be structured by different variables; a complex framework is generated and no single variable could explain the observed patterns. In our study area, the bathymetric gradient coincides with a water temperature gradient: we found shallow and warm waters in coastal areas, which became deeper and colder as we moved forward to the shelf break. Water temperature is considered the main limiting factor in the distribution of marine species (Stuardo 1964, Vannucci 1964, Menni et al. 2010, Okolodkov 2010) and has been the basis of many discussions on the boundaries between biogeographic provinces in the southwestern Atlantic (Ekman 1953, Boltovskoy 1964). The subtropical/sub-Antarctic convergence develops into the Argentinean Province; this mass of water is the product of the mixture of subtropical waters coming from the north transported by the Brazil Current and the sub-Antarctic waters arriving from the south carried by the Malvinas Current (Boltovskoy 1981, Acha et al. 2004).

Changes in the benthic faunal composition at depths greater than 100 m on the Argentinean Continental Shelf have been reported by other authors (Bastida et al. 1992, Escolar et al. 2013). These changes were explained in terms of the high productivity of the shelf break frontal system in the area, which is produced by the meeting of the sub-Antartic shelf waters and the cooler and more productive waters of the Malvinas Current (Acha et al. 2004).

The inventory and analysis of historical information about echinoderms conducted in this paper constitutes the first attempt to validate the preliminary biogeographical observations (see Bernasconi 1964c) and confirms the two main zoogeographic divisions of the study area, the Argentinean and Magellan Provinces (Balech 1954). The results of this study showed that the association of squares that represented the Argentinean Province was characterized by widely distributed species: there are subtropical (Asterina stellifera, Encope emarginata, Astropecten b. brasiliensis) (Tommasi 1970, Tommasi et al. 1988a,b, Martínez 2008) and sub-Antarctic species (Pseudechinus magellanicus, Arbacia dufresnii, Cycethra verrucosa, Porianopsis mira, Cosmasterias lurida) (Bernasconi 1947, 1964c, Tommasi 1965, Escolar 2010). These results show that the Argentinean Province is characterized by low endemism and has high heterogeneity (Balech and Ehrlich 2008) owing to its particular hydrography, as explained above.

Our results confirm the extension of the Magellan Province towards lower latitudes. We found that typically Magellanic species such as Ctenodiscus australis, Acodontaster e. granuliferus, Austrocidaris canaliculata, Sterechinus agassizii and Tripylaster phillippi (Bernasconi 1964c) extend northwards along the Malvinas current up to 36°-37°S, but always at depths greater than 100 m. Von Ihering (1927) was the first to mention the arrival of Magellanic fauna to Cabo Frio (Brazil) and several authors remark that this locality is the boundary between the Magellan and South Brazilian Provinces (Briggs 1974, Boschi 1976). Similar results were obtained with benthic amphipods (López Gappa et al. 2006).

Almost half of the 86 species recorded in the Magellan Province and also half of the 46 species recorded in the Argentinean Province were also recorded by Lancellotti and Vásquez (2000) in Chilean waters. Pérez-Ruzafa et al. (2013) found that echinoderm fauna from Chile is more closely related to Argentina than to Peru. In fact, they established two biogeographical provinces, the Peru-Chilean and the South American or Magellan Provinces. This continuum in the distribution of species between the Pacific and Atlantic Oceans has been found in various groups of marine organisms, and is the main reason for asserting that the Magellan Province extends south from 40º-41ºS in the Pacific Ocean to approximately 30º-31°S in the Atlantic Ocean (Balech 1954, Briggs 1974). Several authors postulate that the opening of the Strait of Magellan 7000 years ago played an important role in the distribution and dispersal of species to create a corridor for the exchange of faunal elements between the two oceans (McCulloch and Davies 2001, Montiel et al. 2005). In contrast, a low similarity (only 7%) was found between the echinoderm fauna from southern Brazil and the Magellan Province; Barboza et al. (2011) postulated that this result suggests a clear turnover of species from the subtropical Brazil towards temperate areas, mainly at Uruguayan latitudes.

The 25% of the echinoderm species recorded in the Magellan Province in this study were registered in Antarctic waters by Bernasconi 1959, 1964c, 1979, Bernasconi and D’Agostino 1978, Dahm 1999, Manjón-Cabeza and Ramos 2003, Chiantore et al. 2006, De Domenico et al. 2006, O’Loughlin et al. 2011. These results confirm that there is a high degree of affinity between Antarctic and sub-Antartic echinoderm fauna previously mentioned by Barboza et al. (2011). The faunal connection between the sub-Antarctic Region of the Magellan Province and the Antarctic Region has also been reported for various groups of benthic invertebrates (Barnes and De Grave 2000, Montiel et al. 2005, Rodríguez et al. 2007). The presence of species on both sides of the Drake Passage provides strong evidence to confirm the faunal exchange between the Magellan Province and the Antarctic Region; therefore, it was inferred that the Polar Front is not a strict barrier to dispersion of many species of benthic invertebrates (Arntz and Brey 2003, Montiel et al. 2005).

It has been stated that echinoderm species could migrate from the Magellan Province through the Malvinas Plateau and shallow seas, following the arc of southern islands (the Scotia Arc) to reach the Antarctic: examples are Cycethra verrucosa, Anasterias antarctica, Arbacia dufresnii, Pseudechinus magellanicus (Bernasconi 1964c). The same pattern but in the opposite direction was reported by Hedgpeth (1969) for several species of Antarctic ophiuroids, such as Astrotoma agassizii, species with circumpolar Antarctic and sub-Antarctic distribution and with a wide dispersion northwards. Bernasconi and D’Agostino (1974) found this species at the northern end of the Antarctic Peninsula, South Georgia, Burdwood Bank and the Malvinas Islands, reaching 42°S in the Pacific Ocean and 39°S in the Atlantic Ocean). Hedgpeth (1969) also mention that the range of distribution of ophiuroids is controlled by depth, so the routes through shallow waters (Scotia Arc) have been of great importance in the spread of this and other classes of echinoderms.

Our results are in agreement with theories that attempt to explain the observed faunal affinities between Antarctica and South America, giving importance to the connection through the Scotia Arc (Arntz et al. 2005, Moyano 2005) and to the Antarctic Circumpolar Current and Antarctic Coastal Current in the case of echinoderms (Pawson 1969, Díaz et al. 2006).