Biodiversity of Porifera in the Southwest Atlantic between 35 o S and 56 o S

The aim of this study is to provide a synthesis of the present knowledge of the marine Porifera in the Southwest Atlantic between 35o S and 56o S, and between the coast of Argentina and 50o W. The analysis of 34 taxonomic publications produced a list of 149 oceanographic stations and 28 coastal localities with records of Porifera for the study area. A total of 196 species, 5 subspecies and 1 variety of marine sponges was recorded, 187 of which belong to the Demospongiae, 10 to the Calcarea and 5 to the Hexactinellida. The most widely distributed sponges in the study area are Dasychalina validissima, Iophon proximum, Mycale magellanica, Tedania massa and Tedania spinata, all Demospongiae. The sampling effort was unequally distributed, reaching a maximum concentration off Buenos Aires Province and around the Malvinas/Falkland Islands. The biodiversity of marine Porifera of Argentina along 21 degrees of latitude is closely related to the concentration of the sampling effort. It appears to be highest in thoroughly surveyed areas, such as around Mar del Plata, Port Stanley and the coast of Tierra del Fuego. No correlation was found between species number and depth. The richest stations were recorded around 130-140 m. Biodiversity of Porifera is weakly but significantly correlated with latitude, since stations between 50o S and 55o S were on average richer than those located off Buenos Aires Province. Knowledge of the geographic distribution of marine Porifera in Argentina is fragmentary. Much more basic information is needed, particularly for the continental shelf off Río Negro and Chubut Provinces.

Therefore, the aim of this study was to provide a synthesis of the current knowledge of the marine sponges in the Southwest Atlantic between 35º S and 56º S, and to discuss whether biodiversity patterns can be recognized along the coast and continental shelf of Argentina.

MATERIALS AND METHODS
A database was compiled after a survey of the taxonomic literature published up to 2004, including oceanographic stations and coastal localities where species of Porifera were recorded.Taxa identified above specific level were not taken into account.Species recorded for imprecise localities were included in the database, but were not used to calculate biodiversity patterns in the study area.
This survey was restricted to an area between 35º S and 56º S, and between the coast of Argen-tina and 50º W. Species recorded for Chilean localities within the Strait of Magellan and south of the Beagle Channel were not included in this study.The whole area was divided into a 1º square grid.Only squares containing oceanographic stations or precise localities with published records of marine sponges were numbered from west to east and from north to south (Fig. 1).In most cases, the geographic coordinates of oceanographic stations were clearly indicated in the taxonomic literature, but in a few exceptions (e.g., several stations of the Discovery and William Scoresby cruises) approximate latitude and longitude were estimated using bathymetric charts.
Correlation between species number and latitude was calculated using 149 oceanographic stations from all cruises.Results did not change when 10 stations from the Pescal II, Prof. W. Besnard and Capitán Cánepa cruises were omitted from the analysis because these surveys (Mothes- de-Moraes & Pauls, 1979;Cuartas, 1992 c) treated only a small proportion of the Porifera.The relationship between species number and depth was analyzed on a subsample of 132 oceanographic stations, since depth data were absent for 17 stations studied by Burton (1940).Mean depth was used for calculations whenever minimum and maximum depths were given in some stations.The study area was then divided by degrees of latitude (35º S to 56º S) and data in Tables 1 and 2 and Appendix 1 were used to calculate the correlation between species number and number of oceanographic stations/ coastal localities sampled at each latitude.Hooper & Van Soest's (2002) systematic classification of the Phylum Porifera was followed for supraspecific taxa.The use of subgenera was avoided throughout this study.The generic placement of more than 50 species listed in Appendix 1 was updated, as 17 genera (Adocia, Anchinoe, Axociella, Dendoryx, Dictyociona, Gymnorossella, Hoplochalina, Leucophloeus, Oligoceras, Pellina, Plumocolumella, Pronax, Pseudanchinoe, Rhaphidophlus, Stylohalina, Stylostichon, Styllotellopsis) could no longer be used since they have been regarded as synonyms (Hooper & Van Soest, 2002), and also because a high proportion of records were relatively old.As a careful examination of the morphology of each of these species was beyond the scope of this study, the best option available was to follow the World List of Extant Porifera compiled by Rob van Soest (available for download at http://www.science.uva.nl/ZMA/Invertebrates/Coel/scirep/index.htm).

RESULTS
According to the information found in 34 taxonomic papers published between 1882 and 2004, marine Porifera were present in 149 oceanographic stations (Table 1) and 28 coastal localities (Table 2) throughout the study area (see distribution in Fig. 2).A total of 196 species, 5 subspecies and 1 variety was recorded, 187 of which belong to the Demospongiae, 10 to the Calcarea and 5 to the Hexactinellida (Appendix 1).
The sampling effort was very unequally distributed, as it was concentrated off Buenos Aires Province and around the Malvinas/Falkland Islands (Fig. 2).Conversely, few stations with records of Porifera exist along vast areas of the continental shelf off Río Negro and Chubut Provinces.
The number of species in each 1º square is indicated in Fig. 3. Species richness was not correlated with depth (Product-moment correlation, r = 0.032, n = 132, P = 0.720).The richest stations were recorded around 130-140 m (Fig. 4a).Species richness was weakly but significantly correlated with latitude (r = 0.273, n = 149, P = 0.0008).Stations between 50º S and 55º S had on average more species of sponges than those located off Buenos Aires Province (Fig. 4b).The most conspicuous exception was Challenger St. 320, where 22 species were found at a depth of 1097 m on the continental slope off Buenos Aires Province.There was a highly significant correlation between the number of species and the number of oceanographic stations/coastal localities with Porifera in each degree of latitude (r = 0.868, n = 21, P = 0.000).

DISCUSSION
The systematic list compiled in Appendix 1 should not be considered as a definitive catalogue of the marine sponges of the study area.Much taxonomic revision needs to be done, particularly on species that were described or recorded in the results of earlier oceanographic cruises to the study area but were not collected again thereafter.Species originally described for other oceans or for the Northern Hemisphere and later recorded for the Southwest Atlantic may actually be new species with more restricted distributions, as was the case of Rhabderemia uruguaiensis (see Van Soest & Hooper, 1993).There are also several species in need of a revision, e.g.Mycale magellanica (Hajdu & Desqueyroux-Faúndez, 1994).
In the present scenario of accelerating change of coastal biotas due to the invasion of non-in-   1 and 2).The 200-m isobath is shown.digenous species (Carlton & Geller, 1993), faunistic compilations such as the present one may prove to be a useful tool in the future.The Southwest Atlantic, formerly regarded as a pristine confine of the world ocean, was shown to be significantly impacted by human-mediated invasions.However, there are no species of Porifera in the area that can be regarded with certainty as non-indigenous, and just 4 species (Cliona celata, Halichondria panicea, Halichondria hirta and Hymeniacidon sanguinea) have been considered as cryptogenic along the coasts of Argentina and Uruguay (Orensanz et al., 2002).
The highest biodiversity of Porifera on the continental shelf was found at a depth of around 130-140 m.Maximum species richness of bryozoans was also recorded in relatively deep areas of the continental shelf (80-120 m; López Gappa, 2000), where the presence of a high-productivity shelf-break front has been documented (Podestá, 1997).
Different groups of invertebrates show contrasting biodiversity trends along the continental shelf of Argentina.The number of decapod crustaceans in the Southwest Atlantic decreases with increasing latitude (Boschi, 1964), being four times higher in the Argentine (i.e., southern Brazil, Uruguay and northern Argentina) than in the Magellanic Biogeographic Province (Boschi, 2000).Only half of the decapods inhabiting the Pacific sector of the Magellanic Province reach the Southwest Atlantic (Vinuesa, 1977).This trend is also observed in the Polyplacophora (Liuzzi, unpublished), where the number of species is twice higher in southern Chile than in the Atlantic sector of the Magellanic Province, but the biodiversity of this group in the southern districts of the Argentine Province (i.e., northern Argentina and Uruguay) is still lower than in the cold waters around Tierra del Fuego (Liuzzi, unpublished).According to biogeographic data in Desqueyroux & Moyano (1987), from a total of approximately 83 species of Demospongiae inhabiting the archipelagos of southern Chile, 21 species (~ 25 %) do not extend their distribution to the Atlantic side of the Magellanic region.
In the Southwest Atlantic from 35º S to 56º S, the biodiversity of bryozoans decreases from south to north, but this trend is overemphasized by a simultaneous decrease in the sampling effort (López Gappa, 2000).Interestingly, the analysis of the distribution of bryozoans, mollusks and echinoderms collected by the R/V Shinkai Maru throughout the continental shelf of Argentina (López Gappa & Lichtschein, 1988;Bastida et al., 1992) demonstrated the existence of a transitional zone between the Argentine and Magellanic assemblages.This intermediate area is inhabited by an impoverished Magellanic fauna, characterized by the absence of many species living around the southern tip of South America.The present study shows a similar pattern for the marine Porifera, as their species number is slightly higher around Tierra del Fuego and the Malvinas/Falkland islands than on the narrow continental shelf off Buenos Aires Province.
According to the existing evidence, the biodiversity of marine Porifera in the Southwest Atlantic from 35º S to 56º S is closely related to the concentration of the sampling effort, as it appears to be highest in the most thoroughly sampled areas, such as around Mar del Plata, Port Stanley and the coast of Tierra del Fuego.The high number of squares with few or no species of Porifera show that the knowledge of this group in this section of the Atlantic Ocean is fragmentary and that vast areas should be more intensely explored in the future, particularly the continental shelf off Río Negro and Chubut Provinces.Table 1.List of oceanographic stations where species of Porifera were recorded for the study area.One-degree squares are numbered as in Fig. 1.
In the WLEP this species is placed in the genus Pyloderma.
[50] Remarks.According to Rützler (2002), Pronax Gray, 1867 is a synonym of Cliona.In the WLEP this species is placed in the genus Phorbas.
Remarks.Levi (1963) and Sarà et al. (1992) recorded this species as Pronax pustulosa.According to the WLEP, this species should be placed in the genus Phorbas.
[75] Remarks.According to Erpenbeck & Van Soest (2002), Leucophloeus Carter, 1883 is a synonym of Ciocalypta.In the WLEP this species is placed in the genus Halichondria.
of Scopalina Schmidt, 1862.In the WLEP this species is placed in the genus Pachychalina.
[75] Remarks.According to Erpenbeck & Van Soest (2002), Pellina Schmidt, 1870 is a synonym of Halichondria.In the WLEP this species is placed in the genus Oceanapia.
[16] Remarks.According to the WLEP this species should be placed in the genus Neopetrosia.
[9] Remarks.According to the WLEP, this species should be placed in the genus Scalarispongia.

Fig. 1 .
Fig. 1.One-degree squares containing records of species of Porifera in the taxonomic literature, numbered from the coast of Argentina to 50º W and from north (35º S) to south (56º S).The 200-m isobath is shown.

Fig. 2 .
Fig. 2. Location of oceanographic stations and coastal localities with precise records of Porifera in the study area (latitude and longitude of each station and locality are given in Tables1 and 2).The 200-m isobath is shown.

Fig. 3 .
Fig. 3. Number of species of Porifera in each 1º square in the study area.The list of species recorded for each square can be compiled from information in Appendix 1.The 200-m isobath is shown.
Fig. 4. a) Relationship between species number and depth (m) in 146 oceanographic stations in the study area.Three Challenger stations (St.317, 320 and 323) were omitted.b) Relationship between species number and latitude (S) in 132 stations in the study area.

Table 2 .
List of coastal localities where species of Porifera were recorded for the study area.Onedegree squares are numbered as in Fig.1.
According to the WLEP, this species should be placed in the genus Auletta.
According to the WLEP, this species should be placed in the genus Dragmacidon.According to the WLEP, this species should be placed in the genus Halicnemia.