Marine invertebrate biodiversity from the Argentine Sea, South Western Atlantic

Abstract The list of marine invertebrate biodiversity living in the southern tip of South America is compiled. In particular, the living invertebrate organisms, reported in the literature for the Argentine Sea, were checked and summarized covering more than 8,000 km of coastline and marine platform. After an exhaustive literature review, the available information of two centuries of scientific contributions is summarized. Thus, almost 3,100 valid species are currently recognized as living in the Argentine Sea. Part of this dataset was uploaded to the OBIS database, as a product of the Census of Marine Life-NaGISA project. A list of 3,064 valid species, grouped into 1,662 genera distributed in 808 families and 23 phyla, was assessed. The best represented taxa were Arthropoda and Mollusca, contributing approximately with the 50% of the mentioned species in the literature. Cumulative species curves were analyzed in order to estimate the percentage of marine invertebrate biodiversity that is currently known. However, no model fit to our data, showing that the recorded species represent less than 50% of the expected marine invertebrate biodiversity for the Argentine Sea. The great surface of the Argentine Marine Platform (6,581,500 km2) and the relative low effort in collecting and studying new species due to economical restrictions could explain the low fraction of described species. The training of new taxonomists, as well as, the support of projects that contribute to the knowledge of marine invertebrate biodiversity from South Western Atlantic is recommended.

Marine Life), or SARCE (South American Research Group on Coastal Ecosystems), contribute to the knowledge of the coastal marine biodiversity.
This work compiles and reviews the available information on marine invertebrate biodiversity in the Argentine Sea gathered after an exhaustive literature search.
Data Analysis: A cumulative species analysis was carried out to estimate the status of knowledge of marine invertebrate biodiversity of Argentine Sea. This study was done by using the Clench model (v2 = (a*v1)/(1+(b*v1)), applied by Jimenez-Valverde and Hortal (2003). In this work, we defined as effort units the number of species described per year from 1758 to 2017. In this analysis, only the valid species were considered. Each dot in Figure 1 represents the year when the valid species was described (and subsequently reported in the literature as living in the Argentine Sea). The number of described valid species per year in the region was tested using the Statistica 5.1 program, with the Simplex & Quasi-Newton adjust model. In case of no data fitting the Clench model, another one would be used.

Discussion
The large surface of the Argentinean Marine Platform and Coasts, together with the low number of valid reported species of marine invertebrates, denote that more research is required to increase the knowledge of this group in the South Western Atlantic Ocean and particularly, in the Argentine Sea. The data here compiled did not fit to the Clench model (y= (a*x)/1+b*x)). The obtained curve was y = ((6.33037*x)/ (1+ ((-0.00198)*x)); R=0.98121. As the value of b is almost zero, the obtained curve could be considered as linear. When fitting the data to a linear curve, the formula was y= 13.578x -24378 (R² = 0.9629). This could be attributable to the fact that species mentioned in the literature for the Argentine Sea would be less than 50% of the expected marine invertebrate species present in the region (Fig. 1).
During the last two centuries, an average of twelve species had been described per year as living in the Argentine Sea. At the beginning of the 19 th century, the descriptions were completely based on material collected by European and North American expeditions (Fig. 2). The creation of the Museo Argentino de Ciencias Naturales (MACN) in 1812 contributed to increase the knowledge and descriptions of marine invertebrates (Penchaszadeh 2012). By the end of the 19 th century and the beginning of 20 th two "golden periods" were observed (1879-1888 and 1899-1908). During these two periods the amount of described species was considerably increased probably associated to global marine expeditions. One of them was undoubtedly the "Challenger Expedition" of 1873-76, which described more that 4,000 new species over the world. The reports of this expedition are considered as one of the greatest progresses in the knowledge of the world´s natural history. By the end of the 20 th century, another pulse, of almost 450 species, was newly described for Argentine waters, in the period 1979-1998 (Fig. 2). This fact could be probably associated to the consolidation of specialists in taxonomy in Argentina and the return of scientists exiled during the military dictatorship (1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983). During these 20 years (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998) 30 % of the Nematoda, Bryozoa and Brachiopoda registered in Argentine waters were described. However, the phyla Mollusca and Arthropoda were still the most represented groups during that period. Finally, in the last years (beginning of 21 st century), new species are being described, mainly promoted by the scientific system of Argentina (MINCyT, CONICET), international projects (Census of Marine Life) and open access databases (OBIS, WoRMS). Nonetheless, the knowledge of marine invertebrate biodiversity is still low in the region.
The Kingdom Animalia comprises 29 invertebrate phyla (WoRMS), however, only six phyla have not been recorded as living in the Argentine Sea (Table 1). These are Cycliophora, Gastrotricha, Gnathostomulida, Orthonectida, Placozoa and Xenacoelomorpha. The phylum Arthropoda and Mollusca constitute around 50 % of the reported marine invertebrates. However, the percentage of Argentinean marine Arthropoda is lower compared to the global knowledge, revealing that this group is far to be resolved in the region. In contrast to that, the mollusks percentage is more consistent. Some groups as Bryozoa, Cnidaria, Porifera and Echinodermata exceed the global registered percentage reported by WoRMS (2017). The observed percentage of the phylum Ne- mertea coincides with the worldwide registered in WoRMS. Nevertheless, only 30 species have been reported as living in the southwestern Atlantic, suggesting that the number of known nemertean is still low. In addition, 70% of Nemertea species was described in the Northern Hemisphere (Kajihara et al. 2008). This could indicate that new Argentinean nemerteans could be described in the future. Research focused on marine invertebrate biodiversity in Argentina is currently growing. Additionally, some young researchers on invertebrate taxonomy are being trained towards a scientific career. On the other hand, the financial support provided by the government is still scarce.
A distribution analysis of the species is a complex issue, due to, in several cases, the literature examined named "Argentine Sea" or "Argentine Coast" as a locality. This is the case of 955 records of species cited for the Argentine Sea without a precise locality. However, distribution patterns by provinces were made excluding those 955 records and estimating the percentage for the main taxonomic groups in order to elucidate hot spots in the Argentine Sea (Fig. 3). It is clear that the Magellan region is the most studied region of the Argentine Sea with 1166 (55%) mentioned species in the literature followed by the Buenos Aires province coast with 526 (25 %). Few records were exclusively mentioned for the Río Negro Province in the literature; only 29 (1,5 %) species were named for this area. Santa Cruz and Chubut provinces, with 251 (12 %) and 137 (6,5 %) reported species respectively, present more species than Río Negro but the number of reported species is still low compared to Tierra del Fuego and Buenos Aires provinces. In general terms, the phylum Mollusca and Arthropoda were the most mentioned groups along the Argentine Sea. Nevertheless, the phylum Nematoda in the Santa Cruz province and Annelida (mostly Polychaeta) in Chubut, were widely studied (Fig. 3). The fact that more species are described in the southern region of the Argentine Sea could be attributable to the concentration of oceanographic campaigns that were performed by international initiatives when travel to Antartica or passing from Pacific to Atlantic Ocean (around Tierra del Fuego and Southern Islands). The major biodiversity encountered in the southern tip of the Southwest Atlantic also could be attributable to an inverse biodiversity pattern that was previously registerd in Southwest Atlantic higher latitudes for some intertidal rocky shore invertebrates (Palomo et al 2011) or other taxa as asellote isopods (Doti et al. 2014). The increasing in biodiversity in high latitudes could also be attributable to the presence of high extentions of hard bottoms that permit the settlement of invertebrates and the fact that most Magellanic species that occur in southern Chile extend to the Southwest Atlantic (Lopez .
In Argentina, the main factors that modify benthic communities are habitat degradation and disturbance, urban development, dredging and resuspension of sediment, establishment of ports, tourism-associated impact, global and local aquatic contamination sources, and fisheries (Bigatti and Penchaszadeh 2008). Notably, bottom trawling dominates coastal and deep-sea fishing in the Argentine platform. This fishery produces a large number of discards of benthic invertebrates, accounting up to 80 % of the catch (Orensanz et al. 2008). In order to provide an adequate management of the natural resources, studies on coastal management, conservation and distribution patterns have been carried out (Sullivan and Bustamante 1999, Barragán et al. 2003, Cusson and Bourget 2005, Cañete et al. 2008, Miloslavich et al. 2011. Finally, biological invasions of different organisms (algae, mollusks, hydroids, bryozoans, ascidiaceans and crustaceans) have negatively affected local marine biodiversity, as well as, regional economy (Orensanz et al. 2002, Penchaszadeh et al. 2005, Bigatti and Penchaszadeh, 2008, Schwindt 2008. A total of 28 marine exotic species and 43 cryptic species have been reported as living in the Argentine Sea (Orensanz et al. 2002), while the number is increasing in the last years. The impact of biological invasions constitutes a serious problem to marine invertebrate biodiversity in Argentine Sea and consequently affects descriptions of new species, even before of their description. The results of this checklist suggest the importance of studies focused on marine invertebrate biodiversity in the southern tip of South America, where some hot spots, as the Protected Marine Area Burdwood bank, harbor great abundance and diversity of endemic species (Miloslavich et al 2011). New studies on marine invertebrate biodiversity will provide consistent data for the generation of management policies tending to create new marine protected areas and the conservation of the species´habitats.