Richness, systematics, and distribution of molluscs associated with the macroalga Gigartina skottsbergii in the Strait of Magellan, Chile: A biogeographic affinity study

Abstract Knowledge about the marine malacofauna in the Magellan Region has been gained from many scientific expeditions that were carried out during the 19th century. However, despite the information that exists about molluscs in the Magellan Region, there is a lack of studies about assemblages of molluscs co-occurring with macroalgae, especially commercially exploitable algae such as Gigartina skottsbergii, a species that currently represents the largest portion of carrageenans within the Chilean industry. The objective of this study is to inform about the richness, systematics, and distribution of the species of molluscs associated with natural beds in the Strait of Magellan. A total of 120 samples from quadrates of 0.25 m2 were obtained by SCUBA diving at two sites within the Strait of Magellan. Sampling occurred seasonally between autumn 2010 and summer 2011: 15 quadrates were collected at each site and season. A total of 852 individuals, corresponding to 42 species of molluscs belonging to Polyplacophora (9 species), Gastropoda (24), and Bivalvia (9), were identified. The species richness recorded represents a value above the average richness of those reported in studies carried out in the last 40 years in sublittoral bottoms of the Strait of Magellan. The biogeographic affinity indicates that the majority of those species (38%) present an endemic Magellanic distribution, while the rest have a wide distribution in the Magellanic-Pacific, Magellanic-Atlantic, and Magellanic-Southern Ocean. The molluscs from the Magellan Region serve as study models for biogeographic relationships that can explain long-reaching patterns and are meaningful in evaluating possible ecosystemic changes generated by natural causes or related to human activities.


Introduction
In the South-eastern Pacific Ocean, the Magellanic biogeographic province (43°S to 56°S) is constituted by a large extension of channels and fjords with diverse coastal environments from glacial influence to direct exposure by the Pacific Ocean (Camus 2001, Spalding et al. 2007. Two biogeographic districts have been categorised for this biogeographic province: the Austral and the Subantarctic. The latter extends from about 52°-53°S to 56°S (Camus 2001); in other words, from the Strait of Magellan to Cape Horn. This territory is characterised by different environmental conditions that determine sub-areas of physiogeology and orography, geology, soils, and differentiated climates (Pisano 1977). Within the Subantarctic biogeographic region, the Strait of Magellan connects the Pacific and Atlantic oceans. For this reason, the Strait of Magellan offers unique characteristics for studying biodiversity and, specifically, aspects related to biogeography (Ríos et al. 2003).
Knowledge about a large part of the marine fauna in the Magellan Region was first attained from scientific expeditions carried out during the 19th century. The historical contributions to the knowledge of molluscs from the Magellanic biogeographic province have been detailed by Reid and Osorio (2000), Cárdenas et al. (2008), and Aldea and Rosenfeld (2011). Currently, many researchers have contributed to the knowledge about these molluscs, principally in descriptive taxonomy and ecology in the Magellan Region (e.g. Linse 2002, Ríos et al. 2003, Zelaya and Ituarte 2003, Pastorino 2005a, 2005b, Linse et al. 2006, Schwabe et al. 2006, Sirenko 2006, Zelaya and Geiger 2007, Ojeda et al. 2010, Signorelli and Pastorino 2011. Recently, new contributions have been made using molecular tools in order to study specific groups of molluscs (e.g. Espoz et al. 2004, Aranzamendi et al. 2009, Gonzalez-Wevar et al. 2010. One crucial aspect of molluscs from these latitudes is their biogeographic relationship that can explain "long reaching patterns" (e.g. Linse et al. 2006, Clarke et al. 2007, Fortes and Absalao 2011. Therefore, molluscs are interesting as a study group to evaluate possible ecosystemic changes generated by natural or human causes.
Although much knowledge exists about molluscs from the Magellan Region, the majority of this knowledge has been centred only on the characterisation of the taxon and not on the search for assemblages and biogeographic patterns. Some contributions to this interaction have come from studies on invertebrates associated with giant kelp, Macrocystis pyrifera (Ojeda and Santelices 1984, Adami and Gordillo 1999, Rios et al. 2007. Currently, the only macroalgae in the Magellan Region with massive commer-cial exploitation corresponds to the carregeanofite Gigartina skottsbergii. This species is distributed from 39°52'S (Romo et al. 2001) toward the Antarctic Peninsula (Wiencke and Clayton 2002). G. skottsbergii is characterised as forming a dense sublittoral bed, reaching a biomass density of around 1773 g/m 2 and densities of 15 individuals/m 2 (Ávila et al. 2004). The extraction of this species has the objective of providing the principal raw material for the production of carrageenan hydrocolloid (carrageenan), a gel with multiple applications in the food and cosmetics industries (Romo et al. 2001;Pujol et al. 2006; Barahona et al. 2012). Due to the growing national and international demand for this raw material, algae beds have suffered significant losses and their restoration has been quite slow, showing largely damaged communities in beds of Puerto Montt (~41°S; Romo et al. 2001). For this reason, a good share of the extractive pressure has moved toward the south, especially in the area of the Gulf of Penas (~47°S) as well as the Magellan Region (~53°S; Romo et al. 2001, Mansilla et al. 2008. Differing from other distribution sites of G. skottsbergii, the Strait of Magellan still has a natural bed of G. skottsbergii (Ávila et al. 2004), and it is important for analysis for two reasons: i) describing the current situation of the fauna present in natural beds and ii) because analysis of the systematics and distribution of molluscs throughout the Strait of Magellan is a good model to characterise possible faunistic connections between different environments (e.g. Atlantic-Pacific). Thus, populations of G. skottsbergii in the Magellan Region constitute an excellent alternative to study the benthic biodiversity.
Here it is possible to study molluscs that are associated with algae and form beds that provide a shelter for associated species (Mansilla 2013), potentially contribute to conservation (Gray 1997, Fernández et al. 2000, Lancellotti and Vásquez 2000 or allow to determine an eventual loss of diversity for the function of the ecosystem (Purvis and Hector 2000). In this sense, the objective of this study is to describe the species richness and distribution of the mollusc species associated with the natural bed of G. skottsbergii in the Strait of Magellan, and to evaluate the biogeographic affinities of all the species.

Material and methods
The study area was localised in two sampling sites with the presence of a bed of G. skottsbergii in the Strait of Magellan: i) Punta Santa Maria, located in Tierra del Fuego (53°21'S -70°27'W), and ii) Punta Santa Ana, located 60 km to the south of Punta Arenas (53°37'S -70°52'W) (Fig. 1). The samples were obtained by SCUBA diving at ~10 m depth in quadrates of 0.25m 2 , which were selected randomly within the bed. In each quadrate, all molluscs were collected ,and also the substrate, where G. skottsbergii settled, was investigated. Subsequently, the rocks were scraped to ensure that all the species and specimens were collected. Fifteen quadrates were sampled during one dive in each site and season, resulting in 60 quadrates per site (2 sites × 4 seasons × 15 quadrates). Sampling was carried out in autumn, winter, and spring of 2010, and in summer of 2011. The samples obtained were deposited in plastic bags, tagged and preserved in Formalin, diluted to 4-5% in seawater, and buffered with sodium borate.

Systematics analysis
Taxonomic identification of the molluscs and the registry of the geographic distribution of each species was based on a complete study of the current literature (e.g. Reid andOsorio 2000, Linse 2002), as well as on classic works (e.g. Smith 1881, Rochebrune and Mabille 1889, Strebel 1905a, 1905b, systematics studies about specific taxa (e.g. Villaroel and Stuardo 1998, Pastorino and Harasewych 2000, Zelaya 2004, and academic databases available on the internet (Morris andRosenberg 2005, USNM 2010).
All of the morphotypes that were identified at species level are included in this report, with the following information presented for each one: a) material examined, b) synonymy, c) remarks, and d) distribution. The material examined is detailed for each bed, showing the number of live specimens collected (spm.) and including the dimensions of the largest and smallest specimens. The synonymy is derived from the last taxonomic study. In the remarks, taxonomic, morphological, and/or ecological aspects are discussed. The distribution shows all previous records of the species, arranged from north to south in both oceans (Pacific and Atlantic). These records were matched into the following marine biogeographic regions (Spalding et al. 2007): Warm Temperate South-eastern Pacific (WTSP), Magellanic, Warm Temperate South-western Atlantic (WTSA) and Southern Ocean (SO). Dimensions of the polyplacophorans refer to their maximum longitude and maximum width. For gastropods, the maximum height is from the ventral umbo of the shell, and the maximum width is perpendicular to the height. Finally, for bivalves, the maximum height is from the umbo on the ventral margin, and the width is between the upper and lower margins.

Statistical analysis
In order to detect whether our sampling effort was able to estimate the total species of molluscs, the linear dependence model was used. This was designed to estimate species richness, depending on the number of samples (Soberon and Llorente 1993). All samples were randomised so as not to affect the shape of the curve (Colwell andCoddington 1994, Moreno andHalffter 2000). The estimation method Simplex and Quasi-Newton of the statistical package STATISTICA 7 was used to estimate the coefficients of the nonlinear regression model.
Possible changes in the assemblage of molluscs throughout the year were determined using a nested design that considered each sampling site and season as sources of variation. For this, a PERMANOVA analysis was performed using species richness (Anderson 2005). Previously, the distance from Bray-Curtis similarity between pairs of observations was calculated, and 9999 permutations were used without data constraints (Anderson 2001). This analysis was developed in the FORTRAN package (Anderson 2005).
Furthermore, we defined species represented by a single individual as "singletons" and species represented by only two individuals as "doubletons" (Colwell and Coddington 1994) as a measure of species rarity.

Analysis of biogeographic aspects
Biogeographic distribution was delimited for the species as "Widespread", "Magellanic-Pacific southeast temperate", "Magellanic", "Magellanic-Atlantic southwest temperate", and "Magellanic-Southern Ocean", following the classification of provinces and biogeographic ecoregions proposed by Spalding et al. (2007) and taking into account previous research (Stuardo 1964, Brattström and Johanssen 1983, Lancelotti and Vásquez 2000, Camus 2001) of the Chilean Coast. In order to estimate the biogeographic affinities of the molluscs recorded in this study, a literature revision was carried out from the different provinces and regions of the South American and Antarctic coasts. A comprehensive review of the bathymetry of each species was performed. All species inhabiting depths less than 30 m were included and considered as "shallow-water species". For the different provinces or regions of the Pacific Coast, the number of species was obtained from the revisions of Valdovinos (1999) and Ramirez et al. (2003). For the Atlantic Coast, the checklists of Scarabino (2003aScarabino ( , 2003bScarabino ( , 2004 and Carcelles (1950) were used. For the province of the Scotia Sea and continental Antarctic, the work of Griffiths et al. (2003) and a personal data compilation were used. Degrees of faunistic affinity between the studied areas were evaluated using the Simpson similarity coefficient (Cheetham and Hazel 1969), and similarities were calculated as quotient between shared species and local richness (SL; see Zelaya 2005).

Results
From a total of 852 mollusc specimens sampled, 42 species were identified, corresponding to 9 orders, 23 families, and 31 genera. Three identities (morphotypes) were identified only at a genus level (Table 1). In terms of richness by class, Gastropoda was represented by 24 species, and Polyplacophora and Bivalvia were each represented by 9 species (Table 1). Of the total species, 38.1% were rare, with 28.6% singletons and 9.5% doubletons (Table 1). Comparing the three classes, Gastropoda had most of the rare species at 45.8% (singletons plus doubletons).
PERMANOVA analysis showed no significant differences (F = 0.9084; p = 0.6835) in the seasonal species composition of the two sites (Table 2). However, it showed significant differences (F = 171.972; p = 0.0001) in species composition between the two study sites.
The species richness associated with sampling effort was determined by the linear dependence model. For Punta Santa Maria, prediction constants were a = 0.126 and b = 4.179; therefore, the expected maximum richness (a / b) was 33 species with an R 2 = 0.96 and slope = 0.002. This value is lower than that observed in the field (S = 36) ( Fig. 2A). Finally, for Punta Santa Ana, prediction constants were a = 1.522 and b = 0.093; therefore, the expected maximum richness (a / b) was 16 species with an R 2 = 0.93 and slope = 0.005. This value is lower than that observed in the field (S = 18) (Fig.  2B). Therefore, in this study, the richness obtained from the model of linear dependence for both sites was lower than that observed in the field.
Remarks. Morphologically, this species is similar to Stenosemus exaratus (G.O. Sars, 1878) but differs by presenting a wider perinotum, black pigmented aesthetes, and different elements of the dorsal perinotum (Schwabe 2009). In relation to its colour, Sirenko (2006) mentioned that it can vary from white to red.
Synonymy. See Kaas et al. (2006). Remarks. Sirenko (2006) mentioned that the coloring of the valves of this species is variable. It is known that this species houses the protozoa parasite Chitonicum simplex Plate 1898 (Schwabe et. al 2006).

Chiton bowenii (King & Broderip, 1831)
Synonymy. See Kaas et al. (2006). Remarks. Sirenko (2006) commented that C. bowenii is a rare species. However, in this study, it was present in two sampling sites. Synonymy. See Kaas and Van Belle (1994). Remarks. Reid and Osorio (2000) mentioned that this species together with the tiny species Leptochiton medinae (Plate, 1899) are the only chitons capable of inhabiting environments with low salinity. Morphologically, this species is distinguished by presenting variable coloring in the valves and in the tegument sculpture (Sirenko 2006).
Remarks. This species presents a morphological similarity to P. aurata but presents longitudinal elevations in the pleural areas, while P. aurata does not possess this sculpture (Schwabe 2009). According to Sirenko (2006), it is a rare species.
Remarks. This species was recorded as a junior synonym of Nacella kerguelensis by Cantera and Arnaud (1985). Nevertheless, Valdovinos and Rüth (2005) commented that morphologically Nacella mytilina is clearly different from the rest of the species. The molecular study carried out by Gonzalez-Wevar et al. (2010) backed the establishment of N. mytilina and N. kerguelensis as different species. N. mytilina is a common component of the epibiontic community associated with Macrocystis pyrifera kelp forests of the Magellan Region (Reid and Osorio 2000). In this study, it was found inhabiting the fronds of G. skottsbergii.
Distribution. Magellanic: Estero Elefantes (Reid and Osorio 2000), Carlos Island in Puerto Edén (Dell 1971), and Guarello Island (Valdovinos and Rüth 2005) Remarks. Waren et al. (2011) studied the species of Lepetidae and concluded that specimens of I. coppingeri and I. emarginuloides are similar. This is concurrent with Strebel (1907) that these species are synonyms, establishing I. copperingeri as a junior synonym of this species.
Remarks. According to McLean (1984), the most similar species is F. radiosa, which is similar in size and presents similar colors and variations. The same author comments that the only distinguishing characteristic between the shells is the presence of primary ribs that are longer than the adjacent ribs present in F. radiosa. These primary ribs are absent in the species F. oriens.
Synonymy. See Dell (1971). Remarks.  commented that the shell of the similar species M. expansa (Sowerby I, 1838) is composed of two well-differentiated layers, with the internal layer being thicker. Also, M. expansa have four pairs of epipodial tentacles and frequently present an additional unpaired tentacle (Zelaya 2004). However, the identification between these species is quite complex due to the extreme morphological similarities (see Zelaya 2004. In this sense, Troncoso et al. (2001, p. 86) recorded and commented on M. violacea for the Kerguelen Islands, but in fact they photographed and mentioned M. expansa (Troncoso et al. 2001, p. 89, Fig. 4).
Remarks. New information about the biology and distribution of the species was presented by . They noted that the records made by Strebel (1908) for the South Georgia and South Sandwich Islands and the Antarctic Peninsula and records made by Smith (1902) for eastern Antarctica have not been commented by any other author in later studies. Because of this, these authors consider their Antarctic distribution points as dubious records, manifesting that this species would be restricted to Subantarctic regions.  (Lamy 1905); Malvinas/Falkland Islands (Melvill and Standen 1898, Strebel 1905a, Castellanos and Landoni 1989, and Burdwood Bank (Melvill and Standen 1907). WTSA: Río de la Plata basin (USNM 2010). SO: Marion and Prince Edward Islands (Watson 1886, Branch et al. 1991, Kerguelen Islands (Smith 1879, Watson 1886, Martens and Thiele 1904, Strebel 1905a, Thiele 1912, Lamy 1915, Powell 1957, Cantera and Arnaud 1985, and Crozet Island (Cantera and Arnaud 1985); probably in South Georgia Island (Strebel 1908), Antarctic Peninsula (Strebel 1908), and Cape Adare (Smith 1902). Fig. 5A Material examined. 1 spm (13 × 12 mm).

Calliostoma nudum (Philippi, 1845)
Synonymy. See Morris and Rosenberg (2005). Remarks. Castellanos and Landoni (1989) commented that this species is a complex variable in which the species C. kophameli Strebel, 1905, C. venustulum (Strebel, 1908), and C. falklandicum (Strebel, 1908) appear to be simply different morphotypes of the species C. nudum. Accordingly, a morphological study is required that details the various examples of the species.
Calliostoma modestula, Castellanos and Fernández 1976: 141, pl. II, figs. 8-9. Remarks. From a morphological point of view, Melvill and Standen (1912) commented that this species presents similarities to Photinula crawshayi (Smith, 1905), although it has more globular whorls. The maximum depth at which it has been recorded is 869 m. However, in this study, a shallower depth was recorded, with specimens found at 10 m in beds of Gigartina skottsbergii.

Photinastoma taeniatum (Sowerby I, 1825)
Synonymy. See Powell (1951). Remarks. Powell (1951) stated that the subspecies Photinastoma taeniatum nivea Cooper & Preston, 1910 presented uncommon characteristics compared to the typical form of the species, not presenting the same color pattern and a higher spire with more globular whorls, but both forms have three spiral whorls in the first whorl of the protoconch. Similarly, Castellanos and Landoni (1989) mentioned that these characteristics had been used by Powell (1951) to identify difference on generic level between those species of Photinastoma and Calliostoma, which were similar. Given this, they estimated that the species should be included within the genus Calliostoma. However, according to , this species should be included under the genus Photinastoma.

Eatoniella nigra (d'Orbigny, 1840)
Synonymy. See Ponder and Worsfold (1994). Remarks. It was described under the name Paludestrina nigra d'Orbigny, 1840 for the north of Chile. Afterwards, Marincovich (1973) described the species E. latina being the first representative Eatoniellidae for the Southeast Pacific. However, Ponder and Worsfold (1994), upon revising the shells of both species, found a common morphology, lower, more ovular, and thinner than the other black-colored species present in South America. Thus, E. latina is considered a junior synonym of this species. Records from South Africa (Rosenberg et al. 2002, OBIS 2014) likely correspond to E. afronigra according to Ponder and Worsfold (1994).

Eumetula pulla (Philippi, 1845)
Synonymy. See Cárdenas et al. (2008). Remarks. Cárdenas et al. (2008) noted that this species is different from the other species in its family because it does not have cords on the base. Morris and Rosenberg (2005) considered Cerithium caelatum (Gould, 1849) as a synonym of Eumetula pulla. However, Zelaya (2009a) considered it a valid species and suggested a significant revision of this complex of species.
Synonymy. See Dell (1972). Remarks. Dell (1972) explained that seven species of the genus Savatieria have been described for the Magellan Region and the Malvinas/Falkland Islands. However, this genus is not well studied, and the Magellanic species are slightly different and not well represented in collections (Dell 1972).

Synonymy. See Cárdenas et al. (2008).
Remarks. This species presents a ruddy-yellow coloring, and one of its most distinguishable characteristics is its smooth texture with one or two stripes under the sutures of each whorl (Cárdenas et al. 2008). Our specimens presented quite eroded shells.   Fig. 6B Material examined. 20 spm (13 × 6 -25 × 12 mm).

Pareuthria plumbea (Philipi, 1844)
Synonymy. See Dell (1971). Remarks. Aldea and Rosenfeld (2011) commented that, differing from other species of the family Buccinidae, it is characterized by direct development during its reproductive cycle by depositing egg masses (Pastorino and Penchaszadeh 2002). Dell (1971) explained that Strebel (1905b), when revising the species of the genus Pareuthria, observed a great similarity between P. plumbea and P. magellanica; however, the original figures did not concur with the distinction that was made by Strebel (1905b).
Synonymy. See Powell (1951). Remarks. This species is similar to the species P. cerealis but differs in that the last whorl is more globular, and it has spiral stripes in the base of the last whorl (Cárdenas et al. 2008). Our examined specimen presented an eroded shell.

Pareuthria janseni
Synonymy. Euthria janseni Strebel 1905b: 622, pl. 21, fig. 7-7a;Strebel 1908: 28. Pareuthria janseni, Forcelli 2000 fig. 265. Remarks. The specimen analyzed in this study had light spiral stripes on the whole surface of the shell, which is characteristic of this species. Similarly, Strebel (1905b) commented that the last whorl presented 30 spiral stripes. This species is very similar to the species P. michaelseni, but it can be distinguished by a more globular last whorl, occupying ¾ of the total shell length (Forcelli 2000).
Remarks. Trophon geversianus is the most well-known species of the genus Trophon. Its morphological variability is evident in the quantity of names proposed for each morphotype of this species (Pastorino 2005b). The rest of the nominal species from the Southern Ocean and adjacent waters displaying a similar morphology were compared by Aldea and Troncoso (2010a).

Synonymy. See Houart (2010).
Remarks. The species that was referred to under the genus Fuegotrophon by Pastorino (2002) that was originally proposed as a subgenus by Powell (1951) based principally on the characteristics of the protoconch and radula. Currently, the name Fuegotrophon pallidus is considered to represent a separate genus (Houart 2010).
Remarks. This species has a similar morphology to Xymenopsis subnodosus (Gray, 1839) in that it presents an external crenulate margin of the aperture, 12-16 axial cords on the last whorl, and 22-24 spiral cords (Pastorino and Harasewych 2000). Xymenopsis muriciformis has a direct development during its reproductive cycle, depositing its egg masses on rocky substrates (Zelaya 2009a).
Remarks. The morphology of this species is similar to Acteon elongatus Castellanos, Rolán & Bartolotta, 1987. However, it can be differentiated because A. elongatus does not have a columellar tooth and has a wider aperture (Aldea et al. 2011b).

Aulacomya atra (Molina, 1782)
Synonymy. See Reid and Osorio (2000). Remarks. Reid and Osorio (2000) noted that this species is easily distinguishable from the other species of mytilids that exist on the Chilean coast, given its radial ribs on valves. However, specimens less than 40 mm could be confused with Perumytilus purpuratus (Lamark, 1819). But at that size, A. atra presents a yellowish or ruddy color, while P. purpuratus has a black periostracum and double the radial ribs (Reid and Osorio 2000).
Remarks. Regarding the current status of this species, Aldea and Rosenfeld (2011) commented that in spite of the genetic and morphological study carried out by Toro (1998), who placed this species in M. edulis chilensis, the taxonomic problem is still not resolved. The study carried out by Cárcamo et al. (2005) on specimens from the Chilean Coast was based on allozymes and compared these specimens with European specimens of M. edulis and M. galloprovincialis (Lamark, 1819). The authors concluded that the Chilean specimens should rather be considered a subspecies of M. galloprovincialis given that it is genetically closer to this species, but having particular and characteristic allele frequencies. Investigating the taxonomy and genetics of Chilean smooth-shelled Mytilus, Borsa et al. (2012) concluded that M. edulis from the northern hemisphere is different from M. edulis from the southern hemisphere in proportion to the nuclear loci and the mitochondrial locus. For this reason they consider them as geographically isolated entities. Thus, the Chilean Blue mussles are considered to represent subspecies of M. edulis. Following the principle of priority, the authors stress that platensis d'Orbigny, 1842 is the correct subspecific name for the southern hemisphere M. edulis, and relegate the name Mytilus chilensis Hupé, 1854 into the synonymy of platensis. Larrain et al. (2012) applied the Me 15-16 marker to samples from sites between 41°S and 51°S and found that the majority of the mussels corresponded to "M. chilensis", and saw no evidence for an occurrence of M. edulis. Additionally, putative hybrids of M. chilensis × M. trossulus and M. chilensis × M. galloprovincialis were detected, and the authors stressed that other markers are needed to differentiate between the southern hemisphere Mytilus species. Concluding it can be said the the correct taxonomic allocation for the southern-hemisphere Mytilus species is still pending. For the time being, we here use the name platensis d'Orbigny, 1842 as a subspecies of M. edulis for the specimens from our samples.  (Dell 1971), Róbalo Bay (Dell 1971, Ojeda et al. 2010, Hermite Islands (Dell 1971), Bertrand Island (Dell 1971), Seno Grandi (Dell 1971), and Orange Bay (Rochebrune and Mabille 1889); Chubut (Carcelles 1944), Malvi-nas/Falkland Islands (Dell 1964), San Sebastián Bay (USNM 2010), and Staten Island (USNM 2010). WTSA: Uruguay (Scarabino 2003b), and Buenos Aires Province (Carcelles 1944). Fig. 7C Material examined. 4 spm (4.5 × 4 -5 × 5 mm).

Zygochlamys patagonica (King & Broderip, 1832)
Synonymy. See Cárdenas et al. (2008). Remarks. Waloszek (1984) reported that the species has a wide variability in its morphological characteristics, presenting different types of sculpture. Coloration can range from white to dark red and yellow. This species is found in shallow waters and on the front of slopes, where it forms large banks (Zelaya 2009b).

Austrochlamys natans (Philippi, 1845)
Synonymy. See Dell (1971). Remarks. Dell (1971) concluded that this species inhabits fronds of the giant kelp Macrocystis pyrifera and that juveniles present a thin shell that is semitransparent, due to an adaptation to this environment. In relation to its comparative morphology, it can be differentiated from Zygochlamys patagonica because of its globular, delicate shell and wider radial cords (Zelaya 2009b).
Remarks. This species is very similar to Carditella tegulata (Reeve, 1843), which has a triangular contour, but its shell is equilateral, with a central umbo and straight upper and lower dorsal margins (Zelaya 2009b). Accordingly, Smith (1881) distinguished the species due to the presence of 14-15 radial ribs and a central umbo. However, the specimens revised by Reid and Osorio (2000) had a corresponding sculpture to Carditella naviformis, but the radial ribs were slightly pronounced from 11 to 13 in number, and the margins of the shell were more similar to C. tegulata.

Tawera elliptica (Lamark, 1818)
Synonymy. See Gordillo (2006). Remarks. The morphology of this species is similar to the smallest specimens of Retrotapes exalbidus. Zelaya (2009b) showed that they can be differentiated in that T. elliptica has wider cords and finer interspaces and the inside of the shell is either purplish or brownish. All specimens collected during this study had a strong violet coloring on the inside of the valves.
Remarks. This species is an epibiont of the giant kelp Macrocystis pyrifera (Ralph and Maxwell 1977), although it can also be found in blocks and by personal observation. It is an incubating species that retains embryos in the gills until they are completely developed.

Biogeography
Of the identified 42 species, 29% have a wide distribution, 9% are distributed in the Warm Temperate South-eastern Pacific-Magellanic provinces, 38% are Magellanic (sensu stricto), and 12% present a Warm Temperate Southwestern Atlantic-Magellanic distribution and Magellanic-Southern Ocean distribution, respectively (Fig. 8).
Taking into account the 9 species of the class Polyplacophora recorded in this study, only the species Callochiton puniceus and Plaxiphora aurata showed a Magellanic-Southern Ocean distribution, while two species were found in the Southeast Temperate Magellanic-Pacific area and four species were distributed only in the Magellan Region (Fig. 8).
Shared species between sampling sites and the different biogeographic areas assessed showed variable values (Table 3). The highest ratio of similarity was observed in Bivalvia from Atlantic Patagonia (0.89), followed by Gastropoda in the same area (0.71). In third place are the Polyplacophora from the intermediate area of the Southeastern Pacific, Bivalvia from Uruguay and the Southern Ocean (0.56, respectively). However, lower values were observed in Gastropoda from Peru and Polyplacophora from Uruguay (0.00, which indicates no species shared with those areas).
The Simpson similarity coefficient showed the greatest value in Polyplacophora from Antarctica with 0.500 (Table 3) and 0.172, respectively (Table 3). Except for areas where there are no shared species, the lowest values were recorded in Bivalvia from Peru with 0.003, and Gastropoda from the Warm Temperate South-eastern Pacific with 0.004 (Table 3).

Number and composition of species
The Magellan Region, defined in the database of Linse (1999) such as the Patagonian platform south of 41°S in the Pacific and Atlantic margins of South America, reports 381 marine species: 250 gastropods and 131 bivalves, not including polyplacophorans due to taxonomic problems with the group. Of the total species reported by Linse (1999), 278 inhabit depths less than 30 m, being considered "shallow-water species": 180 gastropods and 98 bivalves. The 33 species recorded in this study correspond to 12% of the total shallow-water species cited: 13% for Gastropoda and 9% for Bivalvia.  Sirenko (2006) investigated the state of knowledge about the Polyplacophora from the Strait of Magellan and the Malvinas/Falkland Islands, recording a total of 17 species for the Strait of Magellan. However, the author was only able to collect 14 species, due to the rarity of some species, such as Ischnochiton pusio. Additionally, there are 11 other species of polyplacophorans cited for the Magellan Region, but Sirenko (2006) noted that these records were probably erroneous, given that these species are usually present in warmer waters. The 9 polyplacophoran species recorded in this study (2 Ischnochitonidae,1 Callochitonidae,4 Chitonidae, and 2 Mopaliidae) correspond to less than 47% of the species cited for the Strait of Magellan by Sirenko (2006). Nevertheless, the percentages given above should be considered only as a reference, since some species could currently be considered junior synonyms of others following the publication of subsequent taxonomic revisions focused on specific groups (e.g. Pastorino 2005a, 2005b, Aranzamendi et al. 2009, Gonzalez-Wevar et al. 2010). Thus, the number of species varies, tending in some cases to decrease (e.g. Schwabe et al. 2006, Zelaya and Geiger 2007, Signorelli and Pastorino 2011. However, there have been descriptions of new species (e.g. Ituarte 2003, 2004), and a complete taxonomic overview is not possible at the time being.
The mollusc species richness recorded in this study represents a value over the average of those reported in other studies in the last 40 years in sublittoral environments in the Strait of Magellan (Table 4). Similarly, the study that presents the highest number of species (Aldea et al. 2011a) reported a total of 101 species of molluscs, but that study boarded a more extensive zone of the western micro-basin of the Strait of Magellan and some adjacent channels, where diverse substrates were studied. The present study is closer in quantity to the number of species carried out by Ríos et al. (2003), which was contained to the eastern micro-basin of the Strait of Magellan, recording 69 species between 30 and 50 meters (see Table 4). Projecting towards the fjord and canal zone in the Magellanic ecoregion, Dell (1971) reported 73 species in an extensive zone between 42°S and 55°S but did not consider the seafloor of the Strait of Magellan. Reid and Osorio (2000) recorded 62 species of molluscs in the sector of Estero Elefantes and Laguna San Rafael (46°S).
From an ecological point of view, it is very difficult to carry out studies on communities and assemblages and be able to establish trophic groups, due to the lack of biologic studies about most of the mollusc species. For example, Chiton bowenii and Nuttallochiton martiali display unusual autecological aspects (Schwabe 2009). Savatieria meridionale should be compared with other species of the genus (Dell 1972), Calliostoma nudum, C. modestulum and Photinastoma taeniatum have a generic position that needs to be revised due to their similar characteristics (see Castellanos and Landoni 1989), Pareuthria paessleri and P. janseni have unknown developmental strategies (Pastorino and Penchaszadeh 2002). Thus, it is very important to conserve this type of environment, given that it shelters species that are considered by some authors to be "rare" or of low frequency (Dell 1971, Sirenko 2006, Rios et al. 2007. In this sense, algae beds of our sampling sites shelter ~38% of rare species for this habitat (see Table 1).

Distribution aspects of the molluscs
Natural beds of G. skottsbergii are characterized by a high species richness of molluscs. This study showed that the assemblage of molluscs that inhabit beds of G. skottsbergii in the Strait of Magellan are species represented in the Magellanic Biogeographic Province, finding 38% of species that are exclusively distributed within this province. Gastropods in this study presented a high percentage of species with Magellanic distribution sensu stricto (Gastropoda = 45.8% and Bivalvia = 11.1%; see Fig. 8) contradicting Linse et al. (2006), who mentioned that for the Strait of Magellan, bivalves present a higher level of endemism than gastropods (Gastropoda = 13.3% and Bivalvia = 23.2%).
Other biogeographic studies carried out in the channels and fjords of Southern Chile showed that gastropods and bivalves have a higher similarity to molluscs from the Malvinas/Falkland Islands and South Georgia Islands (31% and 37%; Brandt et al. 1999). However, in our study, 74% of the species are present in the Malvinas/Falkland Islands and only 5 species (11%) are present in the South Georgia Islands. Therefore, the biogeographic study done by Zelaya (2005) for gastropods in the South Georgia Islands found that the affinity between the Magellanic Province and the South Georgia Islands is lower than those proposed by Brandt et al. (1999), finding only a 16% similarity with the Magellanic gastropods. Of the 24 species of gastropods recorded in this study, only Iothia emarginuloides, Photinastoma taeniatum, and probably Margarella expansa are reported for the South Georgia Islands. For that reason the affinity is quite low (13%).
In a complementary manner, upon comparison of the composition of the 16 genera of gastropods recorded in this study with those reported by Zelaya (2005) for the South Georgia Islands, only the genera Iothia, Margarella, Photinastoma, Eatoniella, and Trophon are present in both sites. In this manner, the low similarity can be observed between gastropod fauna recorded in this study and those from the South Geor- Table 4. Molluscs recorded in works since 1970 in the Strait of Magellan and adjacent channels. We took into account studies where sublittoral samples were collected.

Source
Latitude and depth Gastropoda Bivalvia Polyplacophora Total species † Linse (2002) 52.9-53.7°S; 8-522m 17 1 0 18 Ríos et al. (2003) 52.6-52.8°S; 30-50m 38 21 10 69 Ríos et al. (2005) 52.3-53.9°S; 24-604m 8 6 1 15 Ríos et al. (2007) 53.0-53.6°S; ~8m 9 5 4 18 Thatje and Brown (2009)  gia Islands. This low affinity between the Magellanic province and the South Georgia Islands not only occurs in molluscs, but data from other groups also supports the idea of including the South Georgia Islands within the Antarctic Region (De Broyer and Jazdewski 1993, Zelaya 2005, Spalding et al. 2007). The differences in the fauna composition can likely be explained by the difference in temperatures caused by the presence of the Antarctic convergence and the deep waters between the South Georgia Islands and the Magellanic Province (Zelaya 2005). However, in this study, only 42 species of molluscs were evaluated (corresponding to 12% of the shallow-water species from the Magellanic Province), and as a result, a larger number of samples and studies in different sectors of Magallanes could give better comparative information about the distribution of different mollusc species.
It is important to note that none of the biogeographic studies mentioned (Brandt et al. 1999, Linse et al. 2006 included the class Polyplacophora in their analysis. In this study, of the 9 species identified, 4 (44%) had a Magellanic distribution and highest similarity with Antarctica (see Table 3). Thus, it would be important to consider this group in future biogeographic research to better understand its current status.
Other biogeographic studies carried out in the Eastern Ocean of South America have demonstrated that the highest rates of endemism are found at high latitudes, principally in the Magellanic and Scotia Sea provinces (Fortes and Absalao 2011). At the same time, Fortes and Absalao (2011) mentioned that these high rates of endemism present in the Scotia Sea could be explained by the influence on the degree of isolation that the Antarctic creates over communities of this region (Clarke et al. 2004).
In general, the assemblage of molluscs recorded in this study showed low affinity with other provinces or regions in South America (see Table 3), and the largest proportion of similarity was presented in molluscs of Atlantic Patagonia and in the intermediate area of the Pacific (see Table 3), while the Simpson similarity coefficient in general presented low values, except for the Antarctic Polyplacophora. These results are important from the point of view of conservation of these benthic Magellanic ecosystems, given that an overexploitation of natural habitats of Gigartina skottsbergii would affect mostly endemic species of the Magellanic biogeographic province as well as other species distributed towards the Atlantic Patagonia and the intermediate area of the Pacific. would like to thank the AM Millennium Scientific Initiative (grant no. P05-002 ICM, Chile) and the Basal Financing Program of the Comisión Nacional de Investigación Científica y Tecnológica (grant no. PFB-23, Chile). The authors would like to thank the people of Patagonia Histórica S.A. for their valuable support to our fieldwork in Punta Santa Ana. The authors also thank Katrin Linse for her contribution in improving the manuscript. This is a contribution to the research programme PMI MAG1203 "Gaia-Antártica: Conocimiento y Cultura Antártica" of the Universidad de Magallanes.