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Continental Shelf Research 28 (2008) 788–796 www.elsevier.com/locate/csr

Assemblages of megabenthic gastropods from Uruguayan and northern Argentinean shelf: Spatial structure and environmental controls Alvar Carranzaa,, Fabrizio Scarabinob,c, Alejandro Brazeirod, Leonardo Ortegac, Sergio Martı´ neze a UNDECIMAR, Facultad de Ciencias, Igua´ 4225, CP 11400, Montevideo, Uruguay Museo Nacional de Historia Natural y Antropologı´a, CC. 399, CP 11000, Montevideo, Uruguay c Direccio´n Nacional de Recursos Acua´ticos, Constituyente 1497, CP 11200, Montevideo, Uruguay d Facultad de Ciencias, Departamento de Ecologı´a, Universidad de la Repu´blica, Igua´ 4225, CP11400, Montevideo, Uruguay e Facultad de Ciencias, Departamento de Evolucio´n de Cuencas, Universidad de la Repu´blica, Igua´ 4225, CP11400, Montevideo, Uruguay b

Received 5 June 2007; received in revised form 25 September 2007; accepted 20 December 2007 Available online 31 December 2007

Abstract We analyzed the structure of the megabenthic gastropod assemblages on the Uruguayan and northern Argentinean shelf and slope. Our analysis determined that there are two major biologically distinct assemblages which occurred in a 210,000 km2 area showing conspicuous environmental gradients and large frontal areas: (a) an assemblage associated with the zone under the inﬂuence of the freshwater discharge of Rı´ o de la Plata and the shallow waters of the inner shelf and (b) an assemblage associated with marine zone in the outer shelf, which includes Magellanic (Subantarctic) and subtropical faunas. A multivariate analysis demonstrated a signiﬁcant correlation between the environmental and biological matrix. This evidence suggests a noticeable effect of the physical environment on the spatial structure of the assemblage. We suggest that the current distribution patterns are caused by two different processes operating together: while processes operating at ecological time scales (e.g. differential tolerances to salinity and depth) determine most of the structure observed at the inner shelf, the presence of two contrasting water masses over the outer shelf determine a biogeographic boundary for the benthic fauna, linked to shifting climatic factors inﬂuencing species niche dynamics over evolutionary time scales. Thus, at the spatial scale here considered, ecological and historical processes must be considered when attempting to understand which factors determine the current structure of benthic assemblages at regional scales. r 2007 Elsevier Ltd. All rights reserved. Keywords: Caenogastropoda; Volutidae; Ranellidae; Nassariidae; Biogeography

1. Introduction The extent to which climate limits distribution ranges of marine species, both at global and ﬁne spatial scales is a major concern due to the impacts of climate change on faunal distributions (e.g. Rivadeneira and Ferna´ndez, 2005). In this vein, the study of the distribution of faunal assemblages will lead to a better understanding of the forces that shape spatial variation in community structure and diversity, an unavoidable issue for effective conservation and management of marine biodiversity. Corresponding author. Tel.: +598 2 5258618.

E-mail address: [email protected] (A. Carranza). 0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.12.010

The overlay of the geographic distribution of a species with the geographic distribution of environmental factors has been the traditional approach to identify which environmental factor(s) coincide with a species border (Parmesan et al., 2005). In the same line, one can identify species assemblages and search for spatial discontinuities in the physical environment, thus providing insight into the processes that regulate patterns of distribution. When the spatial scales considered are large enough, the spatial arrangement of the continents and oceans, combined with the inﬂuence of temperature and latitudinal gradients, and properties of water masses divide the oceans into biogeographic regions with characteristic assemblages. These biogeographic provinces are usually associated with major

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thermal or salinity discontinuities (Levinton, 1995). At small, local scales, ecological processes (i.e. competitive exclusion, habitat heterogeneity) become increasingly important. However, patterns at scales between the localscale and the continental scales of traditional biogeography are less documented. In the south western Atlantic, a malacofaunal transition zone has been reported on the Uruguayan shelf (e.g. Floeter and Soares-Gomes, 1999; Kaiser, 1977a; Olivier and Scarabino, 1972; Scarabino, 1977). This area is characterized by a singular hydrographical system composed of water masses of contrasting thermohaline characteristics, i.e. Tropical Waters (TW), Subtropical Waters (STW), Subantarctic Waters (SAW) and Coastal Waters (CW) deﬁned by salinities o33.2 (Emilsson, 1961; Guerrero et al., 1997a; Sverdrup et al., 1942; Thomsen, 1962). At the northern portion of the Uruguayan shelf, the inﬂuence of TW is restricted to the summer–autumn period, and STW mixes with the colder and relatively fresher SAW between the 100–200 m isobath. This deﬁnes a frontal zone at depths greater than 50 m, and generates a temperature gradient (Ortega and Martı´ nez, 2007) over the outer shelf. This gradient is, however, much less dramatic than the one established from the difference in mean temperature of shallow, estuarine waters (CW) and depth SAW waters. The Brazil/Malvinas currents conﬂuence extends offshore to the oceanic domain, and inshore over the shelf, deﬁning a thermohaline subsurface front between Subtropical Shelf Waters and Subantartic Shelf Waters (Piola et al., 2000). This Subtropical Shelf Front is located near the 50 m isobath at 321S and extends southwards towards the shelf break near 361S (Acha et al., 2004). In addition, the inner shelf is affected by the ﬂuvial discharge of the Rı´ o de la Plata, that ﬂows into the Atlantic Ocean with an average discharge of 22,000 m3 s1 (Framin˜an and Brown, 1996; Guerrero et al., 1997b). Near the coast, CW mixes with SAW forming a water type that dominates the water column up to 50 m (Ortega and Martı´ nez, 2007). All these features makes the Uruguayan shelf an area suitable to examine the effects of meso-scale (100–1000 km) hydrologic process (i.e. discharge of Rı´ o de la Plata,

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thermohaline fronts) on the distribution and structure of benthic biota. Surprisingly, there are only a few studies that analyzed meso-scale distribution patterns of the benthic shelf fauna at this area, and most are restricted exclusively to the inner (Carranza et al., 2008; Giberto et al., 2004) or outer shelf areas (Kaiser, 1977a; Olivier and Scarabino, 1972). In a recent study, Giberto et al. (2007) found three distinctive benthic assemblages along a NW–SE transect of 560 km, corresponding to the freshwater, estuarine, and marine sectors, but restricted their analysis to depths o270 m. In this study, we used the complete list of megabenthic (i.e. 45 cm adult size) gastropods reported from the Uruguayan shelf and slope (0–850 m) aiming to evaluate the overall effect of the strong environmental gradients (i.e. depth, sea bottom salinity and temperature, see Fig. 2) and the meso-scale oceanographic processes (i.e. Subtropical Shelf Front) that determines the boundary between biogeographic provinces, on the spatial structure of the benthic assemblages, and to asses the relative strength of these environmental gradients in determining patterns on beta diversity. 2. Material and methods 2.1. Study area and data gathering The study area comprised ca. 210,000 km2 of the Uruguayan and the northern portion of Argentinean continental shelf, between 331300 and 371300 S. Data of occurrence of large gastropods were obtained from a total of 345 sampling sites from 7.5 to 850 m depth (Fig. 1). These were compiled from two sources: (1) previously unpublished data, obtained from ﬁve research cruises made onboard R.V. ‘‘Aldebaran’’ and (2) published data that included either the complete list of species and the exact geographic location of the operation (Carranza, 2006; Carranza et al., 2008; Juanico´ and Rodrı´ guez-Moyano, 1976; Kaiser, 1977a; Milstein et al., 1976; Olivier and Scarabino, 1972; Quintero, 1986) or mentioned the presence of a given species (Scarabino, 2003, 2004, 1968).

Fig. 1. Study area showing the bathymetry and stations surveyed.

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The bulk of these data correspond to surveys made during the last 40 years. Special care was taken to include only records for live animals. The ﬁshing gear used on the research vessel was an Engel trawl with a 24 m horizontal opening and a 60–100 mm stretched mesh in the cod end. The average soak time was 300 . The exact location of the stations was determined by global positioning system (GPS). Mean operational depth was measured for all the stations. In the commercial trawlers, the gear used was quite similar, but the soak time was in general more that 2 h. Large gastropods collected were identiﬁed to species in situ. Voucher material for each one is deposited at the Museo Nacional de Historia Natural y Antropologı´ a (Montevideo). Presence/absence data for species were then binned in quadrats of 0.51 latitude 0.51 longitude to match the spatial scale of oceanographic data and to achieve a more complete inventory of the megabenthic gastropods within each quadrat. An environmental matrix was constructed using oceanographic data provided by Guerrero et al. (1997b) gathered over 30 years and included minimum, maximum, and mean annual seabed salinities and temperatures as well as it ranges of variation. This was done using seasonal values, with a spatial deﬁnition of 0.51 latitude 0.51 longitude quadrats (ca. 2500 km2). Sediment features were not included due to the lack of available data at an appropriate spatial scale. The study area is dominated by an homogeneous soft bottom body, with an increase on mean grain size towards the continental shelf and slope, and presents little consolidated substrata (Correia et al., 1996). 2.2. Multivariate analysis Hierarchical agglomerative clustering was undertaken using group-average link on Sorensøn association coefﬁcients calculated from presence/absence species matrix (e.g. Clifford and Stephenson, 1975). We thus obtained groups of quadrats based on similarities of species composition. To determine species assemblages we proceed in the following way: (1) a dendrogram based in the Sorensøn distance matrix was constructed and species groups identiﬁed and (2) potential cause of the afﬁnities among quadrats based on the species composition were examined using non-metric multidimensional scaling (NMDS). To test the ordination, the stress coefﬁcient of Kruskal was employed (Kruskal and Wish, 1978). The BIO-ENV procedure (Clarke, 1993; Clarke and Ainsworth, 1993) was used to ﬁnd the suite of environmental variables that best explain the biological structure. This analysis calculated weighted Spearman rank correlation coefﬁcients (r) between the distance matrix calculated for biotic data and the Euclidean distance matrices for all combinations of environmental variables. The null hypothesis that there is no match between two similarity matrices was tested using a Monte Carlo/permutation procedure (RELATE procedure, PRIMER-E version 5, 2001), which

permutes the sample labels from one of the similarity matrices (in this case obtained from the biotic matrix using Sorensøn distance) and recalculates the match (the rank correlation, r) a large number of times. Previous to the analysis, variables were checked for colinearity using Pearson’s product moment correlation coefﬁcient. All variables were retained as none had a mutual r-value greater than 0.95. The remaining variables were standardized and received values ranking from 0 to 1. 2.3. Beta diversity: species continuity, gain, and loss The turnover between two points along a gradient is essentially some measure of the difference between the lists of species present in each point (Janson and Vegelius, 1981). Thus, for a given pair of adjacent points along a gradient, the total number of shared species is the pairwise matching component (continuity, Ai,i+1). The number of species that are present only in the i+1 point is the number of species gained on entering an adjacent quadrat (Bi,i+1), while the number present only in the point i is the species loss (Ci,i+1; Lennon et al., 2001). We thus examined the behavior of these measures along the salinity gradient at the inner shelf and the latitudinal gradient at the outer shelf and slope, the latter correlated with a secondary temperature gradient determined by the Subtropical Shelf Front, in order to asses the strength of these gradients in driving local patterns of beta diversity. 3. Results The assembled database consisted of presence/absence information on 18 large gastropod species, all previously reported for the study area. Hierarchical agglomerative clustering of quadrats based on presence/absence matrix separates two groups (distance 40.8) corresponding grossly to inner (22 quadrats) and outer (19 quadrats) shelf (Fig. 3A). Clustering analysis of species separated those inhabiting the inner from those found in the outer shelf quadrats (distance 40.9; Fig. 4). Within inner shelf species, Rapana venosa was a separate subgroup (subgroup 1). Stramonita haemastoma and Cymatium parthenopeum were subgroup 2, closely associated to another subgroup composed of Pachycymbiola brasiliana, Buccinanops cochlidium, Tonna galea, Zidona dufresnei, and Adelomelon beckii (subgroup 3). The outer shelf species group showed one subgroup with Trophon acanthodes and Provocator corderoi, while the subgroup 5 comprised Adelomelon riosi, Chicoreus beauii, and Ranella olearium and subgroup 6 comprised Americominella duartei, Fissurellidea megatrema, Fusitriton magellanicus, Adelomelon ancilla, and Odontocymbiola magellanica. Species distribution along the dimensional gradient obtained by means of NMDS is shown in Fig. 4. As expected, the groups closely resembled those identiﬁed by clustering. R. venosa, P. brasiliana, B. cochlidium, Z. dufresnei, T. galea, and A. beckii are shallower and common species, whereas A. ancilla, F. magellanicus,

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Fig. 2. Spatial gradients in sea bottom salinity (A) and temperature (B) over the study area. Arrows indicate increasing mean annual salinity and temperature. Note the secondary temperature gradient determined over the outer shelf (50–200 m) associated with the Subtropical Shelf Front (SSF). The Malvinas–Brazil conﬂuence (MBC) and the Subtropical Convergence (STC), occurring off the continental shelf are also illustrated based on average winter sea surface temperatures.

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O. magellanica, F. megatrema, T. acanthodes, and P. corderoi were associated with deeper areas. The latter species occurred in the southern portion of the study area and showed high frequency of occurrence, with the exception of the rare species T. acanthodes and P. corderoi. Shallower, scarce species were placed at the top end of the bi-dimensional plot (S. haemastoma and C. parthenopeum), whereas deeper, rare species occurring exclusively at the northernmost portion of the shelf (C. beauii and R. olearium) were placed at the upper right corner. Finally, two species (A. duartei and A. riosi) showed little association with each other and the remaining groups of species. Biotic and environmental matrices were positively and signiﬁcantly correlated (r ¼ 0.506; p ¼ 0.01). BIOENV analysis conducted on the overall data set showed a maximum correlation of 0.553 for a model including two variables (mean salinity and mean temperature). When only one variable was allowed to enter the model, mean temperature showed the highest correlation (0.523), whereas the best model with three variables included mean depth, mean temperature, and mean salinity (0.536). The correlation coefﬁcient did not increase as additional variables were included. The best results for one to three variables of the analysis of relationships between community composition and the set of environmental variables are given in Table 1. Concerning beta diversity, continuity showed distinct patterns between the gradients studied: along the salinity gradient at the inner shelf, it ranged from 0 to 3 spp. (mean: 0.73 sp.) peaking at salinities from 29.91 to 32.94 and being 0 for salinities from 5.75 to 10.41(Fig. 5). In contrast, continuity averaged 4.12 spp. (range: 3–6 spp.) along the latitudinal gradient at the outer shelf. At this area, species loss (mean: 1.37, range: 0–6 spp.) outweighed species gain (mean: 0.87, range: 0–2 spp.), whereas species loss (mean: 0.09, range: 0.72–0.8 spp.) was lower than species gain (mean: 0.66, range 0–3 spp.) along the inner shelf salinity gradient. It must be noted that species gain and loss are referred to the context of increasing salinity and decreasing latitude (and temperature) (Fig. 6). 4. Discussion

Fig. 3. Hierarchical clustering of quadrats (A) and spatial distributions of the two groups corresponding to inner and outer shelf (B).

Megabenthic gastropod species composition varied across and along the shelf as a response to environmental gradients. Our results indicated a strong effect of environment in the current distribution patterns and assemblage structure, as conﬁrmed by the different analysis employed. Unfortunately, we lack information for several quadrats, but overall, both extremes of the gradients in environmental variables as well as the main frontal zones are well represented. Taking into consideration the coarse spatial deﬁnition of our analyses, as well as the use of oceanographic data averaged on an annual basis and incorporating inter-annual variability in oceanographic conditions, our results constitute an accurate description of the patterns of large gastropod assemblages. Accordingly, the

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Fig. 4. Hierarchical clustering of species. Species abbreviations: AA: Adelomelon ancilla; AB: Adelomelon beckii; AR: Adelomelon riosi; PC: Provocator corderoi; OM: Odontocymbiola magellanica; PB: Pachycymbiola brasiliana; ZD: Zidona dufresnei; BC: Buccinanops cochlidium; AD: Americominella duartei; CB: Chicoreus beauii; CP: Cymatium parthenopeum; FM: Fissurellidea megatrema; FUM: Fusitriton magellanicus; RO: Ranella olearium; RV: Rapana venosa; SH: Stramonita haemastoma; TA: Trophon acanthodes; TG: Tonna galea.

Table 1 Results of the BIOENV analysis showing the set of variables that best explain the biological data models including 1, 2, and 3 variables are shown Maximum number of variables allowed 1

2

3

Selected variables

r

Selected variables

r

Selected variables

r

MT MZ MS TR MINT

0.523 0.447 0.418 0.392 0.371

MS, MT MT, SR MT MZ, MT MT, TR

0.553 0.523 0.523 0.501 0.479

MS, MT MZ, MS, MT MZ, TR, MS MS, MT, SR MT, SR

0.553 0.536 0.533 0.527 0.523

Correlation values represent the Spearman coefﬁcient (r). MZ: mean depth, MS: mean salinity, MT: mean temperature, SR: salinity range, TR: temperature range, and MINT: minimum temperature.

Fig. 5. Bi-dimensional plot of species obtained by NMDS. Abbreviations as in Fig. 4. Stress ¼ 0.06.

faunal patterns here depicted should be only slightly inﬂuenced by the temporal scale spanning species records, since the range of times over which the bulk of samples were collected is close to the life span of most of the longlived species herein studied (Bigatti et al., 2006; Cledo´n et al., 2005). The study area can be divided into two main zones, as shown by the classiﬁcation analysis. These two areas differed not only in terms of its mean depth (276 vs. 42 m), salinity (33 vs. 29) and temperature (14 vs. 7 1C) but also in the annual salinity (2.8 vs. 0.6) and temperature (4.1 vs. 8.7 1C) ranges. The existence of a discrete estuarine assemblage such as the reported by Giberto et al. (2007) is masked by major faunal differences between inner and

outer shelf faunas. However, the dramatic effects of the saline gradient were already reported for soft bottom (Gime´nez et al., 2005; Lercari and Defeo, 2006) and hard substrata (Brazeiro et al., 2006) intertidal invertebrates from the Uruguayan coast and the Uruguayan and Argentinean shelf (Giberto et al., 2007). The afﬁnities between species depicted by the ordination analysis can be explained taken into account some aspects of the macro-scale distribution patterns of the species. The inner shelf was dominated by species occurring along the South Atlantic coast and an exotic, invader Asiatic gastropod (R. venosa) exclusively found in those quadrats under the estuarine regime. Two species (S. haemastoma and C. parthenopeum) are widely distributed in the Atlantic

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Fig. 6. Turnover patterns along the latitudinal and saline gradient. Turnover is expressed as species continuity and as absolute numbers of species gained or lost between two adjacent points.

and Western Paciﬁc and worldwide, respectively, associated with tropical or warm waters (Beu, 1998; Clench, 1947). Both species were represented by unique or few records (Carranza et al., 2008; Juanico´ and Rodrı´ guezMoyano, 1976). However, S. haemastoma is commonly found at or near the shallow intertidal (Scarabino et al., 2006) along the rocky substrata in the Uruguayan Atlantic coast. P. brasiliana, T. galea, Z. dufresnei, A. beckii, and B. cochlidium are much better represented, being mainly associated with depths o50 m. Z. dufresnei was the most ubiquitous species. All these species occurs at the Uruguayan Atlantic coast, Buenos Aires province, Argentina (Castellanos, 1970; Penchaszadeh and de Mahieu, 1976) and Rio Grande do Sul (Brazil; Rios, 1994), the exception being T. galea with only one doubtful record south of Rı´ o de la Plata (Doello-Jurado, 1938). In contrast, the southern portion of the outer shelf was inhabited by mainly Subantarctic or Magellanic species, reaching the northernmost point of its distribution in the study area. This is the case for A. ancilla, O. magellanica, F. magellanicus, A. duartei, T. acanthodes, and P. corderoi

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(Carcelles, 1947; Carranza et al., 2007; Kaiser, 1977a, b; Olivier and Scarabino, 1972; Pastorino, 2005). Closely related species (e.g. subgroup 6) may co-occur due to convergence upon common prey (e.g. Psychrochlamys patagonica, Mytilus edulis) the former being a characteristic species of the cold temperate Magellanic biogeographic Province (Orensanz et al., 1991). The most important scallop beds are located along the 100 m depth isobaths and correspond spatially with the continental shelf-break front near the continental slope (Gutie´rrez and Defeo, 2003). Botto et al. (2006) showed that scallops are the main prey for F. magellanicus, but O. magellanica and A. ancilla also showed evidence of having scallops as part of their diets. Some tropical and/or subtropical species are also represented in the outer shelf: C. beauii and R. olearium (both pelagic developers with long-lived larvae) were found at depths ranging from 140 to 276 m at the northern portion of the study area, constituting the southernmost records for both species (Scarabino, 2003, 1968). A latter, bathyal species, A. riosi is distributed from Rı´ o de Janeiro (Brazil) to Uruguay (231–351400 S, Kaiser, 1977a); the location reported for the holotype, 130 miles east of Mar del Plata lacks precise geographic references, in depths 4600 m. When the midpoints of species latitudinal ranges are averaged and compared, the average midpoint for inner shelf species is 171S, while for outer shelf species is near 331S. If R. olearium and C. beauii (both wide ranging species with only scattered records for the Uruguayan shelf) are excluded, the average midpoint of outer shelf assemblage is near 411S. Thus, this support the idea that species assemblages represent groups of species with different biogeographic history, allowing us to suggest that regional biogeography is a major determinant of overall local community composition and structure. Marine Cenozoic gastropod fauna in the area are known since the Late Miocene (ca. 10 Myr), when a large area of southern South America was occupied by a shallow sea, and was composed by tropical/subtropical elements, considered to be inﬂuenced by a proto-Brazilian warm current that extended at least to Peninsula Valde´s (Argentina). Later, when the Malvinas current begun to fully operate reaching latitudes off the mouth of Rı´ o de la Plata, species and/or lineages adapted to cold temperatures probably dispersed northward (Martı´ nez and del Rı´ o, 2002a, b). In Pleistocene times (ca. 120,000 years), the sea occupied the area presently named Rı´ o de la Plata, and the three current biogeographic groups of molluscs were already established (i.e. species associated with STW, SAW, and endemics from the frontal zone). However, warm water species were represented in a higher proportion than in the present (Martı´ nez et al., 2001), and included some species that today has retracted their limit northwards. This evidence indicates higher temperatures that today. This situation persists in Holocene times at least until ca. 2000 years ago (Martı´ nez et al., 2006).

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Despite the above outlined biogeographic scenario, we used latitude as a proxy to evaluate the effect of large scale oceanographic processes determining a secondary temperature gradient on the current beta diversity of the outer shelf assemblage. This analysis showed that, compared with the structuring effect of the saline gradient at the inner shelf, the latitudinal gradient produced a comparatively weaker effect on the observed beta diversity patterns that can be partially explained for the spatial grain of the analysis. Thus, evidence for a transitional zone, in contrast with a rigid boundary, emerged for our data from megabenthic gastropods, attributable to slight differences in species tolerances to environmental conditions. In this vein, it should be taken into account that no subantarctic species extends its distribution north of 351S, and, conversely, no subtropical species occurs south of that latitude. An exception is the bathyal species A. riosi, associated with deeper waters and displaying an unusual (and not yet fully analyzed) distribution. Records for the subantarctic species here treated for southern Brazil (i.e. Rio Grande do Sul, Rios, 1994) are based on material provided by ﬁshermen, lacked precise geographic references and most likely were collected in southern Uruguayan shelf. In this context, it should be stressed that the marine benthic biogeographical patterns of south western Atlantic is in need of reassessment integrating all existing accurate and veriﬁable data from different taxa. Current schemes are based on sole taxonomical groups and some schemes tend to be particularly inﬂuenced by the patterns observed in pelagic biota. The latter clearly displays strong seasonality at this spatial scale which is not necessarily reﬂected in benthic invertebrate distribution patterns. Despite well-deﬁned boundaries between the two gastropod faunas, a perfect bio-environmental match was not achieved, as observed in the output of the BIOENV analysis. This can be due to: (a) effects of non-measured variables: the inclusion of other physical variables may increase the observed correlation or (b) other ecological processes (current or historical) affecting assemblage structure. Weak relations between measured environmental variables and community structure are expected when depth and sediment characteristics are remarkably uniform over the studied area (Ellingsen, 2001), which is clearly not the case in our study. Several variables have been included in other bio-environmental analysis: for example, surface sediment chlorophyll-a and phaeopigment contents, total organic carbon (TOC) content of surface sediments, carbon:nitrogen (C:N) sediment ratios and dissolved oxygen (DO) were utilized in multivariate analysis for the study of the macrobenthic animal assemblages of the continental margin of Chile (Palma et al., 2005). The number of species and diversity were found to be correlated with changes in bottom-water oxygen concentrations and sediment-bound pigments. A recent study performed on the Gulf of Mexico (o200 m) showed that sediment mean grain size, percentage of clay and organic matter best

explained the macroinfauna spatial patterns, although BIOENV indicated that depth has an overriding role (Herna´ndez-Arana et al., 2003). Concerning the second hypothesis, some biological features of the species involved may affect distribution patterns. Dispersal capability is one of these traits (Carlon and Olson, 1993; Grantham et al., 2003; Heck and McCoy, 1978; Pechenik, 1999; Scheltema, 1971) with broad geographic distributions in marine organisms correlated with the presence of a more or less longlived pelagic larval stage. However, means of dispersal do not always correlate with distribution. Springer (1982) cannot ﬁnd a clear correlation between distribution and dispersal. Thresher and Brothers (1985) found no direct correlation between geographic range size and duration of the pelagic larval stage. Thresher and Brothers also cited Atlantic gastropods (Scheltema, 1971), as other groups which show poor correlation between maximum duration of the larval life and of distribution extent. The authors suggested that some other factor, such as relative speciﬁcity of recruitment sites, is being overlooked or that historical factors are of importance (see Heads, 2005).

5. Conclusions Two main faunistic subunits were determined on the Uruguayan shelf: (a) a zone under the inﬂuence of the freshwater discharge of Rı´ o de la Plata and the shallow waters of the inner shelf and (b) a marine zone in the outer shelf, which includes magellanic and subtropical faunas. Beta diversity was strongly inﬂuenced by the saline gradient operating at the inner shelf, while a species turnover associated with latitude was detected in the outer shelf. Concerning the multivariate structure, a signiﬁcant correlation was found between the environmental and biological matrix. This evidence suggests a noticeable effect of the physical environment on the spatial structure of the assemblage, despite of not being fully explained by the environment. At the spatial scale considered, current (e.g. generation of biogenic substrata like mussel beds) or historical processes including regional biogeography are a major determinant of local community composition.

Acknowledgments The ﬁeldwork was done with the invaluable help of the crew of the R.V. ‘‘Aldebaran’’, specially Pablo Puig, Ernesto Chiesa, and Laura Paesch. Financial support from CSIC of the Universidad de la Repu´blica and PEDECIBA (Uruguay) to A.C. is acknowledged. Special thanks to two anonymous reviewers that helped to substantially improve the manuscript. A.C. thanks Marina and Estela for encouragement and support.

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