Assemblages of megabenthic gastropods from Uruguayan and northern Argentinean shelf: Spatial structure and environmental controls

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Continental Shelf Research 28 (2008) 788–796

Assemblages of megabenthic gastropods from Uruguayan and northernArgentinean shelf: Spatial structure and environmental controls

Alvar Carranzaa,�, Fabrizio Scarabinob,c, Alejandro Brazeirod,Leonardo Ortegac, Sergio Martıneze

aUNDECIMAR, Facultad de Ciencias, Igua 4225, CP 11400, Montevideo, UruguaybMuseo Nacional de Historia Natural y Antropologıa, CC. 399, CP 11000, Montevideo, UruguaycDireccion Nacional de Recursos Acuaticos, Constituyente 1497, CP 11200, Montevideo, Uruguay

dFacultad de Ciencias, Departamento de Ecologıa, Universidad de la Republica, Igua 4225, CP11400, Montevideo, UruguayeFacultad de Ciencias, Departamento de Evolucion de Cuencas, Universidad de la Republica, Igua 4225, CP11400, Montevideo, Uruguay

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 Rıo 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 ofmarine species, both at global and fine spatial scales is amajor concern due to the impacts of climate change onfaunal distributions (e.g. Rivadeneira and Fernandez,2005). In this vein, the study of the distribution of faunalassemblages will lead to a better understanding of theforces that shape spatial variation in community structureand diversity, an unavoidable issue for effective conserva-tion and management of marine biodiversity.

The overlay of the geographic distribution of a specieswith the geographic distribution of environmental factorshas been the traditional approach to identify whichenvironmental factor(s) coincide with a species border(Parmesan et al., 2005). In the same line, one can identifyspecies assemblages and search for spatial discontinuities inthe physical environment, thus providing insight into theprocesses that regulate patterns of distribution. When thespatial scales considered are large enough, the spatialarrangement of the continents and oceans, combined withthe influence of temperature and latitudinal gradients, andproperties of water masses divide the oceans into biogeo-graphic regions with characteristic assemblages. Thesebiogeographic provinces are usually associated with major

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thermal or salinity discontinuities (Levinton, 1995). Atsmall, local scales, ecological processes (i.e. competitiveexclusion, habitat heterogeneity) become increasinglyimportant. However, patterns at scales between the local-scale and the continental scales of traditional biogeographyare less documented.

In the south western Atlantic, a malacofaunal transitionzone has been reported on the Uruguayan shelf (e.g.Floeter and Soares-Gomes, 1999; Kaiser, 1977a; Olivierand Scarabino, 1972; Scarabino, 1977). This area ischaracterized by a singular hydrographical system com-posed of water masses of contrasting thermohalinecharacteristics, i.e. Tropical Waters (TW), SubtropicalWaters (STW), Subantarctic Waters (SAW) and CoastalWaters (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, theinfluence of TW is restricted to the summer–autumnperiod, and STW mixes with the colder and relativelyfresher SAW between the 100–200m isobath. This defines afrontal zone at depths greater than 50m, and generates atemperature gradient (Ortega and Martınez, 2007) over theouter shelf. This gradient is, however, much less dramaticthan the one established from the difference in meantemperature of shallow, estuarine waters (CW) and depthSAW waters. The Brazil/Malvinas currents confluenceextends offshore to the oceanic domain, and inshore overthe shelf, defining a thermohaline subsurface front betweenSubtropical Shelf Waters and Subantartic Shelf Waters(Piola et al., 2000). This Subtropical Shelf Front is locatednear the 50m isobath at 321S and extends southwardstowards the shelf break near 361S (Acha et al., 2004). Inaddition, the inner shelf is affected by the fluvial dischargeof the Rıo de la Plata, that flows into the Atlantic Oceanwith an average discharge of 22,000m3 s�1 (Framinan andBrown, 1996; Guerrero et al., 1997b). Near the coast, CWmixes with SAW forming a water type that dominates thewater column up to 50m (Ortega and Martınez, 2007). Allthese features makes the Uruguayan shelf an area suitableto examine the effects of meso-scale (100–1000 km)hydrologic process (i.e. discharge of Rıo de la Plata,

thermohaline fronts) on the distribution and structure ofbenthic biota.Surprisingly, there are only a few studies that analyzed

meso-scale distribution patterns of the benthic shelf faunaat this area, and most are restricted exclusively to the inner(Carranza et al., 2008; Giberto et al., 2004) or outer shelfareas (Kaiser, 1977a; Olivier and Scarabino, 1972). In arecent study, Giberto et al. (2007) found three distinctivebenthic assemblages along a NW–SE transect of 560 km,corresponding to the freshwater, estuarine, and marinesectors, but restricted their analysis to depths o270m. Inthis study, we used the complete list of megabenthic (i.e.45 cm adult size) gastropods reported from the Urugua-yan shelf and slope (0–850m) aiming to evaluate theoverall effect of the strong environmental gradients (i.e.depth, sea bottom salinity and temperature, see Fig. 2) andthe meso-scale oceanographic processes (i.e. SubtropicalShelf Front) that determines the boundary betweenbiogeographic provinces, on the spatial structure of thebenthic assemblages, and to asses the relative strength ofthese environmental gradients in determining patterns onbeta diversity.

2. Material and methods

2.1. Study area and data gathering

The study area comprised ca. 210,000 km2 of theUruguayan and the northern portion of Argentineancontinental shelf, between 331300 and 371300S. Data ofoccurrence of large gastropods were obtained from a totalof 345 sampling sites from 7.5 to 850m depth (Fig. 1).These were compiled from two sources: (1) previouslyunpublished data, obtained from five research cruises madeonboard R.V. ‘‘Aldebaran’’ and (2) published data thatincluded either the complete list of species and the exactgeographic location of the operation (Carranza, 2006;Carranza et al., 2008; Juanico and Rodrıguez-Moyano,1976; Kaiser, 1977a; Milstein et al., 1976; Olivier andScarabino, 1972; Quintero, 1986) or mentioned thepresence of a given species (Scarabino, 2003, 2004, 1968).

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

A. Carranza et al. / Continental Shelf Research 28 (2008) 788–796 789


The bulk of these data correspond to surveys made duringthe last 40 years. Special care was taken to include onlyrecords for live animals. The fishing gear used on theresearch vessel was an Engel trawl with a 24m horizontalopening and a 60–100mm stretched mesh in the cod end.The average soak time was 300. The exact location of thestations was determined by global positioning system(GPS). Mean operational depth was measured for all thestations. In the commercial trawlers, the gear used wasquite similar, but the soak time was in general more that2 h. Large gastropods collected were identified to speciesin situ. Voucher material for each one is deposited at theMuseo Nacional de Historia Natural y Antropologıa(Montevideo).

Presence/absence data for species were then binned inquadrats of 0.51 latitude� 0.51 longitude to match thespatial scale of oceanographic data and to achieve a morecomplete inventory of the megabenthic gastropods withineach quadrat. An environmental matrix was constructedusing oceanographic data provided by Guerrero et al.(1997b) gathered over 30 years and included minimum,maximum, and mean annual seabed salinities and tem-peratures as well as it ranges of variation. This was doneusing seasonal values, with a spatial definition of 0.51latitude� 0.51 longitude quadrats (ca. 2500 km2). Sedimentfeatures were not included due to the lack of available dataat an appropriate spatial scale. The study area is dominatedby an homogeneous soft bottom body, with an increaseon mean grain size towards the continental shelf andslope, and presents little consolidated substrata (Correiaet al., 1996).

2.2. Multivariate analysis

Hierarchical agglomerative clustering was undertakenusing group-average link on Sorensøn association coeffi-cients calculated from presence/absence species matrix (e.g.Clifford and Stephenson, 1975). We thus obtained groupsof quadrats based on similarities of species composition.To determine species assemblages we proceed in thefollowing way: (1) a dendrogram based in the Sorensøndistance matrix was constructed and species groupsidentified and (2) potential cause of the affinities amongquadrats based on the species composition were examinedusing non-metric multidimensional scaling (NMDS). Totest the ordination, the stress coefficient of Kruskal wasemployed (Kruskal and Wish, 1978).

The BIO-ENV procedure (Clarke, 1993; Clarke andAinsworth, 1993) was used to find the suite of environ-mental variables that best explain the biological structure.This analysis calculated weighted Spearman rank correla-tion coefficients (r) between the distance matrix calculatedfor biotic data and the Euclidean distance matrices for allcombinations of environmental variables. The null hypoth-esis that there is no match between two similarity matriceswas tested using a Monte Carlo/permutation procedure(RELATE procedure, PRIMER-E version 5, 2001), which

permutes the sample labels from one of the similaritymatrices (in this case obtained from the biotic matrix usingSorensøn distance) and recalculates the match (the rankcorrelation, r) a large number of times. Previous to theanalysis, variables were checked for colinearity usingPearson’s product moment correlation coefficient. Allvariables were retained as none had a mutual r-valuegreater than 0.95. The remaining variables were standar-dized 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 isessentially some measure of the difference between the listsof species present in each point (Janson and Vegelius,1981). Thus, for a given pair of adjacent points along agradient, the total number of shared species is the pairwisematching component (continuity, Ai,i+1). The number ofspecies that are present only in the i+1 point is the numberof species gained on entering an adjacent quadrat (Bi,i+1),while the number present only in the point i is the speciesloss (Ci,i+1; Lennon et al., 2001). We thus examined thebehavior of these measures along the salinity gradient atthe inner shelf and the latitudinal gradient at the outer shelfand slope, the latter correlated with a secondary tempera-ture gradient determined by the Subtropical Shelf Front, inorder to asses the strength of these gradients in drivinglocal patterns of beta diversity.

3. Results

The assembled database consisted of presence/absenceinformation on 18 large gastropod species, all previouslyreported for the study area. Hierarchical agglomerativeclustering of quadrats based on presence/absence matrixseparates two groups (distance 40.8) correspondinggrossly to inner (22 quadrats) and outer (19 quadrats)shelf (Fig. 3A). Clustering analysis of species separatedthose inhabiting the inner from those found in the outershelf quadrats (distance 40.9; Fig. 4). Within inner shelfspecies, Rapana venosa was a separate subgroup (subgroup 1).Stramonita haemastoma and Cymatium parthenopeum weresubgroup 2, closely associated to another subgroup composedof Pachycymbiola brasiliana, Buccinanops cochlidium, Tonna

galea, Zidona dufresnei, and Adelomelon beckii (subgroup 3).The outer shelf species group showed one subgroup withTrophon acanthodes and Provocator corderoi, while thesubgroup 5 comprised Adelomelon riosi, Chicoreus beauii,and Ranella olearium and subgroup 6 comprised Americomi-

nella 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 identifiedby clustering. R. venosa, P. brasiliana, B. cochlidium,Z. dufresnei, T. galea, and A. beckii are shallower andcommon species, whereas A. ancilla, F. magellanicus,

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O. magellanica, F. megatrema, T. acanthodes, and P. corderoi

were associated with deeper areas. The latter speciesoccurred in the southern portion of the study area andshowed high frequency of occurrence, with the exception ofthe rare species T. acanthodes and P. corderoi. Shallower,scarce species were placed at the top end of the bi-dimensionalplot (S. haemastoma and C. parthenopeum), whereas deeper,rare species occurring exclusively at the northernmostportion of the shelf (C. beauii and R. olearium) were placedat the upper right corner. Finally, two species (A. duartei

and A. riosi) showed little association with each other andthe remaining groups of species.Biotic and environmental matrices were positively and

significantly correlated (r ¼ 0.506; p ¼ 0.01). BIOENVanalysis conducted on the overall data set showed amaximum correlation of 0.553 for a model including twovariables (mean salinity and mean temperature). Whenonly one variable was allowed to enter the model, meantemperature showed the highest correlation (0.523),whereas the best model with three variables included meandepth, mean temperature, and mean salinity (0.536). Thecorrelation coefficient did not increase as additionalvariables were included. The best results for one to threevariables of the analysis of relationships between commu-nity composition and the set of environmental variables aregiven in Table 1.Concerning beta diversity, continuity showed distinct

patterns between the gradients studied: along the salinitygradient 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 being0 for salinities from 5.75 to 10.41(Fig. 5). In contrast,continuity averaged 4.12 spp. (range: 3–6 spp.) along thelatitudinal gradient at the outer shelf. At this area, speciesloss (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 salinitygradient. It must be noted that species gain and loss arereferred to the context of increasing salinity and decreasinglatitude (and temperature) (Fig. 6).

4. Discussion

Megabenthic gastropod species composition variedacross and along the shelf as a response to environmentalgradients. Our results indicated a strong effect of environ-ment in the current distribution patterns and assemblagestructure, as confirmed by the different analysis employed.Unfortunately, we lack information for several quadrats,but overall, both extremes of the gradients in environ-mental variables as well as the main frontal zones are wellrepresented. Taking into consideration the coarse spatialdefinition of our analyses, as well as the use of oceano-graphic data averaged on an annual basis and incorporat-ing inter-annual variability in oceanographic conditions,our results constitute an accurate description of thepatterns of large gastropod assemblages. Accordingly, the

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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 (50–200m) 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.

Fig. 3. Hierarchical clustering of quadrats (A) and spatial distributions of

the two groups corresponding to inner and outer shelf (B).



faunal patterns here depicted should be only slightlyinfluenced by the temporal scale spanning species records,since the range of times over which the bulk of sampleswere collected is close to the life span of most of the long-lived species herein studied (Bigatti et al., 2006; Cledonet al., 2005).

The study area can be divided into two main zones, asshown by the classification analysis. These two areasdiffered not only in terms of its mean depth (276 vs. 42m),salinity (33 vs. 29) and temperature (14 vs. 7 1C) but also inthe annual salinity (2.8 vs. 0.6) and temperature (4.1 vs.8.7 1C) ranges. The existence of a discrete estuarineassemblage 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 thesaline gradient were already reported for soft bottom(Gimenez et al., 2005; Lercari and Defeo, 2006) and hardsubstrata (Brazeiro et al., 2006) intertidal invertebratesfrom the Uruguayan coast and the Uruguayan andArgentinean shelf (Giberto et al., 2007).The affinities between species depicted by the ordination

analysis can be explained taken into account some aspectsof the macro-scale distribution patterns of the species. Theinner shelf was dominated by species occurring along theSouth Atlantic coast and an exotic, invader Asiaticgastropod (R. venosa) exclusively found in those quadratsunder the estuarine regime. Two species (S. haemastoma

and C. parthenopeum) are widely distributed in the Atlantic

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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 0.523 MS, MT 0.553 MS, MT 0.553

MZ 0.447 MT, SR 0.523 MZ, MS, MT 0.536

MS 0.418 MT 0.523 MZ, TR, MS 0.533

TR 0.392 MZ, MT 0.501 MS, MT, SR 0.527

MINT 0.371 MT, TR 0.479 MT, SR 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.

A. Carranza et al. / Continental Shelf Research 28 (2008) 788–796792


and Western Pacific and worldwide, respectively, asso-ciated with tropical or warm waters (Beu, 1998; Clench,1947). Both species were represented by unique or fewrecords (Carranza et al., 2008; Juanico and Rodrıguez-Moyano, 1976). However, S. haemastoma is commonlyfound at or near the shallow intertidal (Scarabino et al.,2006) along the rocky substrata in the Uruguayan Atlanticcoast. P. brasiliana, T. galea, Z. dufresnei, A. beckii, andB. cochlidium are much better represented, being mainlyassociated with depths o50m. Z. dufresnei was the mostubiquitous species. All these species occurs at theUruguayan Atlantic coast, Buenos Aires province, Argen-tina (Castellanos, 1970; Penchaszadeh and de Mahieu,1976) and Rio Grande do Sul (Brazil; Rios, 1994), theexception being T. galea with only one doubtful recordsouth of Rıo de la Plata (Doello-Jurado, 1938).

In contrast, the southern portion of the outer shelf wasinhabited by mainly Subantarctic or Magellanic species,reaching the northernmost point of its distribution in thestudy area. This is the case for A. ancilla, O. magellanica,F. magellanicus, A. duartei, T. acanthodes, and P. corderoi

(Carcelles, 1947; Carranza et al., 2007; Kaiser, 1977a, b;Olivier and Scarabino, 1972; Pastorino, 2005). Closelyrelated species (e.g. subgroup 6) may co-occur due toconvergence upon common prey (e.g. Psychrochlamys

patagonica, Mytilus edulis) the former being a characteristicspecies of the cold temperate Magellanic biogeographicProvince (Orensanz et al., 1991). The most importantscallop beds are located along the 100m depth isobaths andcorrespond spatially with the continental shelf-breakfront near the continental slope (Gutierrez and Defeo,2003). Botto et al. (2006) showed that scallops are the mainprey for F. magellanicus, but O. magellanica and A. ancilla

also showed evidence of having scallops as part of theirdiets.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 foundat depths ranging from 140 to 276m at the northernportion of the study area, constituting the southernmostrecords for both species (Scarabino, 2003, 1968). A latter,bathyal species, A. riosi is distributed from Rıo de Janeiro(Brazil) to Uruguay (231–351400S, Kaiser, 1977a); thelocation reported for the holotype, 130 miles east of Mardel Plata lacks precise geographic references, in depths4600m.When the midpoints of species latitudinal ranges are

averaged and compared, the average midpoint for innershelf species is 171S, while for outer shelf species is near331S. If R. olearium and C. beauii (both wide rangingspecies with only scattered records for the Uruguayanshelf) are excluded, the average midpoint of outer shelfassemblage is near 411S. Thus, this support the idea thatspecies assemblages represent groups of species withdifferent biogeographic history, allowing us to suggest thatregional biogeography is a major determinant of overalllocal community composition and structure. MarineCenozoic gastropod fauna in the area are known sincethe Late Miocene (ca. 10Myr), when a large area ofsouthern South America was occupied by a shallow sea,and was composed by tropical/subtropical elements,considered to be influenced by a proto-Brazilian warmcurrent that extended at least to Peninsula Valdes(Argentina). Later, when the Malvinas current begun tofully operate reaching latitudes off the mouth of Rıo de laPlata, species and/or lineages adapted to cold temperaturesprobably dispersed northward (Martınez and del Rıo,2002a, b). In Pleistocene times (ca. 120,000 years), the seaoccupied the area presently named Rıo de la Plata, and thethree current biogeographic groups of molluscs werealready established (i.e. species associated with STW,SAW, and endemics from the frontal zone). However,warm water species were represented in a higher propor-tion than in the present (Martınez et al., 2001), andincluded some species that today has retracted their limitnorthwards. This evidence indicates higher temperaturesthat today. This situation persists in Holocene times atleast until ca. 2000 years ago (Martınez et al., 2006).

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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.



Despite the above outlined biogeographic scenario, weused latitude as a proxy to evaluate the effect of large scaleoceanographic processes determining a secondary tem-perature gradient on the current beta diversity of the outershelf assemblage. This analysis showed that, comparedwith the structuring effect of the saline gradient at the innershelf, the latitudinal gradient produced a comparativelyweaker effect on the observed beta diversity patterns thatcan be partially explained for the spatial grain of theanalysis. Thus, evidence for a transitional zone, in contrastwith a rigid boundary, emerged for our data frommegabenthic gastropods, attributable to slight differencesin species tolerances to environmental conditions. In thisvein, it should be taken into account that no subantarcticspecies extends its distribution north of 351S, and,conversely, no subtropical species occurs south of thatlatitude. An exception is the bathyal species A. riosi,associated with deeper waters and displaying an unusual(and not yet fully analyzed) distribution. Records for thesubantarctic species here treated for southern Brazil (i.e.Rio Grande do Sul, Rios, 1994) are based on materialprovided by fishermen, lacked precise geographic refer-ences and most likely were collected in southernUruguayan shelf. In this context, it should be stressed thatthe marine benthic biogeographical patterns of southwestern Atlantic is in need of reassessment integrating allexisting accurate and verifiable data from different taxa.Current schemes are based on sole taxonomical groups andsome schemes tend to be particularly influenced by thepatterns observed in pelagic biota. The latter clearlydisplays strong seasonality at this spatial scale which isnot necessarily reflected in benthic invertebrate distributionpatterns.

Despite well-defined boundaries between the two gastro-pod faunas, a perfect bio-environmental match was notachieved, as observed in the output of the BIOENVanalysis. This can be due to: (a) effects of non-measuredvariables: the inclusion of other physical variables mayincrease the observed correlation or (b) other ecologicalprocesses (current or historical) affecting assemblagestructure. Weak relations between measured environmentalvariables and community structure are expected whendepth and sediment characteristics are remarkably uniformover the studied area (Ellingsen, 2001), which is clearly notthe case in our study. Several variables have been includedin other bio-environmental analysis: for example, surfacesediment chlorophyll-a and phaeopigment contents, totalorganic carbon (TOC) content of surface sediments,carbon:nitrogen (C:N) sediment ratios and dissolvedoxygen (DO) were utilized in multivariate analysis forthe study of the macrobenthic animal assemblages of thecontinental margin of Chile (Palma et al., 2005). Thenumber of species and diversity were found to be correlatedwith changes in bottom-water oxygen concentrations andsediment-bound pigments. A recent study performed onthe Gulf of Mexico (o200m) showed that sediment meangrain size, percentage of clay and organic matter best

explained the macroinfauna spatial patterns, althoughBIOENV indicated that depth has an overriding role(Hernandez-Arana et al., 2003).Concerning the second hypothesis, some biological

features of the species involved may affect distributionpatterns. Dispersal capability is one of these traits(Carlon and Olson, 1993; Grantham et al., 2003; Heckand McCoy, 1978; Pechenik, 1999; Scheltema, 1971) withbroad geographic distributions in marine organismscorrelated with the presence of a more or less long-lived pelagic larval stage. However, means of dispersal donot always correlate with distribution. Springer (1982)cannot find a clear correlation between distributionand dispersal. Thresher and Brothers (1985) found nodirect correlation between geographic range size andduration of the pelagic larval stage. Thresher and Brothersalso cited Atlantic gastropods (Scheltema, 1971), asother groups which show poor correlation betweenmaximum duration of the larval life and of distributionextent. The authors suggested that some other factor, suchas relative specificity of recruitment sites, is being over-looked or that historical factors are of importance (seeHeads, 2005).

5. Conclusions

Two main faunistic subunits were determined on theUruguayan shelf: (a) a zone under the influence of thefreshwater discharge of Rıo de la Plata and the shallowwaters of the inner shelf and (b) a marine zone in the outershelf, which includes magellanic and subtropical faunas.Beta diversity was strongly influenced by the salinegradient operating at the inner shelf, while a speciesturnover associated with latitude was detected in theouter shelf. Concerning the multivariate structure, asignificant correlation was found between the environ-mental and biological matrix. This evidence suggests anoticeable effect of the physical environment on the spatialstructure of the assemblage, despite of not being fullyexplained by the environment. At the spatial scaleconsidered, current (e.g. generation of biogenic substratalike mussel beds) or historical processes including regionalbiogeography are a major determinant of local communitycomposition.

Acknowledgments

The fieldwork was done with the invaluable help of thecrew of the R.V. ‘‘Aldebaran’’, specially Pablo Puig,Ernesto Chiesa, and Laura Paesch. Financial support fromCSIC of the Universidad de la Republica and PEDECIBA(Uruguay) to A.C. is acknowledged. Special thanks to twoanonymous reviewers that helped to substantially improvethe manuscript. A.C. thanks Marina and Estela forencouragement and support.



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Documents

Assemblages of megabenthic gastropods from Uruguayan and northern Argentinean shelf: Spatial structure and environmental controls