Lat. Am. J. Aquat. Res., 37(2): 143-159, 2009
DOI: 10.3856/vol37-issue2-fulltext-3

RESEARCH ARTICLE

 

Biogeography and biodiversity of gastropod molluscs from the eastern Brazilian continental shelf and slope

Biogeografía y biodiversidad de moluscos gastrópodos de la plataforma y talud continental brasileño

 

Gabriela Benkendorfer¹ & Abílio Soares-Gomes¹

¹ Marine Biology Department, Universidade Federal Fluminense, P.O.Box 100.644, Niterói, RJ, Brazil

Dirección para correspondencia

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ABSTRACT. Biogeographic distributional patterns of gastropods are proposed based on the species' geo-graphic and bathymetric distribution. Samples were collected along the Brazilian continental margin between 18° S and 23° S, at 37 stations with depths from 20 m to 1,330 m. The analysis of the biogeographic distribution patterns confirmed the existence of a transitional zone from tropical to subtropical waters in the area of both the continental shelf and slope, suggesting a relationship with water mass circulation. We observed a high species turnover rate between the shelf and slope. The analysis of gastropod species distribution revealed a similar pattern on the shelf and slope and a large difference between shallow and deep-water faunas.

Keywords: macrobenthos, continental margins, geographical distribution, vertical distribution, soft bottoms, Brazil, southwestern Atlantic Ocean.

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RESUMEN. Los patrones de distribución biogeográfica de gastrópodos fueron propuestos basados en la distribución geográfica y batimétrica de las especies. Los muestreos fueron realizados en el margen continental brasileño entre 18°S y 23°S, en 37 estaciones de 20 m a 1.330 m de profundidad. El análisis de los patrones de distribución biogeográfica confirmó la existencia de una zona de transición de aguas tropicales a aguas subtropicales, que se encuentra en la zona de la plataforma continental y también en la zona del talud continental, esto puede sugerir una relación con la circulación de las masas de agua. Se observó una elevada tasa de turnover de las especies entre la plataforma y el talud continental. El análisis de las especies de gastrópodos reveló un patrón similar tanto en la plataforma como en el talud y una gran diferencia entre las faunas de las aguas someras y profundas.

Palabras clave: macrobentos, margen continental, distribución geográfica, distribución vertical, fondos blandos, Brasil, océano Atlántico sudoccidental.

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INTRODUCTION

Understanding the patterns of the geographic distribution of life is a very oíd issue in biology, and one that continues to be debated. In the sea, geographic patterns (e.g., in species assemblages and diversity) have been described for both shallow and deep-sea fauna (Rex, 1993; Rex et al, 1993; Clarke & Crame, 1997; Gray, 1998; Willig et al, 2003; Hillebrand, 2004).

Longitudinal and latitudinal barriers represented by the arrangement of land masses and oceans, by temperatura gradients, and by hydrodynamic patterns and water properties divide the oceans into a series of biogeographic realms with their own characteristic species assemblages (Briggs, 1995; Longhurst, 1998). The sea surface temperature is supposed to be the main forcé limiting the latitudinal distribution of marine species. Therefore, biogeographic realms are not expected to be the same along a depth gradient. In general, the wider a species' vertical distributional range, the wider its geographical distribution (Harley et al, 2003). Eurybathic taxa (those with a broader vertical range) show a wider horizontal distribution than stenobathic taxa (those with a narrow vertical range) (Vinogradova, 1997). Due to the fact that deep-sea species are mainly stenobathic, abyssal and hadal fauna show higher levels of endemism (Vinogradova, 1997; Zezina, 1997). The bulk of shallow-sea fauna is also stenobathic but, according to Menzies et al. (1973), the fauna on the continental slope has a wider geographical distribution than that of any other vertical faunal zone.

Several authors have discussed zoogeographic and diversity patterns for Brazilian shallow waters based on benthic invertebrates (e.g., Briggs, 1974; Semenov, 1978; Kempf, 1979; Palacio, 1982; Floeter & Soares-Gomes, 1999), but few have discussed biogeographic patterns of the neighbouring slope or abyssal zones (Alien & Sanders, 1996 is a good example for abyssal basins).

The effect of the latitudinal gradient is so strong on the species diversity of marine molluscs that it is also evident at the genus and family level (Roy et al., 1998; Crame, 2000) and recognized in fossil assemblages (Crame, 2002; Jablonski et al, 2006). However, in spite of the fact that molluscs are one of the earliest taxa used to investigate latitudinal trends in marine biodiversity, some doubts exist as to whether the latitudinal trends observed in the northern hemisphere also occur in the southern hemisphere (Crame, 2000; Valdovinos et al, 2003; Linse et al, 2006). Some results are conflicting. For example, the patterns found for Pacific Ocean molluscs in the southern hemisphere by Valdovinos et al. (2003) opposed those found by Fortes & Absalão (2004).

The present study aimed to investigate patterns in both the regional and depth distribution of gastropod molluscs, discussing aspects of slope and shelf diversity and provinciality, contributing to the discussion about the latitudinal diversity gradient in the southern hemisphere.

MATERIALS AND METHODS

Study area

The study area comprised the slope and continental shelf between 18°-23°S and 38°-41°W, encompassing an area ranging from the Abrolhos reef bank, situated in the north of Doce River, to the offshore and near-shore region in the vicinity of the Paraíba do Sul River. The study area was divided into two regions: north (18°-19°S), up to 200 km-long, and south (21^o-23°S), where the shelf is narrow and shallow, ranging from 10 to 30 km in width (Fig. 1). The continental slope in both regions is narrow and steep (Emery & Uchuoi, 1984). The oceanographic conditions consist of oligotrophic area s that are associated with the tropical waters of the Brazil Current (BC) and mesotrophic area s due to the seasonal upwelling of the cold, nutri-ent-rich waters of the South Atlantic Central Water (SACW) south of the Doce River (20°S) (Valentín et al, 1987). Primary productivity varíes from 0.3 g C m^-2 d^-1 to 1.1 g C m^-2 d^-1 (Gaeta et al, 1999) and the input of the Doce and Paraíba do Sul rivers is about 900 m s" . The grain-size distribution is not uniform in the area , varying with depth; shallower stations have coarse sediments and deeper stations have finer sedi-ments. The clay and silt concentrations revealed a depth gradient as well, with higher concentrations occurring at deeper stations. Concentration of calcium carbonate exhibited a patchy distribution, with higher percentages in the western sector (Soares-Gomes et al, 1999).

Sampling design and data analysis

Data for this study were obtained in April 1995, dur-ing the Joint Oceanographic Project (JOPS-II/ Leg 8), on board the R/V Victor Hansen from Bremen Uni-versity, Germany. Sampling was carried out at 41 stations, between the depths of 20 m and 1,330 m (Fig. 1 and Table 1). Molluscs were present in 37 of these 41 stations (23 stations in the shelf zone and 14 in the slope zone). The sediment was sampled in triplicate with a 0.1 m² van Veen grab and a 60 x 60 x 30 cm box-corer. Samples were standardized to an area of 0.1 m and 10 L of sediment volume. The macrozoo-benthos was sieved out with a 0.5 mm mesh size, fixed in 70% ethanol, and sorted under a stereomicro-scope for taxonomic identification.

The frequency of occurrence (F_o = number of oc-currences of a species at the shelf or slope stations / total number of shelf or slope stations) was calculated, and species were classified according to their value as constant (F_o > 50%), common (10% ≤F_o ≤50%), or rare (F_o < 10%). Species distribution over the shelf and slope was examined by plotting a histogram of the number of species against the number of stations oc-cupied.

We used Estimates 6.Obi software (Colwell, 1997) to determine whether the species were classified as "unique" (restricted to a single site), "duplicates" (occurring at exactly two sites), "singletons" (represented by a single individual), or "doubletons" (represented by only two individuáis), following the terminology of Colwell & Coddington (1994).

To perform a species richness scale analysis, the total data set was divided into four regions according to their location in geographic area s (north and south) and bathymetric zones (shelf and slope): north shelf, south shelf, north slope, and south slope. Due to dif-ferent sampling efforts, randomized species cumula-tives curves were plotted using Primer 6.1.6 software. Thus, we were able to compare species richness among regions, where the y-axis is the cumulative number of species and the x-axis the station numbers (north shelf: 17, south shelf; 6, north slope: 5, south slope: 9). An area of 0.1 m² was considered for each sampling station. According to Gray (2002), when sample sizes are different, this method is preferable to rarefaction curves as proposed by Sanders (1968). Species diversity was also estimated on a progressive spatial scale, according to Gray's terminology (2000): sample species richness (SR_S) and species richness in large area s (north shelf, south shelf, north slope, south slope) (SR_L). Together, the four regions comprised the total area species richness (SR_T). In order to calcúlate the proportion by which a given region is richer than the average of samples within the total area , Whittaker's (1972) original beta diversity measure (ß_W =(γ/α) - 1) was used, where γ is the total number of species resulting from merging a number of individual samples and α is the average number of species per individual sample. Beta diversity was measured over sectors [ß_W = (SR_L/meanSR_S)] and total area [ß_W = (SR_T/meanSR_S)] scales, where _meannSR_S was the mean sample diversity.

The species' geographic and bathymetric distribution ranges were determined based on information available in the literature (Abbott, 1974; Merlano & Hegedus, 1994; Rios, 1994, among others) and in the Malacolog 3.1 electronic database (Rosenberg, 1993 - www.erato.acnatsci.org/wasp/findsnail.php). Geographic boundaries were established based on the south-western Atlantic biogeographic provinces (Tropical, Paulista, Patagonic, Malvinas) defined by Palacio (1982) using the endemism rate. For a better representation of the geographic distribution in the area , the species were grouped according to their occurrence in the north and south regions. The shelf samples provided 211 species: 91 from the south region, 179 from the north region, and 59 from both regions. From the slope samples, 96 species were used: 72 from the south region, 55 from the north, and 31 from both regions. Depending on the bathymetric distribution, species were designated according to the zonation proposed by Zezina (1997) and Vinogradova (1997) as: shallow species (0-200 m depth), bathyal species (200-3,000 m), and abyssal species (3,000-6,000 m). Species found in both the shelf and bathyal zones were designated eurybathic.

We excluded pelagic species and juveniles (12.25% of the total number of species) from the analyses due, not only to the difficulties in identifying juveniles, but also to the aim of the present study, which is to analyze only benthic species. Only individuáis identified to the species level or ones that could unequivocally be labelled as species were used in the analysis.

RESULTS

Spatial distribution of species

A total of 9,845 specimens, 404 species and morpho-types (including empty shells), 189 genera, and 73 families were collected over the entire study area . A set of 243 species, 93 genera, and 28 families was found exclusively in the shelf zone, whereas 137 species, 43 genera, and 14 families were exclusive to the slope zone. Only 24 species (6%), 20 genera (10.6%), and 16 families (22%) occurred in both zones (Appendix 1 and 2).

Species distribution over the shelf shows that about half the species (130) were restricted to one station and none occurred at all stations. The same pattern was found for the slope, with 110 species restricted to one single site. Considering the shelf and slope, about 3% of the species occurred at more than 50% of all sites (Fig. 2). On the shelf, 2% were constant, 32% common, and 66% rare species. On the slope, the corresponding figures were 1%, 42%, and 57%.

In the whole area , 48% of the species were unique and 18% were duplicates. Singletons and doubletons represented 34% and 13%, correspondingly. The north shelf featured 26% unique species and 17% singletons, and the north slope had 12% unique species and 5% singletons (Table 2).

Diversity gradients

The cumulative dominance on the shelf was similar for both north and south regions. On the slope, there was a great difference in cumulative dominance be-tween the two regions, with the north presenting the highest evenness (Fig. 3). Comparing the cumulative dominance along a vertical gradient, the evenness of the north slope was almost 10% higher than that of the north shelf, whereas the evenness of the south shelf was 40% higher than that of the south slope (Fig. 3).

The estimated species richness on the north shelf was higher than on the south shelf. However, when the sampling area was standardized to 0.6 m² (considering the area of a station as 0.1 m², ), the richness was similar on both shelves (Fig. 4). The south slope displayed a higher total cumulative number of species per area than did the north slope. Conversely, when standardizing the sampling area to 0.5 m², richness was higher on the north slope. In the north, species richness was 0.5 m² higher in the slope zone than in the shelf zone, whereas the opposite pattern was observed in the south (Fig. 4).

In terms of diversity scales, the Alpha diversity (SR_S) values found in the study area ranged from 2 to 84 species. The mean alpha diversity was highest on the north shelf, where 84 species were found at one station, followed by the south shelf, where richness ranged from 15 to 43 species within stations. The highest value of beta diversity (SR_L/meansSR_S) was found on the north shelf (6.66) and the lowest on the north slope (3.22). ß_W values almost doubled on the largest scale (SR_T/meansSR_S) compared to the highest value found on the large area scale (Table 2).

Appendix 1. Taxonomic list of species from the continental shelf (25-200 m depth).

Apéndice 1. Lista taxonómica de especies de la plataforma continental (25-200 m de profundidad).

Family Scissurellidae Gray, 1847
	Scissurella sp.
	Anatoma aedonia Watson, 1886
	Sinezona brasiliensis Mattar, 1987

Family Fissurellidae Fleming, 1822
	Diodora jaumei Aguayo & Rehder, 1936)
	Diodora meta (Ihering, 1927)
	Diodora mirifica Métivier, 1972
	Diodora sayi (Dalí, 1899)
	Diodora sp.
	Emarginula "aff. " phrixodes Dalí, 1927
	Emarginula pumila (A. Adams, 1851)
	Emarginula tuberculosa Libassi,1859
	Lucapinella limatula (Reeve, 1850)
	Puncturella antillana Farfante, 1947
	Puncturella granulata Seguenza, 1863

Family Acmaeidae Carpenter, 1857
	Colisella sp.

Family Trochidae Rafinesque, 1815
	Calliostoma echinatum Dalí, 1881
	Calliostoma gemmosum (Reeve, 1842)
	Calliostoma sp.
	Calliostoma vinosum Quinn, 1992
	Lamellitrochus carinatus Quinn, 1991
	Lamellitrochus lamellosus Verrill & Smith,1880
	Lamellitrochus sp.
	Mirachelus clinocnemus Quinn, 1979
	Solariella staminea Quinn, 1992

Family Skeneidae Thiele, 1929
	Cyclostremiscus caraboboensis Weisbord, 1962
	Cyclostremiscus ornatus (Olsson & McGinty,1958)
	Cyclostremiscus pentagonus (Gabb, 1863)
	Haplocochlias "aff."swiftiVanatta, 1913
	Parviturbo rehderi Pilsbry & McGinty, 1945
	Parviturbo weberi Pilsbry & McGinty, 1945

Family Turbinidae Rafinesque, 1815
	Arene bairdii (Dalí, 1889)
	Arene brasiliana (Dalí, 1927)
	Arene microforis (Dalí, 1889)
	Arene variabilis (Dalí, 1889)
	Arene venusta (Woodring, 1928)
	Astraea latispina (Philippi, 1844)
	Turbo sp.

Family Tricollidae Robertson, 1958
	Gabrielona sulcífera Robertson, 1973
	Tricolia "aff" (C.B. Adams, 1850)
	Tricolia bella (M. Smith, 1937)

Family Seguenziidae Verrill, 1884
	Ancistrobasis costulata (Wattson, 1879)

Family Neritopsidae Gray, 1847
	Smaragdia viridis (Linnaeus, 1785)

Family Phenacolepadidae Thiele, 1929
	Phenacolepas hamillei (Fisher, 1857)

Family Rissoidea Gray, 1847
	Alvania valeriae Absalão, 1993
	Alvania aberrans (C.B. Adams, 1850)
	Alvania auberiana (Orbigny, 1842) Alvania sp.
	Benthonella tenella (Jeffreys, 1883)
	Ceratia rustica (Watson, 1885)
	Folinia bermudezi (Aguayo & Rehder, 1936)
	Rissoina cancellata (Philippi, 1847)
	Rissoina decussata Montago, 1803
	Rissoina fenestrata Schwartz, 1860
	Rissoina princeps (C. B. Adams, 1850)
	Rissoina sp.

Family Barleeidae Gray, 1857
	Amphithalamus vallei Aguayo & Jaume, 1947
	Barleeia rubrooperculata (Castellanus, 1972)
	Caelatura barcellosi Absalão & Rios, 1995
	Caelatura sp.

Family Assimineidae H. & A. Adams, 1856
	Assiminea succinea (Pfeiffer, 1840)
	Assiminea sp.

Family Caecidae Gray, 1850
	Caecum brasilicum Folin, 1874
	Caecum butoti DeYong & Coomans, 1988
	Caecum cornucopiae (Carpenter, 1858)
	Caecum floridanum Stimpson, 1851
	Caecum irregulare Stimpson, 1851
	Caecum meioceras Carpenter, 1858
	Caecum sp.

Family Vitrinellidae Bush, 1897
	Circulus orbignyi (Fischer, 1857)
	Solariorbis "aff " shumoi (Vanatta, 1913)
	Solariorbis infracarinatus Gabb, 1881
	Solariorbis mooreana (Vanatta, 1904)
	Teinostoma cocolitoris Pilsbry & McGinty, 1945
	Vitrinella cupidinensis Altena, 1966

Family Modulidae Fisher, 1884
	Modulus carchedeonius (Lamarck, 1822)

Family Cerithiidae Fleming, 1822
	Bittium sp.
	Bittium varium (Pfeiffer, 1840)
	Cerithium litteratum (Born, 1778)

Family Litiopidae Gray, 1847
	Alaba incerta (Orbigny, 1842)

Family Diastomatidae Cossmann, 1893
	Finella dubia (Orbigny, 1842)

Family Fossaridae Troschel, 1861
	Megalomphalus troubei (Bavay, 1908)

Family Turritellidae Clarke-Woodward, 1851
	Turritella exoleta (Linnaeus, 1758)
	Turritella hookeri Reeve,1849
	Turritella sp.

Family Calyptraeidae Lamarck, 1809
	Calyptraea centralis (Conrad, 1841)
	Calyptraea sp.
	Crucibulum aurícula (Guimelin, 1791)
	Crucibulum striatum (Say, 1824)

Family Xenophoridae Troschell, 1852
	Xenophora conchyliophora (Born, 1780)

Family Cypraeidae Rafinesque, 1815
	Cypraea sp.

Family Triviidae Troschei, 1863
	Trivia candidula (Gaskoin, 1836)
	Trivia nix Schilder, 1922
	Trivia occidentalis Schilder, 1922
	Trivia pediculus (Linnaeus, 1758)
	Trivia sp.
	Trivia suffusa (Gray, 1832)

Family Naticidae Forbes, 1838
	Naticapusilla Say, 1822
	Natica sulcata (Born, 1778)
	Sigatica semisulcata (Gray, 1839)

Family Cerithiopsidae H. & A. Adams, 1853
	Cerithiopsis greenii (C.B. Adams, 1839)
	Cerithiopsis latum (C.B. Adams, 1850)
	Cerithiopsis sp.
	Seila adamsi (H. Lea, 1845)

Family Triphoridae Gray, 1847
	Metaxia exilis (C.B. Adams, 1850)
	Triphora decórala (C.B. Adams, 1850)
	Triphora melanura (C.B. Adams, 1850)
	Triphora ornata (Deshayes, 1823)
	Triphora turristhomae (Holten, 1802)

Family Epitoniidae S.S. Berry, 1910
	Amaea retifera Dalí, 1889
	Epitonium "aff." multistriatum (Say, 1826)
	Epitonium sp.

Family Aclididae G.O. Sars, 1878
	Graphis sp. (Synonym: Aclis Lovén, 1846)

Family Eulimidae Risso, 1826
	Eulima auricincta Abbott, 1959
	Eulima bifasciata (Orbigny, 1842)
	Eulima hypsela (Verril & Bush, 1900)
	Melanella arcuata (C.B. Adams, 1850)
	Scalenostoma sp.

Family Muricidae Rafinesque, 1815
	Aspella castor Radwin & D'Attilio, 1976
	Attiliosa sp.
	Attiliosa striatoides (E. Vokes, 1980)
	Chicoreus tenuivaricosus (Dautzenberg, 1927)
	Dermomurex leali Houart, 1991
	Dermomurex sp.
	Favartia varimutabilis Houart, 1991
	Murexiella glypta (M. Smith, 1938)
	Murexiella sp.
	Muricopsis sp.
	Typhis riosi Bertsch & D'Attilio, 1980

Family Buccinidae Rafinesque, 1815
	Engina sp.
	Engina turbinella (Kiener, 1835)
	Pisania bernardoi P. M. Costa & Gomes, 1998

Family Columbellidae Swainson, 1840
	Aesopus metcalfei (Reeve, 1858)
	Aesopus sp.
	Aesopus stearnsi (Tryon, 1883)
	Amphissa cancellata (Castellanos, 1982)
	Amphissa sp.
	Anachis carloslirai P.M. Costa, 1997
	Anachis fenneli Radwin, 1968
	Anachis isabellei (Orbigny, 1841)
	Anachis obesa (C.B. Adams, 1845)
	Mitrella "aff." Innata (Say, 1826)
	Mitrella albovittata Lopes, Coelho & Cardoso, 1965
	Mitrella sp. 2
	Nassarina minor (C.B. Adams, 1845)

Family Nassariidae Iredale, 1916
	Nassarius albus (Say, 1826)

Family Fasciolariidae Gray, 1853
	Fusinus brasiliensis (Grabau, 1904)
	Fusinus sp.
	Latirus devyanae Rios, P.M. Costa & Calvo,1994
	Latirus sp.

Family Olividae Latreille, 1825
	Ancilla dimidiata (Sowerby, 1850)
	Oliva circinata Marrat, 1870
	Olivancillaria urceus (Roding, 1798)
	Olivella deflorei Klappenbach, 1964
	Olivella minuta (Link, 1807)
	Olivella puelcha (Duelos, 1840)
	Olivella sp.1
	Olivella watermani (McGinty, 1940)

Family Marginellidae Fleming, 1828
	Dentimargo lasallei Talawera & Princz, 1985
	Eratoidea scalaris (Jousseaume, 1875)
	Eratoidea sp. Persicula "aff. " sagittata (Hinds, 1844)

Family Mitridae Swainson, 1831
	Mitra staminea A. Adams, 1853
	Granulina clandestinella Bavay, 1908/1913
	Granula lavalleana Orbigny, 1842

Family Mitrinae Swainson, 1831
	Subcancilla candida (Reeve, 1845)

Family Costellariidae MacDonald, 1860
	Vexilum exiguum (C.B. Adams, 1845)
	Vexilum hendersoni (Dalí, 1927)
	Vexilum lixa Petuchi, 1979
	Vexilum sp.

Family Cancellariidae Forbes & Hanley, 1853
	Cancellaria petuchi Harasewych, Petit & Ver-hecken, 1992
	Tritonoharpa lanceolata (Menke, 1828)
	Tritonoharpa leali Harasewych, Petit & Verhecken, 1992

Family Conidae Rafinesque, 1815
	Conus jaspideus Guimelin, 1791
	Conus mindanus Hwass, 1792

Family Turridae Swainson, 1840
	Acmaturris brisis Woodring, 1928
	Bellaspira sp.
	Benthomangelia macra (Watson, 1881)
	Crassispira cubana Melvill, 1923
	Crassispira fuscescens (Reeve, 1843)
	Crassispira sp.
	Driliola sp.
	Drilliola comatotropis (Dalí, 1881)
	Eucyclotoma stegeri (McGuinty, 1955)
	Fenimorea sp.
	Glyphostoma sp.
	Ithycythara pentagonalis (Reeve, 1845)
	Ithycythara sp.
	Kurtziella dorvillae (Reeve, 1845)
	Leptadrillia cookei (E.A. Smith, 1888)
	Lioglyphostoma jousseaumei (Dautzenberg, 1900)
	Mangelia barbarae (Lyons, 1972)
	Mangelia biconica (Dalí, 1850)
	Mangelia rugurima (Dalí, 1889)
	Mangelia sp.
	Mitrolumna biplicata (Dalí, 1889)
	Nannodiella vespuciana (Orbigny, 1842)
	Neodrillia sp.
	Pilsbryspira sp.
	Polystira formosissima (E.A. Smith, 1915)
	Polystira sp.
	Pyrgocythara candidissima (C.B. Adams, 1845)
	Pyrgocythara guaraní (Orbigny)
	Pyrgospira sp.
	Splendrillia carolinae (Bartsch, 1934)
	Splendrillia lissotropis (Dalí, 1881)
	Splendrillia sp.
	Tenaturrisfulgens (E.A. Smith, 1888)
	Tenaturris gemma (E.A. Smith, 1884)
	Tenaturris sp.
	Veprecula morra (Dalí, 1881)
	Veprecula sp.

Family Terebridae Morch, 1852
	Terebra "aff " duellojuradoi Carcelles, 1953

Family Architectonicidae Gray, 1840
	Architectonica nobilis Roding, 1798
	Heliacus bissulcatus (Orbigny, 1845)

Family Mathildidae Dalí, 1889
	Mathilda barbadensis Dalí, 1881
	Mathilda sp.

Family Pyramidellidae Gray, 1840
	Chrysallida jadisi Olsson & McGuinty, 1958
	Chrysallida sp. 1
	Chrysallida toroensis (Olsson & McGuinty, 1958)
	Eulimastoma canaliculatum (C.B. Adams, 1850)
	Eulimastoma didyma (Verrill & Bush, 1900)
	Eulimastoma sp.
	Eulimastoma weberi (Morrison, 1965)
	Fargoa bushiana Bartsch,1909
	Miralda havanensis (Pilsbry & Aguayo, 1933)
	Odostomia canaliculata C.B. Adams, 1850
	Odostomia laevigata (Orbigny, 1842)
	Odostomia ovuloide C.B. Adams, 1850
	Odostomia seminuda (C.B. Adams, 1837)
	Peristichia agria Dalí, 1889
	Pyramidella crenulata (Holmes, 1859)
	Pyramidella sp.
	Sayella crosseana (Dalí, 1885)
	Turbonilla "aff. " coomansi van Aartsen, 1994
	Turbonilla arnoldoi Jong & Coomans, 1988
	Turbonilla iheringi Clessin, 1900

Family Amathinidae Ponder, 1988
	Iselica anómala (C.B. Adams, 1850)

Family Acteonidae Orbigny, 1842
	Acteon pelecais Marcus, 1981
	"Acteon" vagabundus (Mabille & Rochebrune, 1885)

Family Cylichnidae H. & A. Adams, 1854
	Acteocina bidentata (Orbigny, 1841)
	Acteocina búllala (Kiener, 1834)
	Acteocina candei (Orbigny, 1842)
	Acteocina inconspicua Olsson & McGinty, 1958
	Acteocina lepta Woodring, 1928
	Acteocina sp.
	Cylichna discus Watson, 1883
	Cylichna verrillii Dalí, 1889
	Cylichna sp.
	Scaphander darius Marcus, 1967

Family Hamineidae Pilsbry, 1895
	Atys guildingi (Sowerby, 1869)
	Atys mandrewii E.A. Smith, 1872
	Atys riiseana (Morch, 1875)
	Atys sandersoni Dalí, 1881
	Haminoea elegans (Gray, 1825)

Family Retusidae Thiele, 1926
	Pyrunculus caelatus (Bush, 1885)
	Volvulella paupercula (Watson, 1883)
	Volvulella persimilis (Morch, 1875)
	Volvulella recta (Morch, 1875)
	Volvulella sp. Volvulella texasiana Harry, 1967

Family Siphonariidae Gray, 1840
	Williamia krebsi (Morch, 1877)

Appendix 2. Taxonomic List of species from the Continental Slope (300-1330 m depth).

Apéndice 2. Lista taxonómica de especies de la plataforma continental (300-1330 m de profundidad).

Family Scissurellidae Gray, 1847
	Anatoma aedonia (Watson, 1886)

Family Fissurellidae Fleming, 1822
	Puncturella antillana Farfante, 1947
	Puncturella granulata Seguenza, 1863
	Puncturella sp.

Family Cocculinidae Dalí, 1882
	Cocculina beanii Dalí, 1882

Family Trochidae Rafinesque, 1815
	Calliostoma "aff." coronatum Quinn, 1992
	Euchelus sp.
	Basilissa alta Watson, 1879
	Basilissa sp.
	Calliotropis actinophora (Dalí, 1890)
	Calliotropis "aff " calatha (Dalí, 1927)
	Calliotropis sp.
	Echinogurges "aff" clavatus (Watson, 1879)
	Echinogurges clavatus (Watson, 1879)
	Echinogurges sp.
	Echinogurges sp. 1
	Echinogurges sp. 2
	Echinogurges sp. 3
	Microgaza sp.
	Mirachelus clinocnemus Quinn, 1979
	Solariella lubrica Dalí, 1881
	Solariella sp. 1
	Solariella sp. 2
	Tegula sp.

Family Cyclostrematidae Fisher, 1885
	Brookula cónica (Watson, 1885)
	Brookula pfefferi A.W.B. Powell, 1951
	Brookula sp.
	Brookula spinulata Absalão, Miyaji & Pimenta, 2001
	Granigyra n. sp.

Family Turbinidae Rafinesque, 1815
	Homalopoma boffl Marini, 1975

Family Tricoliidae Robertson, 1958
	Tricolia aff.inis (C.B. Adams, 1850)

Family Seguenziidae Verrill, 1884
	Ancistrobasis costulata (Watson, 1879)
	Hadroconus altus (Watson, 1879)
	Seguenzia hapala Woodring, 1928
	Seguenzia sp. 1
	Seguenzia sp. 2

Family Rissoidae Gray, 1847
	Alvania auberianafaberi Jong & Coomans, 1988
	Alvania xantias (Watson, 1885)
	Benthonella sp.
	Benthonella tenella (Jeffreys, 1883)

Family Barleeidae Gray, 1857
	Barleeia sp.

	Family Vitrinellidae Bush, 1897
	Teinostoma "aff. " obtectum Pils. & Mcg., 1945
	Teinostoma "aff " reclusa Dalí, 1889

Family Diastomatidae Cossmann, 1893
	"Finella"mamillatum (Watson, 1880)

Family Vanikoridae Gray, 1840
	Vanikoro oxychone Morch, 1877

Family Cypraeidae Rafinesque, 1815
	Cypraea cinérea Gmelin, 1791

Family Naticidae Forbes, 1838
	Polinices "aff"fringillus (Dalí, 1881)

Family Bursidae Thiele, 1925
	Bursa sp.

Family Epitoniidae S.S. Berry, 1910
	Amaea retifera Dalí, 1889
	Cylindriscala watsoni (de Boury, 1911)
	Epitonium "aff" angulatum (Say, 1830)
	Epitonium sp. 1
	Epitonium sp.2
	Opaliopsis aff nítida (Verrill & Smith, 1885)
	Solutiscala formosissima de Boury, 1909

Family Janthinidae Leach, 1823
	Recluzia rollandiana Petit, 1853

Family Eulimidae Philippi, 1853
	Eulima sp. 1
	Eulima sp. 2
	Eulima sp. 3
	Eulima sp. 4
	Melanella "aff" arcuata (C.B. Adams, 1850)
	Melanella "aff. " sarissa (Watson, 1883)
	Niso sp.

Family Velutininae Gray, 1840
	Velutina sp. (?)

Family Muricidae Rafinesque, 1815
	Chicoreus tenuivaricosus (Dautzenberg, 1927)
	Trophon sp.

Family Buccinidae Rafinesque, 1815
	Belomitra pourtalesii (Dalí, 1881)
	Belomitra sp.
	Kryptos tholoides (Watson, 1881)

Family Columbellidae Swainson, 1840
	Amphissa cancellata (Castellanos, 1982)
	Anachis n. sp. 1 Anachis n. sp. 2

Family Olividae Latreille, 1825
	Ancilla dimidiata (Sowerby, 1850)
	Olivella amblia Watson, 1882
	Olivella (divina) n.sp1

Family Mitrinae Swainson, 1831
	Mitra sp.

Family Cancellariidae Forbes & Hanley, 1853
	Brocchinia "aff." pustulosa Verhecken, 1991

Family Turridae Swainson, 1840
	Gymnobela sp.
	Bathytoma "aff. " mitrella Dalí, 1881
	Benthomangelia macra (Watson, 1881)
	Compsodrillia sp.
	Leptadrillia sp.
	Drillia "aff. "premorra Dalí, 1881
	Drilliola comatotropis (Dalí, 1881)
	Eubela limacina (Dalí, 1881)
	Eucyclotoma sp.
	Fenimorea "aff."pagodula (Dalí, 1889)
	Kurtziella "aff. " serga (Dalí, 1881)
	Kurtziella sp.
	Leucosyrinx verrillii (Dalí, 1881)
	Leucosyrinx sp.
	Drilliola comatotropis (Dalí, 1881)
	Nannodiella vespuciana (Orbigny, 1842)
	Pleurotomella "aff. " benedicti Verrill & Smith, 1884
	Pleurotomella "aff. " blakeana (Dalí, 1881)
	Pleurotomella "aff " cala (Watson, 1886)
	Pleurotomella "aff" ipara (Dalí, 1881)
	Pleurotomella "aff "porcellana (Watson, 1886)
	Pleurotomella circumvoluta (Watson, 1881)
	Pleurotomella extensa (Dalí, 1881)
	Pleurotomella perparva (Synonym: Philbertia perparva (Watson, 1881))
	Pleurotomella sp. 1
	Pleurotomella sp. 2
	Pleurotomella sp. 3
	Pleurotomella sp. 4
	Pleurotomella sp. 5
	Pleurotomella sp. 6
	Pleurotomella sp. 7
	Pleurotomella sp. 8
	Spirotropis "aff "phaeacra (Watson, 1881)
	Spirotropis sp.

Family Mangeliinae Fischer, 1887
	Mangelia comatotropis Dalí, 1881

Family Pyramidellidae Gray, 1840
	Cingulina babylonia (C.B. Adams, 1845)
	Eulimastoma sp. 1
	Eulimastoma sp. 2
	Eulimella smithii Verrill, 1882
	Eulimella sp. 1
	Eulimella sp. 2
	Odostomia "aff " canaliculata C.B. Adams, 1850
	Sayella crosseana (Dalí, 1885)
	Turbonilla "aff " unilirata Bush, 1899
	Turbonilla sp. 1
	Turbonilla sp. 2
	Turbonilla sp. 32
	Turbonilla sp. 35

Family Acteonidae Orbigny, 1842
	"Acteon" vagabundus (Mabille & Rochebrune, 1885)
	Acteon pelecais Marcus, 1981
	Acteon perforatus Dalí, 1881
	Rictaxis sp.

Family Ringiculidae Philippi, 1853
	Ringiculina nítida Verrill, 1874

Family Cylichnidae H. & A. Adams, 1854
	Cylichna "aff."crispulaWatson, 1883
	Cylichna discus Watson, 1883
	Cylichna verrillii Dalí, 1889
	Cylichna vortex Dalí, 1881
	Scaphander darius Marcus, 1967

Family Diaphanidae Odhner, 1914
	Diaphana seguenzae (Watson, 1886)

Family Bullidae Rafinesque, 1815
	Bulla "aff." abyssicola Dalí, 1881 Bulla "aff." ebúrnea (Dalí, 1881)

Family Hamineidae Pilsbry, 1895
	Haminoea elegans (Gray, 1825)
	Haminoea petitii (Orbigny, 1842)
	Haminoea sp.
	Atys guildingi (Sowerby, 1869)
	Atys mandrewii E.A. Smith, 1872

Family Retusidae Thiele, 1926
	Pyrunculus ovatus (Jeffreys, 1870)
	Volvulellapersimilis (Morch, 1875)

Biogeographic distribution

For the continental shelf samples, 211 species and 89 genera were characterized according to their occur-rence in the southwestem Atlantic zoogeographic provinces: 91 species from the south, 179 from the north, and 59 species from both regions. For the continental slope samples, 96 species and 52 genera were characterized: 72 species from the south, 55 from the north, and 31 from both regions.

In terms of the geographical distribution of taxa, the number of genera with a wide distributional range (occurring in more than three provinces) was lower than the genera with narrower distributions for both shelf and slope stations. However, considering the genera that occurred in both zones, the number of wide-range distributions was higher than the narrow-range ones (Table 3).

At the shelf stations, the number of species co-occurring in both Tropical and Paulista (Tropical-Paulista species) provinces was greater than the number of Tropical species occurring in both regions. In addition, the number of Tropical, Paulista, and Tropi-cal-Paulista species decreased and the number of wide-distribution eurythermic species increased in the southwestem Atlantic provinces (Tropical-Paulista-Patagonic species) towards the south (Fig. 5). At the slope stations, Tropical species were the majority in both regions. The number of Tropical, Paulista, Tropi-cal-Paulista, Tropical-Paulista-Patagonic, and subtropical Paulista-Patagonic species increased towards the south. The number of endemic species was higher in the north for both shelf and slope stations. In addi-tion, a greater number of Tropical endemic species was present at the slope stations in both the south and north, and the shelf stations displayed the highest occurrence of Paulista endemic species. The number of species occurring in all Western Atlantic Provinces was greater on the south shelf (Table 4). Furthermore, shelf stations showed a higher number of eurybathic species (with a wide bathymetrical range) than did slope stations (Table 5).

DISCUSSION

The shelf-slope transition zone is known to have a high species turnover rate (Rex et al, 1977). More-over, species typically found on continental shelves and species from continental slope zones can coexist there, leading to higher species richness. However, evidence from studies done on the Brazilian continental slope showed that the depth where this species turnover begins is variable. In this study, we found high species turnover at station 16 (300 m depth), where some deep-sea species, such as Alvania xantias (Watson, 1885), Brookula spinulata (Absalão, Miyaji & Pimenta, 2001), and Solariella lubrica (Dalí, 1881) showed a high dominance. Miyaji (2001) found a rough change, with 8% of sampled species occurring exclusively at depths greater than 400 m, whereas Sumida & Pires-Vanin (1997) found a different com-munity from shallow area s occurring between 320 m and 500 m depth.

Analyzing the vertical distribution of species found in this study (Table 2), we observed the occurrence of shallow-water and eurybathic species (shallow-bathyal, shallow-bathyal-abyssal, bathyal-abyssal distributions) at the slope stations. This pattern might constitute evidence of the slope's capacity to allow the co-existence of shallow and deep-water species.

The depth gradient differed between the north and south regions: the north shelf showed the highest local, regional, and between-habitat species richness, whereas the south slope had the highest species richness, with values cióse to the shelf ones. Along local gradients, the general pattern observed is that species richness changes with depth, increasing from ca. 200 m to 1500-2500 m or more, and then decreasing towards the abyssal plain (Rex et al, 1993, 2000; Gray, 2002). Nevertheless, those unimodal patterns do not appear to be universal (Rex et al, 1997; Stuart et al, 2001), showing that the change in species richness is not related to depth itself (Gray, 2002).

The highest species richness values observed on the north shelf could be a result of the fact that the highest number of samples were taken in this area , but also of the greater environmental heterogeneity in the area . Larger area s potentially support more species richness on a variety of scales and harbor higher over-all richness, whereas the number of habitats also ulereases, as well as the number of biomes, or of bio-geographic provinces within them (Rosenzweig, 1995; Willig et al, 2003). On the north shelf, species can find a wide continental shelf (200 km) featuring a high variety of bottom types, as well as the presence of coráis and calcareous algae bank habitats. On the other hand, the south region is characterized by a narrow shelf area (10-30 km) and a less heterogeneous bottom (Soares-Gomes et al, 1999). Additionally, because of the constant presence of the Brazil Current, the north region is oligotrophic, which may limit the abundance of species. Conversely, the south region is considered to be mesotrophic due to the seasonal upwelling of cold, nutrient-rich, SACW waters (Gaeta et al, 1999), leading to higher richness.

In addition to the above-mentioned patterns, environmental features also changed with depth. Both north and south slopes are narrow and steep, with more homogeneous bottoms that are dominated by silt fractions and a higher concentration of organic carbon.

The study area is included in a wide transition zone known as the Paulista province (Palacio, 1982). The present study found a lower number of tropical species and a higher number of subtropical ones (species that are common to the Paulista and Patagonic provinces) towards the south. Moreover, a great presence of species common to both the Tropical and Paulista provinces was observed over the entire area . Other studies carried out at more southerly latitudes on the Brazilian coast show the same distribution for molluscs (Mello, 1993; Miyaji, 1995) and polychaetes (Lana, 1987; Attolini, 1997). Furthermore, on the Uruguayan coast (Scarabino, 2004), 16 among 182 gastropods were the same as the species reported in this study (carried out in the Tropical and Paulista provinces). The presence of species that are considered to be endemic to the Paulista Province (i.e., Anachis fenelli Radwin, 1968; Favartia varimutabilis Houart, 1991; Olivella deflorei Klappenbach, 1964) and subtropical species from the Patagonic Province in the north region (Tropical Province) corrobórate the notion of a broader transition zone between tropical and températe waters (Van nucci, 1964; Lana, 1987; Miyaji, 1995; Floeter & Soares-Gomes, 1999).

The disappearance of some tropical species towards the south suggests that the southernmost limit of the Tropical Province is located cióse to 21°S, ac-cording to the results found for gastropods (Floeter & Soares-Gomes, 1999); cirripeds (Young, 1995), and polychaetes (Lana, 1987). Nevertheless, the location of that limit remains uncertain (Absalão, 1989; Mello, 1993; Briggs, 1995). Recently, Joyeux et al. (2001) and Floeter et al. (2008) found that, for tropical reef fishes, the southern limit is 28°S.

Many studies have demonstrated that the bounda-ries of shallow-water faunal distribution are correlated to water masses boundaries (Stevenson et al, 1998; Culver & Buzas, 2000). The presence of species with tropical affinities over the shelf area could be ex-plained by the predominance of the warm and saline water mass of the Brazil Current (BC) (Absalão, 1989; Miyaji, 1995, 2001) that flows southwards (parallel to the shelf break) to 35°S (Emilsson, 1961). In that region, the BC mixes with the cold and less saline water mass of the Malvina Current and the water character-istics become markedly subtropical, with salinity and temperature ranging between 36-35 and 20°-10°C, respectively (Emilsson, 1961). This fact influences the occurrence of cold-water affinity species (Semenov & Berman, 1977; Palacio, 1982).

However, the most significant factor for the main-tenance of cold-water species, as well as of eurybathic species in the shelf zone, particularly in the north region, might be the penetration of the South Atlantic Central Water (SACW) into the continental shelf re-gions. This water mass acts as a vehicle for larval dispersal from cold, deeper regions to warm, shal-lower area s (Absalão, 1989; Miyaji, 1995, 2001), and extends northwards to Espirito Santo State (Gaeta et al, 1999).

Important changes occur in the benthic faunal structure within the large vertical interval of the bathyal zone. The most obvious difference between shallow and deep-bottoms is the reduction in the number of latitudinal or climatic belts, both in terms of biomass and of faunal structures (Zezina, 1997). As there is a simplification in the water mass structure of the continental slope floor region, a reduction in the number of faunal provinces is to be expected (Semenov & Berman, 1977; Zezina, 1997; Culver & Buzas, 2000). Culver & Buzas (2000) found that the differences in benthic foraminifera fauna between shallow (< 200 m) and deep water (> 200 m) provinces at the same latitude were greater than between adjacent shallow water provinces.

As expected, species occurring in the shelf zone were very distinct from those in the slope area , with only 24 (out of 315) species shared between the two zones. When the latitudinal distribution of slope species is analyzed, the pattern is similar to the shelf species distribution. However, the number of Tropical and Tropical-Paulista species increased in the south region.

Studies on geographical distributions of deep-sea species have shown a greater number of species with wider horizontal ranges (Vinogradova, 1997; Zezina, 1997). However, the present study found few species with wide range distributions towards the slope sta-tions. Similar results were observed for the geographic distribution of genera. The restricted-range genera (1 to 2 provinces) were represented by six more genera than the wide-range ones (> 2 provinces). It is, however, possible that the lower number of samples for the slope region (14 stations vs. 23 in the shelf zone) might induce such contradictory results. It is expected that enhanced sampling efforts will not only tend to increase the more sparsely distributed species, but also the number of "rare" endemic species (Alien & Sand-ers, 1996).

Comparing the species found in the present study area (18°-23°S) with their incidence in other geographical regions studied by others authors along the Brazilian continental slope (Merlano & Hegedus, 1994; Sumida & Pires-Vanin, 1997; Miyaji, 2001; Scarabino, 2004) revealed some shared species. Nine-teen species were shared between the present study and works done on the northeast region (11.37%), from a total of 167 species found. Among those, the most dominant species were Brookula cónica (Watson, 1985), Anatoma aedonia (Watson, 1886), Benthonella tendía (Jeffreys, 1883), and Alvania xantias (Watson, 1885). The southeast region shared 10 (7.57%) of a total of 132 species, with the dominant species being Brookula pfefferi A.W.B. Powell, 1951; Seguenzia hapala Woodring, 1928; Nannodiella vespuciana (Orbigny, 1842); and Solariella lubrica (Dalí, 1881). Finally, the south region shared four (3.15%) of a total of 127 species; the most dominant species were Ancilla dimidiata (Sowerby, 1850); Busilis s a alta (Watson, 1886); Pyrunculus ovatus (Jeffreys, 1870); and Puncturella granulata (Seguenza, 1863). As Alien & Sanders (1996) found for the zoogeographic distribution of protobranch bivalves through interbasin comparisons on the percentage of shared species, the species number is higher between adjacent basins. Moreover, the majority of shared species are those firom the Tropical Province and firom both Tropical and Paulista Provinces. Analyses of the biogeographic distribution of bathyal species indicated a transitional pattern, as suggested for the shelf zone. However, the higher number of eurythermic species with tropical affinities found at the slope stations could be an indication that the limit of tropical bathyal species distribution is wider than that of tropical shelf boundaries.

Zezina (1997) proposed biogeographic schemes for the bathyal zone based on a recent brachiopod distribution, which is very similar to the results found in the present study for gastropods firom the continental slope. This scheme proposed, for depths greater than 700 m in the northeast and central Brazilian bathyal zone (similar to the schemes for shallow-water fauna in the southwestern Atlantic), a sub-area named the Atlantic-Central American (divided into Caribbean and Brazilian provinces) and the south Brazilian-Uruguayan subtropical area in the southeast and south bathyal zones. Also, for recent brachiopods living below 700 m, Zezina (1997) defined only one area for the entire southwestern Atlantic Ocean: the Amphi-Atlantic Bathyal area within the central Atlantic Province.

A great species turnover rate was observed between the shelf and slope. An analysis of the gastro-pod species distribution revealed a similar pattern of regional distribution in shelf and slope zones and a great difference between shallow and deep-water faunas. Although the present analysis of biogeographic distribution patterns confirmed the existence of a transitional zone firom tropical to subtropical waters in the case of the slope zone, the sampling effort done on the southeastern Atlantic slope is still too little and those results should be viewed with caution.

 

ACKNOWLEDGMENTS

The authors are indebted to Dr. Bastian Knoppers for the invitation to join in the Joint Oceanographic Project II (JOPS-II), and to Drs. Ricardo Silva Absalão and Paulo Márcio Costa for helping with taxonomic identification. We also thank Dr. Sergio Floeter for some valuable suggestions that improved the manuscript and Carla Mendes for reviewing the English version.

 

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Received: 8 July 2008; Accepted: 2 March 2009

Corresponding author: Abílio Soares-Gomes (abiliosg@vm.uff.br)