Inventory of the marine soft bottom macrofauna of São Sebastião Channel , southeastern Brazilian continental shelf

Analysis of macrofauna from 91 quantitative soft-bottom samples (from five surveys) obtained in the Sao Sebastiao Channel at depths ranging from 2 to 45 m yielded 431 species from a total of 38,630 individuals. The present contribution gives a list of the species found in the area and their depth distribution and sedimentary preferences (granulometric characteristics of the sites where they occurred). This is the first complete inventory of the subtidal invertebrate soft-bottom fauna of the Sao Sebastiao Channel, a peculiar marine area under crescent human impact.

as barrier for the adjacent open sea.The channel itself is about 25 km long with two relatively large openings (6-7 km wide) and a narrow central part of nearly 2 km in width.It presents fan-like features in the south and a well sorted sand barrier in the north.Also, its northern region has a counter-clockwise vortex, which transports fine grains to the south, causing deposition of fine-sediment at the continental margin, an area of low hydrodynamics.In the central deepest parts of the channel no deposition of sediments occurs, probably due to the increase in current speed caused by narrowing (Furtado 1995;Furtado et al. 2008).
Although marine currents in Southern Brazilian shelf are variable in space and time, they mainly move towards the NE in the SSC (Castro Filho 1990), except in summer (Fontes 1995), when a two-layer opposite water flow establishes, the superficial flow directed to the SW and the deep flow towards the NE.The bottom current is not influenced by wind but may be associated with the South Atlantic Central Water (SACW) intrusion and is characterized by low temperature (<18°C) and high salinity (>36).For the rest of the year the prevailing water mass is the Coastal Water (CW) with temperature higher than 20°C and salinity lower than 34.5 (Castro Filho et al. 1987).For detailed physical and biological characterization of the SSC and of the main processes responsible

Introduction
Ecological studies are deeply entangled with the biodiversity knowledge of a given region or system.The São Sebastião Channel (SSC) is a conspicuous area located in the southeastern Brazilian shelf, submitted to a complex oceanographic regime (Castro and Miranda, 1998) and multiple human impacts (Zanardi et al. 1999;Arasaki et al. 2004;da Silva and Bícego, 2010).The channel harbors the main maritime oil terminal in Brazil (Dutos e Terminais Centro Sul - DTCS), the commercial São Sebastião Port, with intense fishery activity, and the São Sebastião city that contributes with large amount of sewage to the area.
In order to describe the species composition of the unconsolidated bottom in the SSC, the present study reports macrofauna species found along the channel during five surveys performed as part of two interdisciplinary projects.The "Oceanography of São Sebastião Channel" (OPISS Project, Pires-Vanin 2008) and the "Assessment of benthic fauna and organic contamination in critical areas of São Sebastião Channel" (CONCASS Project), both supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).The depth range of each species and the dominant sediment type of the sampling stations where they occurred were reported as means of distinguish and define their ecological preferences.This is the first comprehensive inventory of the subtidal species of soft-bottom macrofauna at São Sebastião Channel.In order to create an initial scenario for future work in ecology, monitoring and management it is important to know actual diversity of the area.

Material and Methods
The São Sebastião Channel is located on the northern coast of São Paulo State (23°41′-23°53.5′S, 45°19′-45°30′ W), Brazil (Figure 1).It lays parallel to the coast, separate the continent from the São Sebastião Island and functions Abstract: Analysis of macrofauna from 91 quantitative soft-bottom samples (from five surveys) obtained in the São Sebastião Channel at depths ranging from 2 to 45 m yielded 431 species from a total of 38,630 individuals.The present contribution gives a list of the species found in the area and their depth distribution and sedimentary preferences (granulometric characteristics of the sites where they occurred).This is the first complete inventory of the subtidal invertebrate soft-bottom fauna of the São Sebastião Channel, a peculiar marine area under crescent human impact.The present data are the result of five surveys carried out in the framework of the two interdisciplinary projects mentioned before.For OPISS four seasonal surveys at 15 oceanographic stations were performed (two replicates, 120 samples).They were distributed in five radials perpendicular to the axis of the channel (Figure 1a), and were carried out in November 1993, February, April and August 1994 on board of the boats "Veliger II" and "Albacora" from the Oceanographic Institute of the University of São Paulo.For the CONCASS project one survey was carried out in June 2002 at 31 stations according to figure 1b (three replicates, 93 samples).Stations were positioned with GPS (Global Positioning System) and depths were measured by sounding with "Fish -Finder".Sampling was carried out using a van Veen grab of 0.1 m 2 .The sediment was washed on board through 0.5 mm mesh sieves and the material retained was preserved in 70% ethanol.In the laboratory, sediments were elutriated (Santos et al. 1996) before sorting and identification under stereoscopic microscope.

Results
Table 1 and 2 present geographical coordinates, depth and sediment type of the sampling stations.Depth varied between 2 and 45 m with the deeper places situated in the middle of the channel axis.Sand sediment composed by many classes of grain sizes was patchy distributed along the bottom, except in the channel's central part where muddy sites occur.
Mollusca presented 119 species, from which 75 were bivalves found in mixed sand substrates, especially in very fine grains.Another diversified group was Crustacea, represented by 57 species of Peracarida and 42 species of Decapoda.Amphipoda characterizes half of the Peracarida species, whereas Brachyura did the same for the Decapoda.The last diverse group was Ophiuroidea, encompassing 18 species from a total of 24 found for echinoderms.All the species reported here are from southern Brazilian inner shelf, area under temperatures between 18 and 25°C most of the year (Soares-Gomes and Pires-Vanin 2005; Pires-Vanin 2008).

Discussion
Our present results showed that despite the relative small study area, the channel presented high number of species, similar to that found in the adjacent São Sebastião shelf (Pires-Vanin 2008).However, comparison of diversity results with others reported in the current literature may be done with caution since the sampling methods, units and scales are often different, and moreover the diversity of habitats in tropical and subtropical ecosystems are high (Chardy and Clavier 1988;Alongi 1990).From the high number of species reported (431) nearly 25% cannot be identified, or because they were juveniles, as in the case of mollusks, or were new species, as in the case of crustaceans Isopoda and Cumacea.However, that number reflects a good knowledge of the macrofauna in São Sebastião Channel when contrasted to deeper areas of the adjacent shelf, as Ubatuba.In the outer parts of this last region, at the shelf break, near 50% of the macrobenthic species remains undetermined and at 500 m deep, in the slope, 70% of the Isopoda were new (Pires-Vanin, 2001).The São Sebastião Channel is a depositional environment and has the narrow central area characterized by bottoms of silt and clay.The other sites are sandy places, with well-sorted grains at the South and coarser sands at the North.Sediment pattern deeply influences species diversity of macrofauna in the area, as showed by Pires-Vanin et al. (2013).In the São Sebastião shelf, at the outer side of the channel, the authors found fewer species in the finest-sediment site-group than in the coarser site-groups.
Species richness varied spatially and seasonally (42 to 72 species).The highest values was reported for the northern part of the channel (stations 13 14, 15 of OPISS project, see Figure 1), whereas significant higher mean value were obtained in the winter (41 species) and the lowest in fall (26 species).These findings show the role of sediment type and hydrodynamics in structuring the benthic environment (Loureau 2000).Sandy sediments predominate in areas with high number of species, a frequent result found in other benthic investigations around the world (Gray et al. 2002;Gray and Elliot 2009).In the Southwest Atlantic shelf the oceanographic regime controlling macrobenthic distribution is composed by three water masses, Coastal Water, Tropical Water and South Atlantic Central Water, which fill the shelf and cause striking seasonal change on the environment (Castro and Miranda 1998).As a consequence the destabilization of the benthic fauna structure occurs (Absalão et al. 2006;Venturini et al. 2011).So, the season-controlled distribution of the benthic species was expected, despite the existence of the major constraint represented by sediment characteristics (Muniz and Pires 2000).

Table 1 .
Environmental variables studied at the 15 stations in the four surveys carried out in São Sebastião Channel (OPISS Project).CS = coarse sand; MS = medium sand; FS = fine sand; VFS = very fine sand; SC = coarse silt; SM = medium silt; SF = fine silt.

Table 3 .
Macrobenthic species of invertebrates found in the São Sebastião Channel, northern São Paulo State continental shelf.VCS, Very Coarse Sand;