Distribution and ecology of deep-water mollusks from the continental slope, southeastern Gulf of California, Mexico

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1 Mar Biol (2007) 150: DOI /s RESEARCH ARTICLE Distribution and ecology of deep-water mollusks from the continental slope, southeastern Gulf of California, Mexico P. Zamorano M. E. Hendrickx A. Toledano-Granados Received: 29 August 2005 / Accepted: 5 May 2006 / Published online: 10 August 2006 Springer-Verlag 2006 Abstract Specimens of deep-water mollusks were collected with a bottom dredge in the SE Gulf of California during four cruises (TALUD project) in A total of 24 species (16 Pelecypoda, 5 Gastropoda, and 3 Scaphopoda) were collected. Analyses of environmental data (depth, epibenthic temperature and oxygen content, sediment texture and organic matter content) associated with each record indicated that the community diversity of these deep-water mollusks was related to oxygen concentration and density was correlated with depth. A canonical correspondence analysis (CCA) indicated that the Wve environmental variables explained 53.8% of observed variance in the model. In a multiple regression analysis, density (ind l 1 ) was best correlated with dissolved oxygen concentration (R = 0.25), followed by temperature (R = 0.20), organic matter content (R = 0.15), and depth (R = 0.12). As oxygen best explained variance in the CCA it was selected to perform a single correspondence analysis (CA) using the most abundant and frequent species (two Pelecypoda and three Scapophoda). The analysis shows that pelecypods occur in near anoxic values while scaphopods occur in intermediate oxygen concentrations. Communicated by J.P. Grassle, New Brunswick. P. Zamorano M. E. Hendrickx (&) Laboratorio de Invertebrados Bentónicos, ICML-UNAM, CP Mazatlán, Sinaloa, Mexico michel@ola.icmyl.unam.mx A. Toledano-Granados Unidad Académica Puerto Morelos, ICML-UNAM, Puerto Morelos, Quintana Roo, Mexico Introduction Deep-sea macroinvertebrate communities are characterized by high diversity (Hessler and Sanders 1967; Sanders and Hessler 1969; Grassle 1989; Grassle and Maciolek 1992; Smith et al. 1998). In areas where the oxygen minimum zone (OMZ) intercepts the continental slope, anoxic and severely hypoxic benthic communities are species-poor, but the somewhat less hypoxic zone, which extends into deeper water, is species-rich. Here we dewne anoxia, hypoxia, and moderate hypoxia as having oxygen concentrations of 0, < 0.1 and <0.5ml O 2 l 1, respectively. In the OMZ, depth and dissolved oxygen concentration are the most important factors avecting composition of deep-water communities (Rosenberg et al. 1983; Levin and Gage 1998; Rogers 2000; Hendrickx 2001; Levin et al. 2001; McClain 2004) and species size (Rex and Etter 1998; McClain and Rex 2001; Olabarria and Thurston 2003, 2004). Environmental conditions at the bottom in the deep sea (i.e., muddy sediments, detritus as the food source, and relatively stable values of salinity and temperature) favor settlement and dominance of infaunal polychaetes, peracarids, bivalves, and scaphopods which are often highly diverse (Hessler and Sanders 1967; Sanders and Hessler 1969; Rex et al. 2000; Kröncke and Türkay 2003; Méndez-Ubach, personal communication). Further, composition of sediments (i.e., type and grain size) is known to avect deep-water mollusk distribution (Sanders et al. 1965; Etter and Grassle 1992; Kröncke and Türkay 2003), and the organic matter content (i.e., biogenic deposition and detritus) in sediments avects mollusk diversity and distribution as well as the maximum size reached by a species (Sanders et al. 1965; Turner 1977; Seibold and Berger 1982; Wilson and Shelley 1986).

2 884 Mar Biol (2007) 150: Deep-sea mollusks were Wrst recorded in the Mexican PaciWc during the exploration cruise of the steamer Albatross in 1891 (Dall 1895), which included part of the Gulf of California. One hundred years later, however, the sole comprehensive study of deep-water mollusk distributions (bathymetric and geographic) is attributable to Parker (1964a, b) who reported 37 species from the central and southern continental slope of the Gulf of California: 12 for the upper slope ( m), four for the mid-slope (731 1,799 m) and 21 for the lower slope (1,800 4,122 m). Other records for deep-water mollusks in the Gulf of California are scarce and are mostly found in taxonomic contributions reporting on species or on higher taxonomic categories for the eastern tropical PaciWc (see Keen 1971; Keen and Coan 1975; Skoglund 1991, 1992; Scott et al. 1996; Scott and Blake 1998). A preliminary analysis of this literature and of collections at Scripps Institution of Oceanography indicates that 388 species of mollusks have at least one record below 200 m depth for the Mexican PaciWc (unpublished data). The OMZ that prevails above the external continental shelf and the upper slope in a large portion of the Gulf of California, roughly from 100 to 800 m, with epibenthic dissolved oxygen concentration always <0.5mll 1 and occasionally < 0.1 ml l 1 (Parker 1964a, b; Hendrickx 2001, 2003), limits the occurrence of species that cannot tolerate severe hypoxic conditions: hence it represents a barrier for dispersion of benthic species from the mid-shelf into deeper waters. The present study evaluated the composition and abundance of the infaunal mollusk community in the SE Gulf of California. Relationships between this fauna and certain environmental factors were examined using Weld data and material from a series of stations sampled in with the R/V El Puma (TALUD IV VII cruises). Materials and methods Mollusks were obtained from depths of 731 to 2,250 m on the continental slope ov the State of Sinaloa, SE Gulf of California (Fig. 1) using a modiwed, 85-l capacity Karling dredge (Schlieper 1972) (110 cm 40 cm 20 cm), designed to sample and retain sediments to a depth of 7 cm. In addition to a much larger overall size, modiwcations included lateral stabilizers to prevent the dredge from turning over during deployment, and a removable rear plate (instead of a grid or sieve) allowing for easy collection of sediments after recovery of the gear. A ca. 18 kg weight was attached to the wire ca. 10 m ahead of the dredge to keep the dredge horizontal. The dredge was towed for a minimum distance of ca. 900 m but distance varied during the survey due to changes in depth or bottom prowle. Once recovered from the dredge aboard ship, sediments were measured (volume in liters) and gently washed with sea water on 1,000- and 500-μm sieves. The sampling area was visited four times (TALUD IV, August 2000; TALUD V, December 2000; TALUD VI, March 2001; TALUD VII, 5 9 June 2001) and a total of 44 stations were sampled though mollusks were not always found in the sediments (Table 1). Several stations were sampled during the four cruises, while others were sampled three times, twice, or only once due to adverse conditions (see Table 1). However, each sample (corresponding to a combination of station/date) was considered as an independent set of environmental and biological data due to the low probability of sampling exactly the same spot repeatedly. Positional coordinates for each sampling station were plotted using a GPS navigation system. Depth was measured with an EdoWestern, analogic recorder, epibenthic water temperature and salinity with a CTD, and dissolved oxygen content was estimated with the Winkler method (Strickland and Parson 1972) using water samples collected by rosettemounted, 10-l Niskin bottles 5 10 m above bottom. A subsample of» 200 g of sediment from the Karling dredge was deep-frozen and used for granulometric analyses (Folk 1965) and estimation of organic matter content (weight loss by ignition at 550 C to avoid loss of carbonates; see Dean 1974). Grain size was expressed as percentage of mud, sand, and gravel. Mollusks were Wxed with a 4% formaldehyde sea water solution for at least 1 week, washed with tap water, and preserved in 70% ethanol. The identiwcation guides and taxonomic lists of Keen (1971), Keen and Coan (1975), and Skoglund (1991, 1992) were used for identiwcation. Selected specimens were sent for expert identiwcation. Voucher specimens of all species are deposited in the Regional Marine Invertebrates Collection at UNAM in Mazatlán, Mexico. The following were evaluated/determined for each sampling station: number of species, total abundance (individuals per sample), density, and Shannon Wiener diversity index (H ). Pearson correlation (Primer 5 program 5.2.8, 2001) values were calculated using biotic (i.e., total abundance, species richness, and diversity) and abiotic (i.e., depth, temperature, dissolved oxygen, organic matter content, and sand/mud/ gravel proportion of sediments). Small pebbles (gravel) were rarely found and were therefore not considered in the analysis. To determine which factors inxuence mollusk distribution, a canonical correspondence analysis (CCA) was performed (MVSP program

3 Mar Biol (2007) 150: Fig. 1 Gulf of California sampling stations during TALUD IV VII cruises. Filled circles indicate stations where mollusks were collected, open circles indicate stations where no mollusks were present b 33b 32b Western Mexico b Pacific Ocean , ). Multiple regression analysis (MRA) (Statistica 5 program 5.1, ) was used to rank abiotic variables where the density for each species was standardized and expressed as number of individuals per liter of sediment. Posteriorly, density (ind l 1 ) of dominant and frequently occurring species and the abiotic variable that explained the greatest part of variance, were used to plot a perceptual map based on a single correspondence analysis (CA). Dominance was calculated as Dm = (ni/n) 100, where Dm is the mean dominance index for species i; ni, the number of individuals for species i; N, the total number of individuals for all species (Picard 1965). Frequency was calculated as F =(mi 100)/M, where mi is the number of samples in which species i was found, and M the number of samples (Glémarec 1964). Comparison of species richness was performed using Wve bathymetric ranges of 300 m each (701 1,000, 1,001 1,300, 1,301 1,600, 1,601 1,900, 1,901 2,200 m) and rarefaction curves (Sanders 1968) were obtained using the Primer 5 program. Similarity among depths was calculated by means of a cluster analysis using presence-absence data for each species in each sample (Statistica 5 program 5.1, ). Results Mollusks were collected at 3 of 10 stations on the TALUD IV cruise, at 6 of 9 stations on TALUD V, at 8 out of 12 on TALUD VI, and at 8 of 13 on TALUD VII (Table 1). Most individuals were identiwed to species and the rest (except for one Gastropoda) to genus (Table 2). A total of 24 species, 23 genera, and 22 families were identiwed in the samples: 16 Bivalvia, 5 Gastropoda, and 3 Scaphopoda. Pearson correlation indicated a best Wt between Shannon Wiener diversity and dissolved oxygen concentration at the bottom (R = 0.297); abundance correlated better with depth (R = 0.222), while species richness was correlated with organic matter content (%) (R = 0.158). The CCA indicated that the six variables considered in the analysis explained 54% of the observed variance in the model (Fig. 2). In this analysis, 6 of the 24 species formed a cluster; Wve were collected only once at Sta. 25 on TALUD V (Ma, My, Th, Mi, Ph) and the other (Dp) showed an unusual density (4.667 ind l 1 ) in the same sample. This station was one of the few that had a signiwcant gravel component in the sediment. All three species of Scaphopoda (Da, Gf, Rd) were associated with very similar environmental conditions. Depth, dissolved oxygen content, and temperature were the most important variables of major signiwcance in the model; percentage of sand and mud were of minor signiwcance while organic matter content was intermediate (Fig. 2). The MRA indicated that, as in the case of diversity, density (ind l 1 ) was best correlated with dissolved oxygen concentration (R = 0.25), followed by temperature (R = 0.20), organic matter content (R = 0.15), and depth (R = 0.12). These four variables explained 52% of total variance in density (Table 3). Nature of substrate (i.e., sand, mud) was poorly correlated with density. Considering that dissolved oxygen was the abiotic variable that best explained variance in the CCA, it was used in a single correspondence analysis (CA) for the most abundant and frequent species and their density. Of the 24 species in the samples, only Wve were

4 886 Mar Biol (2007) 150: Table 1 Gulf of California TALUD cruises IV VII dredging stations showing samples with (+) and without ( ) mollusks and environmental parameters measured at bottom level Cruise Station Latitude Longitude Depth (m) Temp ( C) Oxygen (ml l 1 ) OM (%) Mud (%) Sand (%) Sed (l) IV 3 ( ) (+) ND ( ) ( ) ( ) ( ) ( ) (+) (+) ( ) V 3 ( ) (+) ( ) (+) (+) (+) ( ) (+) (+) VI 3 ( ) ( ) ND (+) (+) ( ) (+) ( ) (+) (+) (+) (+) (+) VII 3 ( ) (+) ( ) (+) b ( ) ( ) (+) (+) (+) ( ) b (+) b (+) b (+) Depth corresponds to mean between initial and Wnal haul depths. Gravel was observed only at Sta 25, TALUD V (0.40%). This is the station with the unusual cluster of six species including a high density of D. paciwcum OM organic matter, Sed sediment volume, ND no data selected on the basis of the following criteria: Dm 1 (Picard 1965) and F 10 (Glémarec 1964). These Wve species included two bivalves, Dacrydium paciwcum and Lucinoma heroica, and all three species of Scaphopoda, namely Dentalium agassizi, Gadila fusiformis, and Rhabdus dalli. Four dissolved oxygen intervals were considered (each of 0.5 ml l 1 range) (Table 4). The CA showed that the bivalves were denser in near anoxic conditions (C1) while scaphopods were denser at intermediate oxygen concentrations (C2 and C3). All Wve species had low densities at values > 1.51 ml l 1 (C4) (Fig. 3). D. agassizi occurred across almost the entire dissolved oxygen range measured (Fig. 4a), but higher densities were observed in oxygen concentrations of ml l 1 (Fig. 3). In contrast, bivalves rarely occurred in concentrations > 1 ml l 1 (Fig. 4a); D. paciwcum, for instance, occupied a very narrow dissolved oxygen range ( ml l 1 ) (Fig. 4a).

5 Mar Biol (2007) 150: Table 2 Species of mollusks collected during the TALUD cruises IV VII and density (ind l 1 of sediment) observed at each sampling station Species Code Cruise Station Ind l 1 Bivalvia Acharax johnsoni Dall, 1891 Dacrydium paciwcum (Dall, 1916) Ennucula colombiana (Dall, 1908) Limatula similaris (Dall, 1908) Lucinoma heroica (Dall, 1901) Aj VI VII 32b Dp V V VII Ec V Ls V Lh IV V V VI VI VII 32b Ma V Malletia alata Bernard, 1989 Myonera sp. My V Neilonella ritteri Nr IV (Dall, 1916) V Ennucula cardara Nc V (Dall, 1916) Nuculana sp. 1 N1 V Nuculana sp. 2 N2 VII Periploma carpenteri Pc VI Dall, 1896 Thracia sp. Th V Conchocele excavata Ce VI (Dall, 1901) VI Tindaria sp. Ti VI Vesicomya sp. Ve V Gastropoda Architectonia sp. Ar VI Bathybembyx bairdii Bb V (Dall, 1889) Microglyphis sp. Mi V Philine sp. Ph V Indeterminate Gi VI Scaphopoda Dentalium agassizi Pilsbry and Sharp, 1897 Gadila fusiformis (Pilsbry and Sharp, 1898) Rhabdus dalli (Pilsbry and Sharp, 1897) Da V V V VI VI VII VII VII 33b Gf VII VII VII 33b Rd IV IV VI VI VII VII 34b Rarefaction curves, obtained using bathymetric stratiwcation (300 m), indicated that species richness decreased with depth. Maximum species richness was observed at depths of 1,001 1,300 m (Fig. 5). Unfortunately, no sediment samples were available for the depth range of 1,601 1,900 m. A single linkage cluster analysis based on Euclidian distances indicated that similarity among mollusk assemblages increased with depth: deeper communities (1,601 1,900 and 1,901 2,200 m) were the most similar, while the least similar was observed at 701 1,000 m, the shallowest bathymetric zone in this study (Fig. 6). Of the Wve selected dominant and frequent species, the two bivalves were not found deeper than 1,300 m and the three species of scaphopods did not occur in depths < 1,000 m. D. agassizi had the widest bathymetric range (1,100 2,100 m) while G. fusiformis occurred in a very narrow depth range (1,100 1,250 m) (Fig. 4b). Discussion and conclusions Samples during this study were obtained between 731 and 2,110 m depth (ca. 1,400 depth range). Epibenthic water temperature decreased regularly from 6.2 C at the shallowest station to 2.0 C at the deepest station. Dissolved oxygen varied considerably over the study area (from 0.04 to 2.20 ml l 1 ), and was clearly related to depth; epibenthic water oxygen content was lowest at 815 m, then increased steadily to maximum values at depths of 2,000 2,110 m. Organic matter content was high, although this might be partly due to the ignition technique used (see below). The maximum value (19.90%) was observed at 775 m and minimum (8.26%) at 1,255 m depth. Sediments were mostly muddy (40 of 44 stations with > 90% of mud); sandy sediments (> 70% sand) were observed at only two stations, at 1,165 and 1,255 m depth. The considerable variation in sediment volume reported here is partly a consequence of to the sampling gear. The dredge was pulled over relatively long stretches of sea Xoor, and there was evidence of considerable variation in bottom texture, sometimes over short distances, from very solid, compacted mud to soft mud and occasionally rocky bottom, as observed in other surveys (see Gage 1996). We have no precise records of these variations, but this might explain why some samples were very small. This could also explain the large variation in sediment composition, i.e., percentage of mud, in adjacent stations. Contrary to what has been reported by Levin and Gage (1998) and Sauter et al. (2001), there was no signiwcant relationship between organic matter content of sediments and

6 888 Mar Biol (2007) 150: Fig. 2 Canonical correspondence analysis (CCA) using abiotic variables (depth, epibenthic dissolved oxygen concentration and temperature, percentage of sand and mud, and organic matter content) and mollusk diversity, total abundance, and species richness Table 3 Values of R and R 2 obtained by ranking abiotic variables vs. species density (ind l 1 ) using a multiple regression analysis Independent variable R R accumulated R 2 accumulated Increase of R 2 Disolved O 2 (ml l 1 ) Temperature ( C) Percentage of organic matter Depth (m) Table 4 Total abundance of Wve dominant and frequent species of mollusks using four dissolved oxygen intervals (C1 C4) each 0.5 ml l 1 in range Species Disolved oxygen (ml l 1 ) C1 C2 C3 C Dacrydium paciwcum Lucinoma heroica Dentalium agassizi Gadila fusiformis Rhabdus dalli dissolved oxygen concentration measured close to the bottom (R = 0.055; P = 0.726). We presently have no explanation for this fact. Of the 24 species found in the deep waters of the SE Gulf of California in the present study, only Wve were listed by Parker (1964a, b): Acharax johnsoni, Lima similaris, Nucula cardara, Periploma carpenteri, and D. agassizi, showing how limited our knowledge of deepwater mollusks in the Gulf of California is. Large numbers of the gastropod Bathybembix bairdii were recently trawled ov the coast of El Salvador at m depth (Hendrickx and López, unpublished data), but no infaunal studies have been conducted there. Organic matter (OM) content values recorded during this survey were particularly high. The calcination method used during this survey has been related to a signiwcant loss of water bound to clay components

7 Mar Biol (2007) 150: Fig. 3 Map based on single correspondence analysis (CA) for dominant and frequent species of mollusks collected using epibenthic dissolved oxygen concentration as the abiotic variable (Páez-Osuna et al. 1984); consequently, our OM data could be somewhat overestimated (roughly 20 40%), partly explaining the unusually high values we recorded. Some sediment samples used for meiofaunal studies were collected with a 2.5-mm corer at some of our sampling stations, and C org content was evaluated using the dichromate oxidation method. Maximum values were in the range of 11.4% OM (Gómez-Noguera, personal communication), thus close to the higher values recorded for the Santa Catalina Basin (Levin and Gage 1998). However, data obtained from the dichromate oxidation method did not correlate well with ours, probably because of the big diverences in sampling locations. No mollusks were found in 19 out of the 44 stations where sediment samples were collected. A Student s t test was used to compare two sets of environmental parameter values, the Wrst set including stations with mollusks and the second without mollusks. There was a signiwcant diverence (t 42 = 0.212; P = 0.033) only in the case of organic matter content. However, the higher level of organic matter in stations without mollusks could be partially a consequence of a decrease in the consumption of particulate matter by infaunal species, including mollusks, although patchiness of organic material distribution is also known to cause heterogeneity of bottom sediments (Grassle 1989). Although mean organic matter contents measured at stations without (14.7%) and with (13.1%) mollusks were similar, they were signiwcantly diverent due to the low variation in this parameter. Considering that in stations without mollusks organic matter was not a limiting factor and that there was no signiwcant variation when other parameters were examined, the absence of mollusks in very large samples of sediments (up to 85 l; see Table 1) cannot be explained by sampling bias and may just rexect the spatial heterogeneity of deep-water benthic communities (Grassle and Morse-Porteous 1987; Grassle 1989; Snelgrove et al. 1992). The deep-water environment is generally regarded as food-limited (Grassle 1989) and availability of particulate matter Fig. 4 Range of epibenthic dissolved oxygen concentration (ml l 1 ) (a) and depth (m) (b) recorded for Wve dominant and frequent species of mollusks

8 890 Mar Biol (2007) 150: Fig. 5 Rarefaction curves corresponding to four depth intervals of 300 m. No data available for 1,601 1,900 m depth interval Fig. 6 Cluster analysis based on Euclidian distance linking mollusk species composition with depth from productive surface waters is not constant in time (Witte 2000). Organic enrichment following deposition of autochthonous food is rapidly turned over by oportunistic feeders (Grassle and Morse-Porteous 1987) and patchiness in species richness over a few meters, and rarely over a few kilometers (Grassle 1989), can lead to marked diverences among samples, even over short distances. Deep-water microhabitats tend to persist longer, but are spatially limited and separated by greater distances than shallow water microhabitats (Grassle and Maciolek 1992). Organic matter in sediments is often expressed as percentage of organic carbon (C org ), but these can be expressed as total OM using the transformation coeycient proposed by Platt et al. (1969), i.e., % OM = % C org /0.6. Using this transformation, values recorded in the Santa Maria Basin (565 m depth) reached 4.25% OM (Hyland et al. 1991); ov North Carolina (850 m depth), maximum values of 4.16% OM have been recorded by SchaV et al. (1992); in the Scripps and La Jolla Canyons, ov southern California, USA, Vetter and Dayton (1998) found values up to 5.88% OM (500 m depth). Along the Oman Margin, OM (also reported as percentage of C org ) values depended strongly upon depth, reaching up to 8.32% OM at 400 m, with lower values at 1,000 m (3.21%) and 3,400 m (4.52%) (Levin and Gage 1998: Table 1). Much higher values have been observed in upwelling areas. Values of up to 11% OM (6.5% C org ) have been reported from the Santa Catalina Basin (see Levin and Gage 1998: Table 1). Using the same calcination method as in our study, Rosenberg et al. (1983) reported values of 10.6% OM at 119 m, and up to 15.1% OM at 359 m ov the coast of Peru. The SE Gulf of California is subject to intense upwellings during the winter (Parker 1964b) hence high OM values are not surprising. High OM contents have also been observed in sediments in the Norwegian and Greenland Seas, where it was attributed to unusual hydrographic settings and to hydrodynamic dead zones assumed to cause high accumulation rates of OM (Sauter et al. 2001). We are not aware of a similar mechanism in the SE Gulf of California. The present study suggests that in the SE Gulf of California dissolved oxygen content and depth are important in determining species composition and distribution of mollusks, as previously observed elsewhere for other groups of invertebrates (Rosenberg et al. 1983; Levin and Gage 1998; Hendrickx 2001; Levin et al. 2001; McClain 2004). Food supply, in terms of organic matter, is another major factor avecting the distribution of mollusks as observed previously (Janssen et al. 2000; Witte 2000; Kröncke and Türkay 2003). Our results also indicate that L. heroica has a strong aynity with low dissolved oxygen environments (< 0.5 ml l 1 ). According to Williams et al. (2004), most families included in the Lucinoidea maintain a chemoautotrophic chemosymbiosis with sulphide-oxidizing bacteria, thus allowing for a remarkable tolerance to anaerobic conditions. The present study did not sample at depths < 780 m since we anticipated that the OMZ, which lies between 100 and 800 m depth, would constitute a barrier to dispersal of benthic species from the continental shelf and upper slope to deeper waters. In the OMZ, epibenthic dissolved oxygen concentrations are always < 0.5 ml l 1 and occasionally < 0.1 ml l 1. We recorded dissolved oxygen concentrations < 0.1 ml l 1 in depths as great as 1,160 1,180 m in June The highest densities of mollusks in the southeast Gulf of California were observed at 700 1,000 m depth, where dissolved oxygen rarely exceeded 0.5 ml l 1. In deeper water (> 1,600 m), density and

9 Mar Biol (2007) 150: species richness of mollusks were lower despite a high dissolved oxygen content. Most abundant bivalves dwelt on the upper slope under severe hypoxic conditions, while scaphopods were much more frequently found on the lower slope, in more oxygenated water. Nevertheless, the highest mollusk diversity was observed at stations at > 1,250 m depths where oxygen concentrations were intermediate ( ml l 1 ). Acknowledgments This study was supported by CONACyT project N and partly supported by DGAPA project IN (Mexico) and shiptime was granted by UNAM (Coordinación de la Investigación CientíWca). The authors thank all members of the scientiwc and technical crews for the help provided during the TALUD IV VII cruises. We are particularly thankful to N. Mendéz-Ubach and S. Gómez for providing sediment data. We also thank E. Coan and P. Scott for their assistance with identiwcation of diycult species and S.E. Townsend for reviewing the English. 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