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Hydrobiologia 496: 361–370, 2003. E. Sigvaldad´ottir, A.S.Y. Mackie, G.V. Helgason, D.J. Reish, J. Svavarsson, S.A. Steingr´ımsson & G. Guðmundsson (eds), Advances in Polychaete Research. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Bathymetric distribution and diversity of deep water polychaetous annelids in the Sigsbee Basin, northwestern Gulf of Mexico Alma Yazm´ın P´erez-Mendoza, Pablo Hern´andez-Alc´antara & Vivianne Sol´ıs-Weiss∗ Laboratorio de Ecolog´ıa Costera, Instituto de Ciencias del Mar y Limnolog´ıa, Universidad Nacional Aut´onoma de M´exico. Circuito Exterior S/N Ciudad Universitaria. Apdo. Postal 70-305, M´exico, D. F. C. P. 04510. M´exico E-mail: [email protected] or [email protected] (∗ Author for correspondence) Key words: polychaetes, deep-sea, diversity, Gulf of Mexico, Sigsbee Basin

Abstract The deep water polychaete fauna is analyzed in this study particularly regarding its composition and variations with depth in the Sigsbee Basin, northwestern region of the Gulf of Mexico. Samples were taken at 10 stations along a bathymetric gradient with depth ranges from 200 to 3760 m with a USNEL (0.25 m2 ) corer. A total of 287 individuals were identified, from 21 families and 65 species. The most important families, both in terms of abundance and species richness, were: Paraonidae (65.4 ind./0.25 m2 , 9 spp.), Cirratulidae (28.93 ind./0.25 m2 , 7 spp.) and Spionidae (18.07 ind./0.25 m2 , 7 spp.). In general, density tended to decrease with depth with minima at around 2000 m, although two abundance peaks were detected at 3700 and 3760 m, making the pattern seem an inverted parabolic curve. The Shannon-Wiener diversity values varied from 0.54–0.92 at around 2000 m to 3.39 at 3620 m and 3.34 at 3760 m. These results contrast with what is already reported from the North Atlantic and the Tropical Pacific deep benthic communities, where highest diversities are found at 2000 m. Faunal changes evaluated through Beta Diversity (0.08–0.1) and the low similarity found between the stations, emphasized the high variability in the composition of the fauna in the Sigsee Basin, meaning that the faunal composition is practically different at all the sampling stations. Ten species are newly recorded for the Mexican fauna. Introduction The physical, chemical and sedimentological characteristics affecting the deep-sea marine benthos are more homogeneous and stable over large areas compared to those of shallower coastal environments (Grassle, 1989; Grassle & Macioleck, 1992). In addition, food resources and availability are thought to be less abundant and directly control the development, evolution and persistence of deep-sea communities (Grassle, 1989; Kojima & Ohta, 1989; Grassle, 1991; Tyler, 1995). In these environments, feeding probably depends more on allochthonous organic matter, mostly falling down from the surface in the form of particulate organic carbon or carcasses, than on autochthonous material (local secondary production). Despite this limiting factor, the benthic fauna of deep waters has been found to be highly diversified, though sparsely distributed (Sanders, 1979; Grassle & Maciolek, 1992).

In the classic study by Hessler & Sanders (1967), unexpectedly high species richness was recorded in the deep-sea benthos. Grassle (1991) and Grassle & Maciolek (1992) later confirmed these results pointing out that the number of species in deep zones had been grossly underestimated in earlier studies. Deepsea benthic diversity was now regarded as matching that of the richest known marine environments, those of the shallow tropical seas. Studies on polychaetes in the Gulf of Mexico have focused primarily on the taxonomy of littoral and sublittoral species (Hartman, 1951; Foster, 1971; Uebelacker & Johnson, 1984; Hernández Alcántara & Solis-Weiss, 1991; Solis-Weiss et al., 1994, 1995; Granados Barba & Solis-Weiss, 1994, 1997a,b, 1998; De León González & Solis-Weiss, 1997), with only a few deep-water records. For example, Pettibone (1986) described a commensal polynoid from deep sea mussels and more recently, Desbruyères & Toulmond (1998) gave an account of a new hesionid associated

362 to methane hydrates at 538 m depth. A small number of investigations have been carried out on the deepwater communities (Rowe et al., 1974; Pequegnat et al., 1990; Boland & Rowe, 1991; Carney, 1993; Escobar-Briones & Soto, 1993; Escobar-Briones et al., 1999; Hernández-Robles, 1999; Pérez-Mendoza, 2001). Rowe & Menzel (1971) made biomass measurements and found that polychaetes were the dominant faunal group. In the Gulf of Mexico, the upper limit for the deep-sea benthic environments is the shelf-break, a transitional zone (from 200 m depth) between the shelf and continental slope (Pequegnat, 1983). The objective of the present study was to examine the taxonomic composition and diversity of the polychaete communities along a bathymetric gradient in the Sigsbee Basin and compare the results with other deep-sea studies. Study area The study area is located off Tamaulipas state, northwestern Gulf of Mexico (23◦ 30 –25◦ 30 N; 93◦– 97◦ W), and includes the shelf-break as well as the slope and the abyssal plain at Sigsbee Basin (Fig. 1). The area is within the ‘terrigen province’ (Uchupi, 1975), in which the muddy substrates are primarily derived from the Bravo and Mississippi river discharges (Glover, 1961), even though biogenic fine sediments are also important (Lecuanda & Ramos, 1985). This region is hydrodynamically complex and subjected to the anticyclonic gyres of the Loop current which help generate a high primary productivity (Vidal et al., 1994). Three water layers are recognized covering a 23–4 ◦ C change in temperature: the mixing layer, the thermocline and the deep layer. Mixing layer salinities vary from 34.88 to 36.7‰. Maximal salinity values (36.7‰) are found below this layer and rapidly decrease, forming a halocline at around 400 m. The lowest values (34.88‰) are found at 750 m and are part of the Intermediary Antarctic Water (Nowlin & McLellan, 1967). The other water mass present (4.02 ◦ C; 34.98‰) in the deep zones is part of the Deep North Atlantic waters (Vidal et al., 1990). On the upper slope, a minimum oxygen layer has been detected: Nowlin & MacLellan (1967) recorded a typical distribution in the variations of the oxygen values present, with surface maxima above 4.5 ml/l, a minimum oxygen layer (2.5 ml/l) betweeen 200 and 600 m and an increase of up to 4.5 ml/l at 1250 m. However, the gyres in the area modify this generalized

pattern of oxygen distribution: the anticyclonic gyres known locally as ‘tamaulipecos’ induce the sinking of the oxygen layer as recorded by Moulin (1980).

Materials and methods The polychaetous annelids analysed in this study were collected in June 1997 as part of the Universidad Nacional Autonoma de Mexico (UNAM) ‘SIGSBEE’ project, using the R/V ‘Justo Sierra’. The biological samples were taken at 10 stations along a bathymetric gradient with depth ranges from 200 to 3760 m using a USNEL (0.25 m2 ) corer. Due to time and budget constraints, generally only one core was taken at each location, subdivided in four equal parts, three of which were devoted to biological analyses. From the fourth subdivided part, the following parameters were determined: sediment composition was analysed following Folk (1968) methodology; organic matter content in the sediments was determined by calcination following Trask (1953) methodology and conversion constants. When sediment was insufficient for the analyses, a second core was taken (Stations 1, 2, 3 and 10), and the subdivided parts added to the biological samples. Thus, in order to be able to compare all biological samples, the numerical abundance of each species was adjusted to 0.25 m2 , which is the USNEL core area, and for this reason is here also referred to as ‘density’. ‘Total density’ is the sum of all the values of density calculated for a given species or sets of species at the stations of occurrence. More detailed information on the methodology used in this expedition as well as on physical, chemical and sedimentological results can be found in Hernández-Robles (1999). Bottom temperatures and salinities were measured with a General Oceanics MarkIII WOOCE CTD. Dissolved oxygen concentrations were determined following the Winkler method (Strickland & Parsons, 1972), from bottom water samples taken at each station with a Rosette. The biological samples were screened through a 0.25 mm mesh sieve. Fixation, preservation and identifications were done following standard methodology (Uebelacker & Johnson, 1984; Blake, 1994). In the faunal list (Table 2), the species listed as ‘sp.’ followed by a number or letter are potentially new to science (Pérez-Mendoza, 2001). The identified specimens were deposited in the polychaete collection of the Instituto de Ciencias del Mar y Limnología,

363

Figure 1. Study area with the sampling stations.

UNAM (CPICML-UNAM, DFE.IN.061.0598), Mexico, D.F. The variations in diversity were evaluated with species richness (S = number of Species) and the Shannon–Wiener diversity index: H  (Magurran, 1989). To measure the species’ replacements or biotic changes along the bathymetric gradient, the Beta Diversity Index (β T ) of Wilson & Schmida (1984) was used where   ¯ βT = g(H) /2α, βT Beta Diversity (measurement of the change in composition of species between two localities), α¯ = average number of species in the two localities under comparison, g(H) = number of species ‘incorporated’ in the faunal composition of the two localities under comparison, l(H) = number of species ‘lost’ in the faunal composition of the two localities under comparison. The affinities among stations were evaluated with a Cluster analysis (UPGMA), calculating the similarity coefficient by euclidian distances between stations according to the variations in the densities of the species present (Romesburg, 1990).

Results Environmental conditions The physical and chemical parameters of the water (Table 1) generally followed the patterns of other

deep-sea studies (Lampit, 1992; Newton et al., 1994; Rice et al., 1994; Lambshead et al., 1995; Longhurst et al., 1995; Tyler, 1995). Temperature gradually decreased with depth, from 19.5 ◦ C at 200 m to 4 ◦ C at 3620 m and down, while salinity varied from 34.8‰, at different depths, to 36.2‰ at 3760 m (station 10), with no evident pattern. Dissolved oxygen values were lower between 200 and 500 m. The sediments were predominantly mud (86.32– 99.6% silt-clay), except for station 2 at 498 m where sandy mud (44.7% sand) was found; organic matter content of the sediments varied from 0.92% (again at station 2) to 1.62%, with an average of 1.35%. Faunal composition A total of 287 organisms belonging to 21 families, 41 genera and 65 species were identified. Of these, 10 were new to the Mexican fauna (Table 2). The faunal composition showed that, although several families were present at each station, the number of species per family was usually very low: 15 of the 21 families were only represented by only one or two species (Table 3). The Paraonidae, Nephtydae, Cirratulidae and Spionidae were numerically dominant in the Sigsbee Basin (Table 3); however, only the cirratulids (total density: 28.93 ind./0.25 m2 ) were found at all depths. The paraonids (total density: 65.4 ind./0.25 m2 ) were more abundant at depths more than 2620 m, whereas the spionids (total density: 18.07 ind./0.25 m2 ) were more abundant at depths below 2620 m. Remarkably,

364 Table 1. Geographic positions of the stations, depth and environmental parameters measured Station

1 2 3 4 5 6 7 8 9 10

Latitude (N) 23◦ 23◦ 24◦ 24◦ 24◦ 24◦ 25◦ 24◦ 24◦ 23◦

55 57 03 03 00 12 15 59 03 24

30 06 12 00 12 54 18 30 48 36

Longitude (W)

Depth (m)

Temperature (◦ C)

Salinity (ups)

Dissolved oxygen (mg/l)

Organic matter (%)

Sand (%)

Mud (%)

97◦ 97◦ 97◦ 96◦ 96◦ 96◦ 93◦ 94◦ 94◦ 93◦

200 498 1231 1698 2220 2620 3620 3700 3760 3760

19.5 12.0 10.0 8.0 5.0 5.0 4.0 4.0 4.0 4.0

35.6 34.8 34.6 34.8 34.7 34.8 34.8 35.9 34.8 36.2

2.8 3.0 4.2 5.8 4.8 6.2 6.0 6.0 6.0 5.6

1.68 0.92 1.53 1.30 1.22 1.53 1.30 1.38 1.07 1.22

0.40 44.07 1.28 3.24 1.05 2.69 3.76 5.19 7.98 13.68

99.60 55.93 98.72 96.76 98.95 97.31 96.24 94.81 92.02 86.32

18 12 01 44 34 09 25 46 42 07

12 54 42 24 42 30 42 54 36 54

Figure 3. Values of the Shannon–Wiener Diversity Index, at each sampling station. Figure 2. Density variations and number of species in the sampling stations.

the very high densities recorded in the Nephtyidae (total density: 30.40 ind./0.25 m2 ), were almost exclusively due to one species: N. incisa (28 ind./0.25 m2 ), since the only other species found to belong to this family, A. circinata, was only collected with a density of 2.40 ind./0.25 m2 (Table 3). Numerical abundance and species diversity In general, numerical abundance in the study area was low (average: 20.81 ind./0.25 m2 per station). Furthermore, density variations along the bathymetric gradient indicated a drastic reduction from 64 ind./0.25 m2 at 200 m to 15.2–1.33 ind./0.25 m2 at 498–2200 m (Fig. 2). At greater depths, two density peaks were evident, one at 3700 m (34.0 ind./0.25 m2 ) and another at 3760 m (49.33 ind./0.25 m2 ) (Fig. 2).

The families with the largest number of species were Paraonidae (9 spp.), Spionidae (7 spp.), Syllidae (8 spp.), Cirratulidae (7 spp.) and Lumbrineridae (7 spp.). Species richness values showed the same general trends as those of abundance (Fig. 2), clearly decreasing with depth, from 26 species at 200 m down to a minimum of 2–7 species between 1698 and 2620 m. Two smaller peaks in number of species occurred at 3620 m and 3760 m. Although spatial variations in diversity do not follow any definite pattern, in five stations, H  values are higher than 3 (Fig. 3), similar to those recorded in other deep seas areas (Cosson-Sarradin et al., 1998): highest values are found at depths of 200 – 1231 m (H  = 3.09–3.34), at 3620 m (H  = 3.39) and at 3760 m (H  = 3.34). A significant reduction in diversity was recorded in this study at 1698 m (H  = 0.92) and 2220 m (H  = 0.54), where in both stations only two species were collected (Fig. 3) (Table 2).

365 Table 2. Faunal list, density and frequencies of the Polychaetous Annelids from Sigsbee Basin (∗ = new records for Mexico) Density Frequency (%) (inds./0.25 m2 ) Polychaeta Grube 1850 Scolecida Family Capitellidae Grube, 1862 Decamastus gracilis Hartman, 1963 ∗ Heteromastus filiformis Claparede, 1864 Mediomastus californiensis Hartman, 1944 Notomastus lineatus Claparede, 1870 Decamastus sp. A Uebelacker, 1984 Family Scalibregmatidae Malmgren, 1867 ∗ Pseudoscalibregma aciculata Hartman, 1965 Family Opheliidae Malmgren, 1867 ∗ Tachytrypane jeffreysii McIntosh, 1879 Family Paraonidae Cerruti, 1909 Aricidea (Acmira) simplex Day, 1963 Aricidea (Allia) claudiae Laubier, 1967 Aricidea (Allia) suecica Eliason, 1920 Cirrophorus branchiatus Ehlers, 1908 Cirrophorus furcatus (Hartman, 1957 Cirrophorus lyra (Southern, 1914) Levinsenia gracilis (Tauber, 1879) Levinsenia reducta (Hartman, 1965) Paraonella sp. 1 Family Cossuridae Day, 1963 Cossura delta Reish, 1958 Palpata Aciculata Phyllodocida Family Glyceridae Grube, 1850 ∗ Glycera lapidum Quatrefages, 1865 Hemipodus sp. 1 Family Goniadidae Kinberg, 1866 Goniada maculata Örsted, 1843 Family Paralacydoniidae Pettibone, 1963 Paralacydonia paradoxa Fauvel, 1913 Family Phyllodocidae Örsted, 1843 Eteone longa (Fabricius, 1780) Family Nephtyidae Grube, 1850 Aglaophamus circinata (Verrill, 1874) Nephtys incisa Malmgren, 1865 Family Nereididae Johnston, 1865 Ceratocephale sp. 1 Family Hesionidae Grube, 1850 ∗ Podarkeopsis levifuscina Perkins, 1984 Family Pilargidae Sainth-Joseph 1899 Sigambra tentaculata (Treadwell, 1941) Family Syllidae Grube, 1850 ∗ Parexogone caribensis San Mart´ın, 1991 ∗ Parexogone wolfi San Mart´ın, 1991 Syllis cf. gracilis Grube, 1840 Typosyllis alosae San Mart´ın, 1992 Ehlersia sp. 1 Typosyllis sp. 1 Parexogone sp. 1 Pionosyllis sp. B Uebelacker, 1984 Eunicida sensu stricto Family Lumbrineridae Schmarda 1861

5.00 1.60 0.80 1.33 1.33

30 10 10 10 10

1.00

10

4.00

10

1.80 5.80 1.60 1.60 1.00 3.47 23.87 1.60 23.33

20 20 10 10 10 20 50 10 50

0.80

10

0.80 0.80

10 10

1.33

10

0.80

10

1.33

10

2.40

20

8.80

30

0.80

10

13.00

50

1.33 2.93 1.33 4.00 1.33 1.60 1.33 0.80

10 20 10 10 10 20 10 10

Table 2. contd. ∗ Lumbrinerides jonesi Perkins, 1979

Lumbrineriopsis paradoxa (Saint-Joseph, 1888) Lumbrineris latreilli Audouin & Milne Edwards, 1834 Lumbrineris cf. limicola Hartman, 1944 Scoletoma verrilli (Perkins, 1979) Scoletoma sp. 2 Scoletoma sp. 3 Family Onuphidae Kinberg, 1865 Paradiopatra cf. papillata (Kucheruck, 1979) Paradiopatra sp. 1 Amphinomida sensu stricto Family Amphinomidae Savigny in Lamark, 1818 ∗ Paramphinome jeffreysii (McIntosh, 1868) Canalipalpata Spionida Family Spionidae Grube, 1850 Paraprionospio pinnata (Ehlers, 1901) Prionospio (Prionospio) dubia Day, 1961 ∗ Prionospio (Minuspio) fauchaldi Maciolek, 1985 Prionospio (Prionospio) ehlersi Fauvel, 1928 Prionospio (Prionospio) steenstrupi Malmgren, 1867 Polydora websteri Hartman, 1943 Spiophanes cf. berkeleyorum Pettibone, 1962 Terebellida Family Flabelligeridae Sainth-Joseph, 1894 Diplocirrus capensis Day, 1961 Family Cirratulidae Carus, 1863 Aphelochaeta cf. monilaris (Hartman, 1960) Aphelochaeta cf. phillipsi Blake, 1996 Aphelochaeta cf. williamsae Blake, 1996 Chaetozone cf. columbiana Blake, 1996 Chaetozone cf. commonalis Blake, 1996 Monticellina dorsobranchialis (Kirkegaard, 1959) Monticellina cf. tesselata (Hartman, 1960) Family Ampharetidae Malmgren, 1866 Ampharete sp. A Uebelacker, 1984 Genus 1

1.00 1.33 1.33 1.60 2.40 2.00 0.80

10 10 10 20 20 10 10

0.80 10 0.80 10

5.33 30

6.67 1.00 0.80 3.47 1.00 1.00 1.33

30 10 10 20 10 10 10

0.80 10 2.67 1.00 2.00 3.00 0.80 4.53 5.60

20 10 10 10 10 20 30

0.80 10 0.80 10

Species composition along the bathymetric gradient The variations in the values of density, number of species and diversity were also reflected in species composition along the bathymetric gradient. The reduced number of species was here associated to the abundance of a few polychaetes in those sites. The species Levinsenia gracilis (23.87 ind./0.25 m2 ) and Paraonella sp.1 (23.33 ind./0.25 m2 ), were the most

366 Table 3. Density and number of species per family from Sigsbee Basin Family

Paraonidae Nephtyidae Cirratulidae Spionidae Syllidae Pilargidae Capitellidae Lumbrineridae Nereididae Amphinomidae Opheliidae Ampharetidae Glyceridae Onuphidae Goniadidae Phyllodocidae Scalibregmatidae Flabelligeridae Hesionidae Paralacydoniidae

Total density (inds./0.25 m2 )

No. species

65.4 30.40 28.93 18.07 14.13 13.00 11.39 10.47 8.80 5.33 4.00 1.60 1.60 1.60 1.33 1.33 1.00 0.80 0.80 0.80

9 2 7 7 8 1 5 7 1 1 1 2 2 2 1 1 1 1 1 1

Figure 5. Values of the Beta diversity index along the bathymetric gradient.

Figure 6. Cluster analysis of the sampling stations according to their faunal composition.

Figure 4. Variations of the abundance of the most abundant species, by bathymetric level.

abundant and frequent (50%) (Table 2), followed by Sigambra tentaculata, Paraprionospio pinnata and Monticellina cf. tesselata, all of them important for both their abundance and frequency, but neither considered dominant since their frequency never exceeded 50% at the sampled depths (Table 2). None of them were collected between 1698 and 2620 m and no relationship could be found between species composition at each station and diversity (Fig. 4). L. gracilis (0.8–

14.0 ind./0.25 m2 ) was collected in five stations with highest abundance at 200 and 3700 m, while Paraonella sp. 1 (0.8–13.33 ind./0.25 m2 ), which was also collected at five stations was most abundant at 1231 and 3760 m. The distributions of the other three species with the highest abundance were more restricted, since they were only collected at three stations: S. tentaculata (0.8–6.67 ind./0.25 m2 ) was abundant at 200 and 3760 m; P. pinnata (1.33–4.0 ind./0.25 m2 ) which is usually abundant in soft bottoms of the continental shelf (Hernández-Alcántara et al., 1994) was only found abundant at 200 m, and M. cf. tesselata (1.33– 2.67 ind./0.25 m2 ) was never found abundant at any bathymetric level (Fig. 4). Species composition was rather heterogeneous, characterized by a high proportion of species with low abundances at each bathymetric level (Table 2) and at each station the species with highest abundances were different. Those differences were emphasized when the Beta Diversity analysis was carried out (Fig. 5) since with this method the changes in species compos-

367 ition along the gradient could be evaluated. The high values of Beta Diversity indicated large variations in the faunal composition of adjacent stations, larger at approximately 2000 m. On the other hand, the analysis of similarity between stations based on density fluctuations, highlighted two main groups (Fig. 6): group A, formed by stations 2 (498 m) and 3 (1231 m), where Paraonella sp. 1 was the most abundant species, total abundance values were higher (15.2 and 13.6 ind./0.25 m2 ) and the number of species similar (10 and 11 spp.), and group B, formed by stations 4, 5, 6, 7 and 10. where the number of species and composition were highly heterogeneous, but displayed the lowest densities in the study area (1.33–10.66 ind./0.25 m2 ). Station 1, where the highest diversity was recorded (H  = 3.34) due to high densities and number of species, showed low affinity with the other stations with Nephthy incisa as the most abundant species. Two of the deepest stations: 8 (3700 m) and 9 (3760 m), where L. gracilis and S. tentaculata were found abundantly, are isolated from the other groups since high abundances (34.0 and 49.33 ind./0.25 m2 respectively) but few species (6 and 4 spp.) are present. Thirty eight percent (25 spp.) of the species were collected below 2000 m, which is why the variations in similarity are basically the result of the absence of those species at higher depths.

Discussion In Mexico, studies of deep-sea polychaetes are very scarse so that we were forced to perform the taxonomic identifications with the existing references, which often did not coincide with the study area. For this reason, 10 species from the family Cirratulidae are listed as ‘cf’, (Table 2), even though their anatomical features matched those of the species already described by Blake (1994), since they have not yet been recorded from outside the deep waters of the eastern Pacific (Blake, pers. com.). Clearly, more studies are needed in the area to solve the taxonomic problems raised by this issue and to deal reasonably with the present debate about cosmopolitanism, endemism and regionalism which can only be resolved if comparisons could be carried out on the basis of modern systematics (sibling species have been demonstrated in several areas, several species have been confused under the same name, the bathyal and abyssal fauna remain insufficiently known, etc.), biogeographical

distribution and ecological structure (Dauvin et al., 1994). The distribution of another set of 10 species is extended from other regions of the Gulf of Mexico to the Mexican fauna (Table 2). In general, our results matched those of similar research done on deep seas polychaetes, i.e. Spionidae, Cirratulidae and Paraonidae are ususally the dominant families and represent between 50 and 60% of the total abundance (Paterson et al., 1998; CossonSarradin et al., 1998; Glover et al., 2001). This has been observed both in the Atlantic and in the Pacific Ocean, irrespective of organic enrichment conditions and could show that the functional structure is very similar and dominated by deposit feeding polychaetes (Glover et al., 2001). Predator families such as Syllidae and Lumbrineridae, while not always dominant are regularly present in those zones. Hecker & Paul (1979) and Hessler & Jumars (1974) have found that the most common family in the Pacific Ocean is the Cirratulidae, while in the Atlantic, Cossons-Sarradin et al. (1998) have observed an increase in the abundance of this family between 1590 and 3128 m. In Sigsbee Basin, we found among the most abundant species, the cirratulid Monticellina cf. tesselata; the other important species being the spionid P. pinnata and the paraonid Paraonella sp. 1, which has been recorded as dominant in the Atlantic (Cosson-Sarradin et al., 1998). Another important species is the Pilargid Sigambra tentaculata, similar to Sigambra sp., characterized by Glover et al. (2001) as abundant in the northeastern Atlantic. Even though abundance variations did not show a definite pattern along the bathymetric gradient, in general it tended to decrease with depth. This relationship between abundance and depth has already been recorded by Cosson-Sarradin et al. (1998), who observed in the tropical Atlantic a lineal reduction of the abundance with depth and by Rowe & Pariente (1992) who indicated that biomass and abundance of the benthic fauna decrease exponentially with depth, as a result of the abatement in nutrient flux . The low abundances registered in this study are comparable to those recorded in faunistically ‘poor’ areas of the northeastern Atlantic (17.5 ind./0.25 m2 ), but much lower than those recorded in oligotrophic areas of the Tropical Atlantic (35.75 ind./0.25 m2 ) or in temperate regions of this same ocean (64.5–85.5 ind./0.25 m2 ) (Glover et al., 2001). In two of the stations sampled, we found that at same depth (3760 m: stations 9 and 10), abundance and species composition

368 could be quite different: 49.33 and 10.66 ind./0. 25 m2 and 4 and 16 spp. respectively. However, this can probably be explained by the stations position: station 10 is located to the south of the transect (Fig. 1), with higher salinities, sandy mud substrate, and geographically positioned outside the transect, while station 9 is located in a depression of Sigsbee Basin, with lower salinity and muddy substrate. Those contrasting conditions could be important enough to produce substantial differential effects at the local scale and therefore drastically influence composition and abundance (Fig. 2). Although the low polychaete abundances in the study area are difficult to explain at this point, the low content in organic matter could definitely be a contributing factor (Rowe, 1981; Cosson-Sarradin et al., 1998; Glover et al., 2001). The distribution of the density values recorded in the sampling stations in general, showed that Sigsbee Basin is characterized by lower diversities (with H  values of 2.49 in average) (Fig. 3) than those registered in other deep seas areas (H  > 3) (Nichols & Rowe, 1977; Cosson-Sarradin et al., 1998). These low diversities resulted from the abundance of very few species and are unusual at high depths, but they have already been recorded in the Northeastern Atlantic under strong oceanic currents (Paterson & Lambshead, 1995). They could be indicative of communities under stress, but at this point it is impossible to establish the possible perturbation factor(s) responsible for the presence of species considered to be widely distributed worldwide or their relative high abundance there. The variability observed in specific composition reflected in the high values of Beta Diversity and low similarity among stations suggested that faunal composition was practically different at each station, and also influenced diversity negatively. Diversity variations along the bathymetric gradient were also found to differ somewhat from values recorded in other deep zones, where diversity increases parabolically from the shelf-break to a maximum around medium bathyal depths (400–2800 m) decreasing again towards the abyssal plains (Rex, 1981, 1983; Paterson & Lambshead, 1995; Cosson-Sarradin et al., 1998). Rex et al. (1997) reported that in those communities, the highest diversity values were found between 2000 and 3000 m. Those observations were later confirmed by Etter & Grassle (1992) and Paterson & Lambshead (1995), although the latter authors indicated that geographic position could also have influenced those results.

In Sigsbee Basin, diversity decreased gradually along the bathymetric gradient with initial values of H  =3.34 at 200 m down to H  = 0.91–0.54 at 1698– 2220 m, respectively, and from this depth on, the pattern was irregular but in general tended to increase again to values of 1.92–3.39 at depths of about 3700 m; highest diversity (H  = 3.39) was recorded at 3620 m (Fig. 3). However, excepting the lowest values found at 2220 m (H  = 0.54), the variations of diversity in relation with depth, matched the diversity ranges reported by Nichols & Rowe (1977) for the benthic fauna in deep seas. Conclusions The results obtained in this study show that the polychaetes present in Sigsbee Basin are poorer both in density and species richness than others recorded in deep seas, one of the probable contributing factors being the low organic matter content found in the sediments on which this fauna depends for survival. However, the values of species richness and abundance followed a pattern similar to an inverted parabolic curve not unlike other recorded in similar studies. The polychaete fauna of the area is characterized by the relative abundance of a few species, all different at the different bathymetric levels and by an unusually high number of rare species. At about 2000 m, a reduction in diversity and a change in species composition was detected, together and related to a drastic decrease in the number of polychaetes found at this depth. Acknowledgements Thanks are due to Elva Escobar Briones (Laboratorio de Ecología del Bentos of ICMyL-UNAM) for the donation of the polychaete material used in this study and to D. Hernandez Robles for her assistance in separating the biological samples. A special acknowledgment goes to the anonymous referees whose comments and suggestions greatly improved the manuscript. References Blake, J., 1994. Introduction to the Polychaeta: In Blake, J. B. Hilbig & P. Scott (eds), Taxonomic Atlas of the Benthic Fauna of the Santa Maria basin Bnd Western Santa Barbara Channel, vol. 4. Santa Barbara Mus. nat. Hist., Calif.: 39–101.

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