INTRODUCTIONTop

Cumaceans (more than 1750 known species; see Watling and Gerken 2019Watling L., Gerken S. 2019. World Cumacea Database. World Register of Marine Species. Cumacea. Accessed on 2019-02-03 through: http://www.marinespecies.org/aphia.php?p=taxdetails&id=1137) are small peracarid crustaceans widely distributed from intertidal to hadal bottoms (Jones 1969Jones N.S. 1969. The systematics and distribution of Cumacea from deeps exceeding 200 meters. Galathea Rep. 10: 99-180.) and showing the highest diversity in bathyal environments (Jones and Sanders 1972Jones N.S., Sanders H.L. 1972. Distribution of Cumacea in the deep Atlantic. Deep-Sea Res. 19: 737-745., Reyss 1973Reyss D. 1973. Distribution of Cumacea in the deep Mediterranean. Deep-Sea Res. 20: 1119-1123., Gage and Tyler 1992Gage J., Tyler P. 1992. Deep-sea biology: a natural history of organisms at the deep-sea floor. Cambridge University Press.), a bathymetric distributional pattern also recognized for most benthic taxa (Rex et al. 1997Rex M., Etter R., Stuart C. 1997. Large-scale patterns of species diversity in the deep-sea benthos. In: Ormond R.F.G., Gage J.D., Angel M.V. (eds) Marine biodiversity: patterns and processes, Cambridge University Press, pp 94-121., Rex and Etter 2010Rex M., Etter R. 2010. Deep-sea biodiversity: pattern and scale. Harvard University Press, Cambridge and London.). Furthermore, they represent one of the main faunal components of marine suprabenthic assemblages (Mees and Jones 1997Mees J., Jones M.B. 1997. The hyperbenthos. Oceanogr. Mar. Biol. Annu. Rev. 35: 221-255.) because they live close to the sediment-water interface, burrowing in the top layer substratum or swimming in the near-bottom water (see Foxon 1936Foxon G. 1936. Notes on the natural history of certain sand-dwelling Cumacea. Ann. Mag. Nat. Hist. 17: 377-393., Forsman 1938Forsman B. 1938. Untersuchungen über die Cumaceen des Skagerraks. Zool. Bid. Uppsala 18: 1-161., Dixon 1944Dixon A. 1944. Notes on certain aspects of the biology of Cumopsis goodsiri (Van Beneden) and some other cumaceans in relation to their environment. J. Mar. Biol. Ass. U.K. 26: 61-71.).

The deep cumacean fauna of the NE Atlantic Ocean (including the Bay of Biscay) is probably one of the best known in the world thanks to the taxonomical works of Bonnier (1896)Bonnier J.J. 1896. Édriophthalmes. Résultats Scientifiques de la Campagne du «Caudan» dans le Golfe de Gascogne, août-septembre 1895. Ann. Univ. Lyon 26: 528-562., Fage (1929)Fage L. 1929. Cumacés et Leptostracés provenant des campagnes du Prince Albert 1er de Monaco. Résult. Camp. Sci. Albert 1er Prince Monaco 77: 3-51., Reyss (1974aReyss D. 1974a. Contribution à l’étude des cumacés de profondeur de l’Atlantique nord: le genre Makrokylindrus Stebbing. Crustaceana 26: 5-28., bReyss D. 1974b. Contribution à l’étude des cumacés de profondeur en Atlantique: le genre Diastyloides Sars, 1900. Crustaceana 27: 285-293., 1978)Reyss D. 1978. Cumacés de profondeur de l’Atlantique nord. Famille des Lampropidae. Crustaceana 35: 1-21., Jones (1974Jones N.S. 1974. Campylaspis species (Crustacea: Cumacea) from the deep Atlantic. Bull. Br. Mus. (Nat. Hist.) Zool. 27: 247-300., 1984Jones N.S. 1984. The family Nannastacidae (Crustacea: Cumacea) from the deep Atlantic. Bull. Br. Mus. (Nat. Hist.) Zool. 46: 207-289., 1985)Jones N.S. 1985. Distribution of the Cumacea. In: Laubier L., Monniot C. (eds), Peuplements profonds du golfe de Gascogne: campagnes BIOGAS. IFREMER, Brest, pp 429-433. and Bishop (1981aBishop J.D.D. 1981a. Two new leuconids (Peracarida, Cumacea) of widespread occurrence in the deep Atlantic. Crustaceana 40: 144-159., b)Bishop J.D.D. 1981b. A revised definition of the genus Epileucon Jones (Crustacea, Cumacea), with descriptions of species from the deep Atlantic. Philos. Trans. R. Soc. Lond. B 291: 353-409. but also to the pioneering study of Lagardère (1977)Lagardère J.-P. 1977. Recherches sur la distribution verticale et sur l’alimentation des crustacés décapodes benthiques de la pente continentale du golfe de Gascogne. Analyse des groupements carcinologiques. Bull. Cent. Etud. Rech. Sci. Biarritz 2: 367-440. on their bathymetric distribution in the southern part of the Bay of Biscay. However, quantitative information on the structure of the corresponding taxocœnoses remains very scarce in this area. To fill this gap, a new research programme was initiated in 1984 on the structure of bathyal suprabenthic communities from the southern margin of the Cap Ferret Canyon (SE Bay of Biscay, off Arcachon Bay). During the ESSAIS and ECOFER cruises carried out in 1989, a new set of stations located along a bathymetric gradient was sampled with a suprabenthic sledge. The abundant material collected with this gear was studied at the highest taxonomical level (Dauvin et al. 1995Dauvin J.-C., Sorbe J.-C, Lorgeré J.-C 1995. Benthic Boundary Layer macrofauna from the upper continental slope and the Cap Ferret canyon (Bay of Biscay). Oceanol. Acta 18: 113-122.) as well as at the lowest one for all the suprabenthic fauna collected at three stations (Elizalde et al. 1993aElizalde M., Sorbe J.-C, Dauvin J.-C 1993a. Las comunidades suprabentónicas batiales del golfo de Vizcaya (margen sur del cañón de Cap-Ferret): composición faunística y estructura. Publ. Espec. Inst. Esp. Oceanogr. 11: 247-258., Elizalde 1994Elizalde M. 1994. Les communautés suprabenthiques bathyales de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne). PhD thesis, Université de Bordeaux 1, 212 pp.) and for mysids (Elizalde et al. 1991Elizalde M., Dauvin J.-C, Sorbe J.-C. 1991. Les mysidacés suprabenthiques de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne): répartition bathymétrique et activité natatoire. Ann. Inst. Océanogr. Paris 67: 129-144.) and amphipods (Dauvin and Sorbe 1995Dauvin J.-C., Sorbe J.-C. 1995. Suprabenthic amphipods from the southern margin of the Cap-Ferret canyon (Bay of Biscay, Northeastern Atlantic Ocean): abundance and bathymetric distribution. Pol. Arch. Hydrobiol. 42: 441-460.) according to the species identification progress. According to Dauvin et al. (1995)Dauvin J.-C., Sorbe J.-C, Lorgeré J.-C 1995. Benthic Boundary Layer macrofauna from the upper continental slope and the Cap Ferret canyon (Bay of Biscay). Oceanol. Acta 18: 113-122., cumaceans are the fourth most abundant group in this material (5.6% of the total collected fauna), after isopods (50.8%), amphipods (22.7%) and mysids (14.1%), and their percentage contribution increases with depth in the study area (0.5%-22.4% of the whole fauna collected at each sampling station). The present work is a new contribution to the knowledge of bathyal suprabenthic assemblages from the Cap Ferret area aimed at describing patterns of bathymetric as well as the near-bottom vertical distribution of cumacean species, estimating abundance of individuals and characterizing inter-specific associations.

MATERIALS AND METHODSTop

Study area

The study area is located on bathyal soft bottoms from the southern margin of the Cap Ferret Canyon. In that area, surficial sediments were sampled during previous surveys with box corers for granulometric analyses and estimation of their organic content (see data in Elizalde et al. 1991Elizalde M., Dauvin J.-C, Sorbe J.-C. 1991. Les mysidacés suprabenthiques de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne): répartition bathymétrique et activité natatoire. Ann. Inst. Océanogr. Paris 67: 129-144., 1993bElizalde M., Weber O., Sorbe J.-C. 1993b. Influence des caractères sédimentologiques sur la distribution des crustacés benthiques de la pente atlantique (Golfe de Gascogne; marge sud du canyon du Cap-Ferret). Actes du IIIe Colloque international «Océanographie du Golfe de Gascogne», pp. 269-273., Elizalde 1994Elizalde M. 1994. Les communautés suprabenthiques bathyales de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne). PhD thesis, Université de Bordeaux 1, 212 pp.). Etcheber et al. (1999)Etcheber H., Relexans J.-C., Beliard M., et al. 1999. Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay). Deep-Sea Res. II 46: 2249-2288. synthesized the whole available sedimentological information accumulated in the Cap Ferret region (canyon and its lateral margins). On the southern margin of the canyon, the bathymetric distribution of the sediment fractions is clearly related to the general morphology of the area, showing a narrow sandy lobe extending down to 800 m depth, to the north of the study area. This feature is probably related to the impact of alongslope bottom currents limiting the local deposition of fine particles (see Durrieu de Madron et al. 1999Durrieu de Madron X., Castaing P., Nyffeler F., et al. 1999. Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Res. II 46: 2003-2027.). In the study area, the mean grain size decreases with depth, from 160 µm at the shelf break to a value stabilized around 10 µm from about 600 m down to 3000 m. Conversely, the carbon organic content of sediments (measured in the 1 cm top layer of cores, expressed as % of sediment dry weight) increases with depth, between 0.23% at the shelf break and 1.36% at around 1000 m depth. The dominant fractions (>50%) allow two sedimentary zones to be distinguished according to depth: an upper muddy sand zone characterized by a decreasing dominance of fine sands with depth and a lower mud zone characterized by a dominance of fine silts and clay, and by stabilized values of the mean grain size according to depth. Following the definition given by Stanley et al. (1983)Stanley D.J., Addy S.K., Behrens E.W. 1983. The mudline: variability of its position relative to shelfbreak. In: Stanley D.J., Moore G.T., et al. (eds). The Shelfbreak: Critical Interface on Continental Margins, Special Publications of SEPM., the mud-line is therefore located at about 600 m depth in the study area, a limit representing the erosion-deposition boundary beneath which the organic content of surficial sediments increases in response to the deposition of silty and/or clayey particles (Etcheber et al. 1999Etcheber H., Relexans J.-C., Beliard M., et al. 1999. Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay). Deep-Sea Res. II 46: 2249-2288.).

According to Durrieu de Madron et al. (1999)Durrieu de Madron X., Castaing P., Nyffeler F., et al. 1999. Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Res. II 46: 2003-2027., two superimposed water masses were detected during the ECOFER experiments in the upper part of the Cap Ferret region, inferred from CTD profiles within the Cap Ferret Canyon: (1) a low-salinity water mass extending between 200 and 600 m in the water column, referred to as Eastern North Atlantic Water and characterized by a salinity minimum of 35.51 and a temperature of 10.8°C at 500 m water depth; and (2) a high saline core of Mediterranean Overflow Water detected between 700 and 1300 m water depth, characterized a salinity maximum of 35.76, a temperature of 9.8°C and an oxygen minimum of 3.6 ml L^–1 at about 1000 m water depth. Furthermore, an intermediate nepheloid layer centred at a depth of around 500 m and horizontally detached from the seafloor was detected at the head and on the flanks of the canyon during all five ECOFER experiments (1989-1991). It is supposed that both Eastern North Atlantic Water and Mediterranean Overflow Water impinge on the slope in the study area (canyon southern margin) and therefore impact the structure of the underlying bathyal benthic communities.

Sampling

During the ESSAIS I, ESSAIS II and ECOFER I surveys carried out between April and July 1989, 13 stations ranging from 346 to 1099 m depth (Fig. 1; Table 1) were sampled with a modified Macer-GIROQ suprabenthic sledge (full description in Dauvin et al. 1995Dauvin J.-C., Sorbe J.-C, Lorgeré J.-C 1995. Benthic Boundary Layer macrofauna from the upper continental slope and the Cap Ferret canyon (Bay of Biscay). Oceanol. Acta 18: 113-122.). This gear is equipped with four superimposed nets (0.5 mm mesh size) that simultaneously sample four water layers above the seafloor (N1, 10-40 cm; N2, 45-75 cm; N3, 80-110 cm; N4, 115-145 cm) and with an opening-closing system of these nets acting by contact with the seafloor. Each net is fixed on a rectangular metallic box (height, 30 cm; width, 60 cm) equipped with a TSK flowmeter that estimates the haul length and the bottom area swept by the sledge during each haul (calculated from the mean value of the available flowmeter measurements; see Table 1). Based on these estimates, the standardized abundances of species are expressed in individuals/100 m² (cumulative values from the four sledge nets N1-N4). The sledge was towed over the seafloor at a speed of 1-2 knots. All samplings were carried out during daytime (between 8 and 18 h), except TS06 and TS13, which were carried out at night before midnight (see Table 1). The collected material was fixed on board with a solution of 10% neutral formalin in sea water until sorting into major taxonomical groups at the laboratory. All groups (including cumaceans) were then transferred to and preserved in 70% ethanol until species identification.

Data analyses

Following Brunel (1972)Brunel P. 1972. The Gaspe cod ecosystem in the Gulf of St. Lawrence. III. The daily and seasonal vertical migrations of cod (Gadus morhua) in 1960-62. Nat. Can. 99: 287-357. and Sainte-Marie and Brunel (1985)Sainte-Marie B., Brunel P. 1985. Suprabenthic gradients of swimming activity by cold-water gammaridean amphipod Crustacea over a muddy shelf in the Gulf of Saint Lawrence. Mar. Ecol. Prog. Ser. 23: 57-69., an index K of swimming activity above the bottom was computed for each species, expressed as follows:

K1 = ∑N1/∑Nt
K2 = ∑N2/∑Nt
K3 = ∑N3/∑Nt
K4 = ∑N4/∑Nt

where ∑N1, ∑N2, ∑N3 and ∑N4 are the abundances (number of individuals) in the 10-40, 45-75, 80-110 and 115-145 cm water layers above the bottom, respectively; and ∑Nt is the total number of individuals in the four water layers sampled by the sledge (cumulative values from all available water layers and stations, excluding TS09). This K index can vary from 0 (when a given species is absent at the level considered) to 1 (when all the specimens are sampled in the same water layer).

Abundance data (Dt, ind./100 m²) were analysed using the Plymouth Routine in Multivariate Ecological Research (PRIMER v 5.0) software package (Clarke and Gorley 2001Clark K.R., Gorley R.N. 2001. Primer v5: User Manual/Tutorial. Plymouth Marine Laboratory.). Univariate diversity indices (Shannon-Wiener diversity H’ using log₂, Pielou evenness J’) were calculated from species abundances with the DIVERSE routine. A matrix of similarity between samples was constructed by means of the Bray-Curtis measure applied to square-root transformed species abundances in order to down-weight the contribution of abundant species. From this matrix, the 12 suprabenthic samples (sample TS09 excluded – damaged material) were classified by cluster analysis based on the complete linkage sorting algorithm. A graphical ordination was carried out on the same matrix using non-metric multidimensional scaling. Finally, the SIMPER routine was used to identify species that most contributed to within-group similarity and between-group dissimilarity.

Mean values of data sets were statistically compared using t tests according to Dagnelie (1975)Dagnelie P. 1975. Théorie et méthodes statistiques. Volume II: les méthodes de l’inférence statistique. Les Presses Agronomiques de Gembloux, 463 pp..

RESULTSTop

A total of 1885 cumacean specimens were collected. Of these, 87.7% were identified and classified into 5 families (Nannastacidae, 47.5%; Diastylidae, 30.1%; Leuconidae, 12.7%; Lampropidae, 6.9%; Bodotriidae, 2.8%) and 42 species (Table 2; Supplementary Material Table S1). The number of species (species richness) per station ranged from 5 at stations TS01 and TS02 to 25 at station TS13 (Table 3). The total abundances fluctuated between a minimum of 2.8 ind./100 m² at station TS04 and a maximum of 55.8 ind./100 m² at station TS08. The diversity indexes H’ ranged between 1.98 (TS01) and 4.05 (TS13). All these three structural indices are significantly correlated with depth (species richness r=0.960, p<0.001; abundances r=0.732, p<0.01; diversity r=0.927, p<0.001). Even at station TS07 where the nannastacid Camplylaspis squamifera was highly dominant (45.9% of the individuals), the evenness values, J’, were relatively high (≥0.67) but were correlated neither with depth (r=0.078, p>0.05) nor with H’ values (r=0.093, p>0.05).

At family level, samples from the 0-145 cm water layer were numerically dominated by Nannastacidae or Diastylidae in the shallower part of the study area (although with low abundances between 346 and 485 m), and the Nannastacidae were progressively replaced by Leuconidae below 700 m depth. At species level (Table 2), the highest abundances in the 0-145 cm water layer were recorded for the Nannastacidae Campylaspis squamifera (20.0 ind./100 m², TS08), the Leuconidae Leucon (Epileucon) pusillus (11.1 ind./100 m², TS12) and the Nannastacidae Procampylaspis armata (10.4 ind./100 m², TS08). The Diastylidae Diastyloides serratus showed the widest bathymetric distribution in the study area, being captured between 346 and 1099 m depth (present at 11 stations).

As shown in Figure 2, the near-bottom vertical distribution of the cumacean fauna showed the same pattern at every station: at least 60.0% of the individuals were sampled by the lower net of the sledge, whereas only 0% to 2.7% of them were captured at the uppermost level, demonstrating that these small peracarids are concentrated close to the seafloor during both daytime and night-time (see the case of the night-time samples TS06 and TS13). Furthermore, both families and species showed a drastic abundance decrease between the N1 and N2 lowermost waters layers and species richness showed the same vertical decreasing trend (see Table 4 and S1). Twenty-one species were exclusively sampled in the 10-40 cm water layer and therefore had a K1 index of 1: Bathycuma brevirostre, Cyclaspoides sarsi, Diastylis tumida, Leptostylis longimana, Leptostylis sp.A, Leptostylis sp.B, Makrokylindrus (Adiastylis) anomalus, M. (A.) longipes, Vemakylindrus hastatus, Platysympus typicus, Platytyphlops orbicularis, Leucon (Crymoleucon) tener, Leucon (Epileucon) ensis and Leucon (Leucon) sp., Campylaspis nitens, C. rostrata, Cumella (Cumella) divisa, Cumellopsis puritani, Nannastacus atlanticus, Procampylaspis omnidion and Schizocuma spinoculatum. The other species showed high K1 values (range: 0.50-0.98) in the lowermost level and lower K2-K4 decreasing values (range: 0.33-0) in the upper ones. Only six species were also sampled in the 115-145 cm water layer, with a generally very low K4 index, probably attesting their higher swimming abilities than the preceding ones: Diastylis cornuta, Diastyloides serratus; Hemilamprops normani, Mesolamprops denticulatus; Campylaspis glabra and Procampylaspis armata. Iphinoe serrata, a shelf-origin bodotriid, was sporadically sampled in the 80-110 cm water layer of station TS02 (only one specimen caught).

As presented in Figure 3, the cluster analysis carried out on abundance data reveals the existence of three main groups of stations distributed over depth (station TS02 being outside the groups), and the multidimensional scaling ordination shows similar results to those of the dendrogram, with an excellent stress value of 0.05. As shown in Table 5, the average dissimilarity between these groups was ≥63.9, with the highest value recorded between groups Ia and II. Two nannastacids mainly contributed to the total dissimilarity between groups Ia and Ib: Campylaspis squamifera (20.73%) and C. laevigata (12.10%). Dissimilarity between other paired groups was due to a higher number of contributing species, with Makrokylindrus (Adiastylis) josephinae (9.90%) and Campylaspis squamifera (8.85%) as the top species for paired groups Ia-II and Ib-II, respectively.

Group Ia (3 stations between 346 and 485 m depth; average within-group similarity 63.8) is characterized by the numerical dominance of Nannastacidae (57.2%), the absence of Bodotriidae and Leuconidae and a low total abundance of 6.4±3.7 ind./100 m² (mean±SD). Leptostylis macrura, Diastyloides serratus, Campylaspis glabra, Mesolamprops denticulatus and Campylaspis sulcata accounted for 92.0% of the average within-group similarity (Table 6). Campylaspis sulcata was the most abundant species of this assemblage, with a mean value of 2.2±2.1 ind./100 m² (mean±SD) and a contribution of 34.9% to total group abundance, followed by Leptostylis macrura (28.1%), Styloptocuma gracillimum (9.4%), Diastyloides serratus (8.9%) and Campylaspis glabra (8.3%).

Group Ib (4 stations between 522 and 714 m depth; average within-group similarity 71.3) was also characterized by the dominance of Nannastacidae (66.1%) and the absence of Bodotriidae but marked by the appearance of Leuconidae and a higher total abundance than the preceding one (41.2±14.2 ind./100 m², mea±SD). Campylaspis squamifera, C. laevigata, Leptostylis macrura, Campylaspis glabra, Mesolamprops denticulatus, Styloptocuma gracillimum and Diastyloides serratus accounted for 82.8% of the average within-group similarity (Table 6). Campylaspis squamifera was the most abundant species of this assemblage, with a mean value of 14.2±3.9 ind./100 m² (mean±SD) and a contribution of 34.4% to total group abundance, followed by C. laevigata (11.3%), Leptostylis macrura (10.6%), Mesolamprops denticulatus (7.4%) and Procampylaspis armata (7.0%). Three species, Diastylis cornuta, Leucon (Leucon) affinis and Cumellopsis puritani, were found exclusively in this assemblage.

Group II (4 stations between 790 and 1099 m depth; average within-group similarity 61.3) was characterized by the dominance of Diastylidae (40.3%), the appearance of Bodotriidae and a total abundance of 42.3±8.7 ind./100 m² (mean±SD). This group showed the highest mean values of species richness, total abundance and diversity indices (Table 6). Makrokylindrus (Adiastylis) josephinae, Leucon (Epileucon) pusillus, Diastyloides serratus, Procampylaspis armata, Hemilamprops normani, Cyclaspis longicaudata, Leucon (Macrauloleucon) siphonatus, Vemakylindrus hastatus, Platytyphlops orbicularis, Campylaspis squamifera and Leucon (Crymoleucon) tener accounted for 81.0% of the average within-group similarity (Table 6). Makrokylindrus (A.) josephinae was the most abundant species of this assemblage, with a mean value of 7.3±1.5 ind./100 m² (mean±SD) and a contribution of 17.2% to total group abundance, followed by Leucon (Epileucon) pusillus (12.6%), Diastyloides serratus (10.2%), Procampylaspis armata (6.3%) and Vemakylindrus hastatus (5.4%). 18 species were found exclusively in this assemblage, demonstrating an evident renewal of the cumacean fauna at this bathymetric level: Bathycuma brevirostre, Cyclaspis longicaudata, Cyclaspoides sarsi, Leptostylis longimana, Leptostylis sp.B, Makrokylindrus (Adiastylis) anomalus, M. (A.) longicaudatus, M. (A.) longipes, Hemilamprops normani, Platytyphlops orbicularis, Ithyleucon sorbei, Leucon (Crymoleucon) tener, Leucon (Epileucon) ensis, L. (E.) pusillus, Campylaspis rostrata, Cumella (Cumella) divisa, Procampylaspis omnidion and Schizocuma spinoculatum.

DISCUSSIONTop

Lagardère (1977)Lagardère J.-P. 1977. Recherches sur la distribution verticale et sur l’alimentation des crustacés décapodes benthiques de la pente continentale du golfe de Gascogne. Analyse des groupements carcinologiques. Bull. Cent. Etud. Rech. Sci. Biarritz 2: 367-440. carried out a pioneering study on the bathyal cumacean fauna of the southeastern Bay of Biscay (composition and bathymetric distribution of species in the depth range 200-1400 m). According to modern nomenclature (see Băcescu 1992Băcescu M. 1992. Cumacea II: (Fam. Nannastacidae, Diastylidae, Pseudocumatidae, Gynodiastylidae et Ceratocumatidae). Crustaceorum Catalogus, Pars 8. SPB Academic Publishing, The Hague. 468 pp., Corbera and Sorbe 1999Corbera J., Sorbe J.C. 1999. The problematic cumacean Schizotrema atlanticum from the eastern Atlantic: redescription and ecological notes. J. Crustac. Biol. 19: 123-130.), we suggest the following equivalence between species (Lagardère’s taxa in brackets): Vemakylindrus hastatus (= Diastylis cf. hastata), Eudorella cf. parvula (= E. truncatula), Cumella (Cumella) divisa (= Cumella sp.), Nannastacus atlanticus (= Nannastacus sp.) and Styloptocuma gracillimum (= Cumella gracillimana). Within the restricted depth range 200-1000 m (comparable to the present study), Lagardère listed 37 species belonging to Nannastacidae, Diastylidae, Leuconidae, Bodotriidae and Lampropidae (decreasing order of species richness), slightly lower than the value obtained herein (43 species). Eudorella hirsuta (Sars, 1869) is probably a misidentification (= Eudorella cf. parvula?), as is Campylaspis horrida Sars, 1869 recorded on the upper slope of Arctic waters according to Jones (1984)Jones N.S. 1984. The family Nannastacidae (Crustacea: Cumacea) from the deep Atlantic. Bull. Br. Mus. (Nat. Hist.) Zool. 46: 207-289.. Although not recorded during the present survey, Hemilamprops roseus (Norman, 1863) and Campylaspis macrophthalma Sars, 1878 were sampled on the outer shelf adjacent to our study area (Sorbe 1984Sorbe J.C. 1984. Contribution à la connaissance des peuplements suprabenthiques néritiques sud-Gascogne. Thèse d’Etat, Université de Bordeaux 1.); Vaunthompsonia cristata Bate, 1858, Campylaspis verrucosa Sars, 1866 and Campylaspis vitrea Calman, 1906 were sampled in the Capbreton canyon area (Frutos and Sorbe 2014Frutos I., Sorbe J.C. 2014. Bathyal suprabenthic assemblages from the southern margin of the Capbreton Canyon (“Kostarrenkala” area), SE Bay of Biscay. Deep-Sea Res. II 104: 291-309., 2017Frutos I., Sorbe J.C. 2017. Suprabenthic assemblages from the Capbreton area (SE Bay of Biscay). Faunal recovery after a canyon turbidity disturbance. Deep-Sea Res. Part I 130: 36-46.); and Hemilamprops cristatus (Sars, 1870) and Leucon (Epileucon) longirostris Sars, 1871 were mentioned from bathyal bottoms of the Bay of Biscay (Jones 1985Jones N.S. 1985. Distribution of the Cumacea. In: Laubier L., Monniot C. (eds), Peuplements profonds du golfe de Gascogne: campagnes BIOGAS. IFREMER, Brest, pp 429-433., Frutos and Sorbe 2014Frutos I., Sorbe J.C. 2014. Bathyal suprabenthic assemblages from the southern margin of the Capbreton Canyon (“Kostarrenkala” area), SE Bay of Biscay. Deep-Sea Res. II 104: 291-309.). Therefore, except probable misidentified species, all cumaceans mentioned by Lagardère (1977) Lagardère J.-P. 1977. Recherches sur la distribution verticale et sur l’alimentation des crustacés décapodes benthiques de la pente continentale du golfe de Gascogne. Analyse des groupements carcinologiques. Bull. Cent. Etud. Rech. Sci. Biarritz 2: 367-440.were found again in more recent studies on the bathyal benthic cumacean fauna of the SE Bay of Biscay. Furthermore, a new genus and species, Ithyleucon sorbei, were described from the material collected during the present study (Corbera 2012Corbera J. 2012. Rare and new cumaceans (Crustacea, Peracarida) from the southern margin of the Cap Ferret Canyon (Bay of Biscay). ZooKeys 235: 73-85.), and two apparently undescribed species of the genus Leptostylis remain to be studied.

Previously reported from the NE Atlantic between Norway and British Islands but also from the Azores and the Gulf of Cadiz, Diastylis tumida (740-924 m) is mentioned for the first time in the Bay of Biscay (present study). Originally described from bathyal bottoms of the Gulf of Lion, Mesolamprops denticulatus Ledoyer, 1983 was more recently discovered in the NE Atlantic Ocean as far as the Faeroe–Shetland Channel at 259-753 m (Shalla and Bishop 2007Shalla S.H., Bishop J.D.D. 2007. Lampropidae (Crustacea: Cumacea) from the deep north-east Atlantic and the North Sea, with two new species of Hemilamprops and Mesolamprops. J. Mar. Biol. Ass. U.K. 87: 1191-1200.) as well as in the southern Bay of Biscay (Frutos and Sorbe 2014Frutos I., Sorbe J.C. 2014. Bathyal suprabenthic assemblages from the southern margin of the Capbreton Canyon (“Kostarrenkala” area), SE Bay of Biscay. Deep-Sea Res. II 104: 291-309., Capbreton Canyon; Sorbe and Elizalde 2014Sorbe J.C, Elizalde M. 2014. Temporal changes in the structure of a slope suprabenthic community from the Bay of Biscay (NE Atlantic Ocean). Deep-Sea Res. II 106: 179-191., southern margin of the Cap Ferret Canyon).

Up to now, the structure of deep cumacean assemblages has been poorly investigated. In the southern Bay of Biscay, recent studies on bathyal suprabenthic assemblages (Frutos and Sorbe 2014Frutos I., Sorbe J.C. 2014. Bathyal suprabenthic assemblages from the southern margin of the Capbreton Canyon (“Kostarrenkala” area), SE Bay of Biscay. Deep-Sea Res. II 104: 291-309., 2017Frutos I., Sorbe J.C. 2017. Suprabenthic assemblages from the Capbreton area (SE Bay of Biscay). Faunal recovery after a canyon turbidity disturbance. Deep-Sea Res. Part I 130: 36-46., Sorbe and Elizalde 2014Sorbe J.C, Elizalde M. 2014. Temporal changes in the structure of a slope suprabenthic community from the Bay of Biscay (NE Atlantic Ocean). Deep-Sea Res. II 106: 179-191.) showed that they constitute one of the main components of the near-bottom motile fauna (suprabenthic ecophase), in addition to amphipods, isopods and mysids. However, another part of these assemblages is known to inhabit surficial sediments constituting the endobenthic ecophase of these populations (Fage 1951Fage L. 1951. Cumacés. Faune de France. P. Lechevalier., Jones 1976Jones, N.S. 1976. British cumaceans. Synopses of the British Fauna, 7. Academic Press., Băcescu and Petrescu 1999Băcescu M., Petrescu I. 1999. Ordre des Cumacés (Cumacea Krøyer, 1846). In: Forest, J. et al. Traité de zoologie: anatomie, systématique, biologie: VII. Crustacés: III A. Péracarides. Mém. Inst. Océanogr. (Monaco) 19: 391-428.). These burrowing individuals are inadequately sampled by suprabenthic sledges, mainly designed to sample the near-bottom motile fauna (epi-/suprabenthic ecophase). Therefore, more realistic estimations of benthic cumacean abundances should ideally combine sampling with sledges and grabs/box cores. To our knowledge, such a methodology has never been implemented in the study of these benthic communities. Table 7 shows some structural data on diverse shelf and slope cumacean assemblages, sampled with either suprabenthic sledges or grabs. The abundance data (maximum recorded values for each study) given by suprabenthic sampling are in the same order of magnitude for shelf (range: 1.2-6.5 ind. m^–2) and slope assemblages (range: 0.3-5.4 ind. m^–2), and the corresponding mean values are statistically equal (t_obs=1.256; d.f.=10; p>0.05). In shelf assemblages, these suprabenthic abundances are generally much lower than values obtained with grab sampling (range: 153-24000 ind. m^–2) and far from the worldwide maximum value mentioned by Hawkinson (1992)Hawkinson C.A. 1992. Feeding ecology of gray whales in Agate Bay, California, Summers 1990 and 1991. Master’s thesis, San José State University, 45 pp. in the case of the coastal Diastylidae Diastylopsis dawsoni Smith, 1880 from Agate Bay, California (up to 119881 ind. m^–2; Ekman grab).

According to the structural data presented in Table 7, species richness (maximum values) is generally higher in slope assemblages (range: 7-25 species per station) than in shelf ones (range: 4-15 species per station). Such a trend is also verified when considering the St mean values for slope (29.2±13.0 species per station; mean±SD) and shelf (10.9±6.6 species per station) assemblages of the studied areas (t_obs=3.733; d.f.=15; p<0.001). Furthermore, within the slope assemblages studied herein, the cumulative species richness increases with depth: 13, 22 and 35 species for assemblages Ia, Ib and II, respectively. Although without statistical significance, the diversity values, H’, are also higher in slope assemblages (range: 3.41-4.05) than in the shelf ones (range: 1.24-3.21), corroborating previous observations on bathyal cumacean assemblages (Jones and Sanders 1972Jones N.S., Sanders H.L. 1972. Distribution of Cumacea in the deep Atlantic. Deep-Sea Res. 19: 737-745., Reyss 1973Reyss D. 1973. Distribution of Cumacea in the deep Mediterranean. Deep-Sea Res. 20: 1119-1123.) as well as on all bathyal suprabenthic assemblages (Frutos and Sorbe 2014Frutos I., Sorbe J.C. 2014. Bathyal suprabenthic assemblages from the southern margin of the Capbreton Canyon (“Kostarrenkala” area), SE Bay of Biscay. Deep-Sea Res. II 104: 291-309., 2017Frutos I., Sorbe J.C. 2017. Suprabenthic assemblages from the Capbreton area (SE Bay of Biscay). Faunal recovery after a canyon turbidity disturbance. Deep-Sea Res. Part I 130: 36-46., Sorbe and Elizalde 2014Sorbe J.C, Elizalde M. 2014. Temporal changes in the structure of a slope suprabenthic community from the Bay of Biscay (NE Atlantic Ocean). Deep-Sea Res. II 106: 179-191.).

According to our multivariate analysis (see Fig. 3), two main faunal changes occur in the cumacean fauna collected on the upper bathyal of the southern Bay of Biscay. The first one is observed between 485 and 522 m, and the second one (more clearly marked) between 714 and 790 m. In the upper part of the study area, the substratum consists of muddy fine sands (ca. 400 m depth, median grain size [Mz]=73.9 µm; particles <63 µm [pelites]=38.2%; organic carbon content [C_org]=0.37% of sediment dry weight; see Etcheber et al 1999Etcheber H., Relexans J.-C., Beliard M., et al. 1999. Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay). Deep-Sea Res. II 46: 2249-2288. and Elizalde 1994Elizalde M. 1994. Les communautés suprabenthiques bathyales de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne). PhD thesis, Université de Bordeaux 1, 212 pp.). The edge of the adjacent shelf is characterized by comparable bottoms (ca. 179 m depth: Mz=100.0 µm; pelites=20.0%; see Sorbe 1984Sorbe J.C. 1984. Contribution à la connaissance des peuplements suprabenthiques néritiques sud-Gascogne. Thèse d’Etat, Université de Bordeaux 1.). This similarity in the texture of surficial sediments favours the extension of some shelf species down to the upper bathyal (for instance the Bodotriidae Iphinoe serrata and the Nannastacidae Nannastacus atlanticus). Separating the cumacean assemblages Ia and Ib, the upper ecotone at about 500 m depth is related to the apparition of muddy sediments (characterized by a pelitic fraction ≥50%; see Elizalde 1994Elizalde M. 1994. Les communautés suprabenthiques bathyales de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne). PhD thesis, Université de Bordeaux 1, 212 pp.) that are more fluid than the shallower sandy bottoms. This structural change on the sedimentary coverage has a probable effect on the functioning of the suprabenthic sledge. Primarily designed to slide on compact sandy bottoms thanks to its lateral skates (see Dauvin et al. 1995Dauvin J.-C., Sorbe J.-C, Lorgeré J.-C 1995. Benthic Boundary Layer macrofauna from the upper continental slope and the Cap Ferret canyon (Bay of Biscay). Oceanol. Acta 18: 113-122.), this device probably skims the uppermost sediment layer on fluid muddy bottoms and thus samples surficial infaunal components in addition to the suprabenthic ones. This phenomenon is attested by a significant increase in cumacean mean abundances below 500 m depths: 42.7±10.6 ind./100 m² (mean±SD) versus 5.6±3.4 ind./100 m² at shallower depths (tobs=48.4; d.f.=11; p<0.001). It should be noted that no Leuconidae were observed within the assemblage Ia (346-485 m), although Leucon (Macrauloleucon) siphonatus curiously showed a disjoint bathymetric distribution, being present on both muddy sand bottoms of the outer shelf (91-179 m; Sorbe 1984Sorbe J.C. 1984. Contribution à la connaissance des peuplements suprabenthiques néritiques sud-Gascogne. Thèse d’Etat, Université de Bordeaux 1.) and deeper muddy slope bottoms (608-1099 m; this study).

These new observations corroborate the eurybathic (100-4380 m; Fage 1951Fage L. 1951. Cumacés. Faune de France. P. Lechevalier.) and eurytopic distribution of this strange cumacean with an unusually long branchial siphon (longer than the carapace in some adults; Fage 1951Fage L. 1951. Cumacés. Faune de France. P. Lechevalier.). This morphological peculiarity is probably an adaptive character allowing this species to colonize very diverse benthic habitats. The lower ecotone is imprecisely located between 714 and 791 m depth (due to absence of detailed data for the cumacean fauna of haul TS09). It constitutes the upper limit of the cumacean assemblage II installed on muddy bottoms characterized by a higher organic carbon content of surficial sediments (C_org from 0.84% at 720 m to 1.36% at 995 m) and a pelitic fraction ≥84% (see Etcheber et al. 1999Etcheber H., Relexans J.-C., Beliard M., et al. 1999. Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay). Deep-Sea Res. II 46: 2249-2288.). At ca. 1099 m (the lower limit of the present study area), the surficial sediments show the following features: Mz=12.3 µm; pelites=92.6%, mainly constituted by particles <15 µm (clay=58.0%); C_org=1.30% (see Elizalde 1994Elizalde M. 1994. Les communautés suprabenthiques bathyales de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne). PhD thesis, Université de Bordeaux 1, 212 pp.). This assemblage probably extends on deeper bathyal bottoms because it is composed of many bathyal/abyssal species (not found at shallower depths in the present study), such as Bathycuma brevirostre (350-1700 m), Cyclaspis longicaudata (189-3350 m) and Cyclaspoides sarsi (698-1099 m); Makrokylindrus (Adiastylis) anomalus (950-1550 m), M. (A.) longicaudatus (650-1287 m) and M. (A.) longipes (15-1227 m); Hemilamprops normani (220-3000 m) and Platytyphlops orbicularis (423-1739 m); Leucon (Crymoleucon) tener (790-1445 m), Leucon (Epileucon) ensis (790-2006 m) and L. (E.) pusillus (610-1780 m); and Campylaspis rostrata (220-2338 m), Cumella (Cumella) divisa (610-2864 m), Procampylaspis omnidion (860-4749 m) and Schizocuma spinoculatum (1097-2900 m) (distributional data according to Sars 1899Sars G.O. 1899. An account of the Crustacea of Norway. Vol. III: Cumacea. Part I & II: Cumidae, Lampropidae (part.). Bergen Museum, Christiania 115 pp., Hansen 1920Hansen H.J. 1920. Crustacea Malacostraca IV. Danish Ingolf Exp., Copenhagen, vol. 3, 86 pp., Fage 1951Fage L. 1951. Cumacés. Faune de France. P. Lechevalier., Bishop 1981bBishop J.D.D. 1981b. A revised definition of the genus Epileucon Jones (Crustacea, Cumacea), with descriptions of species from the deep Atlantic. Philos. Trans. R. Soc. Lond. B 291: 353-409., Jones 1984Jones N.S. 1984. The family Nannastacidae (Crustacea: Cumacea) from the deep Atlantic. Bull. Br. Mus. (Nat. Hist.) Zool. 46: 207-289., Gerken 2018Gerken S. 2018. The Lampropidae (Crustacea: Cumacea) of the World. Zootaxa 4428: 1-192.).

In addition to these surficial sedimentary characters that certainly play a major role in the bathymetric distribution and local abundance of cumaceans, the bathyal bottoms investigated during the present study are impacted by the benthic impingement of two superimposed water bodies: the Eastern North Atlantic Water between 200 and 600 m and Mediterranean Overflow Water between 700 and 1300 m (Durieu de Madron et al. 1999Durrieu de Madron X., Castaing P., Nyffeler F., et al. 1999. Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Res. II 46: 2003-2027.). Inferred from CTD profiles carried out in the water column inside the Cap Ferret Canyon (Durieu de Madron et al. 1999Durrieu de Madron X., Castaing P., Nyffeler F., et al. 1999. Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Res. II 46: 2003-2027.) as well as on its southern margin (unpublished data collected during various oceanographic cruises in all seasons except winter), hydrographic conditions in the near-bottom bathyal environment can be supposedly described as follows in order to assess their possible role in the distribution and structuration of the cumacean assemblages detected across the upper slope. Assemblage Ia (346-485 m) is bathed by the Eastern North Atlantic Water, corresponding to near-bottom waters with a salinity range of 35.52 to 5.74, a temperature range of 10.5 to 11.3°C and an oxygen concentration range of 4.16 to 4.48 ml L^–1. Assemblage Ib (522-754 m) is also bathed by the same water body, corresponding to near-bottom waters with a salinity range of 35.54 to 35.85, a temperature range of 10.1 to 10.7°C and an oxygen concentration range of 3.76 to 4.16 ml L^–1. Assemblage II (790-1099 m) is under the influence of the Mediterranean Overflow Water, corresponding to near-bottom waters with a salinity range of 35.69 to 35.77, a temperature range of 9.1 to 10.1°C and an oxygen concentration range of 3.65 to 3.69 ml L^–1. Whatever the bathymetric level in the upper bathyal, near-bottom temperatures show weak fluctuations during the annual cycle (≤1°C; inferred from seasonal CTD profiles). This environmental variable is probably not the triggering factor of reproductive mechanisms in these cumaceans, as is the case with some peracarid and decapod populations from adjacent shelf waters (Sorbe 1984Sorbe J.C. 1984. Contribution à la connaissance des peuplements suprabenthiques néritiques sud-Gascogne. Thèse d’Etat, Université de Bordeaux 1., San Vicente and Sorbe 2013San Vicente C., Sorbe J.-C. 2013. Comparative life-histories, population dynamics and productivity of Schistomysis populations (Crustacea, Mysida) in European shelf environments. J. Sea Res. 81: 13-32.). Furthermore, even in the case of the Mediterranean waters (minimum O2 concentration: 3.60 ml L^–1), the near-bottom waters are far from dysoxia as defined by Levin (2003)Levin L.A. 2003. Oxygen minimum zone benthos: adaptation and community response to hypoxia. Oceanogr. Mar. Biol. Annu. Rev. 41: 1-45. for oceanic waters (≤1.0 ml L^–1), with O₂ saturation values ≥57.24%. As pointed out by Diaz and Rosenberg (1995)Diaz R.J., Rosenberg R. 1995. Marine benthic hypoxia: a review of its ecological effects and the behavioural responses of benthic macrofauna. Oceanogr. Mar. Biol. Annu. Rev. 33: 245-303., the actual O₂ values are probably lower at the water-sediment interface, but the alongslope bottom currents known to periodically occur in the study area (Sorbe and Weber 1995Sorbe J.C., Weber O. 1995. Influence de la profondeur et des sédiments superficiels sur la structure des communautés suprabenthiques bathyales sud-Gascogne. Actas del IV Coloquio Internacional sobre Oceanografía del Golfo de Vizcaya, pp. 183-194., Durieu de Madron et al. 1999Durrieu de Madron X., Castaing P., Nyffeler F., et al. 1999. Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Res. II 46: 2003-2027.) promote the renewal of their near-bottom waters and probably prevent the occurrence of dysoxic bottom events. Finally, it should be noted that the actual impact of these hydrographic variables on the biology and behaviour of benthic peracaridans is far from being well understood.

Although little is known about the diet of cumaceans (see Jones 1976Jones, N.S. 1976. British cumaceans. Synopses of the British Fauna, 7. Academic Press., Băcescu and Petrescu 1999Băcescu M., Petrescu I. 1999. Ordre des Cumacés (Cumacea Krøyer, 1846). In: Forest, J. et al. Traité de zoologie: anatomie, systématique, biologie: VII. Crustacés: III A. Péracarides. Mém. Inst. Océanogr. (Monaco) 19: 391-428., Błażewicz-Paszkowycz and Ligowski 2002Błażewicz-Paszkowycz M., Ligowski R. 2002. Diatoms as food source indicator for some Antarctic Cumacea and Tanaidacea (Crustacea). Antarct. Sci. 14: 11-15.), the distribution of some species could be related to food availability in the bottom environment. The nannastacid species belonging to genera Campylaspis and Procampylaspis are thought to be predators on foraminifers and small crustaceans, based on the structure of their mouthparts modified as piercing organs and on the analysis of their gut contents (Jones 1976Jones, N.S. 1976. British cumaceans. Synopses of the British Fauna, 7. Academic Press., Błażewicz-Paszkowycz and Ligowski 2002Błażewicz-Paszkowycz M., Ligowski R. 2002. Diatoms as food source indicator for some Antarctic Cumacea and Tanaidacea (Crustacea). Antarct. Sci. 14: 11-15.). In the southern Bay of Biscay, the noteworthy numerical dominance of Campylaspis species between 425 and 714 m (see Table S1) is concomitant with the abundance of some benthic agglutinated foraminifers at the same bathymetric level, as reported by Elizalde et al. (1999)Elizalde M., Weber O., Pascual A., et al. 1999. Benthic response of Munnopsurus atlanticus (Crustacea, Isopoda) to the carbon content of the near-bottom sedimentary environment on the southern margin of the Cap-Ferret Canyon (Bay of Biscay, northeastern Atlantic Ocean). Deep-Sea Res. II 46: 2331-2344.. Elizalde et al. (1999)Elizalde M., Weber O., Pascual A., et al. 1999. Benthic response of Munnopsurus atlanticus (Crustacea, Isopoda) to the carbon content of the near-bottom sedimentary environment on the southern margin of the Cap-Ferret Canyon (Bay of Biscay, northeastern Atlantic Ocean). Deep-Sea Res. II 46: 2331-2344. showed that the agglutinated foraminifer Pseudoclavulina mexicana (Cushman, 1922) is preyed upon by the isopod Munnopsurus atlanticus (Bonnier, 1896), crushed by the robust mandibles of this species. Such a trophic link with benthic agglutinated foraminifers remains to be demonstrated in the case of the nannastacid species from the southern Bay of Biscay. The other cumaceans taxa herein mentioned are generally classified as detritivores (see Błażewicz-Paszkowycz and Ligowski 2002Błażewicz-Paszkowycz M., Ligowski R. 2002. Diatoms as food source indicator for some Antarctic Cumacea and Tanaidacea (Crustacea). Antarct. Sci. 14: 11-15. for Antarctic species). The development of their bathyal populations is dependent on periodical inputs of nutritive particles originating from the euphotic zone (overlying pelagial waters and/or adjacent shelf areas).

The present results on the bathyal cumacean fauna can be compared with previous published analyses carried out on the other peracarids from the same collection data (346-1099 m), such as the Mysidacea (Elizalde et al. 1991Elizalde M., Dauvin J.-C, Sorbe J.-C. 1991. Les mysidacés suprabenthiques de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne): répartition bathymétrique et activité natatoire. Ann. Inst. Océanogr. Paris 67: 129-144.; obsolete denomination, grouping current orders Lophogastrida and Mysida) and the Amphipoda (Dauvin and Sorbe 1995Dauvin J.-C., Sorbe J.-C. 1995. Suprabenthic amphipods from the southern margin of the Cap-Ferret canyon (Bay of Biscay, Northeastern Atlantic Ocean): abundance and bathymetric distribution. Pol. Arch. Hydrobiol. 42: 441-460.). In each of these bathyal taxocœnoses, three assemblages have been detected related to depth (assemblages A, B and C for convenience of presentation). The ecotone separating the uppermost assemblages A and B is located around 500 m depth for the three taxocœnoses. As demonstrated herein for cumaceans, these faunal changes are mainly related to a modification of the sedimentary cover (correlated to peculiar hydrodynamic conditions at the shelf break/upper slope), from muddy sands in the upper levels to muddy bottoms below 500 m depth. The upper assemblages A are partly composed of shelf species extending their bathymetric distribution to the muddy sand bottoms of the upper slope (for instance, the amphipods Amphilochoides boecki G.O. Sars, 1892, Rhachotropis integricauda Carausu, 1948, Iphimedia obesa Rathke, 1843, Hippomedon denticulatus (Spence Bate, 1857), Westwoodilla caecula (Spence Bate, 1857), Apherusa bispinosa (Spence Bate, 1857) and A. ovalipes Norman and Scott, 1906; and the mysids Leptomysis gracilis (G.O. Sars, 1864) and Mysideis parva Zimmer, 1915). The bathymetric location of the lower ecotone separating assemblages B and C is variable according to taxocœnoses, probably reflecting different sensibility of taxa /species to near-bottom environmental changes. For cumaceans and amphipods, this ecotone is located between 714 and 754 m (Dauvin and Sorbe 1995Dauvin J.-C., Sorbe J.-C. 1995. Suprabenthic amphipods from the southern margin of the Cap-Ferret canyon (Bay of Biscay, Northeastern Atlantic Ocean): abundance and bathymetric distribution. Pol. Arch. Hydrobiol. 42: 441-460.; this study), suggesting that some amphipods are also affected by the increase in organic carbon in muddy surficial sediments, as mentioned above for cumaceans. For mysids, this lower ecotone is significantly located deeper on the slope (between 924 and 1024 m), probably related to the oxygen minimum zone (OMZ; O₂ concentration minimum =3.6 ml L^–1) detected between 908 and 942 m on CTD profiles carried out in the study area (see Durieu de Madron et al. 1999Durrieu de Madron X., Castaing P., Nyffeler F., et al. 1999. Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Res. II 46: 2003-2027.). According to available distributional data (Nouvel and Lagardère 1976Nouvel H., Lagardère J.-P. 1976. Les mysidacés du talus continental du golfe de Gascogne. I. Tribu des Erythropini (genre Erythrops excepté). Bull. Mus. natn. Hist. nat, Paris, 3e série, n°414, Zoologie 291: 1243-1324., Lagardère and Nouvel 1980Lagardère J.-P., Nouvel H. 1980. Les mysidacés du talus continental du golfe de Gascogne. II. FamilIes des Lophogastridae, Eucopiidae et Mysidae (Tribu des Erythropini exceptée). Bull. Mus. Natn. Hist. Nat, Paris, 4e série, 2, section A, 2: 375-412., Elizalde et al. 1991Elizalde M., Dauvin J.-C, Sorbe J.-C. 1991. Les mysidacés suprabenthiques de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne): répartition bathymétrique et activité natatoire. Ann. Inst. Océanogr. Paris 67: 129-144.), this OMZ actually constitutes the lower bathymetric limit of some upper bathyal mysids such as Amblyops spiniferus Nouvel and Lagardère, 1976, Pseudomma kruppi W. Tattersall, 1909 and Mysidella biscayensis Lagardère and Nouvel, 1980, as well as the upper limit of the deeper bathyal mysid Dactylamblyops thaumatops W. Tattersall, 1907 (the numerically dominant species of the deep mysid assemblage). Surprisingly, this OMZ has apparently no impact on the depth distribution of cumacean assemblages but a slight effect on amphipod assemblages, as suggested by the correspondence analysis of data (see Dauvin and Sorbe 1995Dauvin J.-C., Sorbe J.-C. 1995. Suprabenthic amphipods from the southern margin of the Cap-Ferret canyon (Bay of Biscay, Northeastern Atlantic Ocean): abundance and bathymetric distribution. Pol. Arch. Hydrobiol. 42: 441-460.). Vaquer-Sunyer and Duarte (2008)Vaquer-Sunyer R., Duarte C.M. 2008. Thresholds of hypoxia for marine biodiversity. Proc. Natl. Acad. Sci. U.S.A. 105: 15452-15457. showed that hypoxia thresholds vary greatly across marine benthic organisms, crustaceans being the most sensitive, but without detailed information on peracarids. Due to their known higher swimming activity (see Mauchline 1980Mauchline J. 1980. The biology of mysids and euphausiids. Adv. Mar. Biol. 18, 681 pp., Elizalde et al. 1991Elizalde M., Dauvin J.-C, Sorbe J.-C. 1991. Les mysidacés suprabenthiques de la marge sud du canyon du Cap-Ferret (Golfe de Gascogne): répartition bathymétrique et activité natatoire. Ann. Inst. Océanogr. Paris 67: 129-144.), mysids probably have higher oxygen requirements than most amphipods and cumaceans.

CONCLUSIONSTop

In the upper bathyal of the SE Bay of Biscay, cumaceans constitute an important fraction of the suprabenthic fauna, although they are characterized by a lower diversity and abundance than amphipods (a major group in many near-bottom deep assemblages, as pointed out by Frutos et al. 2017Frutos I., Sorbe J.C. 2017. Suprabenthic assemblages from the Capbreton area (SE Bay of Biscay). Faunal recovery after a canyon turbidity disturbance. Deep-Sea Res. Part I 130: 36-46.). Within the study area, a minimum of 42 cumacean species were censed (some of them putatively new to science), belonging to 5 families (mainly Nannastacidae and Diastylidae). As for other major taxocœnoses, three across-slope cumacean assemblages were detected, mainly structured by depth and surficial sediments (granulometric composition and organic content). This study is a contribution to a better knowledge of the small near-bottom peracarid fauna of the NE Atlantic slope (a major food resource for demersal fishes; see Sorbe 1981Sorbe J.C. 1981. Rôle du benthos dans le régime alimentaire des poissons démersaux du secteur sud-Gascogne. Kieler Meeresforsch., Sonderh. 5: 479-489.), a deep marine area increasingly impacted by many anthropogenic activities (Levin and Dayton 2009Levin L.A., Dayton P.K. 2009. Ecological theory and continental margins: where shallow meets deep. Trends Ecol. Evol. 24: 606-617.).

ACKNOWLEDGEMENTSTop

The authors thank the crews of the RV Côte d’Aquitaine (CNRS) and RV Le Noroit (IFREMER), as well as all participants of the oceanographic surveys for their assistance at sea. They wish to express their gratitude to M. Elizalde, J.C. Dauvin and J.C. Lorgeré for their valuable help during sorting of organisms, to J.M. Jouanneau and O. Weber for the communication of hydrological and sedimentological data and also to J.A. Cuesta, I. Frutos and an anonymous reviewer for his comments that improved our manuscript. This study was partially supported by CNRS-CIRMAT (oceanographic cruises and loan of the Roscoff suprabenthic sledge).

Just after finishing the redaction of the last draft of this manuscript, Jean Claude Sorbe, one of the authors, passed away. Thus, this is one of his last contributions to science, to which he dedicated an important part of his life. Those of us who work with him know his tireless passion for work and his rigour, and we acknowledge all his teachings, but above all we appreciate his friendship. He was a great man, and we will not forget him.

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SUPPLEMENTARY MATERIAL

The following supplementary material is available through the online version of this article and at the following link: http://scimar.icm.csic.es/scimar/supplm/sm050314esm.pdf

Table S1. – Vertical distribution of cu macean individuals in the near-bottom water layers (N1, 10-40 cm; N2, 45-75 cm; N3, 80-110 cm; N4, 115-145 cm above the seafloor) sampled by a suprabenthic sledge at 13 stations on the southern margin of the Cap Ferret Canyon. TS09 samples only partially studied due to bad conservation of material. –, 0; *, damaged specimens.