Bathyal infaunal communities from a deep seamount (Galicia Bank, northeast Atlantic)

INTRODUCTION

Seamounts, underwater mountains or isolated topographic elevations rising steeply from the ocean floor, have summits at least 100 m above the deep-sea floor (hills <500 m; knolls >500 m; and seamounts >1000 m from the seafloor; Yesson et al. 2011YessonC., ClarkM.R., TaylorM.L., RogersA.D. 2011. The global distribution of seamounts based on 30 arc seconds bathymetry data. Deep-Sea Res. Part I58: 442-453. 10.1016/j.dsr.2011.02.004). The total number of deep-sea seamounts is still uncertain, and the studies show great differences in global estimates (Morato et al. 2013MoratoT., KvileK.Ø., TarantoG.H., et al. 2013. Seamount physiography and biology in the north-east Atlantic and Mediterranean Sea. Biogeosciences10: 3039-3054. 10.5194/bg-10-3039-2013). However, seamounts cover a significant portion of the seafloor, forming one of the largest ocean biomes (Rogers 2018RogersA. 2018. The Biology of Seamounts: 25 Years on. Adv. Mar. Biol. 79: 137-224. 10.1016/bs.amb.2018.06.001) and showing high spatial heterogeneity and topographic complexity (Rogers 2004RogersA. 2004. The biology, ecology and vulnerability of seamount communities. IUCN report, 12 pp.). Seamounts offer a variety of environments by combining strong gradients of depth, slope, substrate type, water masses, currents, etc., which are reflected in the physical habitat and biotope distribution (Boehlert and Genin 1987BoehlertG.W., GeninA. 1987. A review of the effects of seamounts on biological processes. In: KeatingB.H. et al. (eds), Seamounts, Islands, and Atolls. Geophys. Monogr. Ser., vol. 43, Washington, D.C, pp. 319-334. 10.1029/GM043p0319, Rogers 1994RogersA. 1994. The biology of seamounts.Adv. Mar. Biol. 30: 305- 350. 10.1016/S0065-2881(08)60065-6). The seamount relief is an obstacle to currents, creating local upwellings and closed circulation cells known as Taylor columns (Boehlert and Genin 1987BoehlertG.W., GeninA. 1987. A review of the effects of seamounts on biological processes. In: KeatingB.H. et al. (eds), Seamounts, Islands, and Atolls. Geophys. Monogr. Ser., vol. 43, Washington, D.C, pp. 319-334. 10.1029/GM043p0319, White et al. 2007WhiteM., BashmachnikovI., ArísteguiJ., MartinsA. 2007. Physical processes and seamount productivity. In: PitcherT.J., MoratoT., HartP.J.B., et al. (eds), Seamounts: Ecology, fisheries & conservation.Wiley-Blackwell, pp. 65-85.). These are related to the arrival of nutrient-rich deep water, which leads to increased productivity in the upper seamount regions (Rogers 2004RogersA. 2004. The biology, ecology and vulnerability of seamount communities. IUCN report, 12 pp.) and has a functional role in increasing local food supply, erosion and sediment deposition (Rogers 1994RogersA. 1994. The biology of seamounts.Adv. Mar. Biol. 30: 305- 350. 10.1016/S0065-2881(08)60065-6). Seamounts also provide essential ecological habitats, thus affecting faunal diversity, offering a large number of microhabitats with particular hydrographic, productivity and substratum characteristics (Ramírez-Llodra et al. 2010Ramirez-LlodraE., BrandtA., DanovaroR., et al. 2010. Deep, diverse and definitely different: unique attributes of the world's largest ecosystem. Biogeosciences7: 2851-2899. 10.5194/bg-7-2851-2010), as well as suitable habitats for fish feeding and spawning grounds (Wessel et al. 2010WesselP., SandwellD.T., KimS.S. 2010. The global seamount census. Oceanography23(1): 24-33. 10.5670/oceanog.2010.60).

Despite the remoteness of deep-sea seamounts and the challenge of accessing them, current knowledge of them is increasing thanks to a reduction in the technical limitations to the exploration of deep-sea environments (Rowden et al. 2010RowdenA.A., SchlacherT.A., WilliamsA., et al. 2010. A test of the seamount oasis hypothesis: seamounts support higher epibenthic megafaunal biomass than adjacent slopes. Mar. Ecol. 31: 95-106. 10.1111/j.1439-0485.2010.00369.x), and research attention is focusing particularly on ecosystem ecology and hydrography (Davies et al. 2015DaviesJ.S., StewartH.A., NarayanaswamyB.E., et al. 2015. Benthic Assemblages of the Anton Dohrn Seamount (NE Atlantic): Defining deep-sea biotopes to support habitat mapping and management efforts with a focus on vulnerable marine ecosystems. PLoS ONE10: e0124815. 10.1371/journal.pone.0124815). Accordingly, seamounts seem not to be ecologically isolated habitats because, though they differ in structure (González-Irusta et al 2021González-IrustaJ.M., De la TorrienteA., PunzónA., et al. 2021. Living at the top. Connectivity limitations and summit depth drive fish diversity patterns in an isolated seamount. Mar. Ecol. Progr. Ser. 670121-137. 10.3354/meps13766), their communities may harbour comparable assemblage compositions to those of adjacent areas (Consalvey et al. 2010ConsalveyM., ClarkM.R., et al. 2010. Life on seamounts. In: McIntyre A.D. (ed), Life in the World's Oceans: Diversity, distribution, and abundance. Wiley Blackwell, United Kingdom, pp. 123-138. 10.1002/9781444325508.ch7, Clark et al. 2012ClarkM.R., SchlacherT.A, RowdenA.A., et al. 2012. Science priorities for seamounts: Research links to conservation and management. PLoS ONE7: e29232. 10.1371/journal.pone.0029232). However, some sampling efforts have inaccurately reported high levels of endemism (Clark et al. 2012ClarkM.R., SchlacherT.A, RowdenA.A., et al. 2012. Science priorities for seamounts: Research links to conservation and management. PLoS ONE7: e29232. 10.1371/journal.pone.0029232, De Forges et al. 2000De ForgesB.R., KoslowJ.A., PooreG.C.B. 2000. Diversity and endemism of the benthic seamount fauna in the southwest Pacific. Nature405: 944-947. 10.1038/35016066).

Seamount research is often biased to study fishes or large suspension feeders, such as corals and sponges. Thus, soft-bottom infauna remains poorly studied in comparison with hard bottom biota (Bongiorni et al. 2013BongiorniL., RavaraA., ParrettiP., et al. 2013. Organic matter composition and macrofaunal diversity in sediments of the Condor Seamount (Azores, NE Atlantic). Deep-Sea Res.Part II98: 75-86. 10.1016/j.dsr2.2013.08.006, Chivers et al. 2013ChiversA.J., NarayanaswamyB.E., LamontP.A., et al. 2013. Changes in polychaete standing stock and diversity on the northern side of Senghor Seamount (NE Atlantic). Biogeosciences10: 3535-3546. 10.5194/bg-10-3535-2013, Rogers 2018RogersA. 2018. The Biology of Seamounts: 25 Years on. Adv. Mar. Biol. 79: 137-224. 10.1016/bs.amb.2018.06.001), despite playing a key role in plankton/benthos interactions and being a fundamental food source for pelagic organisms such as fish (Sautya et al. 2011SautyaS., IngoleB., RayD., et al. 2011. Megafaunal community structure of Andaman Seamounts including the Back-Arc Basin - A quantitative exploration from the Indian Ocean. PLoS ONE6(1): e16162. 10.1371/journal.pone.0016162).

The present paper focused on the Galicia Bank seamount (northwest coast of Spain), which was included in the ninth update of the Sites of Community Importance for the Atlantic Biogeographical Region list in November 2015, as part of the necessary efforts to preserve this deep-sea ecosystem. Also, it is one of the areas under evaluation for habitat monitoring in the European Union Marine Strategy Framework Directive (2008/56/CE). The benthic and pelagic ecosystems of the seamount and the physical processes supporting them, together with its geology and geophysics, were the focus of the INDEMARES (LIFE+) project “Inventory and designation of marine Natura 2000 areas in the Spanish sea” (www.indemares.es; EC contract LIFE 07/NAT/E/000732). Its main objective was to identify, protect and conserve valuable areas under the Habitats Directive, providing the necessary information to establish a network of representative marine protected areas in Spanish waters. Framed within this multidisciplinary investigation, this paper specifically deals with the benthic macroinfauna.

The Galicia Bank has probably been known by Galician fishermen for decades, but the scientific information on its biology and ecology is much more recent, likely due to its difficulty of access and depth (Gofas et al. 2021GofasS., LuqueÁ.A., OliverJ.D., et al. 2021. The Mollusca of Galicia Bank (NE Atlantic Ocean). Eur. J. Tax. 785: 1-114. 10.5852/ejt.2021.785.1605). However, it harbours a diverse soft-bottom megafauna including decapod crustaceans (Cartes et al. 2014CartesJ.E., PapiolV., FrutosI., et al. 2014. Distribution and biogeographic trends of decapod assemblages from Galicia Bank (NE Atlantic) at depths between 700 and 1800 m, with connexions to regional water masses. Deep-Sea Res. Part II106: 165-178. 10.1016/j.dsr2.2013.09.034), fish, corals and other habitat-forming organisms (Serrano et al. 2017aSerranoA., CartesJ.E., PapiolV., et al. 2017a. Epibenthic communities of sedimentary habitats in a NE Atlantic deep seamount (Galicia Bank). J. Sea Res.130: 154-165. 10.1016/j.seares.2017.03.004); a single study on the macroinfauna reports information only at family level (Lourido et al. 2019LouridoA., ParraS., SerranoA. 2019. Preliminary results on the composition and structure of soft-bottom macrobenthic communities of a seamount: the Galicia Bank (NE Atlantic Ocean). Thalassas35: 1-9. 10.1007/s41208-017-0055-9). The nutrient dynamics and the available trophic resources are predominantly pelagic, with the very reduced benthic compartment being conditioned by grain size and organic matter contents, which are driven by the strong currents dominating the area and its isolation from the mainland (Serrano et al 2017aSerranoA., CartesJ.E., PapiolV., et al. 2017a. Epibenthic communities of sedimentary habitats in a NE Atlantic deep seamount (Galicia Bank). J. Sea Res.130: 154-165. 10.1016/j.seares.2017.03.004).

The present paper studied the composition (diversity and abundance patterns), spatial distribution and community structure of the infaunal macrobenthic taxa of the Galicia Bank seamount, comparing them with those from surrounding areas of the North Atlantic and discussing their relationships with the prevailing environmental factors.

MATERIAL AND METHODS

Sample collection

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Within the framework of the INDEMARES (LIFE+) project, two multidisciplinary surveys were carried out in summer on board the RVs Thalassa (2010) and Miguel Oliver (2011). Undisturbed samples were collected using a quantitative USNEL box corer (Hessler and Jumars 1974HesslerR.R., JumarsP.A.1974. Abyssal community analysis from replicate box cores in the central North Pacific. Deep-Sea Res Oceanogr. Abstr. 21: 185-209. 10.1016/0011-7471(74)90058-8, Gage and Tyler 1991GageJ.D., TylerP.A. 1991. Deep-sea biology. A natural history of organisms at the deep-sea floor. Cambridge University Press, UK. 10.1017/CBO9781139163637) of 0.09 m² (30×30 cm) with a subsample of 0.017 m² (17×10 cm) extracted for sediment analyses. Twenty-eight sampling stations were selected in low backscatter areas (i.e. soft sediments), as indicated by a multibeam echosounder, from 683 to 2274 m depth (Fig. 1) (see Lourido et al. 2019LouridoA., ParraS., SerranoA. 2019. Preliminary results on the composition and structure of soft-bottom macrobenthic communities of a seamount: the Galicia Bank (NE Atlantic Ocean). Thalassas35: 1-9. 10.1007/s41208-017-0055-9 for further details).

Fig. 1.- Galicia Bank. A. Geographical location. B. Position of stations and spatial variability in sedimentary type and sediment organic matter (circle sizes are proportional to contents). 

Infaunal samples were carefully sieved on board through a 0.5 mm mesh sieve using sea water. The retained material was anaesthetised with MgCl₂ and preserved with an 8% buffered formaldehyde seawater solution stained with Rose Bengal. All organisms recovered were sorted in the laboratory, identified to the lowest possible taxonomic level and preserved in 70% ethanol. Sipuncula and Nemertea were grouped as “others”. All taxa were then assigned to five trophic categories: carnivores, surface-deposit feeders, subsurface-deposit feeders, suspension feeders and “remaining” (including omnivores, herbivores and scavengers).

Samples for sediment analyses were frozen on board to be later processed in the laboratory. Particle size was analysed through dry sieving for the coarse fraction (>62 μm) and by laser diffraction particle size analyser (Mastesizer 2000) for the finer fraction (<62 μm). The median grain size (Q₅₀) and sorting coefficient (S₀) (Trask 1932TraskP.D. 1932. Origin and environment of source sediments of petroleum. Houston Gulf Publications Co., Houston.) were also determined. Organic matter contents were estimated as losses in weight of dried samples (100°C, 24 h) after combustion (500°C, 24 h) (Buchanan 1984BuchananJ.B. 1984. Sediment analysis. In: Holme N.A., McIntyre A. D., Methods for the Study of Marine Benthos. Blackwell Scientific Publications, Oxford.).

RESULTS

Faunal composition

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Our samples yielded 1397 individuals (19137 ind m^–2) from 182 taxa included in 87 families. Polychaetes accounted for 67.2% of the total, followed by molluscs (13.7%), echinoderms (9.5%), crustaceans (5.2%) and others (4.4%) (Fig. 2). Syllid and spionid polychaetes were the most abundant, whereas ampharetid and syllid polychaetes were the most species-diverse (Table 1). The most dominant taxa were Aurospio dibranchiata (10.0%), Poecilochaetus sp. (4.1%), Limopsis cristata (2.9%), Thyasira succisa (2.9%), Glycera lapidum (2.7%) and Palposyllis prosostoma (2.7%), which accounted for more than 25% of the total, and only the following taxa were present at more than a half of the stations: Nemertea (17), Poecilochaetus sp. (16), Protodorvillea kefersteini (16) and T. succisa (15).

Fig. 2.- Relative abundance of the major macrofaunal taxa at each station of the Galicia Bank. 

Table 1.- Left, families accounting for more than 50% of all individuals; right, families accounting for more than 25% of all species. 

Families	Abundance (%)		Families	Species number	%
Spionidae	12		Ampharetidae	12	6.6
Syllidae	10.3		Syllidae	11	6
Ampharetidae	4.6		Onuphidae	8	4.4
Poecilochaetidae	4.1		Paraonidae	7	3.8
Cirratulidae	3.9		Spionidae	7	3.8
Paraonidae	3.6		Opheliidae	6	3.3
Limopsidae	2.9				27.9
Thyasiridae	2.9
Glyceridae	2.7
Sabeliidae	2.7
Ophiacanthidae	2.6
	52.3

Syllid and poecilochaetid polychaetes, ophiacanthid echinoderms and limopsid and thyasirid bivalves dominated the bank summit, while polychaetes clearly dominated the medium-depth stations (spionids, ampharetids and poecilochaetids) and deepest stations (spionids, glycerids, ampharetids, syllids, cirratulids and paraonids).

Infaunal assemblages

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The cluster analysis distinguished three groups of stations at 25% similarity, which were confirmed by the nMDS and showed significant differences in faunistic composition (ANOSIM, global R=0. 898, p=0. 001): A (1311–1579 m depth), B (765–1092 m depth) and C (1477–2274 m depth) (Fig. 3A–B). Group A included three medium-depth stations with medium and fine sands and showed the lowest species richness and abundance, being dominated by ophiuroids, ampharetids and gastropods. Group B included the shallowest stations with medium sands, moderate to moderate-well sorted sediments, the highest content of coarse sand and low organic matter content. The polychaetes Poecilochaetus sp. and P. prosostoma, the bivalves L. cristata and T. succisa and the ophiuroid Ophiacantha sp. were the most abundant taxa. Group C included the deepest stations, having fine and very fine sands and the highest mud and organic matter content and showing the highest species richness and abundance. The polychaete A. dibranchiata was the most abundant taxon.

Fig. 3.- A. Geographical location of the sampling sites, showing the cluster analysis grouping. B. Non-metric multidimensional scaling ordination of sampling sites showing the cluster groups. Distribution of environmental variables in the ordination space. C. Depth. D. Mud. E. Median grain size (Q₅₀). 

The polychaetes Poecilochaetus sp. and P. prosostoma, and the bivalve T. succisa characterized the shallow Group B (Table 2, SIMPER, average similarity =27.4%), while ampharetids, gastropods, ophiuroids and the holothuroid Labidoplax buskii characterized the medium-depth Group A (Table 2, SIMPER, average similarity =15.3%), and the polychaetes A. dibranchiata and G. lapidum and Nemertea characterized the deepest Group C (Table 2, SIMPER, average similarity =28.9%). The taxa most contributing to the dissimilarity between groups A and B (SIMPER, average dissimilarity =93.63%) and A and C (SIMPER, average dissimilarity =89.77%) were Ampharetidae spp., Phascolion sp. and L. buskii, while the taxa that most contributed to the dissimilarity between groups B and C (SIMPER, average dissimilarity =88.21%) were A. dibranchiata, T. succisa, Poecilochaetus sp., P. prosostoma, and Syllis sp. 1.

Table 2.- Cumulative contributions to the similarity (Cum. %, 50% cutoff) obtained by SIMPER analysis for the benthic infauna, according to the groups obtained in the cluster analysis. 

	Cum %			Cum %
Average similarity: 27.40			Average similarity: 28.90
Group B (765-1092 m)			Group C (1477-2274 m)
Poecilochaetus sp. Claparède in Ehlers, 1875	7.6		Aurospio dibranchiataMaciolek, 1981	23.2
Palposyllis prosostoma Hartmann-Schröder, 1977	12.8		Glycera lapidum Quatrefages, 1866	29.4
Limopsis cristata Jeffreys, 1876	17.9		Nemertea spp.	32.8
Thyasira succisa (Jeffreys, 1876)	23		Ampharetidae spp. Malmgren, 1866	35.5
Ophiacantha sp. Müller & Troschel, 1842	27.9		Syllis sp. Lamarck, 1818	38.1
Protodorvillea kefersteini (McIntosh, 1869)	31.8		Ostracoda spp. Latreille, 1802	40.5
Syllis sp. 1 Lamarck, 1818	35.6		Cirratulidae spp. Ryckholt, 1851	42.6
Jasmineira caudata Langerhans, 1880	39.1		Spiophanes sp. Grube, 1860	44.8
Ophiomyces grandis Lyman, 1879	42.5		Parexogone wolfi (San Martín, 1991)	46.8
Synelmis sp. Chamberlin, 1919	44.8		Antalis agilis (M. Sars, 1872)	48.6
Amphiura chiajei Forbes, 1843	47		Aricidea sp. Webster, 1879	50.4
Aglaophamus malmgreni (Théel, 1879)	49
Eurysyllis tuberculata Ehlers, 1864	50.9
Average similarity: 15.33
Group A (1311-1579 m)
Ampharetidae sp. Malmgren, 1866	36.71
Labidoplax buskii (MacIntosh, 1866)	51.47

Relationship between biotic and environmental variables

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The number of individuals per station ranged from 82 ind m^–2 (1353 m depth, west part of the seamount) to 1822 ind m^–2 (866 m depth, near the bank top), and the number of taxa per station ranged from 6 (1353 m depth, medium sand, west part of the seamount) to 49 (1751 m depth, very fine sand, east part of the seamount) (Table 3). The highest and lowest diversity were observed at medium sand stations from the bank top (station 24, 745 m depth, H’=5.1 bits; station 16, 774 m depth, H’=2.5 bits) (Table 3, Fig. 4).

Total macrofaunal abundance was positively correlated with depth (p<0.05) and mud (p<0.01), and negatively with median grain size and coarse sand (p<0.01). Depth was correlated positively with mud and organic matter (p<0.01) and negatively with median grain size (p<0.05).

Fig. 4.- Bathymetric changes in macrofaunal descriptors: A. Density; B. Taxon richness; C. Diversity. 

Table 3.- Summary of biotic and physical characteristics of the assemblages derived from the cluster analysis (mean with standard deviation and range of values). 

Group (st.)		Depth (m)	Sediment type	TOM (%)	Q50 (mm)	S0	CS (%)	FS (%)	Mud (%)	S	N (ind.m-2)
A (8, 18, 26)	Mean	1414 ± 144.1	MS (2st.)-FS (1st.)	1.5 ± 0.3	0.30 ± 0.04	Mod.-ModW	10.4 ± 6.1	87.6 ± 6.3	2.0 ± 0.8	15 ± 9	306 ± 208
	Range	1311-1579		1.2-1.9	0.24-0.32		3.6-15.3	82.0-94.3	1.2-2.8	6-24	82-493
B (1, 2, 3, 4, 5, 6, 7, 9, 15, 16, 19, 20, 21, 22, 23, 24, 25)	Mean	859 ± 125.5	MS(16st.)-FS(1st.)	1.6 ± 0.2	0.31 ± 0.05	Mod.-ModW	16.9 ± 6.4	82.5 ± 6.3	0.6 ± 0.6	22 ± 11	586 ± 406
	Range	765-1092		1.1-2.0	0.24-0.40		10.1-29.3	70.7-88.8	0-1.8	7-49	164-1822
C (10, 11, 12, 13, 14, 17, 27, 28)	Mean	1764 ± 245.4	FS(5st.)-VFS(3st.)	2.6 ± 0.8	0.15 ± 0.05	Mod.-Poor-Bad	4.9 ± 2.3	75.0 ± 13.0	20.2 ± 14.2	32 ± 9	1033 ± 418
	Range	1477-2274		1.1-3.5	0.07-0.20		1.9-7.7	51.3-90.9	2.7-46.3	20-49	479-1562

Notes: 

TOM, total organic matter; Q₅₀, mean grain size; S₀, sorting coefficient; Mod, moderate sorted; ModW, moderate-well sorted; CS, coarse sand; FS, fine sand; MS, medium sand; VFS, very fine sand; N, infaunal abundance; S, number of species; st., number of stations these values were based on.

Polychaetes were correlated positively with mud (p<0.01) and negatively with median grain size (p<0.05), while crustaceans were correlated positively with depth and mud (p<0.01) and negatively with median grain size (p<0.01) and coarse sand (p<0.05). The others were correlated positively with depth (p<0.05) and mud and negatively with median grain size and coarse sand (p<0.01), whereas echinoderms were correlated negatively with total organic matter (p<0.05) and molluscs showed no significant correlation.

Depth, mud content and median grain size were the major structuring factors of the benthic community (BIO-ENV, p_w=0.596) and showed the highest correlations when considered separately (depth, p_w= 0.540; mud, p_w=0.456; Qp₅₀, p_w=0.374). In the nMDS, the stations were distributed from left to right following increasing values of median grain size and decreasing depths and mud (Fig. 3C–E).

Axes I and II were the most important in the CCA (variance =47.59%). Group C stations were distributed along axis I negative sector, showing the deepest and muddy bottoms, whereas Group B stations appeared distributed along axis I positive sector, and Group A stations were intermediate between those of groups B and C (Fig. 5A). The taxon distribution in the CCA (variance of axis I and II =59.20 %) was consistent with the SIMPER results, showing clear differences between the bank top (Group B, medium sands) and the deeper stations (Group C, fine sand flanks with the highest organic matter) (Fig. 5B).

Fig. 5.- Canonical correspondence analysis ordination. A. Sampling sites. B. Taxa. Q₅₀, median grain size; S₀, sorting coefficient; TOM, total organic matter; FS, fine sand; CS, coarse sand; Agl mal,Aglaophamus malmgreni; Amp sp, Ampharetidae sp.; Amp sp1, Ampharetidae sp. 1; Amp squ,Amphipholis squamata; Amp chi,Amphiura chiajei; Ant agi,Antalis agilis; Aon day,Aonidella dayi; Ari sp,Aricidea sp.; Ari was,Aricidea wassi; Aur dib,Aurospio dibranchiata; Biv sp, Bivalvia sp. ; Cau sp,Caulleriella sp; Cir sp, Cirratulidae sp.; Cus sp, Cuspidariidae sp. ; Euc inc,Euchone incolor; Eur tub,Eurysyllis tuberculata; Gal ocu,Galathowenia oculata; Gal sp1,Galathowenia sp.1; Gas sp, Gastropoda sp.; Gly lap,Glycera lapidum; Har lae,Harpinia laevis; Jas cau,Jasmineira caudata; Lab bus,Labidoplax buskii; Lim sp,Limidae sp.; Lim cris,Limopsis cristata; Lum sp,Lumbriclymene sp.; Lys fra,Lysippe fragilis; Mel sp, Melinninae sp.; Kin dor,Kirkegaardia dorsobranchialis; Nem sp, Nemertea sp.; Ner pun,Nereimyra punctata; Not sp,Nothria sp.; Not lat,Notomastus latericeus; Not sp2,Notomastus sp.2; Oph sp,Ophiacantha sp.; Oph gra,Ophiomyces grandis; Oph dea, Ophiuroidea sp.; Pal pro,Palposyllis prosostoma; Par abr,Paradoneis abranchiata; Pec sp, Pectinoidea sp.; Pha sp,Phascolion sp.; Pho ino,Pholoe inornata; Phy sp,Phyllodoce sp.; Poe sp,Poecilochaetus sp.; Pol sp,Polycirrus sp.; Pro reg,Progoniada regularis; Pro kef,Protodorvillea kefersteini; Sig sp, Sigalionidae sp.; Sol sp, Solenogastres sp.; Spi sp,Spiophanes sp.; Str sp,Streptosyllis sp.; Syl spI,Syllis sp.I; Syl spp,Syllis spp.; Syn sp,Synelmis sp.; Thy suc,Thyasira succisa. 

Trophic structure

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Carnivores dominated the assemblages at the bank top (mobile species with no organic matter requirements), while surface-deposit feeders increased their abundance with depth, where the lower hydrodynamics favoured the deposit of organic matter they require. Carnivores accounted for more than 30% of total abundance on average, but their number decreased from 42% under 1000 m depth to 21% at more than 1500 m depth. In contrast, the abundance of deposit feeders increased with depth, with topography (from 41% at the summit, <1000 m depth) to 62% on the flanks (deep stations, >1500 m) and with granulometry, being positively correlated with mud content (p<0.05). Moreover, surface-deposit feeders dominated at 14 stations (7%–68%), carnivores at 9 (8%–75%), and suspensivores (0%–33%), subsurface-deposit feeders (0%–50%) and remaining (0%–35%) at only one (Fig. 6).

Fig. 6.- Macrofaunal trophic categories at each site (as percentages). C, carnivores; DS, surface-deposit feeders; DSS, subsurface-deposit feeders; R, remaining; S, suspensivores. 

DISCUSSION

Seamounts offer a variety of habitats and environmental conditions to benthic fauna, alternating between hard substrates and soft sediments (Clark et al. 2010ClarkM.R., RowdenA.A., SchlacherT., et al. 2010. The ecology of seamounts: structure, function, and human impacts. Ann. Rev. Mar. Sci. 2: 253-278. 10.1146/annurev-marine-120308-081109). Although there are no identical seamounts, their tops are usually characterized by bioclastic sands and their slopes by basalts with sponges and corals (Somoza et al. 2014SomozaL., ErcillaG., UrgorriV., et al. 2014. Detection and mapping of cold-water coral mounds and living Lophelia reefs in the Galicia Bank, Atlantic NW Iberia margin. Mar. Geol. 349: 73-90. 10.1016/j.margeo.2013.12.017), while the accumulated sands and muds are the perfect habitat for small invertebrate organisms such as polychaete annelids, bivalve molluscs, ophiuroids and crustaceans (Rogers 2004RogersA. 2004. The biology, ecology and vulnerability of seamount communities. IUCN report, 12 pp.).

On the Galicia Bank, sediment grain size decreases with depth, with medium sands characterizing most stations at the bank summit, and fine and very fine sands dominating at the deeper stations, as reported for other deep-sea areas (Levin and Gooday 2003LevinL.A., GoodayA.J. 2003. The deep Atlantic Ocean. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam., Thistle 2003ThistleD. 2003. The deep-sea floor: an overview. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam., Louzao et al. 2010LouzaoM., AnadonN., ArrontesJ., et al. 2010. Historical macrobenthic community assemblages in the Avilés Canyon, N Iberian Shelf: Baseline biodiversity information for a marine protected area. J. Mar. Syst. 80: 47-56. 10.1016/j.jmarsys.2009.09.006). In general, the energy of ocean currents and waves decreases from shallow to deeper waters, therefore favouring settling of smaller particles mainly in the less energetic, deeper waters (Karl 2006KarlH.A. 2006. Sediment of the Sea Floor. United States Geological Survey: Boulder, CO, USA. Retrieved fromhttp://pubs.usgs.gov/circ/c1198/chapters/090-100_Sediment.pdf). The organic matter was low in all our samples (1.1%–3.5%), but the highest values occurred on the bank flanks, likely due to the strong currents winnowing organic particles (Duineveld et al., 2004DuineveldG.C.A., LavaleyeM.S.S., BerghuisE.M. 2004. Particle flux and food supply to a seamount cold-water coral community (Galicia Bank, NW Spain). Mar. Ecol. Progr. Ser. 277: 13-23. 10.3354/meps277013), plus a lack of advective input of organic matter from the continental shelf (Surugiu et al., 2008SurugiuV., DauvinJ.C., GilletP., RuelletT. 2008. Can seamounts provide a good habitat for polychaete annelids? Example of the northeastern Atlantic seamounts. Deep-Sea Res. Part II55: 1515-1531. 10.1016/j.dsr.2008.06.012).

Macrofaunal abundances also tend to decrease with depth in many deep-sea environments (Thistle 2003ThistleD. 2003. The deep-sea floor: an overview. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam.) such as the Gay Head-Bermuda transect (Hessler and Sanders 1967HesslerR.R., SandersH.L. 1967. Faunal diversity in the deep sea. Deep-Sea Res. Oceanogr. Abstr. 14: 65-78. 10.1016/0011-7471(67)90029-0), the northeast Atlantic Goban Spur (Flach et al. 2002FlachE., MuthumbiA., HeipC. 2002. Meiofauna and macrofauna community structure in relation to sediment composition at the Iberian margin compared to the Goban Spur (NE Atlantic). Prog. Oceanogr. 52: 433-457. 10.1016/S0079-6611(02)00018-6), the northwest Atlantic (Levin and Gooday 2003LevinL.A., GoodayA.J. 2003. The deep Atlantic Ocean. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam.) and the Gulf of Mexico (Thistle 2003ThistleD. 2003. The deep-sea floor: an overview. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam.). On seamounts, the particular composition of substrata, often varying along summits, flanks and ridges, combined with other environmental parameters (such as depth) allow each structure to host particular assemblages (Rogers 2018RogersA. 2018. The Biology of Seamounts: 25 Years on. Adv. Mar. Biol. 79: 137-224. 10.1016/bs.amb.2018.06.001). On the Galicia Bank, this is reflected in a bathymetrical increase in macrofaunal abundance. On seamounts, depth is not a linear factor but depends on topography, with the nature of the substratum, the slope and the exposure to currents likely influencing the faunal distribution at smaller scales (Clark et al. 2010ClarkM.R., RowdenA.A., SchlacherT., et al. 2010. The ecology of seamounts: structure, function, and human impacts. Ann. Rev. Mar. Sci. 2: 253-278. 10.1146/annurev-marine-120308-081109, Yesson et al. 2011YessonC., ClarkM.R., TaylorM.L., RogersA.D. 2011. The global distribution of seamounts based on 30 arc seconds bathymetry data. Deep-Sea Res. Part I58: 442-453. 10.1016/j.dsr.2011.02.004). Mud content often tends to increase with depth, as in the northeast Atlantic Senghor Seamount, where this is likely associated with an increasing organic matter availability (Chivers et al. 2013ChiversA.J., NarayanaswamyB.E., LamontP.A., et al. 2013. Changes in polychaete standing stock and diversity on the northern side of Senghor Seamount (NE Atlantic). Biogeosciences10: 3535-3546. 10.5194/bg-10-3535-2013). Therefore, macrofaunal abundances may be favoured by the slope habitat heterogeneity but hindered by the strong bottom currents at the summit (Duineveld et al. 2004DuineveldG.C.A., LavaleyeM.S.S., BerghuisE.M. 2004. Particle flux and food supply to a seamount cold-water coral community (Galicia Bank, NW Spain). Mar. Ecol. Progr. Ser. 277: 13-23. 10.3354/meps277013, Levin and Thomas 1989LevinL.A., ThomasC.L.1989. The influence of hydrodynamic regime on infaunal assemblages inhabiting carbonate sediments on central Pacific seamounts. Deep-Sea Res. Part I36: 1897-1915. 10.1016/0198-0149(89)90117-9).

Among macrofaunal organisms, polychaetes are the most abundant deep-sea taxon (Gage and Tyler 1991GageJ.D., TylerP.A. 1991. Deep-sea biology. A natural history of organisms at the deep-sea floor. Cambridge University Press, UK. 10.1017/CBO9781139163637, Grassle and Maciolek 1992GrassleJ., MaciolekN. 1992. Deep-sea species richness: regional and local diversity estimates from quantitative bottom samples. Am. Nat. 139: 313-341. 10.1086/285329, Ramírez-Llodra et al. 2010Ramirez-LlodraE., BrandtA., DanovaroR., et al. 2010. Deep, diverse and definitely different: unique attributes of the world's largest ecosystem. Biogeosciences7: 2851-2899. 10.5194/bg-7-2851-2010), including seamounts (Surugiu et al. 2008SurugiuV., DauvinJ.C., GilletP., RuelletT. 2008. Can seamounts provide a good habitat for polychaete annelids? Example of the northeastern Atlantic seamounts. Deep-Sea Res. Part II55: 1515-1531. 10.1016/j.dsr.2008.06.012), where they may represent more than 50% of the abundance (Gillet and Dauvin 2000GilletP., DauvinJ.C. 2000. Polychaetes from the Atlantic seamounts of the southern Azores: biogeographical distribution and reproductive patterns. J. Mar. Biol. Ass. U.K.80(6): 1019-1029. 10.1017/S0025315400003088, Glover et al. 2002GloverA.G., SmithC.R., PatersonG.L.J., et al. 2002. Polychaete species diversity in the central Pacific abyss: local and regional patterns, and relationships with productivity. Mar. Ecol. Progr. Ser. 240: 157-170. 10.3354/meps240157). The Galicia Bank was no exception (Table 4). Molluscs also occurred at all depths, representing more than 10% of the total on the Galicia Bank (Table 4), with bivalves being the most numerous (63.5% of the molluscs). All remaining taxa represented less than 10% of the abundance.

Spionid, cirratulid and paraonid polychaetes typically dominate deep-sea bottoms, often contributing up to 25% of all species and individuals in slope or abyssal environments (Schüller and Ebbe 2007SchüllerM., EbbeB. 2007. Global distributional patterns of selected deep-sea Polychaeta (Annelida) from the Southern Ocean. Deep-Sea Res. Part II54: 1737-1751. 10.1016/j.dsr2.2007.07.005). Polychaetes are also dominant on seamounts, being particularly represented by Paraonidae, Cirratulidae, Sabellidae, Syllidae and Ampharetidae (Rogers 1994RogersA. 1994. The biology of seamounts.Adv. Mar. Biol. 30: 305- 350. 10.1016/S0065-2881(08)60065-6), in agreement with our results (Table 4). However, abundances on the bank were low (19137 ind m^–2) compared with other deep-sea areas such as the Aviles Canyon System (56637 ind m^–2) (Lourido et al. 2023LouridoA., ParraS., SánchezF. 2023. Soft-bottom infaunal macrobenthos of the Avilés Canyon System (Cantabrian Sea). Diversity15: 53. 10.3390/d15010053), probably because the bank is a deep, oligotrophic seamount with impoverished infaunal environments because of low mainland advection and strong summit currents (Duineveld et al. 2004DuineveldG.C.A., LavaleyeM.S.S., BerghuisE.M. 2004. Particle flux and food supply to a seamount cold-water coral community (Galicia Bank, NW Spain). Mar. Ecol. Progr. Ser. 277: 13-23. 10.3354/meps277013, Surugiu et al. 2008SurugiuV., DauvinJ.C., GilletP., RuelletT. 2008. Can seamounts provide a good habitat for polychaete annelids? Example of the northeastern Atlantic seamounts. Deep-Sea Res. Part II55: 1515-1531. 10.1016/j.dsr.2008.06.012). The 939 individuals from 33 polychaete families found on the bank resemble those on the northeast Atlantic Senghor Seamount (954/34) (Chivers et al. 2013ChiversA.J., NarayanaswamyB.E., LamontP.A., et al. 2013. Changes in polychaete standing stock and diversity on the northern side of Senghor Seamount (NE Atlantic). Biogeosciences10: 3535-3546. 10.5194/bg-10-3535-2013) but were fewer than on the Condor Seamount (1541/32) (Bongiorni et al. 2013BongiorniL., RavaraA., ParrettiP., et al. 2013. Organic matter composition and macrofaunal diversity in sediments of the Condor Seamount (Azores, NE Atlantic). Deep-Sea Res.Part II98: 75-86. 10.1016/j.dsr2.2013.08.006) and more than on the seamounts studied by Surugiu et al. (2008SurugiuV., DauvinJ.C., GilletP., RuelletT. 2008. Can seamounts provide a good habitat for polychaete annelids? Example of the northeastern Atlantic seamounts. Deep-Sea Res. Part II55: 1515-1531. 10.1016/j.dsr.2008.06.012) (94–567/12–23). However, these authors used dredge and trawl samples and a relatively large mesh size, which may have biased their results, preventing comparisons.

Our results suggested the existence of three different infaunal assemblages on the Galicia Bank, being clearly different at the top than on the surrounding, deeper flanks. (1) The shallowest assemblage at the bank summit (Group B, 765–1092 m depth) showed highly abundant thyasirid and limopsid bivalves, which are common or exclusive deep-sea taxa (Gage and Tyler 1991GageJ.D., TylerP.A. 1991. Deep-sea biology. A natural history of organisms at the deep-sea floor. Cambridge University Press, UK. 10.1017/CBO9781139163637); ophiacantid ophiuroids occurred in an area with megaripples indicating strong currents, so their presence could be due to trophic-hydrographic drivers (Serrano et al. 2017aSerranoA., CartesJ.E., PapiolV., et al. 2017a. Epibenthic communities of sedimentary habitats in a NE Atlantic deep seamount (Galicia Bank). J. Sea Res.130: 154-165. 10.1016/j.seares.2017.03.004); syllids were the predominant polychaete family, as documented on the Condor Seamount, mainly at the summit (Bongiorni et al. 2013BongiorniL., RavaraA., ParrettiP., et al. 2013. Organic matter composition and macrofaunal diversity in sediments of the Condor Seamount (Azores, NE Atlantic). Deep-Sea Res.Part II98: 75-86. 10.1016/j.dsr2.2013.08.006). (2) The deepest assemblage (Group C, 1477–2274 m depth) on the bank flanks was characterized by the dominance of the spionid polychaete A. dibranchiata, which accounted for 10% of the macrofauna, as shown for the NW Atlantic (Grassle and Maciolek 1992GrassleJ., MaciolekN. 1992. Deep-sea species richness: regional and local diversity estimates from quantitative bottom samples. Am. Nat. 139: 313-341. 10.1086/285329) and the Mid-Atlantic Ridge (Shields and Blanco-Pérez 2013ShieldsM.A., Blanco-PerezR. 2013. Polychaete abundance, biomass and diversity patterns at the Mid-Atlantic Ridge, North Atlantic Ocean. Deep-Sea Res. Part II98: 315-325. 10.1016/j.dsr2.2013.04.010); Spionidae, one of the most frequent polychaete families in deep-sea soft sediments (Glover et al. 2002GloverA.G., SmithC.R., PatersonG.L.J., et al. 2002. Polychaete species diversity in the central Pacific abyss: local and regional patterns, and relationships with productivity. Mar. Ecol. Progr. Ser. 240: 157-170. 10.3354/meps240157, Shields and BlancoPérez 2013ShieldsM.A., Blanco-PerezR. 2013. Polychaete abundance, biomass and diversity patterns at the Mid-Atlantic Ridge, North Atlantic Ocean. Deep-Sea Res. Part II98: 315-325. 10.1016/j.dsr2.2013.04.010), may alternate between surface-deposit and suspension feeding, a competitive advantage that let them feed both in still waters and when current speed increases suspended food fluxes (Shields and Blanco-Pérez 2013ShieldsM.A., Blanco-PerezR. 2013. Polychaete abundance, biomass and diversity patterns at the Mid-Atlantic Ridge, North Atlantic Ocean. Deep-Sea Res. Part II98: 315-325. 10.1016/j.dsr2.2013.04.010). (3) The third assemblage (Group A,1311–1579 m depth) included stations with intermediate positions between the other two and was characterized by the presence of ampharetids, gastropods and ophiuroids.

The sedimentary habitats of the Galicia Bank showed four faunal assemblages defined by bathymetric, geomorphologic, granulometric, hydrographic dynamic and biological (including fishing impacts) data (Serrano et al. 2017bSerranoA., González-IrustaJ.M., PunzónA., et al. 2017b. Deep-sea benthic habitats modeling and mapping in a NE Atlantic seamount (Galicia Bank). Deep-Sea Res. Part I126: 115-127. 10.1016/j.dsr.2017.06.003). Among them, “Summit Sands” matched with our Group B in both environmental characteristics and faunal composition, with highly abundant sand dwelling ophiacanthid (Ophiacantha sp.) and ophiohelid (Ophiomyces grandis) ophiuroids and limopsid bivalves (Limopsis minuta and L. cristata), while the deeper than 1400 m “Bank Flanks Sands” matched with our Group C. Depth and substrate type, together with depth-related water mass influences, were key factors in sedimentary habitats. This included seamounts, where depth was the strongest environmental proxy for the assemblage-structuring processes, giving rise to communities generally distributed as bands encircling the seamounts (Du Preez et al. 2016Du PreezC., CurtisJ.M.R., ClarkeM.E. 2016. The structure and distribution of benthic communities on a shallow seamount (Cobb Seamount, Northeast Pacific Ocean). PLoS ONE11: e0165513. 10.1371/journal.pone.0165513). These bands also occurred on the Galicia Bank, and this depth-related zonation was more evident on sedimentary than on rocky habitats (Serrano et al. 2017aSerranoA., CartesJ.E., PapiolV., et al. 2017a. Epibenthic communities of sedimentary habitats in a NE Atlantic deep seamount (Galicia Bank). J. Sea Res.130: 154-165. 10.1016/j.seares.2017.03.004). Therefore, depth, topography, current distribution (i.e. with the strongest ones in the summit area) and isolation from the mainland emerged as key factors controlling species distribution on the Galicia Bank.

Nevertheless, diversity was not correlated with depth, although a somewhat increasing trend could be observed. Many seamounts show a mid-slope peak (Cosson-Sarradin et al. 1998Cosson-SarradinN., SibuetM., PatersonG.L.J., VangriesheimA. 1998. Polychaete diversity at tropical Atlantic deep-sea sites: environmental effects. Mar. Ecol. Progr. Ser. 165: 173-185. 10.3354/meps165173, Maciolek and Smith 2009MaciolekN.J., SmithW. 2009. Benthic species diversity along a depth gradient: Boston Harbor to Lydonia Canyon. Deep-Sea Res. Part II56: 1763-1774. 10.1016/j.dsr2.2009.05.031, Probert et al. 2009ProbertP.K., GlasbyC.J., Grove S.L., PaavoB.L. 2009. Bathyal polychaete assemblages in the region of the Subtropical Front, Chatham Rise, New Zealand. N. Z. J. Mar. Fresh. Res. 43:5: 1121-1135. 10.1080/00288330.2009.9626535), most likely being caused by factors other than depth, such as nutrient input, temperature, hydrostatic pressure and current dynamics (Gage and Tyler 1991GageJ.D., TylerP.A. 1991. Deep-sea biology. A natural history of organisms at the deep-sea floor. Cambridge University Press, UK. 10.1017/CBO9781139163637) or by changes in sediment characteristics (Etter and Grassle 1992EtterR.J., GrassleJ.F. 1992. Patterns of species diversity in the deep sea as a function of sediment particle size diversity. Nature360: 576-578. 10.1038/360576a0). On the Galicia Bank, diversity might be affected by the seamount morphology, particularly the numerous slope microhabitats and the hostile summit environment. Accordingly, the shallowest summit stations of Group B differed in infaunal species composition from the deepest flank stations of Group C, a pattern resembling that of decapod crustaceans, which also showed a generalized bathymetric species substitution (Cartes et al. 2014CartesJ.E., PapiolV., FrutosI., et al. 2014. Distribution and biogeographic trends of decapod assemblages from Galicia Bank (NE Atlantic) at depths between 700 and 1800 m, with connexions to regional water masses. Deep-Sea Res. Part II106: 165-178. 10.1016/j.dsr2.2013.09.034).

The trophic structure also showed bathymetric patterns, with carnivores and filter feeders (e.g. ophiacanthids and limopsids) dominating the bank summit on the Galicia Bank, the latter taking advantage of the currents to feed on the more abundant suspended particles. In agreement with Probert et al. (2009ProbertP.K., GlasbyC.J., Grove S.L., PaavoB.L. 2009. Bathyal polychaete assemblages in the region of the Subtropical Front, Chatham Rise, New Zealand. N. Z. J. Mar. Fresh. Res. 43:5: 1121-1135. 10.1080/00288330.2009.9626535), predators tended to be more abundant at shallower stations on the Galicia Bank, with their energetic profit decreasing with depth because of the amount of energy required to find their preys, which tend to be more distant in deeper environments (Thistle 2003ThistleD. 2003. The deep-sea floor: an overview. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam.). On the Galicia Bank, surface-deposit feeders increased their abundance with depth, showing maxima at the deepest part, as in the deep-sea Goban Spur transect (Levin and Gooday 2003LevinL.A., GoodayA.J. 2003. The deep Atlantic Ocean. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam.). The organic matter reaching the deep sea is advantageously processed by deposit feeders (e.g. spionids, ampharetids, cirratulids or paraonids), which tend to be dominant in this environment (Thistle 2003ThistleD. 2003. The deep-sea floor: an overview. In: TylerP.A., Ecosystems of the deep oceans. Ecosystems of the world, 28. Elsevier, Amsterdam.).

CONCLUSIONS

Exploring seamount macroinfaunal assemblages provides key information that contributes to our understanding of the ecosystem distribution drivers. Therefore, our work provides an important environmental baseline information on the infaunal community of the Galicia Bank, while addressing the lack of studies on deep seamounts and the taxonomic bias towards larger animals. Our results agree with previous studies in showing that depth and substrate type combine with topography as the key factors driving the infaunal benthic community structure and distribution on the Galicia Bank.