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. 2011). The total number of deep-sea seamounts is still uncertain, and the studies show great differences in global estimates (Morato et al. 2013). However, seamounts cover a significant portion of the seafloor, forming one of the largest ocean biomes (Rogers 2018) and showing high spatial heterogeneity and topographic complexity (Rogers 2004). 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 1987, Rogers 1994). The seamount relief is an obstacle to currents, creating local upwellings and closed circulation cells known as Taylor columns (Boehlert and Genin 1987, White et al. 2007). These are related to the arrival of nutrient-rich deep water, which leads to increased productivity in the upper seamount regions (Rogers 2004) and has a functional role in increasing local food supply, erosion and sediment deposition (Rogers 1994). 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. 2010), as well as suitable habitats for fish feeding and spawning grounds (Wessel et al. 2010).

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. 2010), and research attention is focusing particularly on ecosystem ecology and hydrography (Davies et al. 2015). Accordingly, seamounts seem not to be ecologically isolated habitats because, though they differ in structure (González-Irusta et al 2021), their communities may harbour comparable assemblage compositions to those of adjacent areas (Consalvey et al. 2010, Clark et al. 2012). However, some sampling efforts have inaccurately reported high levels of endemism (Clark et al. 2012, De Forges et al. 2000).

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. 2013, Chivers et al. 2013, Rogers 2018), despite playing a key role in plankton/benthos interactions and being a fundamental food source for pelagic organisms such as fish (Sautya et al. 2011).

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. 2021). However, it harbours a diverse soft-bottom megafauna including decapod crustaceans (Cartes et al. 2014), fish, corals and other habitat-forming organisms (Serrano et al. 2017a); a single study on the macroinfauna reports information only at family level (Lourido et al. 2019). 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 2017a).

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.

Seamounts offer a variety of habitats and environmental conditions to benthic fauna, alternating between hard substrates and soft sediments (Clark et al. 2010). 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. 2014), while the accumulated sands and muds are the perfect habitat for small invertebrate organisms such as polychaete annelids, bivalve molluscs, ophiuroids and crustaceans (Rogers 2004).

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 2003, Thistle 2003, Louzao et al. 2010). 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 2006). 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., 2004), plus a lack of advective input of organic matter from the continental shelf (Surugiu et al., 2008).

Macrofaunal abundances also tend to decrease with depth in many deep-sea environments (Thistle 2003) such as the Gay Head-Bermuda transect (Hessler and Sanders 1967), the northeast Atlantic Goban Spur (Flach et al. 2002), the northwest Atlantic (Levin and Gooday 2003) and the Gulf of Mexico (Thistle 2003). 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 2018). 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. 2010, Yesson et al. 2011). 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. 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. 2004, Levin and Thomas 1989).

Among macrofaunal organisms, polychaetes are the most abundant deep-sea taxon (Gage and Tyler 1991, Grassle and Maciolek 1992, Ramírez-Llodra et al. 2010), including seamounts (Surugiu et al. 2008), where they may represent more than 50% of the abundance (Gillet and Dauvin 2000, Glover et al. 2002). 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 2007). Polychaetes are also dominant on seamounts, being particularly represented by Paraonidae, Cirratulidae, Sabellidae, Syllidae and Ampharetidae (Rogers 1994), 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. 2023), 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. 2004, Surugiu et al. 2008). The 939 individuals from 33 polychaete families found on the bank resemble those on the northeast Atlantic Senghor Seamount (954/34) (Chivers et al. 2013) but were fewer than on the Condor Seamount (1541/32) (Bongiorni et al. 2013) and more than on the seamounts studied by Surugiu et al. (2008) (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 1991); ophiacantid ophiuroids occurred in an area with megaripples indicating strong currents, so their presence could be due to trophic-hydrographic drivers (Serrano et al. 2017a); syllids were the predominant polychaete family, as documented on the Condor Seamount, mainly at the summit (Bongiorni et al. 2013). (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 1992) and the Mid-Atlantic Ridge (Shields and Blanco-Pérez 2013); Spionidae, one of the most frequent polychaete families in deep-sea soft sediments (Glover et al. 2002, Shields and BlancoPérez 2013), 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 2013). (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. 2017b). 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. 2016). 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. 2017a). 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. 1998, Maciolek and Smith 2009, Probert et al. 2009), most likely being caused by factors other than depth, such as nutrient input, temperature, hydrostatic pressure and current dynamics (Gage and Tyler 1991) or by changes in sediment characteristics (Etter and Grassle 1992). 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. 2014).

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. (2009), 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 2003). 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 2003). 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 2003).

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.