The Alboran Sea, located in the westernmost part of the Mediterranean Sea, is a transitional area between the Atlantic Ocean and the Mediterranean Sea that has been reported as a self-standing ecoregion harbouring a large marine biodiversity and a wide variety of habitats (Spalding et al. 2007, Rueda et al. 2021, Templado et al. 2021). Because of its distinctive geomorphological and oceanographical features, the Alboran Sea has been identified as a specific biogeographical sector for several marine groups (Real et al. 2021), including demersal fish (Gaertner et al. 2005, González et al. 2021), molluscs (Gofas et al. 2011, Ciércoles et al. 2018) and crustaceans (Abelló et al. 2002).

Crustaceans are one of the most morphologically diverse taxonomic groups of the aquatic ecosystems, where they are one of the top dominant groups (Martin and Davis 2001). Within the crustaceans, the order Decapoda constitutes a dominant component of Mediterranean benthic and demersal communities of both the continental shelf and slope (Maynou and Cartes 2000, Guijarro 2012). Their relative importance in the Mediterranean Sea has been hypothesized because of their very high competitive trophic strategy (Cartes and Sardà 1992, Maynou and Cartes 2000). Moreover, decapods are a key taxon linking lower and higher trophic levels (Cartes 1998, Fanelli 2007). Several decapod species are of commercial interest and form an important component of the catches of the bottom trawl fishery in the Alboran Sea and the Gulf of Vera. In fact, some decapod species, such as the deep-water rose shrimp, Parapenaeus longirostris, the Norway lobster, Nephrops norvegicus, and the red shrimp Aristeus antennatus are economically valuable target species in demersal fisheries, so their populations are regularly assessed in the Alboran Sea (González et al. 2021), as well as in other parts of the Mediterranean Sea (Guijarro 2012; Regulation (EU) 2019/1022). Knowledge of the distribution and abundance of species along environmental gradients has traditionally been important to characterize and understand the role of biological communities in aquatic systems (Wenner and Boesch 1979). Studies of biological communities are also essential tools for understanding the dynamics of exploited species from an ecosystem point of view, which is a key element in considering separate management units (Abelló et al. 2002). Additionally, species-specific studies on decapod species have revealed geographical variability patterns throughout the Mediterranean Sea, reflecting the importance of studies at a small-scale geographical level. This approach has been shown to be more efficient for the management of certain species in contrast to large regional approaches (Gaertner et al. 2005, Guijarro et al. 2019) and it is particularly important for scientifically sustaining spatial management. The Alboran Sea, together with the Gulf of Vera, can be considered a transition zone between the Mediterranean and Atlantic biota of crustaceans, since it also constitutes a semipermeable barrier for genetic population exchanges between the Mediterranean Sea and the Atlantic Ocean (Abelló et al. 2002, Mateo-Ramírez et al. 2015, Pascual et al. 2016).

Several studies on decapod assemblages have been carried out in different habitats of the Alboran Sea, particularly those of the infralittoral zone (soft bottoms, García Muñoz et al. 2008; seagrass meadows, García-Raso 1990, García Raso et al. 2006, Mateo-Ramírez et al. 2016; macroalgal communities, Mateo-Ramírez et al. 2018). However, despite their ecological and economical interest, few studies of decapod assemblages in circalittoral and bathyal soft bottoms have been carried out in the Alboran Sea using beam-trawl samples (Mateo-Ramírez et al. 2015) or including the Alboran Sea as part of studies with a wider scope covering the Spanish Mediterranean waters (Abelló et al. 2002).

The aims of this study were to (1) update faunistic inventories of decapod crustacean species in the Alboran Sea and the adjacent Gulf of Vera; (2) identify and characterize the main decapod assemblages; (3) analyse significant differences between assemblages in the ecological indices considered as community descriptors (decapod abundance, species richness, Pielou’s evenness and the Shannon-Wiener diversity index) in order to study the spatial and temporal changes of the composition and structure of the decapod assemblages; and (4) model spatio-temporal trends of ecological indices in terms of potential environmental driving.

The present study reports a high number of decapod species on circalittoral and bathyal soft bottoms of the northern Alboran Sea, including Alboran Island and the Gulf of Vera, and increases the number of previously recorded species for these sedimentary habitats (15 additional species to those reported by Abelló et al. 2002). Nevertheless, all the species detected in the present study were previously listed in the Alboran Sea by Marco-Herrero et al. (2017) and García-Raso et al. (2018) in a bibliographic compilation of marine decapod crustaceans from the Iberian Peninsula. A high decapod richness for the Alboran Sea was already reported by García Muñoz et al. (2008) in infralittoral sedimentary habitats, and this was related to a high diversity of bottom and habitat types and the confluence of Atlantic and Mediterranean waters and organisms. Abelló et al. (2002) highlighted that the exceptional and highly diverse decapod assemblages of the shelf and slope of the northern Alboran Sea, with a biogeographical boundary around Cabo de Palos, made this area very different from other western Mediterranean areas.

Comparing the geographical sectors of the study area, the highest number of species was detected in the northern Alboran Sea (86 spp.), whereas the lowest number was found at Alboran Island and in the Gulf of Vera (55 and 54 spp., respectively). It is important here to reiterate that no sampling could be carried out at depths shallower than 100 m around Alboran Island because it is a Marine and Fishing Reserve and bottom trawling is not allowed because of the occurrence of vulnerable marine ecosystems (Mateo-Ramírez et al. 2021). Species with Atlantic affinities, most of them with a low frequency of occurrence (F<0.73%) (such as P. henslowii, Ergasticus clouei, and Pagurus mbizi), were recorded in the northern Alboran Sea and/or Alboran Island sectors but not in the Gulf of Vera sector, in agreement with Abelló et al. (2002). This could be due to the effect of the semi-permanent Almeria-Oran oceanographic front that acts as an effective barrier for gene flow and/or species dispersion for these species, as has also been detected for some other decapod species (García Raso et al. 2014, García Lafuente et al. 2021). It is worth mentioning the occurrence of the subtropical Atlantic hermit crab P. mbizi (Forest 1955). It was initially reported in 2014 in the westernmost sector of the northern Alboran Sea (an exclusive occurrence in European waters and the Mediterranean Sea) by García Raso et al. (2014) in a depth range of 50 to 150 m. In the present study, specimens of P. mbizi were collected in the central and eastern parts of the northern Alboran Sea over a wider bathymetric range (50-361 m), showing a similar depth range to that recorded in Atlantic waters (30 -650 m) (Forest 1961). However, P. mbizi was absent in the Gulf of Vera, so its expansion is so far restricted to the Alboran Sea. On the other hand, the crab Scyramathia carpenteri (previously known as Rochinia carpenteri) was first recorded in the northern Alboran Sea sector (Abelló et al. 2002), but the present study provides records for the Gulf of Vera sector, thus suggesting a possible expansion to other western Mediterranean areas. The presence and, in some cases, potential eastward expansion of typical Atlantic species (e.g. S. carpenteri) and African species (e.g. P. mbizi) through the Alboran Sea could be a result of climate change effects, favouring a northwards expansion of thermophilic Atlantic species along the Atlantic coasts and into the Mediterranean Sea (i.e. meridionalization) (García Raso et al. 2014, 2018), once they manage to surpass the colder waters of the Alboran Sea.

Marine species distribution is generally related to dynamic environmental variables such as temperature and other variables such as food availability (Snelgrove and Butman 1994, Cartes et al. 2004, Martins et al. 2014), but it is also related to structural properties such as depth and the substrate type of the seabed. As in previous studies (Cartes and Sardà 1993, Follesa et al. 2009, Deval et al. 2017, among others), depth was the main structuring factor for decapod crustacean distribution in the present study, probably because of a combination of other depth-related factors such as light, temperature and food availability (Cartes et al. 2004). In the present study, four decapod assemblages were detected in relation to depth, in agreement with previous studies on the northern Alboran Sea (Mateo-Ramírez et al. 2015), adjacent Mediterranean areas (Abelló et al. 2002, García-Rodríguez et al. 2011, Deval et al. 2017, among others) and adjacent Atlantic waters (López de la Rosa 1997, Muñoz et al. 2012, Castillo et al. 2014). A similar segregation has also been detected for cephalopods (Quetglas et al. 2000, González and Sánchez 2002, Ciércoles et al. 2018), fish (Gouraguine et al. 2011, García-Ruiz et al. 2015, Ramírez-Amaro et al. 2015) and megabenthic fauna (Abad et al. 2007), highlighting the importance of depth as a structuring factor in marine communities of the Mediterranean Sea.

The continental shelf assemblages (IS and OS) showed a high level of overlap between each other because they shared the most dominant species, but the two slope assemblages (US and MS) displayed marked differences and different dominant species. These greater differences in the slope assemblages could be attributed to some hydrological differences between the two depth strata (Abelló et al. 1988). Although a thermally stable environment can be assumed below 200 m in the Mediterranean Sea (Hopkins 1985), small changes in some environmental parameters (such as salinity) must be present at the species level, affecting their distribution and abundance, as shown in other studies (García-Rodríguez et al. 2011, Keller et al. 2017, Quattrocchi et al. 2020). Additionally, the dominant decapod species in each detected assemblage were similar to those found in the same area with the same sampling gear (Abelló et al. 2002). This similarity could be related to the stability of the demersal structures over time, as shown by other authors in the Mediterranean Sea (Gaertner et al. 2005).

The decapod assemblages displayed geographical differences in composition and structure between the sectors of the study area. Although sampling effort was different between sectors, the annual repeat of the same sampling stations and the long temporal database reinforce the idea that the differences between these sectors are mainly driven by differences in the composition and structure of decapod assemblages between sectors. The main differences were due to the pandalid shrimp P. heterocarpus, which was dominant in the OS assemblage of the northern Alboran Sea and Gulf of Vera sectors, but not at Alboran Island. Also, P. heterocarpus was far less abundant in the Alboran Island sector, and seemed to have a deeper and more restricted bathymetric range (328 to 566 m depth). This may be related to the influence of the nutrient-rich coastal upwellings present along the northern Alboran Sea sector, which allow the species to reach shallower waters (Carbonell et al. 2003). In contrast, P. edwardsii was more abundant in the US assemblage of the Gulf of Vera than in the same assemblage of other sectors, in agreement with García-Rodríguez et al. (2000). Finally, a higher abundance of A. antennatus was detected in the MS assemblages of the Alboran Island and Gulf of Vera, probably because of the greater presence of submarine canyons in both sectors, which are a preferred habitat for this species (Martínez-Baños 1997). In fact, the presence of populations of A. antennatus attracts a large deep-water trawling fishery to Alboran Island, the northeastern part of the Alboran Sea and the Gulf of Vera (Sardà et al. 2004; García-Rodríguez 2003, Fernandez-Arcaya et al. 2019).

Although no sampling could be carried out in the present study at depths shallower than 100 m around Alboran Island (owing to the presence of a marine reserve), when we compared the fauna from shallow depths around the island (<100 m depth) with those in other geographical sectors found in other studies based on beam-trawl samples (Gofas et al. 2014), we observed that the hermit crab D. arrosor is the only dominant species shared by all three sectors. The rest of the dominants decapods in the shallow sedimentary habitats around the island, such as Scyllarus pygmaeus, the hermit crab Paguristes eremita and several species of crabs of the genus Pisa, were absent or displayed low abundances on the continental shelf of the northern Alboran Sea. These differences in the dominant species are probably due to the fact that the sedimentary habitats in the island are mainly composed of bioclastic and coarse sands and rhodoliths, which are less common on the continental shelf, where muddier bottoms generally occur (Gofas et al. 2014).

In the present study, the composition of the decapod assemblages of the shallowest strata (shelf) of the Alboran Sea is similar to those of other parts of the Mediterranean Sea and adjacent Atlantic waters. Similar dominant species (but with a different percentage of contribution) were found for the shelf (IS, OS) and US assemblages of the Alboran Sean in comparison with other Mediterranean areas (Abelló et al. 1988, 2002, García-Rodríguez et al. 2011, Deval et al. 2017). One exception was detected in those of the Italian margin of the Tyrrhenian sea, where D. arrosor was scarce or not recorded at all (Fanelli et al. 2007). Moreover, it is worth mentioning that P. longirostris showed a different bathymetric range between the Alboran Sea and the Tyrrhenian Sea, being more abundant in shallower waters characterizing the OS assemblages (<200 m) in the latter (Fanelli et al. 2007). Some authors have suggested that the main drivers of high abundances of P. longirostris are the increase in water temperature (Quattrocchi et al. 2020), the interaction between wind and current circulations (Ligas et al. 2011), high salinity (Benchoucha et al. 2008) and high primary productivity (Colloca et al. 2004). Regarding the deepest assemblages, differences were detected between those of the northern Alboran Sea and others parts of the Mediterranean and adjacent Atlantic water. The dominant species in the deepest assemblage (MS) of the present study were mostly different from those of the central and eastern Mediterranean Sea, where A. antennatus, N. norvegicus, Aristaeomorpha foliacea and P. martia are dominant (Fanelli et al. 2007, Deval et al. 2017, Fernandez-Arcaya et al. 2019). This fact contributes to the faunistic differentiation of the Alboran Sea, which is important for fisheries management. The rare presence of A. foliacea in the Alboran Sea, with only one individual captured in the present study, is noteworthy and similar to the observations made in the southern Alboran Sea, where just one record was observed some decades ago (Maurin 1962). The small populations of A. foliacea in the study area could be related to environmental drivers because its abundance declines with temperature and salinity rises, as detected in the western Mediterranean basin (Balearic Basin) (Cartes et al. 2011, 2014). The continental slope decapod assemblages of the northern Alboran Sea are also well differentiated from those in adjacent areas of the Atlantic Ocean. Only one decapod, S. membranacea, was shared with the slope assemblages of southern Spain (Gulf of Cádiz), which are dominated by N. norvegicus, Philocheras echinulatus, S. membranacea, Processa canaliculata and P. sivado (López de la Rosa 1997). In the Atlantic waters of northwest Africa, the slope assemblages (200-500 m depth) displayed a lower contribution of P. longirostris and P. heterocarpus than those found in the present study (Muñoz et al. 2012). Moreover, two dominant species, P. narval and D. arrosor, were shown to share the US assemblages and another two (P. martia and Polycheles typhlops) share the MS assemblage in the Canary Islands (González-Pajuelo et al. 2006).

Regarding ecological indices, GAM analyses showed clear depth trends for the four ecological indices: abundance, species richness, Shannon-Wiener diversity and Pielou’s evenness. Decapod abundance increased with depth, peaking around 200 m, which is related to the high abundance of some gregarious dominant species, such as P. heterocarpus. Species richness also increased with depth, peaking at 400 to 500 m, thus making the US assemblage a transition zone with the coexistence of characteristic species from adjacent shallow and deep assemblages, in agreement with the findings for other faunal groups in the northern Alboran Sea, such as molluscs (Ciércoles et al. 2018). However, Shannon-Wiener diversity and Pielou’s evenness decreased with depth, with a minimum at 200 m, but increased afterwards. These results could be related to the fact that the OS assemblage was characterized by few, very abundant species, while the slope assemblages showed a higher number of species which were less dominant. Similar results were found by Mateo-Ramírez et al. (2015) in the northern Alboran Sea, by Cartes and Sardà (1992) and Fanelli et al. (2007) in the western and Central Mediterranean Sea, and by López de la Rosa (1997) and Muñoz et al. (2012) in Atlantic waters. The increased environmental stability of the deep areas is likely to be a potential explanation for a high diversity (Sanders and Hessler 1969, Rex 1973), which may allow the development of a more mature and complex assemblage, as indicated by Abelló et al. (1988).

The four GAM models also revealed geographical effects (a combination of longitude and latitude), with a greater abundance, species richness and Shannon diversity index in the westernmost part of the Alboran Sea, probably related to the high productivity associated with the major nutrient-rich coastal upwellings in this area (Rubín et al. 1997, Sarhan et al. 2000), the nutrient-rich water transport through the strait of Gibraltar, and the cyclonic vorticity that accumulates nutrients and produces permanent fertilization in the westernmost part of the Alboran Sea (Skliris and Beckers 2009, Vargas-Yáñez et al. 2019). Similarly, a high diversity was detected in the easternmost part, which is a transition area with the coexistence of Mediterranean and Atlantic species. Slight temporal patterns were found for abundance and species richness. However, the present study showed the absence of consistent temporal trends and the presence of annual oscillations of the decapod assemblages in the study area. Temporary changes in the abundance of benthic and demersal species could be the result of biological processes, such as recruitment, mortality and predation, in addition to variations in the fishing impact (increased or decreased fishing effort) in some commercial decapod species or even in commercial species feeding on decapods (Snelgrove and Butman 1994, Ólafsson et al. 1994).

The results of the present study improve knowledge of the decapod assemblages inhabiting the trawlable bottoms of a specific part of the European margin and the western Mediterranean Sea. The study area corresponds to the geographical subareas GSA01 and GSA02 established by the General Fisheries Commission for the Mediterranean (GFCM) for fishing management. Faunistic studies are an essential tool in managing fisheries, especially because both fisheries management and conservation policies apply to an ecosystem-based approach (Pikitch et al. 2004, Bellido et al. 2011). Moreover, this study provides a baseline for characterizing specific components (decapod crustaceans in this case) of benthic and demersal communities on sedimentary habitats under the framework of the Marine Strategy Framework Directive (MSFD, 2008/56/EC). This is especially important in areas with a high marine diversity that are regarded as self-standing ecoregions, such as the Alboran Sea (Abelló et al. 2002, Spalding et al. 2007, Coll et al. 2010).