The composition and structure of circalittoral and bathyal soft bottom echinoderm assemblages along the northern Alboran Sea, including the Alboran Ridge, have been studied in relation to sediment and near-bottom water masses characteristics. Samples were taken using a beam-trawl (50 sampling stations from 40 to 774 m depth) during 2014, 2015 and 2016 MEDITS surveys (30 to 800 m depth range). A total of 99,390 individuals were collected and 53 taxa of the five classes of echinoderms were found, with ophiuroids and holothuroids being the most diverse and abundant taxa. A high abundance of Ophiocten abyssicolum was detected in some samples, probably related to the high productivity of the Alboran Sea. Multivariate analyses using echinoderm abundance data indicated the presence of six assemblages in relation to depth, salinity and sediment type. The geographic location of the Alboran Sea, its singular oceanographic and hydrological conditions and diverse sediment types promoting habitat heterogeneity, as well as the wide depth range may promote the high echinoderm biodiversity reported in the present study. The information provided for this important benthic component improves the current knowledge on the biodiversity of circalittoral and bathyal soft bottoms of the northern Alboran Sea, and could be used as a baseline for further monitoring programs under the framework of different directives such as the Marine Strategy Framework Directive.

Centrostephanus longispinus (Philippi, 1845) is a sea urchin widely distributed across the tropical and temperate Atlantic Ocean (including the Caribbean) and Mediterranean Sea. Although it is present along the Alboran Sea coastline (Western Mediterranean), it is generally considered rare and is listed under conservation and protection lists and conventions due to fragmented populations threatened by seabed degradation. This study provides the first density and size distribution data for this echinoid in the circalittoral and bathyal bottoms of the Alboran Sea, aiming to relate its presence to seabed features, environmental variables, and human pressures. A series of 131 (62 ROV and 69 TASIFE transects) underwater image transects were collected during CIRCAESAL expeditions (2021, 2023, 2024) using a ROV and a photogrammetric sledge from infralittoral to bathyal bottoms (17–856 m depth). Images were processed with OFOP software to quantify and classify individuals by size classes, depth, substrate, seafloor roughness, micro-habitat, and coverage of key benthic structuring species. A total of 524 individuals of C. longispinus were detected in 13 transects, with the highest densities recorded at 48–100 m depths in rough, rocky substrates with crevices and a moderate to low coverage of key benthic structuring species. Differences in habitat use were also observed across depth strata: individuals in shallower zones tend to remain hidden within crevices and structurally complex substrates, displaying a more cryptic behaviour, whereas those in deeper strata rely less on refuge and occupy less complex habitats. The largest aggregations occurred near the Guadiaro Canyon, outside the “Estrecho Oriental” Special Area of Conservation (SAC), suggesting this area may serve as a population reservoir deserving conservation. Despite these findings, ecological knowledge of C. longispinus remains limited, and future studies should improve the knowledge gaps, particularly in the eastern and southern Alboran Sea.

Seafloor elevations (e.g., seamounts, banks, mounds) are essential underwater features for supporting and maintaining global marine biodiversity. Such geomorphological features might be of particular relevance to preserve biodiversity in the Mediterranean Sea, where a high number of anthropogenic impacts threaten deep-sea ecosystems. Using imagery from a remotely operated vehicle (ROV), deep-sea megabenthic and demersal fish assemblages were identified and quantified in some of the less studied areas of the Seco de los Olivos Bank (also known as Chella Bank), a seafloor elevation considered a hotspot of biodiversity in the Alboran Sea, Western Mediterranean. The 62 taxa identified were grouped into five well-defined benthic and demersal assemblages, mainly influenced by substrate type and typified by massive sponges, cold-water corals, sea pens, and ray-finned fishes. Nine taxa were identified as indicators of vulnerable marine ecosystems (VMEs) and/or endangered species. The heterogeneous distribution of substrate types plays a key role in assemblage composition, with hard substrates enhancing biodiversity at the local scale. A wide variety of indicators of anthropogenic activities were found, including bottom trawling marks, remains of fishing gears, and other types of marine litter, whose presence seemed to reduce the diversity of the observed assemblages. The results presented here improve the still scarce quantitative assessments of deep-sea benthic ecosystems to implement effective management measures in the framework of the main European policies (e.g., Habitats Directive, Marine Strategy Framework Directive) to reach 2030 conservation goals.

The Alboran Sea is the westernmost sub-basin of the Mediterranean Sea, and it is connected to the Atlantic Ocean through the Strait of Gibraltar. The Alboran Ridge is located in the middle of the Alboran Sea and represents a hotspot of biodiversity in the Mediterranean Sea. Besides their critical importance, there are few studies on the communities and changes in biodiversity, and they mostly concentrate on infralittoral and circalittoral bottoms. In this work, the composition, structure and bathymetric and temporal changes of megafauna of the Alboran Ridge were examined. Samples were collected from MEDITS surveys carried out between 2012 and 2022 at depths ranging from 100 to 800 m. Analyses were performed separately for each of the taxonomic groups: osteichthyes, chondrichthyes, crustaceans, molluscs, echinoderms and “other groups”. There was no common spatial organization for each of the faunistic groups studied, although most of them displayed differences between the shelf and the slope. The continental shelf was characterized by the highest values of community metrics such as abundance, biomass, species richness and mean weight of species for all groups except for chondrichthyes and crustaceans. Decreasing trends of some community metrics were detected in some of the faunistic groups throughout the study period.

Since Tagoro volcano erupted in 2011, several impacts have been associated to the volcano formation process, some of which are still present to date. This chapter is a review of the marine environmental perturbations caused by Tagoro volcano as a new geological structure, but thoroughly onto the partly annihilated benthic and demersal pre-existing biota, and the colonizing dynamics during the recovery process. Shallow recent volcanic activity in the NE Atlantic is uncommon, thus, Tagoro provides a unique opportunity to study, from the very beginning, the evolution of the unusual shallow hydrothermal systems, and the establishment of new marine habitats and associated biota. Distinct habitat types, with different associated deposit products (volcaniclastic aprons, lava balloons, lava ponds, etc.), have been described, and a description of the colonizing biota has been made from the available published works as well as from underwater imagery and benthic dredge samples taken during several field expeditions (Vulcano0313, 1013, 0314 and Vulcana0417). Those habitats included hard (rocky) and mixed (loose) substrate habitats, but also extreme habitats with hydrothermal vents and bacterial mats, accompanied by significant physical and chemical anomalies. Habitat preference by the observed taxa among the volcanic edifice has been explored through nMDS analyses, and a comparative analysis with published data of the typical fauna of the region (La Restinga, Mar de Las Calmas, Marine Reserve, El Hierro Island), permitted to foresee the very first steps and direction of the recovery of the benthic and demersal communities. The impact caused by Tagoro onto the nearest littoral benthic communities, the ichthyofauna and the local fisheries have been described as well. Finally, some recommendations and further steps are given in order to adequately monitor the successional trend and environmental status of the benthic and demersal communities.

The arrival of a new invasive alga, Rugulopteryx okamurae, in the Strait of Gibraltar (SoG) in 2015 marked an unprecedented milestone in the North African and, later, in the European marine ecosystems. Nowadays, it is colonising vast infralittoral areas and significantly modifying some habitats and associated communities of the southern Iberian Peninsula. In recent expeditions, a high amount of free drifted thalli of this alga has been detected in different circalittoral and bathyal habitats of the northern SoG and the Alboran Sea. The present study combines quantitative data of this alga obtained with the use of a remotely operated vehicle (ROV) and a bottom otter trawl. The coverage–entanglement level of the drifted thalli on circalittoral and bathyal benthic invertebrates (e.g., not covering, covering only the basal part, covering one-third of the invertebrate, etc.) was also annotated from picture frames taken in locations with abundant drifted thalli. In underwater images, drifted thalli were mainly detected in circalittoral and bathyal bottoms of the northern SoG and the north-western Alboran Sea, between 50 to ca. 450 m depth. Nevertheless, abundant drifted thalli were also detected in bottom otter trawl samples from circalittoral bottoms of the north-central and north-eastern Alboran Sea. Small benthic organisms (e.g., encrusting sponges, hydrozoans, etc.) generally displayed low coverage–entanglement levels of drifted thalli. Nevertheless, large sessile and colonial benthic organisms with a complex three-dimensional morphology (e.g., gorgonians, colonial scleractinians) reached high levels of R. okamurae thalli entangled in different parts of their colonies. The drifted R. okamurae thalli entangled in these colonial suspension feeding organisms may hinder their feeding capability in the long term, resulting in habitat deterioration in the near future.

The shallow Tagoro submarine volcano monitoring represents a unique opportunity not only for improving our sparse understanding of submarine volcanic processes in specific scientific fields as physical and chemical oceanography or marine geology but also its interactions over the marine biology in one of the richest marine ecosystems in Europe. This chapter aims to summarize the most relevant physical–chemical, geological and biological changes that occurred in the marine ecosystem of El Hierro island, at the Marine Reserve Punta de La Restinga—El Mar de Las Calmas, due to the genesis of the new underwater volcano Tagoro (27º37′07″N–017º59′28″W) in October 2011. During the first six months of the eruption, extreme physical–chemical perturbations caused by this event, comprising thermal increase from up to + 18.8 °C, water acidification with a pH decrease of 2.9 units, deoxygenation to anoxic levels and extremely high metal enrichment among others, resulted in significant and dramatic alterations of the marine ecosystem. After March 2012, once the eruptive phase was finished, the new submarine volcano entered an active hydrothermal phase involving the release of heat with smaller but still significant and important thermal anomalies of up to + 2.55 °C around the craters, density decrease of − 1.43 kg m−3, pH decrease of − 1.25 units, and high concentrations of metals and inorganic nutrients similar to upwelling zones. These enrichments are still active up to date, producing clear signs of marine recovery not only in the benthonic strata but also in the whole water column compared with pre-eruptive data. Since its eruption ten years ago, an unprecedented monitoring effort has turned into the longest and most complete multidisciplinary time-series for the study of a shallow submarine volcano, with the realization of 31 oceanographic multidisciplinary expeditions that systematically measure more than 40 different physical–chemical and biological variables. All this information and the results obtained during the evolution of the process could serve as a baseline for better understanding future or similar submarine eruptions worldwide.

Remains of molluscs were collected from the seafloor on the north-eastern margin of the Gulf of Cadiz, between 300 and 1000 m water depth, using different sampling methods (e.g. dredging, trawling and box-coring), during several deep-sea expeditions. Samples contained a suite of species which nowadays mostly occur northwards of the English Channel, together with other widespread species. Species now locally extinct in the Gulf of Cadiz and restricted to northern latitudes, which unequivocally indicate a faunal shift, include the gastropods Buccinum undatum, Colus gracilis, Liomesus ovum and Neptunea antiqua, the bivalves Arctica islandica, Chlamys islandica, Modiolus modiolus, Mya truncata and Nuculana pernula and the scaphopod Antalis entalis. These species represent "Boreal Guests" of marked palaeoclimatic significance, some of which are reported for the first time in the Gulf of Cadiz. The boreal species collected were mostly large (>5 cm) whereas smaller boreal species were extremely scarce, probably winnowed away by strong bottom currents. The pteropod Limacina retroversa, at present restricted to water masses northwards of the Iberian Peninsula but widespread in Mediterranean sediments of the last glaciation, was also recorded. Accelerator mass spectrometry (AMS) 14C dates obtained from nine specimens of molluscs ranged between 26.1 and 14.6 kyr B.P., thus confirming their attribution to a last glacial assemblage. The abundance of these molluscan remains in the present Mediterranean Outflow Water pathway could be explained if this outflow was reduced in intensity or more likely shifted to a deeper level, leaving the upper slope in contact with suitable Atlantic intermediate waters. The findings of Boreal Guests in the Gulf of Cadiz document the continuity of the faunal shift which is wellknown in the Mediterranean basin. Species still living in the Gulf of Cadiz and the Alboran Sea nevertheless account for 84.6% of specimens among the larger species. & COPY; 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Molluscs of Chella Bank and its surroundings were studied from 21 samples collected with a van Veen grab in the depth range 95-729 m. A total of 299 taxa were identified (77 live-taken), thus increasing by more than 95% the species of molluscs reported in the recently declared site of community importance “Sur de Almería-Seco de los Olivos”. Two of the species are new records to Spanish waters and one to the Alboran Sea. The high species richness observed could be related to the location, the hydrological characteristics and the topographical heterogeneity of the area within the Alboran Sea. Four significant groups of samples were discriminated through multivariate analysis of quantitative data of live-taken molluscs: (I) bathyal muddy bottoms with buried rhodoliths; (II) bathyal muddy bottoms with coral rubble; (III) bathyal hemipelagic muddy bottoms and (IV) bathyal sandy bottoms. Molluscs were more diverse on coral framework bottoms than on sedimentary bottoms around Chella Bank. Most of the live-taken species are widely distributed along the Atlantic and Mediterranean Sea, and a few are strictly Mediterranean. The most striking feature was the occurrence of two species with planktotrophic larval development for which Chella Bank is the sole recorded locality in the Mediterranean (Episcomitra angelesae and Mitrella templadoi) and which elsewhere extremely rare (Mathilda spp.).

The invasive seaweed Rugulopteryx okamurae has recently arrived in Europe from the western Pacific. Its explosive spread on coastal areas of the Gulf of Cadiz (GoC), Strait of Gibraltar and Alboran Sea is spoiling native coastal ecosystems and inflicting heavy losses on ecosystem services. We discovered for the first time large amounts (up to 17 g m(-2)) of detached R. okamurae thalli on deep-sea bottoms of the GoC that are being dragged from the Strait of Gibraltar shores into the NE Atlantic by the Mediterranean Outflow Water (MOW). Laboratory experiments revealed that collected unattached macroalgae from deep-sea locations were alive and healthy and maintained intact photosynthetic capacity after long dark periods, suggesting a tremendous resilience and invasive potential. Given the rapid transport of healthy thalli by the MOW and massive accumulation of them in the GoC basin, R. okamurae could represent a major threat to NE Atlantic ecosystems, affecting not only coastal but also deep-sea habitats.

Meadows of the seagrass Posidonia oceanica inhabit most infralittoral bottoms of the Mediterranean Sea and are considered one of the main climax stages of the infralittoral environment. This seagrass has its western distributional limit along the coast of the Alboran Sea. Taking into account the decline of P. oceanica meadows and the global scenario of ocean warming, it becomes essential to know the structure, temporal dynamics, sexual reproduction and conservation status of this seagrass, across its geographical distribution, including the distribution boundaries where the meadows withstand limiting environmental conditions. In the present work, we studied the structure, phenology and flowering events of four P. oceanica meadows located in the northwestern Alboran Sea (close to the Strait of Gibraltar). Results indicate a decreasing trend of patch size, bathymetric range and number of leaves per shoot towards the Strait (and the Atlantic Ocean), as well as an increasing trend of shoot density and leaf height. Phenological parameters of the northwestern Alboran Sea P. oceanica meadows presented temporal dynamics similar to meadows from other locations within the biogeographical distribution of this seagrass, with similar or even less annual variability in the former. Although most of the studied P. oceanica meadows seem to present a good health status (BiPo index ~0.6) with high shoot densities and some flowering events, some of them showed evidence of regression.

Carbonate mounds clustering in three fields were characterized on the upper continental slope of the northern Alboran Sea by means of a detailed analysis of the morphosedimentary and structural features using high-resolution bathymetry and parametric profiles. The contemporary and past benthic and demersal species were studied using ROV underwater imagery and some samples. A total of 325 mounds, with heights between 1 and 18 m, and 204 buried mounds were detected between 155 to 401 m water depth. Transparent facies characterize the mounds, which root on at least six erosive surfaces, indicating different growth stages. At present, these mounds are covered with soft sediments and typical bathyal sedimentary habitat-forming species, such as sea-pens, cerianthids and sabellid polychaetes. Nevertheless, remains of colonial scleractinians, rhodoliths and bivalves were detected and their role as potential mound-forming species is discussed. We hypothesized that the formation of these mounds could be related to favorable climatic conditions for cold-water corals, possibly during the late Pleistocene. The occurrence on top of some mounds of abundant rhodoliths suggests that some mounds were in the photic zone during minimum sea level and boreal guest fauna (e.g., Modiolus modiolus), which declined in the western Mediterranean after the Termination 1a of the Last Glacial (Late Pleistocene).

Multi-species bottom trawl fisheries are one of the human activities with a great impact on the benthic habitats and their associated biota.This study provides estimates of the bottom trawling activity (effort), catches and landings of the main commercial species as well as an estimation of the total revenue (TR) generated inside a mud volcano field located in the Spanish margin of the Gulf of Cádiz, during a time series from 2007 to 2012.To date, no studies have been carried out to analyse the temporal evolution of bottom trawling activity and TR in a mud volcano fied, or the economic consequences of possible potential bottom trawling regulation of certain sectors harbouring vulnerable and/or threatened habitats.In this study, Vessel Monitoring System data, logbooks and sales slips were used.The spatial distribution of the bottom trawling activity, catches and TR were related to the seafloor morphology and specific bottom types of the mud volcano field.During the time series, a high bottom trawling activity and associated catches was detected in flat sandy and muddy bottoms, including the Anastasya sector and between the Guadalquivir and Cádiz Diapiric Ridges.Low bottom trawling activity and catches were detected in the deepest areas but also in areas with hard and detritic bottoms such as Gazul and Chica sectors as well as in the Diapiric Ridges.A similar spatial pattern was detected for the TR asociated with these bottom trawling fisheries.An increase in bottom trawling activity was detected during the time series, mainly at the end, probably for increasing the TR and mantaining the economic profit due to the instability and increases in fuel prices and offset the increased costs.Based on the obtained information, bottom trawling regulations should be implemented in certain sectors harboring singular and/or threatened habitats and species.In some of these sectors, a low TR from bottom trawling was detected and, bottom trawling regulation may potentially have a low socioeconomic impact.This specific bottom trawling regulation could provide a sustainable balance between bottom trawling activities and habitat conservation in this mud volcano field according to the aims of the Habitats Directive (92/43/EEC) and the European Marine Strategy Framework Directive (2008/56/EEC).

This article offers an overview of the main sedimentary systems defining the geomorphology of deep sea environments from low to high latitudes. Mass-transport deposits, turbidite systems, contourites, volcaniclastic aprons, glacial trough mouth systems, carbonate mounds and other bathyal systems, such as pelagites, hemipelagites, mid-ocean channels and polymetallic mineral deposits, are presented with special attention to their morphology, sediments, processes and controlling factors. The integration of the main systems on the continental margins and adjacent abyssal plains in the North Atlantic and westernmost Mediterranean allows to characterize different sedimentation models.

This study focused on the assessment and quantification of discards generated by clam fisheries along the northern Alboran Sea (western Mediterranean). Discard samples (n = 278) were collected throughout one year on board nine commercial vessels. A total of 129 species were identified, mostly represented by molluscs (72 spp.), arthropods (20 spp.) and echinoderms (12 spp.). Molluscs dominated in terms of abundance (67.5%) and biomass (94.2%). The superfamily Paguroidea (i.e. hermit crabs), together with undersized target individuals, were the most abundant taxa. The abundance and biomass of discards displayed significant maximum values in winter, which could be partly related to biotic factors including population dynamics of some dominant species. Multivariate analyses indicated the presence of different assemblages related to the targeted bivalve species, reflecting the transition between a fine surface-sands biocoenosis exposed to wave action and a well-sorted fine sands biocoenosis below 5 m depth. Analysis of biogeographical affinities showed that most discarded species (73.2%) have an extensive Atlantic range, whereas 7.1% have a restricted distribution within the Mediterranean. The presence of subtropical species highlights the uniqueness of this area (the Atlantic–Mediterranean transition) in European seas. The usefulness of discard analysis for biodiversity assessment is discussed.

In this work, we integrate five case studies harboring vulnerable deep-sea benthic habitats in different geological settings from mid latitude NE Atlantic Ocean (24–42° N). Data and images of specific deep-sea habitats were acquired with Remoted Operated Vehicle (ROV) sensors (temperature, salinity, potential density, O2, CO2, and CH4). Besides documenting some key vulnerable deep-sea habitats, this study shows that the distribution of some deep-sea coral aggregations (including scleractinians, gorgonians, and antipatharians), deep-sea sponge aggregations and other deep-sea habitats are influenced by water masses’ properties. Our data support that the distribution of scleractinian reefs and aggregations of other deep-sea corals, from subtropical to north Atlantic could be dependent of the latitudinal extents of the Antarctic Intermediate Waters (AAIW) and the Mediterranean Outflow Waters (MOW). Otherwise, the distribution of some vulnerable deep-sea habitats is influenced, at the local scale, by active hydrocarbon seeps (Gulf of Cádiz) and hydrothermal vents (El Hierro, Canary Island). The co-occurrence of deep-sea corals and chemosynthesis-based communities has been identified in methane seeps of the Gulf of Cádiz. Extensive beds of living deep-sea mussels (Bathymodiolus mauritanicus) and other chemosymbiotic bivalves occur closely to deep-sea coral aggregations (e.g., gorgonians, black corals) that colonize methane-derived authigenic carbonates.

Lists of species include in conservation agreements or relevant by their singularity present in MPA and KBA of Alboran Sea and adjacent areas. This dataset is part of the Chapter 25 Marine Protected Areas and Key Biodiversity Areas of the Alboran Sea and Adjacent Areas. In J. C. Baéz et al. (eds.), Alboran Sea - Ecosystems and Marine Resources, https://doi.org/10.1007/978-3-030-65516-7_25

DATA REPORT article Front. Mar. Sci., 23 March 2021Sec. Deep-Sea Environments and Ecology https://doi.org/10.3389/fmars.2021.637078

Benthic habitats, as well as their associated biota, have been studied in the Alboran Sea since the nineteenth century, with a very significant increase of knowledge in the last five decades. The geographical location of the Alboran Sea between three different biogeographical regions, the complex oceanography and the heterogeneous seafloor promote the coexistence of a wide diversity of habitat-forming species and, therefore, of habitat types. Some of these habitats host very complex communities in comparison to similar ones that are located northwards in the Atlantic Ocean or eastwards in the Mediterranean Sea. Moreover, some of those habitats are considered to be threatened or are experiencing very strong declines during this last decade and are probably extinct nowadays (e.g. Zostera marina beds). General knowledge of the associated biota is larger for those habitats of shallow waters in comparison to those of the deep sea and for those located in the northern sector in relation to those of the southern sector of the Alboran Sea. In most habitats, only some components of the associated biota have been studied, and there is a general lack of information for specific groups (e.g. meiofauna, Platyhelminthes, etc.).