Threats to macroalgal coralligenous assemblages in the Mediterranean Sea

Investigating the outcomes of a threatened gorgonian in situ transplantation: Survival and microbiome diversity in Paramuricea clavata (Risso, 1827)

Gorgonian octocorals are threatened by global and local stressors that can act synergistically to affect their health. In recent years, mass mortality events triggered by marine heatwaves have caused demographic declines in Mediterranean gorgonian populations that may lead to their collapse. Potential changes in microbiome composition under stressful conditions may further increase the susceptibility of the gorgonian holobiont to disease. Given the low recovery capacity of gorgonians, restoration approaches using transplantation are becoming an increasingly attractive option to counteract their decline. Here, we compared the survival and microbiome diversity of Paramuricea clavata colonies transplanted to sites differing in depth and local environmental conditions. Gorgonians sampled at a greater depth than the transplantation site were more likely to suffer necrosis after 1 year of monitoring. Gorgonian transplantation into environments disturbed by an anthropogenic source of pollution resulted in an imbalance of the microbiome with potential consequences on the success of restoration initiatives.

First baseline assessment of seafloor litter on Italian coralligenous assemblages (Mediterranean Sea) in accordance with the European Marine Strategy Framework Directive

A monitoring programme for coralligenous assemblages was carried out in Italy for implementation of the Marine Strategy Framework Directive (MSFD), aiming to characterise the coralligenous' environmental status and litter distribution. A standardised monitoring protocol, based on ROV-imaging, has been applied; this paper analyses the data obtained during the first MSFD cycle (2015-2019). Ninety-five areas were monitored at depths ranging between 14 and 199 m in the three subregions bordering Italy. Overall, 4316 macrolitter items were recorded. The median litter density was 2 items 100 m^-2 (range: 0-120 items 100 m^-2), but significant subregional differences emerged in litter quantities and composition. Fishery-related litter (mainly lines and ropes) was the most common type (86 %), affecting vulnerable coralligenous reefs. A first baseline is proposed for assessing achievement toward Good Environmental Status and evaluation of the efficacy of the Italian MSFD measures aiming to eliminate the negative effects of marine litter on vulnerable reefs.

Warming may increase the vulnerability of calcareous algae to bioinvasions

Understanding the interactions between various stressors, and the resulting cumulative impacts they exert, is essential in order to predict the potential resilience of marine habitats to climate change. Crustose coralline algae (CCA) are a major calcifying component of marine habitats, from tropical to polar oceans, and play a central role as ecosystem engineers in many rocky reefs. These species are increasingly threatened by the stress of climate change. However, the effects of other stressors linked to global change, such as invasive species, have scarcely been addressed. We have studied the interactive effects of invasive algae and global warming on CCA, combining observational and experimental approaches. CCA sensitivity to invasive algae is heightened when they are concurrently exposed to elevated seawater temperature, and the interaction between these two stressors triggers drastic synergistic effects on CCA. The reduction and eventual disappearance of these "ecosystem foundation species" may undermine ecological functioning, leading to the disappearance and/or fragmentation of the communities associated with them.

Diversity Loss in Coralligenous Structuring Species Impacted by Fishing Gear and Marine Litter

Coralligenous structuring species (CSS) form a group of marine megabenthic species with an engineering capacity. Since they are highly vulnerable to anthropogenic activities, they have been selected for the Marine Strategy Framework Directive (MSFD) monitoring programs. The pressure and impact of fishing gear and marine litter on these species were evaluated through the image analysis of 54 remotely operated vehicle (ROV) routes along the Campania coasts (Tyrrhenian Sea, Italy). CSS density was calculated as the number of colonies/100 m2. Anthropogenic pressure was estimated as the frequency of frames showing longline, nets, other gear, plastic objects, metal objects, and other litter; while the impact was expressed as the frequency showing necrosis/epibiosis, broken/upturned and covered/entangled colonies. Cnidaria dominate in the Napoli, Campanella and Capri areas, while Bryozoa dominate in Cilento N and Cilento S areas. Campanella and Capri appeared to be the least heterogeneous despite their higher CSS densities, which was possibly related to the dominance of a few species. These areas were the most affected by showing the highest numbers of fishing gear (longlines) and marine litter (metal objects) recorded, amongst which longlines are the most abundant. In addition, these fishing areas are either close to a large urban center or located along popular touristic routes. In all the areas, colonies with necrosis/epibiosis (CNE) impact are present with low-moderate values, while the category gears covering/entangling (GCE) impact prevails in the Campanella and Capri areas, and this is strictly connected to the high presence of fishing gear.

Double Trouble: Synergy between Habitat Loss and the Spread of the Alien Species Caulerpa cylindracea (Sonder) in Three Mediterranean Habitats

The role of habitat degradation on the spread of the alien green alga Caulerpa cylindracea is reported here by comparing observations achieved through a multi-year assessment on three Mediterraneans habitats, namely Posidonia oceanica meadows, Phyllophora crispa turf, and coralligenous reefs. Due to the peculiarity of the study site, both natural-reference and impacted conditions were investigated. C. cylindracea occurred in all the studied habitats under impacted conditions. High susceptibility to the invasion characterized impacted P. oceanica, where Caulerpa cover reached 70.0% in summer months. C. cylindracea cover did not differ significantly among conditions in P. crispa turf, where values never exceeded 5.0%. Conversely, the invasive green algae was low in abundance and patchily distributed in coralligenous reefs. Our results confirmed that habitat loss enhances the spread of C. cylindracea, although with different magnitudes among habitats. Dead matte areas of P. oceanica represented the most vulnerable habitat among those analyzed, whereas coralligenous reefs were less susceptible to the invasion under both the studied conditions.

Extreme events and conservation of subtidal habitats: Effects of a rainfall flood on coralligenous reefs

The increase of the intensity and frequency of rainfall-dominated flood is considered a main effect of climate change. The present study evaluated the effect of a rainfall flood event on coralligenous reefs. The flooded site was compared to three control sites using a Before/After-Control/Impact (BACI) design. Sites were sampled using the STAR (STAndaRdized coralligenous evaluation procedure) approach and three ecological indices (ESCA, COARSE and ISLA) were calculated. At the disturbed site the number of species per sample, beta diversity, sensitivity levels of assemblages and the values of the three indices were lower after the flooding event, while the same variables did not decrease at the control sites. Algal turf and Dictyotales increased at the disturbed sites after the flood event, while Udoteaceae, erect sponges, bryozoans and Corallium rubrum decreased. This study provides evidence for identifying floods as a further cause of degradation for the coralligenous reef assemblages.

Ecological Status of Coralligenous Macroalgal Assemblages in the Marine Protected Area (MPA) Isole Ciclopi (Ionian Sea)

The coralligenous habitat represents one of the most important hotspots of Mediterranean biodiversity. However, along the Ionian coast of Sicily (Italy) the coralligenous macroalgal assemblages have always been poorly studied. The present study was carried out in the coralligenous habitat of the Marine Protected Area (MPA) Isole Ciclopi, located along the central-eastern coast of Sicily. Previously, only a few floristic studies, including some data on the coralligenous flora, were conducted within this MPA in the 1970s and 2001. Therefore, the present study aimed to gain an updated knowledge on the coralligenous flora and to compare the current data with data derived from the previous floristic studies, to observe if in the last 50 years environmental changes occurred and to monitor the effectiveness of the MPA in protecting this habitat. In particular, the coralligenous flora of the MPA was analyzed through remotely operated vehicles (ROV) surveys and destructive samples. ROV surveys allow us to observe that the coralligenous assemblages of the MPA are well-structured, especially regarding the encrusting Rhodophyta, which showed the highest percent cover among the main morphological groups/taxa. Through the sample analysis in the laboratory, a total of 92 taxa has been recorded. Comparing the floristic lists of the present research and the past studies, an increase of local biodiversity was highlighted. Nevertheless, an increment of Non-Indigenous Species (NIS), warm-water species, and Rhodophyta with wide ecological valence was also noted. The main causes of these variations in the coralligenous flora have been traced back to reduced water transparency, maybe due to sedimentation, and a rise in the seawater temperature. Therefore, although the coralligenous assemblages of the MPA Isole Ciclopi appear to be well-structured, future studies will be necessary to continue monitoring this habitat to evaluate whether the MPA is effective in safeguarding this hotspot of biodiversity.

Assessment of Marine Litter in the Coralligenous Habitat of a Marine Protected Area along the Ionian Coast of Sicily (Central Mediterranean)

In the Mediterranean Sea, the coralligenous is an extremely important habitat for its biodiversity and role in carbon sequestration. However, coralligenous biocenosis is sensitive to many anthropogenic impacts, among which one of the major threats is the marine litter. The aim of the present work was to evaluate the marine litter present in the coralligenous habitat of the Marine Protected Area (MPA) Isole Ciclopi (Sicily, Italy). Through the analysis of frames obtained by Remotely Operated Vehicles (ROV) videos, data on the type of marine litter, grade of organisms’ coverage, litter-organism interactions, and seabed coverage of items were gained. Through ROV surveys, a low number of marine litter items was observed and most of them were fishing gears. On the found items there was a high degree of organisms’ coverage, which suggests that probably the marine litter was abandoned or lost since a long time. Overall, it was observed that in recent years the fishing activity in the MPA has not affected the coralligenous habitat. The present study represents a baseline for future monitoring programmes, which will be useful to develop protection measures and sustainable fishing, in order to preserve the coralligenous habitat of the MPA.

Characterization of North-Western Mediterranean coralligenous assemblages by video surveys and evaluation of their structural complexity

Thanks to several European directives and conventions there is a general increase of awareness regarding the key ecological role of coralligenous habitats in the Mediterranean Sea, addressing several research projects to standardize protocols for the description of its integrity. Here we surveyed 13 stations along the Italian coasts of the Western Mediterranean Sea, using video-transects technique, comparing the biological structure of coralligenous assemblages and testing the importance of their three-dimensional complexity as a proxy to define their health conditions. We considered the diversity of taxa, fishing impacts and the entity of damage on gorgonian's choenenchyme due to thermal stress, to evidence a gradient in the coralligenous health conditions. Here we developed a method to evaluate coralligenous complexity, selecting categories of taxa particularly sensitive to multiple stressors, named Structural Descriptors to describe the three-dimensional structure of the bioconcretions and to assess a unique Index of 3D - Structural Complexity.

Application of Hyperspectral Imaging to Underwater Habitat Mapping, Southern Adriatic Sea

Hyperspectral imagers enable the collection of high-resolution spectral images exploitable for the supervised classification of habitats and objects of interest (OOI). Although this is a well-established technology for the study of subaerial environments, Ecotone AS has developed an underwater hyperspectral imager (UHI) system to explore the properties of the seafloor. The aim of the project is to evaluate the potential of this instrument for mapping and monitoring benthic habitats in shallow and deep-water environments. For the first time, we tested this system at two sites in the Southern Adriatic Sea (Mediterranean Sea): the cold-water coral (CWC) habitat in the Bari Canyon and the Coralligenous habitat off Brindisi. We created a spectral library for each site, considering the different substrates and the main OOI reaching, where possible, the lower taxonomic rank. We applied the spectral angle mapper (SAM) supervised classification to map the areal extent of the Coralligenous and to recognize the major CWC habitat-formers. Despite some technical problems, the first results demonstrate the suitability of the UHI camera for habitat mapping and seabed monitoring, through the achievement of quantifiable and repeatable classifications.

Coralline algae on hard and soft substrata of a temperate mixed siliciclastic-carbonatic platform: Sensitive assemblages in the Zannone area (western Pontine Archipelago; Tyrrhenian Sea)

In the Mediterranean Sea, coralline algae assemblages (i.e. rhodolith beds and coralligenous assemblages) are considered biodiversity hotspots comparable to tropical reefs. However, information regarding their environmental distribution is still poor. In this view, relevant international actions have been adopted by the European Union to fill this gap. This work represents one of a few cases of predictive (fine-scale) habitats distribution map obtained through an integrated semi-automatic approach based on bathymetry, backscatter, seismic profiles, video, and sampling data. The used method has permitted the identification of nine morphological zones, four backscatter facies, and four benthic habitats distributed on the Zannone seafloor (western Pontine Archipelago; Tyrrhenian Sea). In particular, the finding of widespread sensitive habitats (i.e. coralligenous assemblages and rhodolith beds) reveals as the marine area off the western Pontine Archipelago (Tyrrhenian Sea) is highly suitable for their development (distance from the mainland, lack of river mouths), confirming the relevant ecological value of the Zannone area. Therefore, such information constitutes an update to the Mediterranean habitats distribution inventory, highlighting the need for the application of protection actions possibly targeted in the establishment of a Marine Protected Area.

Biodiversity loss in a Mediterranean ecosystem due to an extreme warming event unveils the role of an engineering gorgonian species

Stochastic perturbations can trigger major ecosystem shifts. Marine systems have been severely affected in recent years by mass mortality events related to positive thermal anomalies. Although the immediate effects in the species demography affected by mortality events are well known, information on the mid- to long-term effects at the community level is much less documented. Here, we show how an extreme warming event replaces a structurally complex habitat, dominated by long-lived species, by a simplified habitat (lower species diversity and richness) dominated by turf-forming species. On the basis of a study involving the experimental manipulation of the presence of the gorgonian Paramuricea clavata, we observed that its presence mitigated the effects of warming by maintaining the original assemblage dominated by macroinvertebrates and delaying the proliferation and spread of the invasive alga Caulerpa cylindracea. However, due to the increase of sediment and turf-forming species after the mortality event we hypothesize a further degradation of the whole assemblage as both factors decrease the recruitment of P.clavata, decrease the survival of encrusting coralligenous-dwelling macroinvertebrates and facilitate the spreading of C. cylindracea.

Mediterranean Lithophyllum stictiforme (Corallinales, Rhodophyta) is a genetically diverse species complex: implications for species circumscription, biogeography and conservation of coralligenous habitats

Lithophyllum species in the Mediterranean Sea function as algal bioconstructors, contributing to the formation of biogenic habitats such as coralligenous concretions. In such habitats, thalli of Lithophyllum, consisting of crusts or lamellae with entire or lobed margins, have been variously referred to as either one species, L. stictiforme, or two species, L. stictiforme and L. cabiochiae, in the recent literature. We investigated species diversity and phylogenetic relationships in these algae by sequencing three markers (psbA and rbcL genes, cox2,3 spacer), in conjunction with methods for algorithmic delimitation of species (ABGD and GMYC). Mediterranean subtidal Lithophyllum belong to a well-supported lineage, hereby called the L. stictiforme complex, which also includes two species described from the Atlantic, L. lobatum and L. searlesii. Our results indicate that the L. stictiforme complex consists of at least 13 species. Among the Mediterranean species, some are widely distributed and span most of the western and central Mediterranean, whereas others appear to be restricted to specific localities. These patterns are interpreted as possibly resulting from allopatric speciation events that took place during the Messinian Salinity Crisis and subsequent glacial periods. A partial rbcL sequence from the lectotype of L. stictiforme unambiguously indicates that this name applies to the most common subtidal Lithophyllum in the central Mediterranean. We agree with recent treatments that considered L. cabiochiae and L. stictiforme conspecific. The diversity of Lithophyllum in Mediterranean coralligenous habitats has been substantially underestimated, and future work on these and other Mediterranean corallines should use identifications based on DNA sequences.

Benthic mucilage blooms threaten coralligenous reefs

Mucilaginous aggregates produced by planktonic or benthic algae are considered ecological threats to marine systems. The study evaluated the effects of the spread of benthic mucilaginous aggregates on the structure of coralligenous assemblages. The assemblage and the quality of a site subjected to a benthic mucilage bloom were compared to those of two reference sites using a Before/After-Control/Impact (BACI) design. Results showed the α and β-diversity, ESCA and COARSE quality ecological indices and the cover of encrusting algae and bryozoans were lower at the impact site after the mucilage event than at the control sites and at the impact site before the mucilage event. An opposite pattern was observed for the necrosis of gorgonians and the cover of algal turf. This study describes for the first time the impacts of ephemeral mucilage blooms on the whole coralligenous assemblage, identifying a further threat of this habitat and the need of adequate monitoring programs.

Mediterranean Bioconstructions Along the Italian Coast

Marine bioconstructions are biodiversity-rich, three-dimensional biogenic structures, regulating key ecological functions of benthic ecosystems worldwide. Tropical coral reefs are outstanding for their beauty, diversity and complexity, but analogous types of bioconstructions are also present in temperate seas. The main bioconstructions in the Mediterranean Sea are represented by coralligenous formations, vermetid reefs, deep-sea cold-water corals, Lithophyllum byssoides trottoirs, coral banks formed by the shallow-water corals Cladocora caespitosa or Astroides calycularis, and sabellariid or serpulid worm reefs. Bioconstructions change the morphological and chemicophysical features of primary substrates and create new habitats for a large variety of organisms, playing pivotal roles in ecosystem functioning. In spite of their importance, Mediterranean bioconstructions have not received the same attention that tropical coral reefs have, and the knowledge of their biology, ecology and distribution is still fragmentary. All existing data about the spatial distribution of Italian bioconstructions have been collected, together with information about their growth patterns, dynamics and connectivity. The degradation of these habitats as a consequence of anthropogenic pressures (pollution, organic enrichment, fishery, coastal development, direct physical disturbance), climate change and the spread of invasive species was also investigated. The study of bioconstructions requires a holistic approach leading to a better understanding of their ecology and the application of more insightful management and conservation measures at basin scale, within ecologically coherent units based on connectivity: the cells of ecosystem functioning.

The dynamics of a Mediterranean coralligenous sponge assemblage at decennial and millennial temporal scales

This paper concerns the changes occurred over both decennial and millennial spans of time in a sponge assemblage present in coralligenous biogenic build-ups growing at 15 m depth in the Ligurian Sea (Western Mediterranean). The comparison of the sponge diversity after a time interval of about 40 years (1973–2014) showed a significant reduction in species richness (about 45%). This decrease affected mainly the massive/erect sponges, and in particular the subclass Keratosa, with a species loss of 67%, while the encrusting and cavity dwelling sponges lost the 36% and 50%, respectively. The boring sponges lost only one species (25%). This changing pattern suggested that the inner habitat of the bioconstructions was less affected by the variations of the environmental conditions or by the human pressures which, on the contrary, strongly affected the species living on the surface of the biogenic build-ups. Five cores extracted from the bioherms, dating back to 3500 YBP, allowed to analyse the siliceous spicules remained trapped in them in order to obtain taxonomic information. Changes at generic level in diversity and abundance were observed at 500/250-years intervals, ranging between 19 and 33 genera. The number of genera showed a sharp decrease since 3500–3000 to 3000–2500 YBP. After this period, the genera regularly increased until 1500–1250 YBP, from when they progressively decreased until 1000–500 YBP. Tentatively, these changes could be related to the different climatic periods that followed one another in the Mediterranean area within the considered time span. The recent depletion in sponge richness recorded in the Ligurian coralligenous can be considered relevant. In fact, the analysis of the spicules indicated that the sponges living in these coralligenous habitats remained enough stable during 3000 years, but could have lost a significant part of their biodiversity in the last decades, coinciding with a series of warming episodes.

The two facets of species sensitivity: Stress and disturbance on coralligenous assemblages in space and time

Marine coastal ecosystems are affected by a vast array of human-induced disturbances and stresses, which are often capable of overwhelming the effects of natural changes. Despite the conceptual and practical difficulty in differentiating between disturbance and stress, which are often used interchangeably, the two terms bear different ecological meanings. Both are external agents, but the former causes mortality or physical damage (subtraction of biomass), whereas the latter causes physiological alteration (reduction in productivity). Sensitivity of marine organisms may thus have a dual connotation, being influenced in different ways by disturbance and by stress following major environmental change. Coralligenous assemblages, which shape unique biogenic formations in the Mediterranean Sea, are considered highly sensitive to change. In this paper, we propose a method to differentiate between disturbance and stress to assess the ecological status of the coralligenous assemblages. Disturbance sensitivity level (DSL) and stress sensitivity level (SSL) of the sessile organisms thriving in the coralligenous assemblages were combined into the integrated sensitivity level of coralligenous assemblages (ISLA) index. Changes in the coralligenous status were assessed in space, along a gradient of stress (human-induced pressures) at several sites of the western Mediterranean, and in time, from a long-term series (1961-2008) at Mesco Reef (Ligurian Sea) that encompasses a mass mortality event in the 1990s. The quality of the coralligenous assemblages was lower in highly urbanised sites than that in sites in both marine protected areas and areas with low levels of urbanisation; moreover, the quality of the assemblages at Mesco Reef decreased during the last 50years. Reduction in quality was mainly due to the increase in stress-tolerant and/or opportunist species (e.g. algal turfs, hydroids and encrusting sponges), the disappearance of the most sensitive macroalgae (e.g. Udoteaceae and erect Rhodophyta) and macro-invertebrates (e.g. Savalia savaglia, Alcyonium coralloides and Smittina cervicornis), and the appearance of invasive alien algal species. Although the specific indices of SSL or DSL well illustrated the changes in the spatial or temporal datasets, respectively, their integration in the ISLA index was more effective in measuring the change experienced by the coralligenous assemblages in both space and time.

Structure and biodiversity of coralligenous assemblages dominated by the precious red coral Corallium rubrum over broad spatial scales

Data on species diversity and structure in coralligenous outcrops dominated by Corallium rubrum are lacking. A hierarchical sampling including 3 localities and 9 sites covering more than 400 km of rocky coasts in NW Mediterranean, was designed to characterize the spatial variability of structure, composition and diversity of perennial species inhabiting coralligenous outcrops. We estimated species/taxa composition and abundance. Eight morpho-functional groups were defined according to their life span and growth to characterize the structural complexity of the outcrops. The species composition and structural complexity differed consistently across all spatial scales considered. The lowest and the highest variability were found among localities (separated by >200 km) and within sites (separated by 1–5 km), respectively supporting differences in diversity indices. The morpho-functional groups displayed a consistent spatial arrangement in terms of the number, size and shape of patches across study sites. These results contribute to filling the gap on the understanding of assemblage composition and structure and to build baselines to assess the response of this of this highly threatened habitat to anthropogenic disturbances.

Ecological change, sliding baselines and the importance of historical data: lessons from Combining [corrected] observational and quantitative data on a temperate reef over 70 years

Understanding the effects of environmental change on ecosystems requires the identification of baselines that may act as reference conditions. However, the continuous change of these references challenges our ability to define the true natural status of ecosystems. The so-called sliding baseline syndrome can be overcome through the analysis of quantitative time series, which are, however, extremely rare. Here we show how combining historical quantitative data with descriptive ‘naturalistic’ information arranged in a chronological chain allows highlighting long-term trends and can be used to inform present conservation schemes. We analysed the long-term change of a coralligenous reef, a marine habitat endemic to the Mediterranean Sea. The coralligenous assemblages of Mesco Reef (Ligurian Sea, NW Mediterranean) have been studied, although discontinuously, since 1937 thus making available both detailed descriptive information and scanty quantitative data: while the former was useful to understand the natural history of the ecosystem, the analysis of the latter was of paramount importance to provide a formal measure of change over time. Epibenthic assemblages remained comparatively stable until the 1990s, when species replacement, invasion by alien algae, and biotic homogenisation occurred within few years, leading to a new and completely different ecosystem state. The shift experienced by the coralligenous assemblages of Mesco Reef was probably induced by a combination of seawater warming and local human pressures, the latter mainly resulting in increased water turbidity; in turn, cumulative stress may have favoured the establishment of alien species. This study showed that the combined analysis of quantitative and descriptive historical data represent a precious knowledge to understand ecosystem trends over time and provide help to identify baselines for ecological management.

Ecoregion-based conservation planning in the Mediterranean: dealing with large-scale heterogeneity

Spatial priorities for the conservation of three key Mediterranean habitats, i.e. seagrass Posidonia oceanica meadows, coralligenous formations, and marine caves, were determined through a systematic planning approach. Available information on the distribution of these habitats across the entire Mediterranean Sea was compiled to produce basin-scale distribution maps. Conservation targets for each habitat type were set according to European Union guidelines. Surrogates were used to estimate the spatial variation of opportunity cost for commercial, non-commercial fishing, and aquaculture. Marxan conservation planning software was used to evaluate the comparative utility of two planning scenarios: (a) a whole-basin scenario, referring to selection of priority areas across the whole Mediterranean Sea, and (b) an ecoregional scenario, in which priority areas were selected within eight predefined ecoregions. Although both scenarios required approximately the same total area to be protected in order to achieve conservation targets, the opportunity cost differed between them. The whole-basin scenario yielded a lower opportunity cost, but the Alboran Sea ecoregion was not represented and priority areas were predominantly located in the Ionian, Aegean, and Adriatic Seas. In comparison, the ecoregional scenario resulted in a higher representation of ecoregions and a more even distribution of priority areas, albeit with a higher opportunity cost. We suggest that planning at the ecoregional level ensures better representativeness of the selected conservation features and adequate protection of species, functional, and genetic diversity across the basin. While there are several initiatives that identify priority areas in the Mediterranean Sea, our approach is novel as it combines three issues: (a) it is based on the distribution of habitats and not species, which was rarely the case in previous efforts, (b) it considers spatial variability of cost throughout this socioeconomically heterogeneous basin, and (c) it adopts ecoregions as the most appropriate level for large-scale planning.