[Massive mortality of marine invertebrates: an unprecedented event in northwestern Mediterranean]

Lethal and sublethal effects of thermal stress on octocorals early life-history stages

The frequency and severity of marine heatwaves causing mass mortality events in tropical and temperate coral species increases every year, with serious consequences on the stability and resilience of coral populations. Although recovery and persistence of coral populations after stress events is closely related to adult fitness, as well as larval survival and settlement, much remains unknown about the effects of thermal stress on early life-history stages of temperate coral species. In the present study, the reproductive phenology and the effect of increased water temperature (+4°C and +6°C above ambient, 20°C) on larval survival and settlement was evaluated for two of the most representative Mediterranean octocoral species (Eunicella singularis and Corallium rubrum). Our study shows that reproductive behavior is more variable than previously reported and breeding period occurs over a longer period in both species. Thermal stress did not affect the survival of symbiotic E. singularis larvae but drastically reduced the survival of the non-symbiotic C. rubrum larvae. Results on larval biomass and caloric consumption suggest that higher mortality rates of C. rubrum exposed to increased temperature were not related to depletion of endogenous energy in larvae. The results also show that settlement rates of E. singularis did not change in response to elevated temperature after 20 days of exposure, but larvae may settle fast and close to their native population at 26°C (+6°C). Although previous experimental studies found that adult colonies of both octocoral species are mostly resistant to thermal stress, our results on early life-history stages suggest that the persistence and inter-connectivity of local populations may be severely compromised under continued trends in ocean warming.

Marine heatwaves drive recurrent mass mortalities in the Mediterranean Sea

Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015-2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning. Overall, we show that increasing the resolution of empirical observation is critical to enhancing our ability to more effectively understand and manage the consequences of climate change.

Integration assay of bacteriological risks in marine environment using salmonella spp and multimarker response in the bivalve Donax trunculus: Novel biomonitoring approach

Microbiological contamination is one of the riskiest forms of human contamination in seawater, which threaten the stability of ecosystems and human health. In this study, we study the accumulation of a pathogenic bacteria Salmonella spp; isolated from the marine environment, in the soft tissue of Donax trunculus (Mollusca, Bivalvia), a commonly used as a bioindicators species for aquatic ecosystems monitoring, under laboratory conditions during both exposure and recovery periods. These bacteria were added in seawater at three concentrations previously determined against sentinel specie at three exposure periods (24, 48 and 96 h). In a second series of experiments, exposed specimens were afterward transplanted to clean water to assess the recovery pattern. The mortality rate of bivalves was determined as biomarker of general stress. Our findings suggest that microbiological contamination by Salmonella spp was gradually incorporated into the body of D. trunculus causing a significant induction of enzymatic activity of acetylcholinesterase (AChE), Catalase (CAT), glutathione-S-transferase (GST) and malondialdehyde (MDA) levels, as a function of time and concentration. Exposure to a bacterial concentration of 5.10⁴ bacteria/liter resulted in the mortality of more than 80% of the specimens. This study is to test the pathogenicity of Salmonella strains at concentrations close to those of the marine environment, and their effects on biomarkers, thus deducing the existence of an exponential relationship between bacterial concentrations and enzymatic response. The principal component analysis shows that the four biomarkers had similar variation with bacterial concentrations, while two groups were obtained to change following the exposure time (CAT-GST and AChE-MDA). This study provides new findings on the potential accumulation of pathogenic bacteria associated with neurotoxicity and oxidative stress in the wedge clam Donax trunculus.

Animal Forest Mortality: Following the Consequences of a Gorgonian Coral Loss on a Mediterranean Coralligenous Assemblage

In this work, the consequences of a local gorgonian coral mortality on the whole coralligenous assemblage were studied. A Before/After-Control/Impact sampling design was used: the structure of the coralligenous assemblage was compared before and after the gorgonian mortality event at the mortality site and two control sites. At the mortality site, a relevant decrease in alpha and beta diversity occurred, with a shift from a stratified assemblage characterized by gorgonians and other invertebrates to an assemblage dominated by algal turfs; conversely, neither significant variations of the structure nor decrease in biodiversity were observed at the control sites. The assemblage shift involved the main taxa in different times: in autumn 2018, a large proportion of the plexaurid coral Paramuricea clavata died, but no significant changes were observed in the structure of the remaining assemblage. Then, in autumn 2019, algal turfs increased significantly and, one year later, the abundance of the gorgonian Eunicella cavolini and bryozoans collapsed. Although the mechanisms of the assemblage shift following gorgonian loss will remain uncertain and a cause-effect relationship cannot be derived, results suggest the need for detecting signs of gorgonian forests stress in monitoring programs, which should be considered early indicators of their condition. in the coralligenous monitoring programs for detecting any sign of gorgonian forests stress which should be considered an early indicator of the assemblage condition.

Oceanographic features and limited dispersal shape the population genetic structure of the vase sponge Ircinia campana in the Greater Caribbean

Understanding population genetic structure can help us to infer dispersal patterns, predict population resilience and design effective management strategies. For sessile species with limited dispersal, this is especially pertinent because genetic diversity and connectivity are key aspects of their resilience to environmental stressors. Here, we describe the population structure of Ircinia campana, a common Caribbean sponge subject to mass mortalities and disease. Microsatellites were used to genotype 440 individuals from 19 sites throughout the Greater Caribbean. We found strong genetic structure across the region, and significant isolation by distance across the Lesser Antilles, highlighting the influence of limited larval dispersal. We also observed spatial genetic structure patterns congruent with oceanography. This includes evidence of connectivity between sponges in the Florida Keys and the southeast coast of the United States (>700 km away) where the oceanographic environment is dominated by the strong Florida Current. Conversely, the population in southern Belize was strongly differentiated from all other sites, consistent with the presence of dispersal-limiting oceanographic features, including the Gulf of Honduras gyre. At smaller spatial scales (<100 km), sites showed heterogeneous patterns of low-level but significant genetic differentiation (chaotic genetic patchiness), indicative of temporal variability in recruitment or local selective pressures. Genetic diversity was similar across sites, but there was evidence of a genetic bottleneck at one site in Florida where past mass mortalities have occurred. These findings underscore the relationship between regional oceanography and weak larval dispersal in explaining population genetic patterns, and could inform conservation management of the species.

Detect coastal disturbances and climate change effects in coralligenous community through sentinel stations

This study was implemented to assess the Sessile Bioindicators in Permanent Quadrats (SBPQ) underwater environmental alert method. The SBPQ is a non-invasive and low-cost protocol; it uses sessile target species (indicators) to detect environmental alterations (natural or anthropic) at either the local or global (i.e., climate change) scale and the intrusion of invasive species. The SBPQ focuses on the monitoring of preselected sessile and sensitive benthic species associated with rocky coralligenous habitats using permanent quadrats in underwater sentinel stations. The selected target species have been well documented as bioindicators that disappear in the absence of environmental stability. However, whether these species are good indicators of stability or, in contrast, suffer variations in long-term coverage has not been verified. The purpose of this study was to assess the part of the method based on the hypothesis that, over a long temporal series in a highly structured and biodiverse coralligenous assemblage, the cover of sensitive sessile species does not change over time if the environmental stability characterising the habitat is not altered. Over a ten-year period (2005–2014), the sublittoral sessile biota in the Straits of Gibraltar Natural Park on the southern Iberian Peninsula was monitored at a 28 m-deep underwater sentinel stations. Analyses of the coverages of target indicator species (i.e., Paramuricea clavata and Astroides calycularis) together with other accompanying sessile organisms based on the periodic superimposition of gridded images from horizontal and vertical rocky surfaces allowed us to assess the effectiveness of the method. We conclude that no alterations occurred during the study period; only minimal fluctuations in cover were detected, and the method is reliable for detecting biological changes in ecosystems found in other geographical areas containing the chosen indicator species at similar dominance levels.

Nodular-like growth and axial thickening in gorgonians are a defensive response to endolithic cyanobacteria, involving amyloid deposition

An accurate approach to coral disease study is critical for understanding the global decline of coral populations. Such an approach should involve the proper use of medical concepts and terminology to avoid confusion and promote clarity in the coral disease literature. Inflammatory and neoplastic disorders have been frequently confused in corals. They are both reported as 'growth anomalies' because of their possible gross similarity, but in fact they are very different types of lesions and pathologic phenomena. In this work, we assessed the distribution and prevalence of growth anomalies, externally visible as nodular-like lesions, in the soft corals Eunicella cavolinii and E. singularis in 2008-2009 in 3 different areas along the Campanian coastline of Italy. Histopathology revealed them as chronic inflammatory lesions, resembling chronic inflammatory lesions of vertebrates, encapsulating an unidentified pathogen. Congo red and Masson Fontana histochemistry highlighted an amoebocyte infiltration with the presence of new apposition of melanin coupled with amyloid sheets intended as part of the defensive response, as reported in other invertebrates. A parallel molecular analysis of 16S rRNA of the lesions suggested that the causative agent is an endolithic cyanobacterium belonging to the order Nostocales. This is the first study assessing the presence of amyloid fibrils in corals.

Historical record of Corallium rubrum and its changing carbon sequestration capacity: A meta-analysis from the North Western Mediterranean

Background

There is a scarcity of long time-span and geographically wide research on the health status of Corallium rubrum, including limited research on its historical ecology and carbon sequestration capacity.

Objectives

To reconstruct the temporal trends of the most reported C. rubrum population parameters in the Northwestern Mediterranean Sea and to determine the changes in total carbon sequestration by this species.

Data sources

Quantitative and qualitative, academic and grey documents were collected from scientific web browsers, scientific libraries, and requests to scientists.

Study eligibility criteria

Documents with original information of basal diameter, height and/or weight per colony, with a depth limit of 60 m in the Catalan and Ligurian Seas were analyzed.

Synthesis methods

We calculated yearly average values of C. rubrum biometric parameters, as well as estimated total weight, carbon flux, and carbon fixation in the structures of C. rubrum’s colonies.

Results

In both study areas, the values of the selected morphometric parameters for C. rubrum decreased until the 1990s, then increased from the 2000s, with average values surpassing the levels of the 1960s (Ligurian Sea) or reaching levels slightly lower than those of the 1980s (Catalan Sea). The difference in carbon sequestered between the oldest (1960s: Ligurian Sea; 1970s: Catalan Sea) and the lowest (1990s) biomass value of colonies is nearly double.

Limitations

Quantitative data previous to the 1990s are very limited. Information on recent recovery trends in C. rubrum parameters is concentrated in a few areas and biased towards colonies in marine protected areas, with scarce quantitative information from colonies in other areas.

Conclusions

The halt in the C. rubrum decreasing trend coincided with the exhaustion of tree-like colonies and the first recovery response due to effective protection measures in some areas. Nevertheless, C. rubrum climate change mitigation capacity through carbon sequestration can be drastically reduced from its potential in only a few decades.

Response of the temperate scleractinian coral Cladocora caespitosa to high temperature and long-term nutrient enrichment

Anthropogenic nutrient enrichment and increased seawater temperatures are responsible for coral reef decline. In particular, they disrupt the relationship between corals and their dinoflagellate symbionts (bleaching). However, some coral species can afford either high temperatures or nutrient enrichment and their study can bring new insights into how corals acclimate or adapt to stressors. Here, we focused on the role of the nutrient history in influencing the response of the Mediterranean scleractinian coral Cladocora caespitosa to thermal stress. Colonies living naturally in nutrient-poor (<0.5 µM nitrogen, <0.2 µM phosphorus, LN) and nutrient-rich (ca. 10–20 µM nitrogen, 0.4 µM phosphorus, HN) locations were sampled, maintained under the right nutrient conditions, and exposed to a temperature increase from 17 °C to 24 °C and 29 °C. While both HN and LN colonies decreased their concentrations of symbionts and/or photosynthetic pigments, HN colonies were able to maintain significant higher rates of net and gross photosynthesis at 24 °C compared to LN colonies. In addition, while there was no change in protein concentration in HN corals during the experiment, proteins continuously decreased in LN corals with increased temperature. These results are important in that they show that nutrient history can influence the response of scleractinian corals to thermal stress. Further investigations of under-studied coral groups are thus required in the future to understand the processes leading to coral resistance to environmental perturbations.

Anthropogenic impact is negatively related to coral health in Sicily (Mediterranean Sea)

Shallow-water marine organisms are among the first to suffer from combined effects of natural and anthropogenic drivers. The orange coral Astroides calycularis is a shallow-water bioconstructor species endemic to the Mediterranean Sea. Although raising conservation interest, also given its special position within the Dendrophylliidae, information about the threats to its health is scant. We investigated the health status of A. calycularis at five locations in northwestern Sicily along a gradient of cumulative human impact and the most probable origin of the threats to this species, including anthropogenic land-based and sea-based threats. Cumulative human impact appeared inversely related to the performance of A. calycularis at population, colony, and polyp levels. Sea-based human impacts appeared among the most likely causes of the variation observed. The reduction in polyp length can limit the reproductive performance of A. calycularis, while the decrease of percent cover and colony area is expected to impair its peculiar feeding behaviour by limiting the exploitable dimensional range of prey and, ultimately, reef functioning. This endangered habitat-forming species appeared susceptible to anthropogenic pressures, suggesting the need to re-assess its vulnerability status. Creating microprotected areas with specific restrictions to sea-based human impacts could be the best practice preserve these bioconstructions.

Genetic diversity increases with depth in red gorgonian populations of the Mediterranean Sea and the Atlantic Ocean

Background

In the ocean, the variability of environmental conditions found along depth gradients exposes populations to contrasting levels of perturbation, which can be reflected in the overall patterns of species genetic diversity. At shallow sites, resource availability may structure large, persistent and well-connected populations with higher levels of diversity. In contrast, the more extreme conditions, such as thermal stress during heat waves, can lead to population bottlenecks and genetic erosion, inverting the natural expectation. Here we examine how genetic diversity varies along depth for a long-lived, important ecosystem-structuring species, the red gorgonian, Paramuricea clavata.

Methods

We used five polymorphic microsatellite markers to infer differences in genetic diversity and differentiation, and to detect bottleneck signs between shallow and deeper populations across the Atlantic Ocean and the Mediterranean Sea. We further explored the potential relationship between depth and environmental gradients (temperature, ocean currents, productivity and slope) on the observed patterns of diversity by means of generalized linear mixed models.

Results

An overall pattern of higher genetic diversity was found in the deeper sites of the Atlantic Ocean and the Mediterranean Sea. This pattern was largely explained by bottom temperatures, with a linear pattern of decreasing genetic diversity with increasing thermal stress. Genetic differentiation patterns showed higher gene flow within sites (i.e., shallow vs. deeper populations) than between sites. Recent genetic bottlenecks were found in two populations of shallow depths.

Discussion

Our results highlight the role of deep refugial populations safeguarding higher and unique genetic diversity for marine structuring species. Theoretical regression modelling demonstrated how thermal stress alone may reduce population sizes and diversity levels of shallow water populations. In fact, the examination of time series on a daily basis showed the upper water masses repeatedly reaching lethal temperatures for P. clavata. Differentiation patterns showed that the deep richer populations are isolated. Gene flow was also inferred across different depths; however, not in sufficient levels to offset the detrimental effects of surface environmental conditions on genetic diversity. The identification of deep isolated areas with high conservation value for the red gorgonian represents an important step in the face of ongoing and future climate changes.

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.

Response diversity in Mediterranean coralligenous assemblages facing climate change: Insights from a multispecific thermotolerance experiment

Climate change threatens coastal benthic communities on a global scale. However, the potential effects of ongoing warming on mesophotic temperate reefs at the community level remain poorly understood. Investigating how different members of these communities will respond to the future expected environmental conditions is, therefore, key to anticipating their future trajectories and developing specific management and conservation strategies. Here, we examined the responses of some of the main components of the highly diverse Mediterranean coralligenous assemblages to thermal stress. We performed thermotolerance experiments with different temperature treatments (from 26 to 29°C) with 10 species from different phyla (three anthozoans, six sponges and one ascidian) and different structural roles. Overall, we observed species-specific contrasting responses to warming regardless of phyla or growth form. Moreover, the responses ranged from highly resistant species to sensitive species and were mostly in agreement with previous field observations from mass mortality events (MMEs) linked to Mediterranean marine heat waves. Our results unravel the diversity of responses to warming in coralligenous outcrops and suggest the presence of potential winners and losers in the face of climate change. Finally, this study highlights the importance of accounting for species-specific vulnerabilities and response diversity when forecasting the future trajectories of temperate benthic communities in a warming ocean.

Brown Rot Syndrome and Changes in the Bacterial Сommunity of the Baikal Sponge Lubomirskia baicalensis

Mass mortality events have led to a collapse of the sponge fauna of Lake Baikal. We describe a new Brown Rot Syndrome affecting the endemic species Lubomirskia baicalensis. The main symptoms are the appearance of brown patches at the sponge surface, necrosis, and cyanobacterial fouling. 16S rRNA gene sequencing was used to characterize the bacterial community of healthy versus diseased sponges, in order to identify putative pathogens. The relative abundance of 89 eubacterial OTUs out of 340 detected has significantly changed between healthy and diseased groups. This can be explained by the depletion of host-specific prokaryotes and by the appearance and proliferation of disease-specific OTUs. In diseased sponges, the most represented OTUs belong to the families Oscillatoriaceae, Cytophagaceae, Flavobacteriaceae, Chitinophagaceae, Sphingobacteriaceae, Burkholderiaceae, Rhodobacteraceae, Comamonadaceae, Oxalobacteraceae, and Xanthomonadaceae. Although these families may contain pathogenic agents, the primary causes of changes in the sponge bacterial community and their relationship with Brown Rot Syndrome remain unclear. A better understanding of this ecological crisis will thus require a more integrative approach.

Heatwaves diminish the survival of a subtidal gastropod through reduction in energy budget and depletion of energy reserves

Extreme climatic events, such as heatwaves, are predicted to be more prevalent in future due to global climate change. The devastating impacts of heatwaves on the survival of marine organisms may be further intensified by ocean acidification. Here, we tested the hypothesis that prolonged exposure to heatwave temperatures (24 °C, +3 °C summer seawater temperature) would diminish energy budget, body condition and ultimately survival of a subtidal gastropod (Thalotia conica) by pushing close to its critical thermal maximum (CT_max). We also tested whether ocean acidification (pCO₂: 1000 ppm) affects energy budget, CT_max and hence survival of this gastropod. Following the 8-week experimental period, mortality was markedly higher at 24 °C irrespective of pCO₂ level, probably attributed to energy deficit (negative scope for growth) and concomitant depletion of energy reserves (reduced organ weight to flesh weight ratio). CT_max of T. conica appeared at 27 °C and was unaffected by ocean acidification. Our findings imply that prolonged exposure to heatwaves can compromise the survival of marine organisms below CT_max via disruption in energy homeostasis, which possibly explains their mass mortality in the past heatwave events. Therefore, heatwaves would have more profound effects than ocean acidification on future marine ecosystems.

The roles of endolithic fungi in bioerosion and disease in marine ecosystems. II. Potential facultatively parasitic anamorphic ascomycetes can cause disease in corals and molluscs

Anamorphic ascomycetes have been implicated as causative agents of diseases in tissues and skeletons of hard corals, in tissues of soft corals (sea fans) and in tissues and shells of molluscs. Opportunist marine fungal pathogens, such as Aspergillus sydowii, are important components of marine mycoplankton and are ubiquitous in the open oceans, intertidal zones and marine sediments. These fungi can cause infection in or at least can be associated with animals which live in these ecosystems. A. sydowii can produce toxins which inhibit photosynthesis in and the growth of coral zooxanthellae. The prevalence of many documented infections has increased in frequency and severity in recent decades with the changing impacts of physical and chemical factors, such as temperature, acidity and eutrophication. Changes in these factors are thought to cause significant loss of biodiversity in marine ecosystems on a global scale in general, and especially in coral reefs and shallow bays.

An integrated method to evaluate and monitor the conservation state of coralligenous habitats: The INDEX-COR approach

A new method based on photographic sampling coupled with in situ observations was applied to 53 stations along the French Mediterranean coast, to assess the integrity of coralligenous reefs affected by different levels of anthropogenic pressure. The conservation state of the assemblages characterizing these habitats was then assessed by an index - the INDEX-COR - that integrates three metrics: (i) the sensitivity of the taxa to organic matter and sediment deposition, (ii) the observable taxonomic richness, and (iii) the structural complexity of the assemblages. The sensitivity of INDEX-COR was tested and showed good correlation with the Level of Pressure calculated for each station according to expert judgment and field observations.

Correlation between pollution and decline of Scleractinian Cladocora caespitosa (Linnaeus, 1758) in the Gulf of Gabes

During an expedition in 2014 in the Gulf of Gabes that aimed to evaluate the impact of the pollution of the phosphate industry on the marine environment, numerous dead coral fragments were retrieved from several stations along a 18 km long transect in front of the industry complex of Gabes. Detailed taxonomy of these coral fragments shows clearly that all fragments belong to the species Cladocora caespitosa (Linnaeus, 1758). Quantitative analysis of the coral fragments indicates a positive correlation with stations characterized by positive bathymetric anomalies. We suggest the presence of probable small-scaled (up to 4 m high) biogenic (palaeo-) build-ups composed mainly of coral colonies and bryozoans. Radiocarbon dating of three coral fragments show ages as old as 1897, 1985 and 1986 AD and suggests the presence of living C. caespitosa as close as 6 km to the phosphate treatment industry of Gabes at least until 1986 AD. This latter age coincides with the construction of the ammonium phosphate production plant, in 1979, in the Gulf of Gabes with an increase of the natural phosphate production. The higher impact of pollution on the marine environment in the inner part of the Gulf of Gabes likely induced the decline of C. caespitosa. This is well in agreement with enhanced siltation processes suggested by the sedimentary facies and grain-size analyses presently characterizing the Gulf of Gabes nowadays.

Population structure and connectivity in the Mediterranean sponge Ircinia fasciculata are affected by mass mortalities and hybridization

Recent episodes of mass mortalities in the Mediterranean Sea have been reported for the closely related marine sponges Ircinia fasciculata and Ircinia variabilis that live in sympatry. In this context, the assessment of the genetic diversity, bottlenecks and connectivity of these sponges has become urgent in order to evaluate the potential effects of mass mortalities on their latitudinal range. Our study aims to establish (1) the genetic structure, connectivity and signs of bottlenecks across the populations of I. fasciculata and (2) the hybridization levels between I. fasciculata and I. variabilis. To accomplish the first objective, 194 individuals of I. fasciculata from 12 locations across the Mediterranean were genotyped at 14 microsatellite loci. For the second objective, mitochondrial cytochrome c oxidase subunit I sequences of 16 individuals from both species were analyzed along with genotypes at 12 microsatellite loci of 40 individuals coexisting in 3 Mediterranean populations. We detected strong genetic structure along the Mediterranean for I. fasciculata, with high levels of inbreeding in all locations and bottleneck signs in most locations. Oceanographic barriers like the Almeria-Oran front, North-Balearic front and the Ligurian-Thyrrenian barrier seem to be impeding gene flow for I. fasciculata, adding population divergence to the pattern of isolation by distance derived from the low dispersal abilities of sponge larvae. Hybridization between both species occurred in some populations that might be increasing genetic diversity and somewhat palliating the genetic loss caused by population decimation in I. fasciculata.

Snapshot of a Bacterial Microbiome Shift during the Early Symptoms of a Massive Sponge Die-Off in the Western Mediterranean

Ocean warming is affecting marine benthic ecosystems through mass mortality events that involve marine invertebrates, in particular bivalves, corals, and sponges. Among these events, extensive die-offs of Ircinia fasciculata sponges have been recurrently reported in western Mediterranean. The goal of our study was to test whether the temperature-related mass sponge die-offs were associated with or preceded by an early unbalanced bacterial microbiome in the sponge tissues. We took advantage of the early detection of disease and compared the microbiomes of healthy vs. early diseased I. fasciculata tissues. Our results showed a microbiome shift in early diseased tissues. The abundance of Gammaproteobacteria and Acidobacteria increased and that of Deltaproteobacteria decreased in diseased vs. healthy tissues. The change in community composition was also noticeable at the operational taxonomic unit (OTU) level. Diseased tissues contained more bacterial sequences previously identified in injured or stressed sponges and corals than healthy tissues. Bacterial diversity increased significantly in diseased tissues, which contained a higher number of low abundance OTUs. Our results do not support the hypothesis of one particular pathogen, whether a Vibrio or any other bacteria, triggering the Northwestern Mediterranean mass mortalities of I. fasciculata. Our data rather suggest an early disruption of the bacterial microbiome balance in healthy sponges through a shift in OTU abundances, and the purported consequent decline of the sponge fitness and resistance to infections. Opportunistic bacteria could colonize the sponge tissues, taking benefit of the sponge weakness, before one or more virulent pathogens might proliferate ending in the mass sponge die-off.

Experimental evidence of the synergistic effects of warming and invasive algae on a temperate reef-builder coral

In the current global climate change scenario, stressors overlap in space and time, and knowledge on the effects of their interaction is highly needed to understand and predict the response and resilience of organisms. Corals, among many other benthic organisms, are affected by an increasing number of global change-related stressors including warming and invasive species. In this study, the cumulative effects between warming and invasive algae were experimentally assessed on the temperate reef-builder coral Cladocora caespitosa. We first investigated the potential local adaptation to thermal stress in two distant populations subjected to contrasting thermal and necrosis histories. No significant differences were found between populations. Colonies from both populations suffered no necrosis after long-term exposure to temperatures up to 29 °C. Second, we tested the effects of the interaction of both warming and the presence of invasive algae. The combined exposure triggered critical synergistic effects on photosynthetic efficiency and tissue necrosis. At the end of the experiment, over 90% of the colonies subjected to warming and invasive algae showed signs of necrosis. The results are of particular concern when considering the predicted increase of extreme climatic events and the spread of invasive species in the Mediterranean and other seas in the future.

Thermally tolerant corals have limited capacity to acclimatize to future warming

Thermal stress affects organism performance differently depending on the ambient temperature to which they are acclimatized, which varies along latitudinal gradients. This study investigated whether differences in physiological responses to temperature are consistent with regional differences in temperature regimes for the stony coral Oculina patagonica. To resolve this question, we experimentally assessed how colonies originating from four different locations characterized by >3 °C variation in mean maximum annual temperature responded to warming from 20 to 32 °C. We assessed plasticity in symbiont identity, density, and photosynthetic properties, together with changes in host tissue biomass. Results show that, without changes in the type of symbiont hosted by coral colonies, O. patagonica has limited capacity to acclimatize to future warming. We found little evidence of variation in overall thermal tolerance, or in thermal optima, in response to spatial variation in ambient temperature. Given that the invader O. patagonica is a relatively new member of the Mediterranean coral fauna, our results also suggest that coral populations may need to remain isolated for a long period of time for thermal adaptation to potentially take place. Our study indicates that for O. patagonica, mortality associated with thermal stress manifests primarily through tissue breakdown under moderate but prolonged warming (which does not impair symbiont photosynthesis and, therefore, does not lead to bleaching). Consequently, projected global warming is likely to cause repeat incidents of partial and whole colony mortality and might drive a gradual range contraction of Mediterranean corals.

Gorgonian disease outbreak in the Gulf of Naples: pathology reveals cyanobacterial infection linked to elevated sea temperatures

In recent years, mass mortality events of benthic invertebrates in the Mediterranean Sea have been documented to coincide with the increased seawater temperatures associated with global climate change. Following a disease outbreak in gorgonians during the summer seasons of 2008 and 2009 in the Gulf of Naples (Tyrrhenian Sea), we conducted gross and microscopic analyses of healthy and diseased specimens of Eunicella cavolinii and E. singularis using both light and electron microscopy (SEM). Macroscopically, diseased colonies exhibited evident tissue thinning, and dead colonies showed a complete loss of polyps and coenenchyme, exposing their skeletons to settlement by fouling organisms. Histopathology revealed chronic inflammatory lesions at the polyp and axial level, characterized by amoebocyte infiltration of tissue accompanied by new apposition of melanin/gorgonin sheets. We interpreted this response as a defense against different kinds of pathogens-identified as mainly a heterogeneous consortium of filamentous cyanobacteria-and which gradually led to enlargement and hardening of the coral axis, which resembled a wood-like structure at the final stage of the disease. These processes elicited the formation of multiple inflammatory nodules and capsules, some of which were macroscopically visible. A parallel 16S rRNA and ITS analysis of the diseased tissue identified Synechococcus, Arthrospira and other uncultured cyanobacteria grouped within the Oscillatoriales. These results suggest that a cyanobacterial consortium is involved in the pathogenesis of the inflammatory disease leading to the mortality of Gorgoniaceae in the area. Finally, there were anomalously high temperatures (up to 25°C) between 10 and 20 m depth during the sampling period, particularly in June 2009. This supports the hypothesis that the coral skeleton may serve as a reservoir for the pathogens in cooler seasons, with warmer conditions leading to pathogen reactivation and recurring mortality events.

A time-domain nuclear magnetic resonance study of Mediterranean scleractinian corals reveals skeletal-porosity sensitivity to environmental changes

Mediterranean corals are a natural model for studying global warming, as the Mediterranean basin is expected to be one of the most affected regions and the increase in temperature is one of the greatest threats for coral survival. We have analyzed for the first time with time-domain nuclear magnetic resonance (TD-NMR) the porosity and pore-space structure, important aspects of coral skeletons, of two scleractinian corals, Balanophyllia europaea (zooxanthellate) and Leptopsammia pruvoti (nonzooxanthellate), taken from three different sites on the western Italian coast along a temperature gradient. Comparisons have been made with mercury intrusion porosimetry and scanning electron microscopy images. TD-NMR parameters are sensitive to changes in the pore structure of the two coral species. A parameter, related to the porosity, is larger for L. pruvoti than for B. europaea, confirming previous non-NMR results. Another parameter representing the fraction of the pore volume with pore sizes of less than 10-20 μm is inversely related, with a high degree of statistical significance, to the mass of the specimen and, for B. europaea, to the temperature of the growing site. This effect in the zooxanthellate species, which could reduce its resistance to mechanical stresses, may depend on an inhibition of the photosynthetic process at elevated temperatures and could have particular consequences in determining the effects of global warming on these species.

Thermal threshold and sensitivity of the only symbiotic Mediterranean gorgonian Eunicella singularis by morphometric and genotypic analyses

The only symbiotic Mediterranean gorgonian, Eunicella singularis, has faced several mortality events connected to abnormal high temperatures. Since thermotolerance data remain scarce, heat-induced necrosis was monitored in aquarium by morphometric analysis. Gorgonian tips were sampled at two sites: Medes (Spain) and Riou (France) Islands, and at two depths: -15 m and-35 m. Although coming from contrasting thermal regimes, seawater above 28 °C led to rapid and complete tissue necrosis for all four populations. However, at 27 °C, the time length leading to 50% tissue necrosis allowed us to classify samples within three classes of thermal sensitivity. Irrespectively of the depth, Medes specimens were either very sensitive or resistant, while Riou fragments presented a medium sensitivity. Microsatellite analysis revealed that host and symbiont were genetically differentiated between sites, but not between depths. Finally, these genetic differentiations were not directly correlated to a specific thermal sensitivity whose molecular bases remain to be discovered.

Long-term responses of the endemic reef-builder Cladocora caespitosa to Mediterranean warming

Recurrent climate-induced mass-mortalities have been recorded in the Mediterranean Sea over the past 15 years. Cladocora caespitosa, the sole zooxanthellate scleractinian reef-builder in the Mediterranean, is among the organisms affected by these episodes. Extensive bioconstructions of this endemic coral are very rare at the present time and are threatened by several stressors. In this study, we assessed the long-term response of this temperate coral to warming sea-water in the Columbretes Islands (NW Mediterranean) and described, for the first time, the relationship between recurrent mortality events and local sea surface temperature (SST) regimes in the Mediterranean Sea. A water temperature series spanning more than 20 years showed a summer warming trend of 0.06°C per year and an increased frequency of positive thermal anomalies. Mortality resulted from tissue necrosis without massive zooxanthellae loss and during the 11-year study, necrosis was recorded during nine summers separated into two mortality periods (2003–2006 and 2008–2012). The highest necrosis rates were registered during the first mortality period, after the exceptionally hot summer of 2003. Although necrosis and temperature were significantly associated, the variability in necrosis rates during summers with similar thermal anomalies pointed to other acting factors. In this sense, our results showed that these differences were more closely related to the interannual temperature context and delayed thermal stress after extreme summers, rather than to acclimatisation and adaption processes.

Does thermal history influence the tolerance of temperate gorgonians to future warming?

To date, several studies have provided evidence that thermal stress affects the growth, survival and physiology of tropical and temperate macroinvertebrate species. However, few studies have focused on subtidal temperate species and the potential differential thermal tolerances of populations dwelling under contrasting temperature conditions. To assess the role that environmental history has on the response of the temperate gorgonian Eunicella singularis to thermal stress, we compared populations dwelling in the coldest and warmest areas of the NW Mediterranean Sea. Our results show that E. singularis populations from both areas exhibited a high resistance to thermal stress; however, populations from warmer areas had an increased tolerance to thermal stress. Specifically, the upper thermal limits found for cold and warm populations were 28 and 29 °C, respectively. The higher resistance of E. singularis colonies to thermal stress found in this study compared to the field temperature conditions during recent mass mortality events highlights that performing further thermotolerance experiments under contrasting levels of feeding is necessary to fully assess the tolerance thresholds displayed by both study populations. To our knowledge, this study provides the first evidence for the role of thermal history in shaping the thermotolerance responses of Mediterranean marine invertebrates dwelling under contrasting temperature environments.

The response of the Mediterranean gorgonian Eunicella singularis to thermal stress is independent of its nutritional regime

Over the last few decades, sessile benthic organisms from the Mediterranean Sea have suffered from the global warming of the world's oceans, and several mass mortality events were observed during warm summers. It has been hypothesized that mortality could have been due to a nutrient (food) shortage following the stratification of the water column. However, the symbiotic gorgonian Eunicella singularis has also presented a locally exceptional mortality, despite its autotrophic capacities through the photosynthesis of its dinoflagellate symbionts. Thus, this study has experimentally investigated the response of E. singularis to a thermal stress (temperature increase from 18 to 26°C), with colonies maintained more than 2 months under four nutritional diets: autotrophy only (AO), autotrophy and inorganic nitrogen addition (AN), autotrophy and heterotrophy (AH), heterotrophy only (HO). At 18°C, and contrary to many other anthozoans, supplementation of autotrophy with either inorganic nitrogen or food (heterotrophy) had no effect on the rates of respiration, photosynthesis, as well as in the chlorophyll, lipid and protein content. In the dark, heterotrophy maintained the gorgonian's metabolism, except a bleaching (loss of pigments), which did not affect the rates of photosynthesis. At 24°C, rates of respiration, and photosynthesis significantly decreased in all treatments. At 26°C, in addition to a decrease in the lipid content of all treatments, a bleaching was observed after 1 week in the AO treatment, while the AH and AN treatments resisted three weeks before bleaching. These last results suggest that, temperatures above 24°C impair the energetic reserves of this species and might explain the mortality events in the Mediterranean.

Inferred calcification rate of a Mediterranean azooxanthellate coral is uncoupled with sea surface temperature along an 8° latitudinal gradient

INTRODUCTION

Correlations between sea surface temperature (SST) and growth parameters of the solitary azooxanthellate Dendrophylliid Leptopsammia pruvoti were assessed along an 8° latitudinal gradient on western Italian coasts (Mediterranean Sea), to check for possible negative effects of increasing temperature as the ones reported for a closely related, sympatric but zooxanthellate species.

RESULTS

Calcification rate was correlated with skeletal density but not with linear extension rate, indicating that calcium carbonate deposition was preferentially allocated to keep a constant skeletal density. Unlike most studies on both temperate and tropical zooxanthellate corals, where calcification rate is strongly related to environmental parameters such as SST, in the present study calcification rate was not correlated with SST.

CONCLUSIONS

The lower sensitivity of L. pruvoti to SST with respect to other sympatric zooxanthellate corals, such as Balanophyllia europaea, may rely on the absence of a temperature induced inhibition of photosynthesis, and thus the absence of an inhibition of the calcification process. This study is the first field investigation of the relationship between SST and the three growth parameters of an azooxanthellate coral. Increasing research effort on determining the effects of temperature on biological traits of the poorly studied azooxanthellate scleractinians may help to predict the possible species assemblage shifts that are likely to occur in the immediate future as a consequence of global climatic change.

Marine caves of the Mediterranean Sea: a sponge biodiversity reservoir within a biodiversity hotspot

Marine caves are widely acknowledged for their unique biodiversity and constitute a typical feature of the Mediterranean coastline. Herein an attempt was made to evaluate the ecological significance of this particular ecosystem in the Mediterranean Sea, which is considered a biodiversity hotspot. This was accomplished by using Porifera, which dominate the rocky sublittoral substrata, as a reference group in a meta-analytical approach, combining primary research data from the Aegean Sea (eastern Mediterranean) with data derived from the literature. In total 311 species from all poriferan classes were recorded, representing 45.7% of the Mediterranean Porifera. Demospongiae and Homoscleromorpha are highly represented in marine caves at the family (88%), generic (70%), and species level (47.5%), the latter being the most favored group along with Dictyoceratida and Lithistida. Several rare and cave-exclusive species were reported from only one or few caves, indicating the fragmentation and peculiarity of this unique ecosystem. Species richness and phylogenetic diversity varied among Mediterranean areas; the former was positively correlated with research effort, being higher in the northern Mediterranean, while the latter was generally higher in caves than in the overall sponge assemblages of each area. Resemblance analysis among areas revealed that cavernicolous sponge assemblages followed a pattern quite similar to that of the overall Mediterranean assemblages. The same pattern was exhibited by the zoogeographic affinities of cave sponges: species with Atlanto-Mediterranean distribution and Mediterranean endemics prevailed (more than 40% each), 70% of them having warm-water affinities, since most caves were studied in shallow waters. According to our findings, Mediterranean marine caves appear to be important sponge biodiversity reservoirs of high representativeness and great scientific interest, deserving further detailed study and protection.

Growth and demography of the solitary scleractinian coral Leptopsammia pruvoti along a sea surface temperature gradient in the Mediterranean Sea

The demographic traits of the solitary azooxanthellate scleractinian Leptopsammia pruvoti were determined in six populations on a sea surface temperature (SST) gradient along the western Italian coasts. This is the first investigation of the growth and demography characteristics of an azooxanthellate scleractinian along a natural SST gradient. Growth rate was homogeneous across all populations, which spanned 7 degrees of latitude. Population age structures differed between populations, but none of the considered demographic parameters correlated with SST, indicating possible effects of local environmental conditions. Compared to another Mediterranean solitary scleractinian, Balanophyllia europaea, zooxanthellate and whose growth, demography and calcification have been studied in the same sites, L. pruvoti seems more tolerant to temperature increase. The higher tolerance of L. pruvoti, relative to B. europaea, may rely on the absence of symbionts, and thus the lack of an inhibition of host physiological processes by the heat-stressed zooxanthellae. However, the comparison between the two species must be taken cautiously, due to the likely temperature differences between the two sampling depths. Increasing research effort on determining the effects of temperature on the poorly studied azooxanthellate scleractinians may shed light on the possible species assemblage shifts that are likely to occur during the current century as a consequence of global climatic change.

Temperature anomalies and mortality events in marine communities: insights on factors behind differential mortality impacts in the NW Mediterranean

Two large-scale mass mortality events (MMEs) of unprecedented extent and severity affecting rocky benthic communities occurred during the summers of 1999 and 2003 along the coasts of the NW Mediterranean Sea. These mortality outbreaks were associated with positive thermal anomalies. In this study, we performed an analysis of inter-regional and inter-annual differences in temperature (T) conditions associated with MMEs of the red gorgonian Paramuricea clavata by analyzing high resolution T time series (hourly records for 3 to 8 years) from four regions of the NW Mediterranean with differing hydrological conditions and biological impacts. High resolution records allowed a detailed analysis using classical and new descriptors to characterize T anomalies. We were able to determine that the MMEs were triggered by two main types of positive thermal anomalies, with the first type being characterized by short periods (2 to 5 days) with high Mean T reaching more than 27°C in some regions and being associated with high intra-day and intra-period variability, while the second type of anomaly presented long duration (near one month) at warm T (24°C) with low intra-period variability. Inter-regional patterns arose; some regions displayed both types of anomalies, while others exhibited only one type. The results showed that T conditions should be considered as the main factor that explains the observed inter-regional and inter-annual differences in mortality impacts. In explaining these differences, the late timing of T anomalies, in addition to their magnitude was found to be determinant. Finally, by combining thermotolerance experimental data with the maximal T stress conditions observed in the four regions, we were able to determine the differential risk of mass mortality across regions. We conclude that expanding high resolution T series is important for the development of sound management and conservation plans to protect Mediterranean marine biodiversity in the face of climate change.

Low dynamics, high longevity and persistence of sessile structural species dwelling on Mediterranean coralligenous outcrops

There is still limited understanding of the processes underlying benthic species dynamics in marine coastal habitats, which are of disproportionate importance in terms of productivity and biodiversity. The life-history traits of long-lived benthic species in these habitats are particularly poorly documented. In this study, we assessed decadal patterns of population dynamics for ten sponge and anthozoan species that play key structural roles in coralligenous outcrops (∼25 m depth) in two areas of the NW Mediterranean Sea. This study was based on examination of a unique long-term photographic series, which allowed analysis of population dynamics over extensive spatial and time spans for the very first time. Specifically, 671 individuals were censused annually over periods of 25-, 15-, and 5-years. This long-term study quantitatively revealed a common life-history pattern among the ten studied species, despite the fact they present different growth forms. Low mortality rates (3.4% yr⁻¹ for all species combined) and infrequent recruitment events (mean value of 3.1±0.5 SE recruits yr⁻¹) provided only a very small fraction of the new colonies required to maintain population sizes. Overall, annual mortality and recruitment rates did not differ significantly among years; however, some species displayed important mortality events and recruitment pulses, indicating variability among species. Based on the growth rates of these 10 species, we projected their longevity and, obtained a mean estimated age of 25–200 years. Finally, the low to moderate turnover rates (mean value 0.80% yr⁻¹) observed among the coralligenous species were in agreement with their low dynamics and persistence. These results offer solid baseline data and reveal that these habitats are among the most vulnerable to the current increases of anthropogenic disturbances.

Sea urchins predation facilitates coral invasion in a marine reserve

Macroalgae is the dominant trophic group on Mediterranean infralittoral rocky bottoms, whereas zooxanthellate corals are extremely rare. However, in recent years, the invasive coral Oculina patagonica appears to be increasing its abundance through unknown means. Here we examine the pattern of variation of this species at a marine reserve between 2002 and 2010 and contribute to the understanding of the mechanisms that allow its current increase. Because indirect interactions between species can play a relevant role in the establishment of species, a parallel assessment of the sea urchin Paracentrotus lividus, the main herbivorous invertebrate in this habitat and thus a key species, was conducted. O. patagonica has shown a 3-fold increase in abundance over the last 8 years and has become the most abundant invertebrate in the shallow waters of the marine reserve, matching some dominant erect macroalgae in abundance. High recruitment played an important role in this increasing coral abundance. The results from this study provide compelling evidence that the increase in sea urchin abundance may be one of the main drivers of the observed increase in coral abundance. Sea urchins overgraze macroalgae and create barren patches in the space-limited macroalgal community that subsequently facilitate coral recruitment. This study indicates that trophic interactions contributed to the success of an invasive coral in the Mediterranean because sea urchins grazing activity indirectly facilitated expansion of the coral. Current coral abundance at the marine reserve has ended the monopolization of algae in rocky infralittoral assemblages, an event that could greatly modify both the underwater seascape and the sources of primary production in the ecosystem.

Patterns of chemical diversity in the Mediterranean sponge Spongia lamella

The intra-specific diversity in secondary metabolites can provide crucial information for understanding species ecology and evolution but has received limited attention in marine chemical ecology. The complex nature of diversity is partially responsible for the lack of studies, which often target a narrow number of major compounds. Here, we investigated the intra-specific chemical diversity of the Mediterranean sponge Spongia lamella. The chemical profiles of seven populations spreading over 1200 km in the Western Mediterranean were obtained by a straightforward SPE-HPLC-DAD-ELSD process whereas the identity of compounds was assessed by comparison between HPLC-MS spectra and literature data. Chemical diversity calculated by richness and Shannon indexes differed significantly between sponge populations but not at a larger regional scale. We used factor analysis, analysis of variance, and regression analysis to examine the chemical variability of this sponge at local and regional scales, to establish general patterns of variation in chemical diversity. The abundance of some metabolites varied significantly between sponge populations. Despite these significant differences between populations, we found a clear pattern of increasing chemical dissimilarity with increasing geographic distance. Additional large spatial scale studies on the chemical diversity of marine organisms will validate the universality or exclusivity of this pattern.

Sponge mass mortalities in a warming Mediterranean Sea: are cyanobacteria-harboring species worse off?

Mass mortality events are increasing dramatically in all coastal marine environments. Determining the underlying causes of mass mortality events has proven difficult in the past because of the lack of prior quantitative data on populations and environmental variables. Four-year surveys of two shallow-water sponge species, Ircinia fasciculata and Sarcotragus spinosulum, were carried out in the western Mediterranean Sea. These surveys provided evidence of two severe sponge die-offs (total mortality ranging from 80 to 95% of specimens) occurring in the summers of 2008 and 2009. These events primarily affected I. fasciculata, which hosts both phototrophic and heterotrophic microsymbionts, while they did not affect S. spinosulum, which harbors only heterotrophic bacteria. We observed a significant positive correlation between the percentage of injured I. fasciculata specimens and exposure time to elevated temperature conditions in all populations, suggesting a key role of temperature in triggering mortality events. A comparative ultrastructural study of injured and healthy I. fasciculata specimens showed that cyanobacteria disappeared from injured specimens, which suggests that cyanobacterial decay could be involved in I. fasciculata mortality. A laboratory experiment confirmed that the cyanobacteria harbored by I. fasciculata displayed a significant reduction in photosynthetic efficiency in the highest temperature treatment. The sponge disease reported here led to a severe decrease in the abundance of the surveyed populations. It represents one of the most dramatic mass mortality events to date in the Mediterranean Sea.

Human health benefits supplied by Mediterranean marine biodiversity

This paper summarizes the overall benefits supplied by Mediterranean marine biodiversity to human health and highlights the anthropogenic and environmental causes that are threatening these benefits. First, the Mediterranean Sea is a valuable source of seafood, which is an important component of the so-called "Mediterranean diet". This type of diet has several health benefits, including cardio and cancer protective effects, which are attributed to the high intake of seafood-derived n-3 (omega-3) fatty acids. Second, the Mediterranean marine organisms, particularly the benthic ones, have furnished a large variety of bioactive metabolites, some of which are being developed into new drugs to threat major human diseases such as cancer. Third, the Mediterranean coastal areas provide environments for practising maritime leisure activities that provide physical and psychological benefits to users. Despite all this, fishing, tourism, contamination and sea warming are deteriorating this rich marine ecosystem, which needs to be protected to assure human welfare.

The biodiversity of the Mediterranean Sea: estimates, patterns, and threats

The Mediterranean Sea is a marine biodiversity hot spot. Here we combined an extensive literature analysis with expert opinions to update publicly available estimates of major taxa in this marine ecosystem and to revise and update several species lists. We also assessed overall spatial and temporal patterns of species diversity and identified major changes and threats. Our results listed approximately 17,000 marine species occurring in the Mediterranean Sea. However, our estimates of marine diversity are still incomplete as yet-undescribed species will be added in the future. Diversity for microbes is substantially underestimated, and the deep-sea areas and portions of the southern and eastern region are still poorly known. In addition, the invasion of alien species is a crucial factor that will continue to change the biodiversity of the Mediterranean, mainly in its eastern basin that can spread rapidly northwards and westwards due to the warming of the Mediterranean Sea. Spatial patterns showed a general decrease in biodiversity from northwestern to southeastern regions following a gradient of production, with some exceptions and caution due to gaps in our knowledge of the biota along the southern and eastern rims. Biodiversity was also generally higher in coastal areas and continental shelves, and decreases with depth. Temporal trends indicated that overexploitation and habitat loss have been the main human drivers of historical changes in biodiversity. At present, habitat loss and degradation, followed by fishing impacts, pollution, climate change, eutrophication, and the establishment of alien species are the most important threats and affect the greatest number of taxonomic groups. All these impacts are expected to grow in importance in the future, especially climate change and habitat degradation. The spatial identification of hot spots highlighted the ecological importance of most of the western Mediterranean shelves (and in particular, the Strait of Gibraltar and the adjacent Alboran Sea), western African coast, the Adriatic, and the Aegean Sea, which show high concentrations of endangered, threatened, or vulnerable species. The Levantine Basin, severely impacted by the invasion of species, is endangered as well. This abstract has been translated to other languages (File S1).

Global warming-enhanced stratification and mass mortality events in the Mediterranean

Summer conditions in the Mediterranean Sea are characterized by high temperatures and low food availability. This leads to "summer dormancy" in many benthic suspension feeders due to energetic constraints. Analysis of the most recent 33-year temperature time series demonstrated enhanced stratification due to global warming, which produced a approximately 40% lengthening of summer conditions. Many biological processes are expected to be affected by this trend, culminating in such events as mass mortality of invertebrates. Climatic anomalies concomitant with the occurrence of these events represent prolonged exposure to warmer summer conditions coupled with reduced food resources. Simulation of the effects of these conditions on a model organism demonstrated a biomass loss of >35%. Losses of this magnitude result in mortality similar to that noted in field observations during mass mortality events. These results indicate that temperature anomalies are the underlying cause of the events, with energetic constraints serving as the main triggering mechanism.