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The blue crab Callinectes sapidus has shown an immense capacity to adapt to new habitats in the Mediterranean Sea. Followingthe numerous reports of its proliferation along western Sicily, an investigation was conducted to identify any existing populations. In August 2021, a population of blue crabs was found in the natural reserve of Trapani which includes a large area of restored saltmarshes. In this study, by developing a tracking system using Unmanned Aerial Vehicles (UAV), we studied (i) the behavior of blue crabs on an hourly scale, and (ii) the weekly position of blue crabs in the saltmarshes to determine substrate preference. This study provides a new approach to better understand C. sapidus’ habitat use and their potential impact on local biodiversity. Blue crab activity was found to increase with temperature and tidal height, with a peak in activity observed at high tide and at maximum temperatures: the mean speed (m h-1) was higher (12.1 ± 8.7 m h-1) at T > 28 °C than at T = 23.7 °C (2.0 ± 3.2 m h-1). Considering habitat use, in 49 ± 13 % of the cases, blue crabs were observed on the sand, while 21 ± 14 % in the Cymodocea meadows, and 30 ± 15 % in the Ruppia meadows. These microhabitats provide a refuge for C. sapidus and should be prioritized and studied before management plans are designed and implemented to manage this important invasive species.

Temperature is a fundamental variable for all biological processes. It influences the metabolism and tolerance limits of all living organisms, affecting species phenology and distribution patterns. It also facilitates the spread of non-indigenous species and the proliferation and expansion of native outbreak-forming species. Pennaria disticha is a colonial benthic cnidarian reported to be invasive in different Indian and Pacific coastal areas, as well as a harmful member of fouling communities found in Mediterranean marine aquaculture farms. Using the most basal functional trait (i.e., thermal tolerance), we explored the potential of P. disticha to colonize different habitats across the Mediterranean Sea in future warming scenarios. Respiration rate was measured as a proxy of P. disticha metabolism under 12 different experimental temperatures. The obtained thermal tolerance dataset was used to create a thermal performance curve (TPC). We then scaled modeled curve to occurrence probability to map species potential metabolic habitat suitability and phenological shifts within the Mediterranean Sea when subjected to different warming scenarios. Prediction maps for future climatic conditions showed a potential temporal and spatial expansion of P. disticha in the Western and Central Mediterranean. The present data increases our understanding of the ecological performance and potential distribution of an invasive and outbreak-forming species. This information will contribute to the development of early warning systems and to the design and implementation of risk assessment and management plans.

51 pages, 6 figures, Global change is striking harder and faster in the Mediterranean Sea than elsewhere, where high levels of human pressure and proneness to climate change interact in mod- ifying the structure and disrupting regulative mechanisms of marine ecosystems. Rocky reefs are particularly exposed to such environmental changes with ongoing trends of degradation being impressive. Due to the variety of habitat types and associated marine biodiversity, rocky reefs are critical for the functioning of marine ecosystems, and their decline could profoundly affect the provision of essential goods and services which human populations in coastal areas rely upon. Here, we provide an up-to-date overview of the status of rocky reefs, trends in human-driven changes undermining their integrity, and current and upcoming management and conservation strategies, attempting a pro- jection on what could be the future of this essential component of Mediterranean marine ecosystems.

International audience

Biodiversity-Ecosystem Function experimental stu- dies focusing on marine systems are few and produce contrasting outcomes. Here, we provide experimental proofs on how communities, on a natural gradient of diversity, respond under stressful conditions in inter- tidal habitats. In August 2017 a field experiment was performed in Western Sicily (Italy) in a site charac- terized by tidal pools whose biodiversity structure and composition change as a function of the distance from the low tide mark. The pools closer to the sea are more stable from a thermal and oxic point of view and characterized by high algal biodiversity. Those further from the sea are highly variable with extreme conditions and are poorer in species. We chose 12 pools – homogeneous in size and geometry – and, to know the environmental fluctuations, we measured, for 24 hours at the minute resolution, the oxygen concentration (through an oximeter) and the tem- perature (using thermos-loggers). Then, we carried out a manipulative experiment, follow the same field procedures, to investigate the effects of a climatic event (heat wave) in combination with anthropogenic stress (increased gasoline concentra- tion). We used community metabolism (oxygen consumption, mg/L) as a proxy of functioning. At the end of the treatments, we characterized the bio- diversity contained in each pool by scraping 3 quad- rats 10 x 10 cm and related it to community metabolism. No difference was observed in the metabolism between treated and controlled in high diversity pools; whilst significant differences have been found in low diversity pools for the climatic stressor. Regarding to the chemical stressor, no remarkable difference has been found. Interestingly, these results show that under the action of a disturbance, the functioning – as expressed by oxy- gen consumption – can be altered over time. The richer the community increases the ability to restore the control conditions, while the poorer pools were unable to do so.

Impacts of climate change involve the interactions of multiple stressors on intertidal organisms, but rarely are the impacts of these stressors examined together. Many tests are also conducted in artificial, controlled laboratory conditions, without making use of natural opportunities to test performance of organisms under different environmental stressors. Given its small tidal range, the Mediterranean Sea provides such an opportunity, with a very fine scale environmental gradient and species living very close to each other over the tidal gradient. The vertical distribution of the limpets, Patella rustica and P. caerulea overlap in Palermo, Sicily, but despite this they have different thermal windows. The higher shore P. rustica had a lower metabolic rate than the mid shore P. caerulea, and was also more tolerant of thermal stress, being able to maintain heart function up to 38°C as opposed to 36°C in P. caerulea. When heat stressed, P. rustica also induced heat shock protein expression, whereas expression in P. caerulea was very low. Utilizing a natural gradient of seawater pH (caused by CO2 vents off Vulcano Island, Sicily), the combined effects of ocean acidification and thermal stress were investigated, which revealed an intraspecific gradient in species response. In general both species were more tolerant of thermal stress, maintaining heart rates between 41- 43°C, but individuals stressed by lower pH were less tolerant as compared to those further from the vent which experienced natural seawater pH. Heat shock protein expression and function of metabolic enzymes were also analyzed. These preliminary findings highlight the need for testing the impacts of multiple stressors, but also the benefits of utilizing naturally occurring habitats to investigate the interactions between environmental stressors.

Patella rustica e Patella caerulea sono due specie congeneriche di patelle che vivono lungo le coste rocciose del Mediterraneo. La prima è dominante nella zona intertidale superiore, mentre la seconda è più abbondante nella parte inferiore della stessa zona. Entrambe le specie si trovano periodicamente esposte a condizioni fisiche fortemente stressanti come l’emersione e il riscaldamento. Gli effetti da stress fisico sono stati valutati analizzando la risposta fisiologica degli organismi utilizzando l’osmolarità dell’emolinfa e come marker biochimico le heat shock proteins (Hsp70).I trattamenti in laboratorio hanno simulato le condizioni naturali alle quali questi animali possono periodicamente trovarsi. L’emersione e il conseguente riscaldamento costituiscono un fattore molto stressante. Un dato interessante si evince invece dall’analisi dell’Hsp70 (isoforma costitutiva Hsp73), i valori di queste proteine in entrambe le specie non variano significativamente rispetto ai controlli dimostrando che questi animali posseggono già sufficienti quantità di Hsps come strategia di difesa primaria agli stress acuti. Questo potrebbe spiegare la loro termo-tolleranza che permette la sopravvivenza in ambienti tidali.

Ecologically dominant species often define ecosystem states, but as human disturbances intensify, their subordinate counterparts increasingly displace them. We consider the duality of disturbance by examining how environmental drivers can simultaneously act as a stressor to dominant species and as a resource to subordinates. Using a model ecosystem, we demonstrate that CO2-driven interactions between species can account for such reversals in dominance; i.e., the displacement of dominants (kelp forests) by subordinates (turf algae). We established that CO2 enrichment had a direct positive effect on productivity of turfs, but a negligible effect on kelp. CO2 enrichment further suppressed the abundance and feeding rate of the primary grazer of turfs (sea urchins), but had an opposite effect on the minor grazer (gastropods). Thus, boosted production of subordinate producers, exacerbated by a net reduction in its consumption by primary grazers, accounts for community change (i.e., turf displacing kelp). Ecosystem collapse, therefore, is more likely when resource enrichment alters competitive dominance of producers, and consumers fail to compensate. By recognizing such duality in the responses of interacting species to disturbance, which may stabilize or exacerbate change, we can begin to understand how intensifying human disturbances determine whether or not ecosystems undergo phase shifts. Refereed/Peer-reviewed

Global warming, through increasing temperatures, may facilitate the spread and proliferation of outbreak-forming species which may find favourable substrate conditions on artificial aquaculture structures. The presence of stinging organisms (cnidarian hydroids) in the facilities fouling community are a source of pollution that can cause critical problems when in-situ underwater cleaning processes are performed. Multiple stressor experiments were carried out to investigate the cumulative effect on farmed mussels' functional traits when exposed to realistic stressful conditions, including presence of harmful cnidarian cells and environmental conditions of increasing temperature and short-term hypoxia. Exposure to combined stressors significantly altered mussels' performance, causing metabolic depression and low filtering activity, potentially delaying, or inhibiting their recovery ability and ultimately jeopardizing organisms' fitness. Further research on the stressors properties and occurrence is needed to obtain more realistic responses from organisms to minimize climate change impacts and increase ecosystem and marine economic activities resilience to multiple stressors.

Thunnus thynnus (Atlantic bluefin tuna, ABT) and other tuna species reproduce in the Mediterranean Sea during the summer period. Despite the Central Mediterranean Sea, the Strait of Sicily in particular, being a key spawning site for many tuna species, little is known on the effects of oceanographic variability on their larval distribution in this area. The abundance and presence-absence of larval specimens for three tuna species (ABT, bullet tuna and albacore) were modeled in order to examine their relationships with environmental factors, by analysing historical in situ information collected during seven annual surveys (2010–2016). The results revealed that most tuna larvae for the three species were found in the easternmost part of the study area, south of Capo Passero. This area is characterized by a stable saline front and warmer nutrient-poor water, and it has different environmental conditions, compared with the surrounding areas. The models used to investigate the presence-absence and abundance of the three species showed that ABT was the most abundant, followed by bullet tuna and albacore. The presence and abundance data collected are comparable with those of other spawning areas in the Mediterranean. Regarding biological and physical parameters, the results suggest that temperature, salinity, and day of the year are the key factors for understanding the ecological mechanisms and geographical distribution of these species in this area. Temperature affects the presence of ABT larvae and salinity, which, with a physical barrier effect, is a key factor for the presence-absence of bullet and albacore and for albacore abundance.

Biodiversity plays a key role for our planet by buffering ongoing and future changes in environmental conditions. We tested if canopy-forming algae enhancing biodiversity (CEB) in a Mediterranean intertidal reef ecological community could alleviate the effect of stressors (heat waves and pollution from sewage) on community metabolic rates (as expressed by oxygen consumption) used as a proxy of community functioning. CEB exerted a buffering effect related to the properties of stressor: physical-pulsing (heat wave) and chronic-trophic (sewage). After a simulated heat wave, CEB was effective in buffering the impacts of detrimental temperatures on the functioning of the community. In reefs exposed to chronic sewage effluents, benefits derived from CEB were less evident, which is likely due to the stressor's contextual action. The results support the hypothesis that ecological responses depend on stressor typology acting at local level and provide insights for improving management measures to mitigate anthropogenic disturbance.

Estimated larval backward trajectories of three Tuna species, namely, Atlantic Bluefin Tuna (Thunnus thynnus, Linnaeus, 1758), Bullet Tuna (Auxis Rochei, Risso, 1801) and Albacore Tuna (Thunnus alalunga, Bonnaterre, 1788) in the central Mediterranean Sea, were used to characterize their spawning habitats, and to assess the impact of changes due to the major environmental parameters (i.e., sea surface temperature and chlorophyll-a concentration) on larval development during their advection by surface currents. We assumed that the environmental variability experienced by larvae along their paths may have influenced their development, also affecting their survival. Our results showed that the Tuna larvae underwent an accelerated growth in favorable environmental conditions, impacting on the notochord development. In addition, further updated information on spawning and larval retention habitats of Atlantic Bluefin Tuna, Bullet and Albacore Tunas in the central Mediterranean Sea were delivered.

Around 350 million tonnes of plastics are annually produced worldwide. A remarkable percentage of these products is dispersed in the environment, finally reaching and dispersed in the marine environment. Recent field surveys detected microplastics’ concentrations in the Mediterranean Sea. The most commonly polymers found were polyethylene, polypropylene and viscose, ethylene vinyl acetate and polystyrene. In general, the in-situ monitoring of microplastic pollution is difficult and time consuming. The main goals of this work were to spectrally characterize the most commonly polymers and to quantify their spectral separability that may allow to determine optimal band combinations for imaging techniques monitoring. The spectral signatures of microplastics have been analysed in laboratory, both in dry condition and on water surface, using a full spectrum spectroradiometer. The theoretical use of operational satellite images for remote sensing monitoring was investigated by quantifying the spectral separability achievable by their sensors. The WorldView-3 sensor appears the most suitable for the monitoring but better average spectral separability are expected using the recently released PRISMA images. This research was preparatory to further outdoor experiments needed to better simulate the real acquisition condition.

The Mediterranean Sea is subject to multiple human pressures increasingly threatening its unique biodiversity. Spatially explicit information on the ecological status of marine ecosystems is therefore key to an effective maritime spatial planning and management, and to help the achievement of environmental targets. Here, we summarized scientific data on the ecological status of a selection of marine ecosystems based on a set of ecological indicators in more than 700 sites of the Mediterranean Sea. For Posidonia oceanica seagrass beds, rocky intertidal fringe, and coastal soft bottoms, more than 70% of investigated sites exhibited good to high ecological conditions. In contrast, about two-thirds of sites for subtidal rocky reefs were classified to be in moderate to bad conditions, stressing the need for prioritizing conservation initiatives on these productive and diverse environments. Very little quantitative information was available for the southern Mediterranean Sea, thus monitoring programs and assessments in this area are essential for a representative assessment of the health of marine coastal ecosystems in the whole basin. This overview represents a first step to implement a baseline that, through georeferenced data on ecological status, could help identifying information gaps, directing future research priorities, and supporting improvements to spatial models of expected cumulative impacts on marine ecosystems.

Macroalgal forests are one of the most productive and valuable marine ecosystems, but yet strongly exposed to fragmentation and loss. Detailed large-scale information on their distribution is largely lacking, hindering conservation initiatives. In this study, a systematic effort to combine spatial data on Cystoseira C. Agardh canopies (Fucales, Phaeophyta) was carried out to develop a Habitat Suitability Model (HSM) at Mediterranean scale, providing critical tools to improve site prioritization for their management, restoration and protection. A georeferenced database on the occurrence of 20 Cystoseira species was produced collecting all the available information from published and grey literature, web data portals and co-authors personal data. Data were associated to 55 predictor variable layers in the (ASCII) raster format and were used in order to develop the HSM by means of a Random Forest, a very effective Machine Learning technique. Knowledge about the distribution of Cystoseira canopies was available for about the 14% of the Mediterranean coastline. Absence data were available only for the 2% of the basin. Despite these gaps, our HSM showed high accuracy levels in reproducing Cystoseira distribution so that the first continuous maps of the habitat across the entire basin was produced. Misclassification errors mainly occurred in the eastern and southern part of the basin, where large gaps of knowledge emerged. The most relevant drivers were the geomorphological ones, followed by anthropogenic variables proxies of pollution and urbanization. Our model shows the importance of data sharing to combine a large number of spatial and environmental data, allowing to individuate areas with high probability of Cystoseira occurrence as suitable for its presence. This approach encourages the use of this modelling tool for the prediction of Cystoseira distribution and for supporting and planning conservation and management initiatives. The step forward is to refine the spatial information of presence-absence data about Cystoseira canopies and of environmental predictors in order to address species-specific assessments.

The green lipped mussel, Perna viridis, is an important aquaculture species throughout the Indo-Pacific region where production is often impacted by environmental degradation. To predict the impacts and mitigate against environmental problems due to various kinds of anthropogenic pollution, such as heavy metals and eutrophication, on P. viridis aquaculture a Dynamic Energy Budget (DEB) model was constructed. By integrating species-specific parameters and regional-specific environmental data the DEB model determined how the life history traits of P. viridis respond to changing environmental conditions. Using various levels of basal maintenance costs and food availability to elucidate the energetic costs due to environmental pollution, the DEB model predicted that a 20% increase in basal maintenance cost due to environmental pollution such as heavy metals will result in a subsequent decrease in both lifetime reproductive output and ultimate body size of P. viridis by ~18% and ~8%, respectively. Increasing food availability can, however, mitigate the energetic constraints due to increased basal maintenance cost. The time to reach commercial size, for example, will be longer by 13% and 3% under lower and higher food availability conditions, respectively when there is a 20% increase in maintenance cost due to environmentally induced stress, which would significantly increase the operational cost of an aquaculture facility. In light of the increased importance of P. viridis as an aquaculture species, the P. viridis DEB model can, therefore, be used to illustrate the effects of varying environmental conditions on P. viridis life history traits which are relevant to the success of aquaculture facilities, and contribute towards better management of this species.

Fisheries currently represent the main source of animal protein intake worldwide, although catches of most commercial species are at or beyond maximum sustainable yields. Increasing production would require an excess of exploitation levels and aquaculture is expected to become crucial in sustaining a growing seafood demand. Nonetheless, many threats are expected to affect aquaculture and the increased production must evolve in a way that minimizes environmental and socio-economic impacts. The claimed sustainable development of human activities at sea (blue growth and economy) seeks for new joint analyses and solutions at (trans-)national systemic level should be planned and applied. To meet a sustainable development, both production and management approaches should evolve. Here we propose a conceptual framework to integrate a “downscaling approach” based on functional features of cultivated organisms to accommodate multiple stressors in setting sustainable development standards to design adaptive solutions fitting with the management of marine space.

Connecting the nonlinear and often counterintuitive physiological effects of multiple environmental drivers to the emergent impacts on ecosystems is a fundamental challenge. Unfortunately, the disconnect between the way “stressors” (e.g., warming) is considered in organismal (physiological) and ecological (community) contexts continues to hamper progress. Environmental drivers typically elicit biphasic physiological responses, where performance declines at levels above and below some optimum. It is also well understood that species exhibit highly variable response surfaces to these changes so that the optimum level of any environmental driver can vary among interacting species. Thus, species interactions are unlikely to go unaltered under environmental change. However, while these nonlinear, species-specific physiological relationships between environment and performance appear to be general, rarely are they incorporated into predictions of ecological tipping points. Instead, most ecosystem-level studies focus on varying levels of “stress” and frequently assume that any deviation from “normal” environmental conditions has similar effects, albeit with different magnitudes, on all of the species within a community. We consider a framework that realigns the positive and negative physiological effects of changes in climatic and nonclimatic drivers with indirect ecological responses. Using a series of simple models based on direct physiological responses to temperature and ocean pCO2, we explore how variation in environment-performance relationships among primary producers and consumers translates into community-level effects via trophic interactions. These models show that even in the absence of direct mortality, mismatched responses resulting from often subtle changes in the physical environment can lead to substantial ecosystem-level change. Refereed/Peer-reviewed

Aim: Understanding and quantifying the seasonal patterns in biodiversity of phyto- benthos, macro-zoobenthos and fishes in Mediterranean coastal lagoons, and the species dependence upon environmental factors. Location: The study was carried out in the “Stagnone di Marsala e Saline di Trapani e Paceco,” the largest coastal lagoon system in the central Mediterranean Sea (Sicily, Italy), a Special Protection Area located along one of the central ecological corridors joining Africa and Europe. Methods: The coastal lagoon system was selected as a model ecosystem to investi- gate the seasonal variations in biodiversity indices and dominance–diversity relation- ships in phytobenthos, macro-zoobenthos and fishes, and how seasonal variations in temperature, salinity, depth, inorganic and organic suspended matter affect the abundance of the species constituting these communities. Models of ecosystem structure, describing the interactions among functional groups and environmental variables, were also developed using confirmatory path analysis and artificial neural networks to exemplify their application in predicting temperature-driven alterations. Results: Wide seasonal variations in biodiversity indices and dominance–diversity relationships across the communities of the coastal lagoon system were observed, driven by the dynamics in climate and resource availability. The effects of the envi- ronmental variables on taxon abundances varied in relation to the community, with the widest responses elicited in phytobenthos and fishes. Temperature was the main variable affecting taxon abundances in macro-zoobenthos and was also the major driver of shallow water ecosystem structure. Main conclusions: This research shed light on the seasonal variations in biodiversity of Mediterranean coastal lagoons, elucidating also the tight dependence of phytob- enthos, macro-zoobenthos and fish diversity upon environmental factors. The find- ings and the methodological approach proposed may be crucial in developing models able to predict future climate-driven alterations in communities inhabiting these im- portant and threatened ecosystems.

Shellfish aquaculture represents a worldwide valuable segment of the aquaculture market, spreading along the Mediterranean coasts, and is sensitive to the still unforeseen, poorly-known effects of climate change. Threats due to temperature rise can threaten the deployment and development of this sector, up until now recognised as the best candidate to mitigate the effects of fishery overexploitation. Here, we investigate the effects of temperature increase on the model species, Mytilus galloprovincialis, measuring outcomes from valve fragility (thickness) and condition index. Evidence of a reduction in the thickness of valves and the modulation condition of the mussels along with temperature increase have been gathered from simulations of a natural temperature gradient changing along latitude (the Italian Peninsula) and temperature risen (mesocosm trial). The obtained results offer a baseline to help the next generation of managers and stakeholders when assessing the reliability and feasibility of shellfish culture in a changing sea that can generate undetected and underestimated impacts on the sector.

Defining sustainability goals is a crucial but difficult task because it often involves the quantification of multiple interrelated and sometimes conflicting components. This complexity may be exacerbated by climate change, which will increase environmental vulnerability in aquaculture and potentially compromise the ability to meet the needs of a growing human population. Here, we developed an approach to inform sustainable aquaculture by quantifying spatio-temporal shifts in critical trade-offs between environmental costs and benefits using the time to reach the commercial size as a possible proxy of economic implications of aquaculture under climate change. Our results indicate that optimizing aquaculture practices by minimizing impact (this study considers as impact a benthic carbon deposition≥1g Cm−2day−1) will become increasingly difficult under climate change. Moreover, an increasing temperature will produce a poleward shift in sustainability trade-offs. These findings suggest that future sustainable management strategies and plans will need to account for the effects of climate change across scales. Overall, our results highlight the importance of integrating environmental factors in order to sustainably manage critical natural resources under shifting climatic conditions.

Marine litter is a major source of pollution in the Mediterranean basin, but despite legislative requirements, scant information is available for the ongoing assessment of this threat. Using higher size classes as proxy for litter distribution, this study gave a synoptic estimation of the amount, composition, and distribution of floating macro-litter in the Mediterranean. The average amount of macro-litter was in a range of 2-5 items/km2, with the highest in the Adriatic basin. Seasonal patterns were present in almost all study areas and were significant in the Ligurian Sea, Sardinian-Balearic basin, and Central Tyrrhenian Sea. Plastic accounted for > 80% of litter in all areas and seasons, with the highest proportion in the Adriatic Sea, Ligurian Sea, and Sicilian-Sardinian Channels; in the Bonifacio Strait, Tyrrhenian Sea, and Sardinian-Balearic basin, litter composition was instead more diverse. Spatial analysis suggested an almost homogeneous distribution of litter without evident regular aggregation zones.

Chlorophyll- a ( CHL-a ) and sea surface temperature ( SST ) are generally accepted as proxies for water quality. They can be easily retrieved in a quasi-near real time mode through satellite remote sensing and, as such, they provide an overview of the water quality on a synoptic scale in open waters. Their distributions evolve in space and time in response to local and remote forcing, such as winds and currents, which however have much finer temporal and spatial scales than those resolvable by satellites in spite of recent advances in satellite remote-sensing techniques. Satellite data are often characterized by a moderate temporal resolution to adequately catch the actual sub-grid physical processes. Conventional pointwise measurements can resolve high-frequency motions such as tides or high-frequency wind-driven currents, however they are inadequate to resolve their spatial variability over wide areas. We show in this paper that a combined use of near-surface currents, available through High-Frequency (HF) radars, and satellite data ( e.g. , TERRA and AQUA/MODIS), can properly resolve the main oceanographic features in both coastal and open-sea regions, particularly at the coastal boundaries where satellite imageries fail, and are complementary tools to interpret ocean productivity and resource management in the Sicily Channel.

9 pages, 5 figures, 4 tables, The increasing frequency of jellyfish outbreaks in coastal areas has led to multiple ecological and socio-economic issues, including mass mortalities of farmed fish. We investigated the sensitivity of the European sea bass (Dicentrarchus labrax), a widely cultured fish in the Mediterranean Sea, to the combined stressors of temperature, hypoxia and stings from the jellyfish Pelagia noctiluca, through measurement of oxygen consumption rates (MO2), critical oxygen levels (PO2crit), and histological analysis of tissue damage. Higher levels of MO2, PO2crit and gill damage in treated fish demonstrated that the synergy of environmental and biotic stressors dramatically impair farmed fish metabolic performances and increase their health vulnerability. As a corollary, in the current scenario of ocean warming, these findings suggest that the combined effects of recurrent hypoxic events and jellyfish blooms in coastal areas might also threaten wild fish populations, This work has received funding from the European Union’s projects MED-JELLYRISK (grant n. I-A/1.3/098 - ENPI CBCMED programme), VECTORS (Vectors of Change in Oceans and Seas Marine Life, Impact on Economic Sectors, grant n. 266445, FP7th programme), CERES (Climate Change and European Aquatic Resources, grant n. 678193, Horizon 2020 programme) and from the Italian Ministry of Research and University project PRIN TETRIS 2010 (grant n. 2010PBMAXP_003)