Marta Coll; Chiara Piroddi; Jeroen Steenbeek; Kristin Kaschner; Frida Ben Rais Lasram; Jacopo Aguzzi; Enric Ballesteros; Carlo Nike Bianchi; Jordi Corbera; Thanos Dailianis; Roberto Danovaro; Marta Estrada; Carlo Froglia; Bella S. Galil; Josep M. Gasol; Ruthy Gertwagen; João Gil; François Guilhaumon; Kathleen Kesner-Reyes; Miltiadis-Spyridon Kitsos; Athanasios Koukouras; Nikolaos Lampadariou; Elijah Laxamana; Carlos M. López-Fé de la Cuadra; Heike K. Lotze; Daniel Martin; David Mouillot; Daniel Oro; Saša Raicevich; Josephine Rius-Barile; Jose Ignacio Saiz-Salinas; Carles San Vicente; Samuel Somot; José Templado; Xavier Turon; Dimitris Vafidis; Roger Villanueva; Eleni Voultsiadou

“ Explanations for the observed heterogeneity of species richness in the Mediterranean Sea include the threshold of the Siculo-Tunisian Strait that divides the Mediterranean into two basins, and the paleo-biogeographical history of the Mediterranean Sea. The western basin shows more biological similarity with the Atlantic Ocean, hosting a higher number of cold-temperate species, while the eastern basin shows more biological similarities with the Indo-Pacific, and hosts a larger number of subtropical species. ”

“ Biodiversity patterns for the marine mammals contrasted with patterns for fishes and invertebrates in that many species were also predicted to occur in the offshore western and central basin waters, and particularly in slope waters (Figure 5e) . The biodiversity patterns of sea turtles broadly mimic those of the other more species-rich taxa in that there was a concentration in coastal areas and a decline in species richness from the northwest to the southeast (Figure 5f) . ”

“ If the southern sectors of the Mediterranean are being “tropicalized” (higher occurrence of tropical aliens) and the northern sectors “meridionalized” (increased proportion of indigenous thermophilic species) , it is uncertain what will happen to those species of boreo-Atlantic origin, which entered the Mediterranean during glacial periods and have been established in the northern and colder areas of the basin. Because they cannot move farther northward, they may dramatically decrease [324] or even be at risk of extirpation. ”

“ The export of fish and fish products, including salt-fish (tarichos) and fish sauce (garum) mainly from European anchovy to the Aegean Sea, continue into the Roman period [205] . These products are exported from the western Mediterranean, but garum is forgotten in the west by the tenth century, although it is still prepared in Constantinople in the fifteenth and sixteenth centuries [206] . Naval trade traffic becomes intense, and invasions of islands from the mainland are already common, and they result in the beginning of the introduction of alien species in those ecosystems. ”

“ Coastal development, sediment loading, and pollution reduced the extent of important habitats for marine diversity, such as seagrass meadows, oyster reefs, maërl, and macroalgal beds, and affected Mediterranean ecosystem functioning well before the 1900s [271] – [272] . Most species depend strongly on their habitats (such as bryozoans, sponges, echinoderms, benthic decapods, and organisms of the suprabenthos and meiobenthos) ; hence, its loss and degradation have major effects on marine diversity. ”

“ These results depict a region of high diversity and heterogeneity, but they also evidence the need for further study of geographical areas that are largely unexplored, mainly the African coasts and certain zones of the southeastern basin and the deep sea.
Our study illustrates that the Mediterranean is a complex region where ecological and human influences meet and strongly interact, posing a large and growing potential impact to marine biodiversity. Although much is known about individual threats, knowledge is very limited about how multiple impacts will interact. ”

“ At bathyal and abyssal depths, levels of nematode genera and species richness are similar to those reported from other deep-sea areas of the world oceans [155] . In the deep sea of the Mediterranean, small-bodied taxa (e.g., meiofauna) can reach a high diversity, and with the presence of a high prokaryotic diversity in the sediments of the deep-sea Mediterranean [156] , this may change the view that the Mediterranean deep-sea biota is impoverished in comparison with its Atlantic counterpart. ”

“ But there may also be habitat fragmentation and local extinction of species unable to undertake migrations. Lack of (evidence of) species extinctions, coupled with establishment of alien species, is apparently leading to an increased species richness of the Mediterranean, a much debated issue [332] . Richness is increasing at the whole-basin scale (γ-diversity) , but it is difficult to establish what is happening at local scales (α-diversity) in coastal areas. ”

“ Marine research in the Mediterranean has been regionally biased, reflecting sparse efforts along the southern and easternmost rim. It has even been suggested that the relative species richness of different taxa by sector of the Mediterranean is a better indicator of the level of research effort than of true species richness [256] . Therefore, as new species are assessed in the eastern and southern areas, patterns may be modified. Moreover, the diversity in the eastern end is more influenced by species introductions. ”

“ Seawater acidification may also be a threat to Mediterranean marine biodiversity [344] . The most obvious consequence of the increased concentration of CO2 in seawater is a reduced rate of biogenic calcification in marine organisms [345] , [346] . This could affect both planktonic and benthic communities. Calcifying phytoplankton (coccolithophores) play a significant role in the primary productivity of the oligotrophic Mediterranean Sea, whereas many benthic habitats are engineered by sessile organisms that lay down carbonate crusts. ”

“ Endobenthic species display rhythms of emergence, including burying or burrowing within the substrate and sheltering in natural holes [358] . Marine species react to light intensity cycles, which may include movements in and out of our sampling windows [359] . Information gathered without attention to such rhythmicity will affect perceived population distribution, biomass, and estimated biodiversity [358] . These issues have been integral to land ecology since the early twentieth century [360] but have been rarely considered in the marine environment. In the Mediterranean, Sardà et al. ”

“ Exploitation of marine species
This study also illustrates that the oldest and one of the most important maritime activities that has become a threat to diversity is human exploitation of marine resources. People around the Mediterranean have exploited marine resources since earliest times. Maybe not surprisingly, negative effects of the exploitation of the Mediterranean marine biodiversity were first reported in the fourth century B.C. by Aristotle. ”

“ These thermophilic aliens have a distinct advantage over the native Mediterranean biota. Though no extinction of a native species is yet attributable to invasion of new species, sudden declines in abundance, concurrent with proliferation of aliens, have been recorded [167] . Examination of the profound ecological impacts of some of the most conspicuous invasive alien species underscores their role, among many anthropogenic stressors, in altering the infralittoral benthic communities. ”

“ Food availability depends almost entirely on the supply of energy from the water column and decreases with depth, which may explain most of the variability between the observed spatial patterns of the benthic biodiversity in the deep Mediterranean Sea. ”

“ Bathymetric patterns
Because seaweeds and seagrasses are photosynthetic organisms, their development is limited to shallow areas where there is enough light for growth. They are distributed between the mediolittoral zone and the deepest limit of the circalittoral zone, situated at 110 m in the clearest waters of the western Mediterranean [179] and a bit deeper in the even more oligotrophic waters of the eastern part [180] . Their growth occurs only on the continental shelves and the uppermost parts of seamounts above 150 m depth. ”

“ The numbers of alien species that have been recorded over the past century have increased in recent decades. The increasing role of the Mediterranean as a hub of international commercial shipping, a surge in the development of marine shellfish farming over the last 25 years, and the continued enlargement of the Suez Canal have contributed to the resurgence of introductions since the 1950s. ”

“ As shown above, anthropogenic factors have influenced the general patterns and temporal trends of Mediterranean marine diversity with varying degrees of intensity. Quantifying the importance of each threat is essential for future analysis.
Lotze et al. [73] provided data to evaluate the human impacts that caused or contributed to the depletion or extirpation of species in the north Adriatic Sea over historical time scales. Exploitation stood out as the most important factor causing or contributing to 93% of depletions and 100% of local extinctions or extirpations (Figure 8c) . ”

“ While both methods identified areas around Sicily, the coast of Tunisia, and the Western Mediterranean as high diversity hot spots, the Adriatic and Aegean seas showed up as high in species richness only in the predicted maps. Both types of analyses arrived at similar patterns for marine mammals, although the lack of distinction between resident and visitor species in the AquaMaps analysis hampered the direct comparison of diversity patterns for these taxa. ”

“ Human impacts on marine biodiversity grow increasingly stronger as the Mediterranean cities and ports continue to grow and more recent centuries witnessed substantial advances in technology. It is assumed that since the fourteenth century, the adoption of new fishing methods (such as the tonnara, a sort of drift net mainly used for tuna fishing) in the Western Mediterranean, their spread to southern Italy [217] , [218] , and their introduction to the Adriatic in the seventeenth century [217] , [219] increase fishing catches. ”

“ In the western basin, where the depth does not exceed 3,000 m, the abyssal fauna is less abundant than in the deeper eastern basin, where abyssal species are dominant in the Matapan trench, which is more than 5,050 m deep [141] . The close affinity between Mediterranean and Atlantic congeneric deep-water species suggests that the ancestors of the Mediterranean bathyal endemic species moved from the Atlantic when conditions were favorable (i.e. when larvae of deep Atlantic fauna was able to enter in the Western Mediterranean due to hydrodynamic and physico-chemical conditions allowed it) . ”

“ By their nature, expert-drawn maps or sightings often represent underestimates of total species distributions because of the absence or lack of effort in certain areas (in our case the southern shorelines of Mediterranean along the coasts of northern Africa and the eastern sites) and the inability to detect rarer species without sufficient efforts. On the other side, AquaMaps model predictions do not currently factor human impacts or ecological interactions and may be closer to fundamental or historical niche rather than realized niche. ”

“ However, the populations of the most common benthic mollusks in depths greater than 1,000 m off the Israeli coast are composed of both adult and juvenile specimens, and one species, Yoldia micrometrica, the most common and abundant species in the eastern Mediterranean, is unrecorded from the westernmost part of the sea. ”

“ Benthic biodiversity is estimated as the total number of meiofaunal taxa, and as nematode species richness (expected number of nematode species for a theoretical sample of 51 specimens) . Additional information from the literature on spatial patterns of Mediterranean marine diversity suggests that the measurement of local α-diversity is not sufficient to draw a clear picture for the whole Mediterranean basin. ”

“ Our understanding of the mechanisms driving deep-sea biodiversity patterns is still limited, but some of the factors frequently invoked are (a) sediment grain size and substrate heterogeneity [188] ; (b) productivity, organic content, or microbial activity [267] ; (c) food resources [189] ; (d) oxygen availability [268] ; (e) water currents [144] ; and (f) occasional catastrophic disturbances [172] . Thus, the spatial distribution of available energy may influence the distribution of benthic abundance, biomass, and biodiversity [8] , [143] , [154] , [172] , [269] – [270] . ”

“ Since the fifth century B.C., humans have exploited marine resources. Aristotle, in his zoological works dating to the fourth century B.C., focuses his scientific interest on fish and invertebrates exploited by humans in various ways [201] . Fisheries in the Aegean communities by that period are characterized by variability both in the nature and abundance of the exploited fish and in the manner of their exploitation [202] . ”

“ We produced AquaMaps of predicted patterns of biodiversity for different taxa in the Mediterranean by overlaying the respective subsets of the 685 available distribution maps for Mediterranean species and counting all species predicted to occur in a given cell. We assumed a species to be present in each cell for which the species-specific predicted relative probability of occurrence was greater than zero. ”

“ Industrialized fishing had severe impacts on species, habitats, and ecosystems [221] . Several studies also show historical changes in fish communities of different regions of the basin [e.g., [21] , [81] , [222] – [223] . These findings point to a general severe depletion of top predators in the basin, including Atlantic bluefin tuna, which is considered critically endangered according to the declining trend observed in the Atlantic and the Mediterranean in the last 50 years. ”