Spatial patterns of benthic biodiversity in the deep sea are poorly known in comparison with other ecosystems. Available information is scarce and our maps and estimates include only approximations for the deep sea. In this context, metazoan meiofauna and, in particular, nematodes can be used to describe the biodiversity patterns in the deep sea. Deep-sea nematode diversity appears to be related to that of other benthic components such as foraminifers  , macrofauna  , and the richness of higher meiofauna taxa in the deep sea . Results for the deep sea of the Mediterranean show a clear longitudinal biodiversity gradient that also occurs along the open slopes, where values decrease eastward, from Catalonia to the margins of southern Crete Figure 4a.
The analysis of the Nematoda indicates that at equally deep sites, nematode diversity decreases from the western to the eastern basin and longitudinal gradients are evident when comparing sites at 3, m or 1, m depth . Complementary information on spatial patterns of the deep Mediterranean fauna can be found in .
A Longitudinal patterns, and B bathymetric patterns of benthic nematodes along the open slopes of the European margins. 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.
These findings indicate that each region is characterized by the presence of a specific assemblage and species composition. However, these patterns may not hold for all the taxonomic groups  , and a broader comparison is needed. Predicted patterns of overall species richness based on AquaMaps showed a concentration of species in coastal and continental waters most pronounced in the Western Mediterranean, Adriatic, and Aegean seas Figure 5. Less than half of the species were predicted to occur in the deeper waters of the central Mediterranean, and biodiversity was particularly low in offshore waters at the eastern end.
Given the overall proportion of ray-finned fishes in AquaMaps dataset File S2 , overall biodiversity patterns from these figures were largely dominated by Actinopterygii Figures 5a and b. The concentration in coastal waters was more pronounced in the map focusing on these taxa Figure 5b.
Predicted species richness of elasmobranchs was similar to that for Actinopterygii , but rays and sharks occurred farther offshore, especially in the waters of Tunisia and Libya Figure 5c. The Aegean Sea, especially its northern sector, also showed high invertebrate species richness, which was otherwise low in most of the remaining central and eastern basin Figure 5d. 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. Therefore, there were similarities and differences between expert-drawn maps Figures 2 and 4 and modeling results Figure 5.
The pattern describing species richness of ray-finned fish was similar overall Figures 2b and 5b , but for the elasmobranchs there were some noticeable differences Figures 2c and 5c. 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. Nevertheless, differences could be seen around the Aegean and Alboran seas Figures 3c and 6e. Maps of sea turtle diversity showed peaks in the western region based on both types of analysis, but there were a few discrepancies regarding the eastern Mediterranean Figures 3e and 6f.
AquaMaps analysis of predicted species richness of invertebrates also showed a geographical gradient Figure 5d. Latitudinal transects corresponding to cross sections through the species richness map Figure 5a highlighted the importance of coastal habitats for fishes and invertebrates. These habitats were represented by peaks in species numbers in areas corresponding to shelf waters Figure 6a.
Cross-section gradients followed a similar pattern for fishes and invertebrates; large variations were mostly determined by depth changes along the respective transects. There was also an overarching trend of decreasing species richness from western to eastern waters, a trend that became particularly pronounced in the southern transects. Marine mammal transects diverged from the general trend in that species richness was less directly linked to depth variation.
Changes in fish and invertebrate species richness along three different longitudinal cross sections again followed similar depth contours Figure 6b. Marine mammal longitudinal biodiversity patterns in the Western Mediterranean followed a different trend with highest numbers predicted to occur in deeper waters, such as the southern Tyrrhenian Sea. There appeared to be a general decrease of diversity from northern to southern regions. A Latitudinal transects, and B Longitudinal transects.
The contribution of fishes, invertebrates, and marine mammals to geographic gradients in biodiversity is shown. 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 m in the clearest waters of the western Mediterranean  and a bit deeper in the even more oligotrophic waters of the eastern part .
Their growth occurs only on the continental shelves and the uppermost parts of seamounts above m depth. Seaweeds, which have a limited distribution across the whole bathymetric gradient, show an increase in species richness from the highest levels of the mediolittoral rocks down to the lower infralittoral and upper circalittoral communities. There they display the highest species richness, as many as species reported in a surface of 1, cm 2 at 18 m depth . Species richness then decreases along the circalittoral zone from the shallowest down to the deepest parts  , becoming nil at the beginning of the bathyal zone.
The pattern of a generally decreasing diversity with increasing depth was also documented here for invertebrate and fish species Figures 3 , 4 , 7 , and 8 and is consistent with previous studies [e. Diversity was concentrated in coastal areas and continental shelves, mainly above m depth. However, patterns did not necessarily show a monotonic decrease with depth.
For example, more polychaete species inhabited shallow waters than deep waters, particularly below 1, m deep, but this pattern was less clear when looking at maximum ranges of depth Figure 7a , File S2. It is not clear whether this is a real pattern of lower deep-sea diversity or a result of the lack of proper faunistic studies in the Mediterranean at those depths.
Larger numbers of cumacean species were found in shallow waters of 0—99 m depth 48 species and between m and 1, m depth, but species richness decreased below this depth Figure 7b , references in File S2. The highest endemism The largest number of mysidaceans 54 species was also found in shallow waters less than m deep. At depths between m and 1, m, 27 species were found, and below 1, m, 21 species. The level of endemism was also higher in the 0— m depth interval 29 species, The circalittoral zone was the region with highest anthozoan species richness Half of the total number of species were restricted to one of the infra-, circa-, or bathyal zones, and 9.
We also found exceptions to the pattern of decreasing diversity with depth. The bathymetric range of Mediterranean sipunculans was generally quite wide .
Most of the Mediterranean records were bathyal, whereas there were few sublittoral records File S2. A Bathymetric ranges of distribution for Mediterranean polychaete species at minimum and maximum depths where they have been reported File S2 , and B number of Mediterranean cumaceans recorded in each m depth interval Endemic species are plotted in gray. For nonendemic species only records from the Mediterranean Sea are considered, File S2.
Temporal trends for alien species refer to recorded exotic mollusks in the whole Mediterranean Sea . B Shifts in species diversity of the North Adriatic Sea over historical time scales. Species depletions and extirpations occurred mostly in larger species groups, while invasions occurred in smaller and lower trophic-level species [data from ].
C Threats to diversity in the North Adriatic Sea over historical time scales. Shown is the percent of recorded species depletions and extinctions caused by, or attributed to, different human impacts. Also shown is whether human impacts acted as single or multiple causes. Data were adapted from Lotze et al.
More recent studies, however, have demonstrated that such patterns are not always recognizable [e. In open slope systems, bathymetric gradients of species diversity have been widely documented [e. In the Mediterranean, nematode diversity also decreases with depth Figure 4b , but the degree of species decrease is limited and ample ranges of biodiversity are observed at the same depth. These results suggest that the eurybathy of the Mediterranean fauna 3, species could be lower than previously reported . For example, analysis of all the existing nematode diversity data from the Aegean Sea showed that there is a gradual increase of diversity with depth from the littoral zone down to the bathyal areas 2, m N.
Lampadariou, personal observation. Complementary information on bathymetric patterns of the deep Mediterranean fauna are explored with detail in . Available data from the literature show that environmental factors have led to profound changes in the abundance, distribution, and composition of Mediterranean marine species in the distant past [e. For example, during the Cretaceous, the Mediterranean Sea called Tethys was connected to the Atlantic on its western side and the Indo-Pacific on its eastern side. The two oceans contributed very different faunas to the Tethys.
During the Miocene, the Tethys was isolated from the Indo-Pacific Ocean and at the Messinian stage, the connection with the Atlantic Ocean was also closed. During this Messinian salinity crisis, the Mediterranean underwent severe desiccation that drove most species to extinction. Although some shallow areas remained on the two sides of the Siculo-Tunisian Strait, and there were many allopatric speciations  ,  ,  , the reopening of the Strait of Gibraltar 5 million years ago led to restocking of the Mediterranean with fauna and flora from the Atlantic.
Up to the nineteenth century, the Mediterranean had been connected with the eastern Atlantic Ocean only. In this section, however, we summarized main changes since the end of the last ice age approximately 12, years ago. During this time there were notable climate-driven fluctuations but also human-induced changes due to the long periods of exploration and exploitation, and more recently the reopening to the Red Sea through the Suez Canal, the globalization of commerce and trade, increasing pollution and eutrophication of coastal areas, habitat modification and loss, and finally the looming climate change.
Early evidence of human interaction with marine fauna in the Mediterranean Sea comes from the Paleolithic period and continues through the Mesolithic and Neolithic periods approximately 20,— B. In Greece, fish bones of large tuna, Sparidae and Mugillidae , were found. Zooarchaeological remains in Spain include 20 taxa and show changes in mean fish size and range over time that have been considered as indication of overfishing.
In Gibraltar, remains of Mediterranean monk seals and mollusks consumed by humans were found. However, stable isotope analyses of human bones show that between 10, and B. Since the fifth century B. Aristotle, in his zoological works dating to the fourth century B. 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 .
Mollusks and other invertebrates are part of the diet of ancient Greeks, and their consumption is connected with the treatment or prevention of various health problems and diseases . Bath sponges of the genera Spongia and Hippospongia , collected by skillful divers, are widely exploited for household and personal hygiene purposes, and play a principal role in medical practice . Commercial fishing and fish processing activities play an important role in the Pontic economy.
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 . These products are exported from the western Mediterranean, but g arum is forgotten in the west by the tenth century, although it is still prepared in Constantinople in the fifteenth and sixteenth centuries . 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.
Some of these introductions rats, carnivores trigger the extirpation of many seabird colonies, and they have shaped the current distribution of several seabird species  , . Seafood becomes increasingly popular toward the end of Roman domination, probably because of the proximity of, and access to, marine resources. There is historical evidence of overfishing in some parts of the Western Mediterranean in the early Imperial period .
Even then, certain fishing techniques are prohibited to manage or counteract the decline in fish stocks such as fishing by torch lights at night , and efforts are made to boost natural availability with introduced fish and shellfish stocks. For example, the parrot fish Sparisoma cretense is captured in the Aegean Sea and released in the Tyrrhenian Sea  , .
There are also pictorial remains that show fishing gear and a large variety of targeted species during Roman times. Gastropods  , the red coral Corallium rubrum  , and several species of sponges  were exploited on an industrial scale. Fishing, fish processing, industrial exploitation of several marine species, and development of improved fishing gear continue during the Byzantine period . Various literary sources point out that targeted species, among them the currently overfished tuna, are conspicuous. There is a year gap between the Moslem conquest of the Near East and northern Africa and the appearance in the ninth century of the first Arabic written sources .
In northern Africa, the first written evidence dates from the tenth century and refers to fishing gear used to catch mullets, Atlantic bluefin tuna with large spears , and fish in shallow waters . Zooarchaeological material from the Israeli coastline dating from the Byzantine through the Moslem Crusader and Mamluk periods fourteenth century points to a high consumption of marine and freshwater fish that are still fished in Israel today, such as the thin-lipped grey mullet Liza ramada , Sparidae , and the parrot fish .
There is noticeable fishing activity dating from the Byzantine, Moslem tenth century , and later Norman periods eleventh to thirteenth centuries in southern Italy and in Sicily, where Atlantic bluefin tuna is the main target species exploited by traps tonnara .
Harvesting of the gastropods Hexaplex trunculus and Bolinus brandaris is an example of the successive exploitation of marine resources from the Iron Age until the thirteenth century in the Eastern Mediterranean. These species are specifically harvested for the purple pigments extracted from their shells and used to dye clothes. This harvest disappear from the Levantine area in the late twelfth century, and from Greece a century later, although both species are still abundant to this day .
Another example of human exploitation of marine resources from historical times is the hunting of seabirds on islands, particularly of shearwaters, which probably constituted the only source of protein in periods of scarcity especially on small islands. In places such as Formentera Balearic Islands , humans contribute to the depletion, and partial extinction, of Balearic shearwaters Puffinus mauretanicus , with consequences at the level of the marine trophic web .
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  ,  , and their introduction to the Adriatic in the seventeenth century  ,  increase fishing catches.
Fishing catches increase to an extent that even the early fishermen organizations sixteenth century , such as Cofradias in Catalonia  and the Prud ' homies in Provence  , are concerned about possible negative effects on exploited stocks. Such effects are further intensified by the increasing industrialization in the nineteenth century, with an increase in the efficiency of existing fishing gear e. Industrialized fishing had severe impacts on species, habitats, and ecosystems . Several studies also show historical changes in fish communities of different regions of the basin [e.
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. Historical fluctuations in the abundance of this species have been described on the basis of a centuries-long time-series of tuna trap catches, starting in the seventeenth century, and suggested to be linked to climate fluctuations . Despite this comparative wealth of historic information about temporal trends mainly linked to the history of human exploitation of Mediterranean marine biodiversity, many unknowns remain in spatial and chronological gaps from prehistoric periods to the present.
Ancient, medieval, and early modern records contain qualitative rather than quantitative data, and it is difficult to depict general diversity trends at either a species or ecosystem level at the scale of the whole Mediterranean. Interesting results do emerge from analyses of specific regions.
Berlin: Springer. Iranian Journal of Ichthyology 2 2 : Oxbow Books, Oxford. Cobitis amphilekta sp. Earth History.
The overall trends reported by Lotze et al. This changed during the Classical period B.
It is possible that marine species recovered from heavy exploitation after the collapse of the Roman Empire, as has been documented for terrestrial resources . However, human population increased during the Medieval period approximately A. With the onset of the industrialization in Europe in the nineteenth century, signs of species depletions and rareness increased and accelerated throughout the twentieth century, when the first extirpations of species were also recorded.
Biodiversity did not decrease, however, because some species were newly introduced into the Adriatic Sea .
No temporal trend is known for alien species in the Adriatic Sea, so we showed Figure 8a a timeline of mollusk invasions in the Mediterranean as a whole  , which started in the late nineteenth century and accelerated during the twentieth century. The depletion of formerly abundant species and the invasion of new species caused a shift in species composition and diversity in the north Adriatic Sea .
Local species depletions and extirpations mostly occurred among large species, including marine mammals, birds, reptiles, and commercial fish and invertebrates, while species invasions were mainly by smaller species at lower trophic levels, such as invertebrates and algae Figure 8b. Such fundamental changes in species composition had effects on the structure and functioning of food webs and ecosystems  , . Population declines have also been noted among marine mammals throughout the Mediterranean.
The Mediterranean monk seal, in particular, was deliberately hunted during the Roman period  , and it disappeared in the greatest part of the Mediterranean basin during the early s  , . Currently, it mainly occurs in small, isolated areas of the Greek and Turkish coasts, and northwest African coastal waters Figure 9 , but the presence of Mediterranean monk seal in some of these areas is uncertain. There are fewer loggerhead and green turtles throughout the Mediterranean, although historical records were available to determine the severity of their population decline  , .
Known nesting sites especially for the loggerhead turtle disappeared in several areas of the basin  Figure 9. Present red areas and historical yellow areas distribution of the Mediterranean monk seal  ,  ,  ,  ,  —  , and nesting sites for loggerhead turtle and green turtle [modified from 22]. Green and red triangles, respectively, are the former nesting sites for loggerhead turtle and green turtle; green and red dots are the present sites. Question marks represent sites where one or a few Mediterranean monk seals have been recently seen.
Although the population trends for most seabird species are not well known, all reliable long-term information suggests that most seabird species have recovered on the European coasts during the last three decades. This recovery is due to more restrictive conservation policies at national and international levels. With the exception of shearwaters, seabird species show relatively stable population trends. Sparse data on shags suggest a slow recovery in the last two decades.
Storm petrel populations are stable at the few long-term monitored sites  , but many suitable breeding sites have been destroyed since historical times along coastlines. Paleontological records confirm that the distribution of many species was much larger, even occupying habitats in the interior of large islands relatively far from the sea, where recolonization is now impossible .
Population recoveries of Mediterranean seabirds must be considered only partial, and only occurring where protection is effective . 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. Habitat loss or destruction was the second-most-important human impact, followed by eutrophication, introduced predators, disease, and general disturbance. This highlights the importance of cumulative human impacts, especially in coastal ecosystems, with emphasis on species with commercial interest. Recently, anthropogenic drivers and threats to diversity increased and further diversified in the Mediterranean, as observed elsewhere . Published information and the opinion by experts identified and ranked current threats to diversity in the Mediterranean Figure 10 , and File S2.
The sum of the ranking 0—5 for each threat showed that for 13 large taxonomic groups, habitat loss and degradation are considered the primary impact on diversity, followed by exploitation, pollution, climate change, eutrophication and species invasions. These were the most conspicuous threats and also affect the greatest number of taxonomic groups.
Other threats to diversity were maritime traffic collisions with vessels and aquaculture. Within 10 years from now, habitat degradation and exploitation were predicted to retain the predominant roles, while pollution and climate change will likely increase in importance, followed by eutrophication. Of all current threats to biodiversity in the Mediterranean, climate change was predicted to show the largest growth in importance within the next 10 years We used published data on specific taxa and expert opinion. Threats to diversity were ranked from 0 to 5 for 13 taxonomic groups and results are shown as the percentage of the ranking to the maximum values File S2.
Figure 11 shows past changes and projected future increases in sea surface temperature SST in the Mediterranean Sea. This can imply that a number of tropical Atlantic species that entered the Mediterranean during the last interglacial , to , years ago will reenter the Western Mediterranean in the near future  — . The southern sectors of the Mediterranean harbor many native warm-water species that do not occur or get much rarer in the northern sectors. By —, the Mediterranean is projected to warm by 3.
Taking into account data regarding marine biodiversity and threats, we mapped vertebrate endangered species and have tried to locate potential hot spot areas of special concern for conservation in the Mediterranean Figure The first attempt included fish, marine mammals, and sea turtles, which are considered important sentinels for ocean health.
The identified hot spots highlighted the ecological importance of most of the western Mediterranean shelves. The Strait of Gibraltar and adjacent Alboran Sea and African coast were identified as representing important habitat for many threatened or endangered vertebrate species. The most threatened invertebrate species in the Mediterranean, the limpet Patella ferruginea , is also distributed along this area . Both the northern Adriatic and Aegean seas also showed concentrations of endangered, threatened, or vulnerable species. Other equally species-rich waters along the northeast African coast, and the southern Adriatic Sea, were of lesser concern for the protection of endangered species.
This figure includes critically endangered, endangered, vulnerable, or near threatened species. Colors express species occurrence from blue little occurrence to red highest occurrence. Our estimate of 17, species for marine biodiversity in the Mediterranean updated and exceeded previous values, which were on the order of 8,—12, species Table 3. In comparison with the estimate  , the total number of recorded species has increased substantially.
As a result of recent efforts and improvements in analytical methods and instruments, our estimates of invertebrates and protists, in particular, have undergone an upward revision in recent years. Current estimates of sponges, cnidarians, polychaetes, mollusks, arthropods, echinoderms, ascidians, and other invertebrates all exceed those dating back to the early s.
However, since most microbial diversity is basically unknown, global numbers and their evolution are uncertain. They covered vertebrate taxa fairly comprehensively, but other taxonomic groups were underrepresented. Total estimates of Mediterranean species of macrophytes and metazoans represented 6. Macrophytes showed the highest percentage of shared species with global estimates, and Heterokontophyta and Magnoliophyta scored the highest Among metazoans, Mediterranean sponges showed the highest percentage Other groups represented much lower percentages of the total, such as echinoderms 2.
Previous studies claim the existence of a gradient of species richness from the northwest to the southeast Mediterranean [e. Our results confirmed this general decreasing trend and showed that the distribution of marine diversity in the Mediterranean is highly heterogeneous. The Western Mediterranean displays the highest values of species richness, likely owing to the influx of Atlantic species and the wide range of physicochemical conditions. The central Mediterranean, Adriatic, and Aegean seas are areas of second-highest species richness, although with exceptions.
The Adriatic Sea sometimes displays lower species numbers because of restricted exchange with the western basin, decreasing depth toward the north, the presence of fresh water, and the larger amplitude of temperature variations  , . However, this basin shows a large number of endemics possibly owing to its higher isolation.
The Aegean Sea normally follows the western areas, mainly because of its more direct exchange with the western basin and its higher habitat diversity  ,  , . The Levantine Basin and southeastern side have in general the lowest species richness, which is due to the unfavorable conditions prevailing in the area such as high salinity as well as the less intensive sampling effort  , . In fact, a lack of data is evident in several eastern and southern regions of the Mediterranean basin.
This may have strongly influenced some of our results regarding spatial patterns, so generalizations have to be made carefully. 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 . 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. The Suez Canal, opened in , has restored the connection between the Mediterranean and the Indian Ocean  , and in recent years we have witnessed an exponential increment in the number of Indo-Pacific species recorded in the Eastern Mediterranean [e.
This trend will continue to influence the biodiversity of the Mediterranean Sea. In addition, the data used to draw spatial patterns were collected from the s to s, so results may differ from the current situation and may represent potential ranges and values rather than current ones. However, similarities exist between results achieved with distribution maps drawn with expert data and predicted results using AquaMaps models.
These similarities indicated that the species richness maps resulting from this study are a useful first attempt to represent comprehensive species richness patterns at the Mediterranean scale. Differences encountered using both methods may be due to limitations of the data. 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. Therefore some AquaMaps predictions may represent overestimates a good example is the Mediterranean monk seal; see www. Besides, the relative probability of occurrence calculated from AquaMaps does not distinguish between a rare species that might only have been sighted once in a given cell, and a more abundant species that might be sighted every day.
Therefore, for many species, occurrence was inferred from habitat use outside of the Mediterranean. Because the Mediterranean environment represents some environmental extremes such as salinity and temperature records , occurrences in the eastern part may not have been captured adequately by AquaMaps, and this could partially explain the low values in this region. These limitations are extended to our first attempt to depict hot spot areas in the Mediterranean.
The eastern region hosts important populations of elasmobranchs and marine mammals that are currently threatened, but their probability of occurrence estimated by AquaMaps model is lower than 0.
Further studies should be able to reconcile both mapping sources and confirm or correct patterns. 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.
The Siculo-Tunisian Strait still partially acts as a barrier to the dispersal of many species between the two basins and constitutes their meeting point. Diversity differences between areas may also reflect changes in water masses and circulation  ,  as well as changes in temperature and salinity . The diversity of some groups is definitively influenced by this temperature gradient. For the sipunculans, richness may be linked to the temperature of the water masses during the year  , which reflects a physiological barrier between cold and warm water for cold- and warm-water species.
For example, Golfingia margaritacea is mainly a temperate and boreal species  , and its presence in the Mediterranean may indicate the prevalence of colder water masses. In contrast, other thermophilic species, such as Phascolion convestitum and Aspidosiphon elegans , have been proposed as Lessepsian migrants  , . Diversity distribution in the Mediterranean is also associated with a productivity gradient. Higher productivity areas show higher diversity partially because they are important feeding and reproductive sites for several taxa.
Most of these areas occur in the Western Mediterranean and the northern Adriatic that, for example, host many species of fish, seabirds, marine mammals, and turtles [e. Their distribution is associated with feeding habits [e. Moreover, some fish, seabirds, sea turtles, and mammals show opportunistic feeding behavior, exploiting discards from trawling and purse seines, and to a lesser extent from artisanal long-lining [e. Most Mediterranean marine mammals are predominantly offshore and prefer deep-water habitats, but a few species can venture to inshore waters and scavenge fishery discards  ,  , .
The three main categories explaining the drivers of biodiversity in the deep Mediterranean are i bathymetric gradients, which are associated with increasing pressure and decreasing food availability in deeper sediments; ii geographical and physicochemical features, which are responsible for the north-northwest—south-southeast gradient in trophic conditions; and iii environmental heterogeneity e.
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  ; b productivity, organic content, or microbial activity  ; c food resources  ; d oxygen availability  ; e water currents  ; and f occasional catastrophic disturbances . Thus, the spatial distribution of available energy may influence the distribution of benthic abundance, biomass, and biodiversity  ,  ,  ,  ,  — .
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. In the past, geological and physical changes lie at the root of the most dramatic changes in biodiversity in the Mediterranean Sea. Today, human activities are essential elements to consider as well, and several of them threaten marine diversity. The most important threats in this region are habitat loss, degradation and pollution, overexploitation of marine resources, invasion of species, and climate change.
Our results show that habitat degradation and loss is currently the most widespread threat and was also important in the past. Human interventions, such as coastal modification, that can be traced back to before the Roman period  , have important consequences for diversity. 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. Cultural eutrophication, in particular in semienclosed basins such as the Adriatic Sea, can also be traced back for centuries  , .
This phenomenon reached its peak in the late s  and, in addition to fishing, may be the cause of the sequence of jellyfish outbreaks, red tides, bottom anoxia events leading to benthic mass mortalities, and mucilage events that have occurred in recent ecological history of the Adriatic Sea . Direct and indirect pollution is generated directly from the coast, or through fluvial contributions, and ends up in the sea . Pollution affects a wide range of marine species [e.
The main threats for most seabirds and marine turtles in the Mediterranean arise from habitat degradation and loss  , . The breeding habitat for seabirds is relatively well protected along the northern Mediterranean shore, but the protection of many seabird colonies and hot spots is less effective along the southern shore because of limited resources.
Marine wind farms, which are expected to increase in some countries, may represent a new conservation concern for seabird populations . Marine turtles are also affected primarily by degradation of habitats but also by marine pollution, driftnets, gillnet and longline by-catches, and boat strikes  ,  , .
The continuing increase of coastal settlements is important for the region's economic activity, but it is also causing intense environmental degradation through excessive coastal development, further pollution, and consumption of natural resources, all of which add pressure to coastal areas and the marine environment . 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. He mentioned that scallops had vanished from their main fishing ground Gulf of Kalloni, in Lesvos Island since fishermen began using an instrument that scratched the bottom of the sea .
Early records of overfishing and depletion of coastal resources become evident during Roman and medieval times and are driven by human population growth and increasing demand and the increasing commercialization and trade of food and products  , . The current high demand for marine resources continues and has resulted in high levels of fishing or harvesting intensity.
Several fish resources are highly exploited or overexploited [e. Other organisms that are exploited or affected by exploitation in the Mediterranean include macrophytes, sponges, cnidarians, echinoderms, mollusks, arthropods, polychaetes, ascidians, and other invertebrates File S2 [e. The threats to currently endangered marine mammals and sea turtles include unwanted by-catch  ,  as well as historical exploitation.
For sea turtles, the overall mortality rate caused by entanglement in fishing gear and by habitat degradation is poorly known  , but for marine mammals the major threats clearly derive from human activities: direct or indirect effects of exploitation, such as prey depletion, direct killing, and fishery by-catch  ,  ,  ,  ,  — . At sea, threats to seabirds mainly come from fisheries  —  , particularly by-catch in longlining  , . Fishing is being expanded toward deeper areas and is threatening several ecosystems [e.
Fishing activity may also be the cause of ecosystem structural and functional changes and ecosystem degradation [e. A few Mediterranean invasive aliens have drawn the attention of scientists, managers, and media for the conspicuous impacts on the native biota attributed to them. A pair of coenocytic chlorophytes, Caulerpa taxifolia  and Caulerpa racemosa var. Other work  has traced the impacts of invasive aliens that entered the Mediterranean from the Red Sea through the Suez Canal and displaced native species. Tropical species have been entering the Mediterranean through either the Suez Canal Lessepsian migration or the Strait of Gibraltar for decades, and mainly by ship transportation.
The Mediterranean is highly susceptible to ship-transported bioinvasions: one-fifth of the alien species recorded in the Mediterranean were first introduced by vessels . The increase in shipping-related invasions may be attributed to the increase in shipping volume throughout the region, changing trade patterns that result in new shipping routes, improved water quality in port environments, augmented opportunities for overlap with other introduction vectors, and increasing awareness and research effort  — . The swarms of the vessel-transported American comb jelly Mnemiopsis leidyi that spread across the Mediterranean from Israel to Spain in raise great concern because of their notorious impacts on the ecosystem and fisheries [ansamed.
Moreover, with the development of large-scale marine aquaculture mariculture in the late twentieth century, the commercially important alien shellfish Crassostrea gigas and Ruditapes philippinarum were intentionally introduced to the Mediterranean. The high permeability of aquaculture facilities, transport, and transplantation of these species have resulted in many unintentional introductions: oyster farms have become veritable gateways into Mediterranean coastal waters for alien macrophytes .
Segments of the industry may still resort to illegal importation: neither the Turkish authorities nor the UN Food and Agricultural Organization were aware of the importation of the bilaterally ablated female banana prawn Fenneropenaeus merguiensis that was found in the Bay of Iskenderun, Turkey .
Although some aliens are responsible for reducing the population of some native species  , others have become locally valuable fishery resources . Some Erythrean aliens were exploited commercially almost as soon as they entered the Levantine Sea, and their economic importance was quickly acknowledged . Levantine fisheries statistics record the growing prominence of the Erythrean aliens: the Erythrean prawns are highly prized and, beginning in the s, a shrimp fishery developed in the Levantine Sea.
Nearly half of the trawl catches along the Levantine coast consist of Erythrean fish, but the commercially exploitable species were accompanied each summer by swarms of the scyphozoan jellyfish Rhopilema nomadica , washed ashore along the Levantine coast. The shoals of jellyfish adversely affect tourism, fisheries, and coastal installations, and severe jellyfish envenomations require hospitalization.
The recent spread of the silver stripe blaasop Lagocephalus sceleratus and the striped catfish Plotosus lineatus pose severe health hazards. Pronounced thermal fluctuations and a significant increase in the average temperature of the waters in the Mediterranean during the past two decades have coincided with an enlarged pool of warm-water alien species that have become established and expanded their distributions see next section.
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 .
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. Local population losses and niche contraction of native species may not induce immediate extirpation, but they may trigger reduction of genetic diversity and loss of ecosystem functions and processes, and habitat structure.
Climate change is exerting a major effect on Mediterranean marine biodiversity through seawater warming [e. The increase in seawater temperature has affected the distribution and abundance of native and alien species, and has had both direct and indirect effects on invertebrates and fish [e. The increase in water temperature in the Mediterranean also alters jellyfish population dynamics [e.
Seawater of the Mediterranean Sea has been warming since at least the s  , . Rising temperature enlarges the pool of alien species that could establish themselves, enables the warm-water species native and alien present in the sea to expand beyond their present distributions, and provides the thermophilic aliens with a distinct advantage over the native Mediterranean biota.
Although tropical invaders have been recorded in the northernmost sectors of the Mediterranean [e. Tropical species have been entering the Mediterranean through either the Suez Canal Lessepsian migration or the Strait of Gibraltar for decades  ,  , but they used to remain in the eastern or western basin, respectively. Thus it conformed to the traditional physiographic and biogeographic subdivision of the Mediterranean . However, in the last two decades, the number of tropical species that have also spread through the entire basin is growing.
Examples of Erythrean aliens that crossed the Strait of Sicily include algae, a seagrass, many invertebrates and fish [e. Species coming from the tropical Atlantic have traveled the opposite way to reach the Levantine Sea [e. The Strait of Sicily is today a crossroad for species of distinct tropical origins Atlantic and Indo-Pacific , expanding their range longitudinally within the Mediterranean  , . Because they cannot move farther northward, they may dramatically decrease  or even be at risk of extirpation. Although the total extinction of flora and fauna from a basin as wide as the Mediterranean may be unrealistic, the signs of increased rarity or even disappearance of cold-water species deserve further investigation  ,  — .
An example is the deep-water white coral, Lophelia pertusa , reefs of which have become rare in the Mediterranean . These coldest parts of the Mediterranean Gulf of Lions, northern Adriatic could act as a sanctuary for cold-temperate species, but if warming intensifies, those areas may act as traps without any cooler water for escape .
Global warming may cause thermophilic species of the southern Mediterranean to appear more frequently in the northern and colder parts [e. 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 .
Instances of species replacement [e. Thus, the relationship between tropicalization, meridionalization, and biodiversity is not straightforward. In general, the establishment of tropical invasive aliens may cause Mediterranean communities to lose their particular character  and to become similar to their tropical analogs, especially in the southern portions of the basin . Cladocora caespitosa , the most important shallow-water zooxanthellate species living in the Mediterranean, was more abundant and built more conspicuous formations during periods of the Quaternary, when the Mediterranean climate was subtropical .
However, warming episodes in recent summers coincided with mass-mortality events of this coral [e. Hence, it is unlikely that the Mediterranean in the future will contain significant coral constructions. The overwhelming number of Lessepsian immigrants will move the composition of the biota more and more like that of the Red Sea, but Mediterranean communities will probably look like those that today characterize southern Macaronesia and the Cape Verde region, with scanty coral and abundant algae [e.
Seawater acidification may also be a threat to Mediterranean marine biodiversity . The most obvious consequence of the increased concentration of CO 2 in seawater is a reduced rate of biogenic calcification in marine organisms  , . 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. Calcareous red algae are the builders of coralligenous reefs, one of the most important Mediterranean ecosystems, and seawater acidification will probably impair their role .
However, noncalcifying photosynthetic plants, such as frondose algae and seagrasses, may take advantage of a greater availability of CO 2. But large, erect species of brown algae as well as Mediterranean seagrass are now in decline because of the environmental degradation, induced primarily by human activities  , . The study of Mediterranean marine diversity over many years has produced a significant amount of information. Yet this information remains incomplete with the discovery and description of new species, especially of smaller, less conspicuous and cryptic biota Table 1 and File S2.
The biodiversity in the Mediterranean Sea may be in fact much higher than is currently known. We do not have credible measures of microbial richness, but development of new technologies will allow us to decide whether this is knowable or not.
The description of microbial diversity is probably better approached through the continued study at selected sites, such as the Microbial Observatories, for which data exist on both identification methodologies and the functioning of the ecosystem. Sites in the southern and eastern Mediterranean are still to be added.
Phycological surveys are also required in Croatia, because several species and even genera described from the Adriatic have never been found again and require taxonomic reevaluation. We do not expect a significant increase in the rate of description of new species, but the description of new macroalgal species continues [e. A large number of species are poorly known, and our checklist includes several taxa inquirenda see File S2.
Accurate morphological studies, and new molecular tools, are required to decipher the taxonomy of several genera, including Ectocarpus , Cystoseira , Acrochaetium , Polysiphonia , and Ulva. A similar situation exists for the invertebrates see File S2. Most of the small fauna of the Mediterranean are typical of current scientific knowledge: in one of the best-known geographic areas of the world, there are many regions and habitats that remain insufficiently studied, and several taxonomic groups in deep-sea areas and portions of the southern region are still poorly known.
The description of new species is still a high priority. As illustrative examples, the accumulation curves for cumaceans, mysids, polychaetes, and ascidians discovered described or first recorded Figure 13 show that no asymptote has been reached, and there has been no slowing in the rate of discovery for less conspicuous species in the Mediterranean, as it is observed when analyzing accumulation curves in other parts of the world .
The shortage of taxonomists for many groups is a particularly serious problem worldwide, and it also applies to the Mediterranean Sea. Lake Hula : reconstruction of the fauna and hydrobiology of a lost lake by Ch Dimentman Book 2 editions published in in English and held by 45 WorldCat member libraries worldwide. Mare nostrum : Neogene and anthropic natural history of the Mediterranean basin, with emphasis on the Levant by Francis Dov Por Book 5 editions published in in English and held by 24 WorldCat member libraries worldwide.
Lake Hula : reconstruction of the fauna and hydrobiology of a lost lake by Ch Dimentman Book 2 editions published in in English and held by 4 WorldCat member libraries worldwide. The legacy of Tethys : an aquatic biogeography of the Levant by Francis Dov Por 1 edition published in in English and held by 1 WorldCat member library worldwide. TheLevant as biogeographic bridge : land, sea and air, with additional papers on the Levant fauna Book 1 edition published in in English and held by 1 WorldCat member library worldwide. Audience Level. Related Identities. Associated Subjects.