T<\/big>he precursor oceans of the Mediterranean, the Paleo- and Neotethys, were tropical oceans located near the equator. Accordingly, as fossils show, they were inhabited by a tropical flora and fauna similar to the species found today for example in the Indian Ocean. During the Jurassic and Cretaceous, huge coral reefs existed in the then extensive shelf areas of Africa and in the relatively shallow ocean arms, as well as on the flooded terranes that were later stacked up to form the land masses of present-day Greece. In the shallow sea, thick sediment deposits formed from precipitating lime and the calcareous remains of the organisms of the rich tropical sea, such as calcareous algae and ammonites and other molluscs.<\/p>\n
A<\/big>lmost all of the tropical inhabitants of the Mediterranean died out in the Miocene when the connection to the Atlantic and Indian Oceans was severed and the Mediterranean dried up, leaving behind vast desert areas and salt lakes. Only a few species were able to adapt to the changed environmental conditions and survived in the salt lakes. After the Strait of Gibraltar reopened, the Mediterranean Sea was filled from the Atlantic Ocean, so that most of its current inhabitants are of Atlantic origin and related to species of the Atlantic ocean, some of them originating from (sub)tropical regions and others from the colder northern seas. Despite the higher temperatures, 70% of the fauna species of the North Sea are also found in the Mediterranean.<\/p>\n
A<\/big> large number of marine animals and plants are cosmopolitan, meaning that they are found almost everywhere on Earth (due to their ability to easily move over long distances in the sea, to the interconnectedness of all oceans, and to the relatively uniform environmental conditions in the water). In the Mediterranean the percentage of cosmopolitan species is lower than in most oceans. These species include many species of jellyfish<\/a> and some whales and dolphins<\/a>, several of which appear only sporadically in the Mediterranean during their migrations, but also some algae<\/a>, sponges<\/a>, worms<\/a> and species such as goose barnacles<\/a>, which have been spread around the world by ships.<\/p>\n T<\/big>he Mediterranean Sea shows a surprisingly high percentage of endemic species (almost 30%), i.e. organisms whose occurrence is limited to the Mediterranean (and in some cases the directly adjacent areas of the Atlantic Ocean). Some of these species are old endemics, i.e. surviving inhabitants of the Tethys, such as the seagrass species<\/a> Posidonia oceanica<\/em>. Others are neo-endemics resulting from the repopulation of the Mediterranean Sea during the Miocene (as it was filling up from the Atlantic ocean), when single species split up into several species during the process of adapting to the newly conquered habitats, such as the algae of the genus Cystoseira<\/a> or the blennies<\/a>.<\/p>\n A<\/big>bout 12,000 species are known to exist in the Mediterranean today, including 1,300 plant species. Given our limited knowledge of some animal and plant groups, especially microscopic plankton species, the actual number of species is probably much higher. This means that the biodiversity, i.e. the abundance of species in the Mediterranean Sea, is very high, far higher than in other seas. For example, there are over 600 species of sponges, 420 sea anemones, corals and jellyfish, 500 bryozoans, 800 worms, 1,200 snails, 390 bivalvia, 53 cephalopods (octopus, squids, etc.), over 1,000 species of crustaceans, 25 sea urchins, 60 starfish and related species, 100 sea squirts, 80 cartilaginous fish (sharks and rays), over 500 bony fish (osteichthyes), 5 sea turtles, one species of seal and 8 regularly occurring whales and dolphins.<\/p>\n see also: Marine animals<\/a><\/p>\n T<\/big>oday, numerous animal and plant species in the Mediterranean have become rarer or have disappeared in parts of their range. Entire animal groups are endangered by human influence, overfishing and pollution of the sea. Many species have been placed on the Red List. However, no species has yet been confirmed as completely extinct.<\/p>\n A<\/big> disastrous effect on the native flora and fauna of the Mediterranean has for example the toxic alga Caulerpa taxifolia<\/em> from Australia, which was accidentally released into the sea from a research institute in Monaco and has proven to be almost impossible to control. It has spread across large areas of the western Mediterranean and the Adriatic Sea, displacing and destroying all other life. Fortunately the alga has not yet appeared in the Aegean Sea. Recently, a slight decline in the spread of the alga has been observed, which is probably due to the introduction of a snail that feeds on this species.<\/p>\n I<\/big>n recent decades, an increase in the immigration of animal and plant species can be seen, primarily via the Suez Canal, which was (re)opened in 1869. Since then, 500 species from the Red Sea have settled in the eastern Mediterranean and are continuing to spread. Some of these species, such as the pretty bivalve Pinctada imbricata<\/em>, have migrated into the Mediterranean since the opening of the Suez Canal and have established themselves without a major impact on the existing species. Other species reproduce rapidly and displace the native fauna, such as the dusky spinefoot (Siganus luridus)<\/em>, a fish species that has also become quite common in our region over the last twenty years. Other tropical species have appeared only in recent years with the rising sea temperatures, such as the invasion of the common lionfish (Pterois miles)<\/em>, which arrived in the southern Aegean in 2022 and by now has been seen also around Naxos. Another example is the sea urchin Diadema setosum<\/em>, which first appeared a few years ago and is already arriving in large numbers in parts of the southern Aegean, displacing the native species. However, a much greater threat to the sea urchins is a disease that probably was brought by the immigrating species, a parasitic single-celled ciliate, which has caused in recent years the extinction of entire sea urchin populations in many parts of the world.<\/p>\n W<\/big>hile sometimes only a small change or influence is needed to damage an ecosystem, it is often not only very difficult, but next to impossible to reverse the damage. Even though nature can adapt to practically anything, it takes thousands or millions of years to do so \u2013 the time scales of evolution are very different from those we humans think in and are concerned with.<\/p>\n continue: Environmental factors in the Mediterranean Sea<\/a><\/p>\nToday’s changes<\/h3>\n