section_path: Biodiversity
section_name: Biodiversity
paragraph_text: Biodiversity
  
  Unlike the vast multidirectional ocean currents in open oceans within their respective oceanic zones; biodiversity in the Mediterranean Sea is stable due to the subtle but strong locked nature of currents which is favourable to life, even the smallest macroscopic type of volcanic life form. The stable marine ecosystem of the Mediterranean Sea and sea temperature provides a nourishing environment for life in the deep sea to flourish while assuring a balanced aquatic ecosystem excluded from any external deep oceanic factors. It is estimated that there are more than 17,000 marine species in the Mediterranean Sea with generally higher marine biodiversity in coastal areas, continental shelves, and decreases with depth.
  
  As a result of the drying of the sea during the Messinian salinity crisis, the marine biota of the Mediterranean is derived primarily from the Atlantic Ocean. The North Atlantic is considerably colder and more nutrient-rich than the Mediterranean, and the marine life of the Mediterranean has had to adapt to its differing conditions in the five million years since the basin was reflooded.
  
  The Alboran Sea is a transition zone between the two seas, containing a mix of Mediterranean and Atlantic species. The Alboran Sea has the largest population of bottlenose dolphins in the Western Mediterranean, is home to the last population of harbour porpoises in the Mediterranean and is the most important feeding grounds for loggerhead sea turtles in Europe. The Alboran Sea also hosts important commercial fisheries, including sardines and swordfish. The Mediterranean monk seals live in the Aegean Sea in Greece. In 2003, the World Wildlife Fund raised concerns about the widespread drift net fishing endangering populations of dolphins, turtles, and other marine animals such as the spiny squat lobster.
  
  There was a resident population of orcas in the Mediterranean until the 1980s, when they went extinct, probably due to long-term PCB exposure. There are still annual sightings of orca vagrants.