The Mediterranean Sea, a body of water nearly landlocked by three continents, represents a highly distinct marine environment. Its connection to the Atlantic Ocean through the Strait of Gibraltar creates a unique oceanographic profile. The sea is characterized by high salinity and relatively high temperatures, with deep-sea temperatures remaining warm. This semi-enclosed nature and lack of substantial nutrient inflow contribute to its generally oligotrophic, or low-nutrient, status. Despite these conditions, the Mediterranean is a recognized hot spot of marine life, hosting a remarkable biodiversity where an estimated 25 to 30 percent of the species are endemic, meaning they are found nowhere else.
Iconic Marine Mammals
A small but significant population of air-breathing mammals resides in the Mediterranean waters, primarily composed of various cetaceans and one seal species. The Fin Whale (Balaenoptera physalus), the second-largest animal globally, is the largest resident cetacean, with a distinct population that concentrates in the northwestern basin during summer feeding periods.
Smaller toothed whales and dolphins are more numerous and widely distributed. The Striped Dolphin (Stenella coeruleoalba) is the most abundant cetacean, frequently observed in large offshore groups, while the Common Bottlenose Dolphin (Tursiops truncatus) is typically found closer to coastal shelf areas. The Pelagos Sanctuary, a transboundary marine protected area, was established to safeguard these populations and their habitat.
The critically endangered Mediterranean Monk Seal (Monachus monachus) is the only seal species native to the sea, representing one of the most threatened marine mammals worldwide. Historically widespread, its remaining populations are now fragmented and confined to isolated coastal areas and remote caves.
Pelagic Fish and Cartilaginous Species
The open water column supports a diverse assemblage of bony fish (Teleosts) and cartilaginous fish (Elasmobranchs), which are commercially and ecologically significant. The Atlantic Bluefin Tuna (Thunnus thynnus) uses the sea as a breeding ground, though its stocks have been severely impacted by overfishing. Smaller pelagic species, such as sardines (Sardina pilchardus) and European anchovies (Engraulis encrasicolus), form vast schools that serve as the foundation of the marine food web.
In the deeper waters, species like European Hake (Merluccius merluccius) and the demersal Red Mullet (Mullus barbatus) are targeted by commercial fisheries. A significant aquaculture industry focuses on species such as the Gilthead Seabream (Sparus aurata). The Mediterranean is home to nearly half of the world’s cartilaginous fish species, including various rays and sharks.
These Elasmobranchs, such as the Giant Devil Ray (Mobula mobular) and the occasional Great White Shark (Carcharodon carcharias), act as apex predators. Nearly half of the Mediterranean’s shark and ray species are threatened with extinction, largely due to high rates of accidental capture, or bycatch, in fishing gear.
Diverse Invertebrate and Benthic Life
The seabed and shallow coastal zones are rich with non-vertebrate life, ranging from complex colonial organisms to mobile mollusks. The endemic seagrass Posidonia oceanica forms extensive underwater meadows that act as crucial nurseries and habitat for countless invertebrates. These meadows are often home to cephalopods like the Common Octopus and Cuttlefish, which are prized for their intelligence.
Sessile organisms like sponges and corals provide essential structure to the benthic environment. Approximately 700 species of sponges, characterized by a high degree of endemism, live in the Mediterranean, filtering water and creating microhabitats for other fauna. The Red Coral (Corallium rubrum), a slow-growing, long-lived colonial organism, forms dense assemblages on vertical cliffs and in dark caves.
Echinoderms are also a prominent group, including various species of sea urchins, starfish, and sea cucumbers. About 57 species of sea cucumbers are reported in the sea, with species like Holothuria tubulosa and the deep-water Parastichopus regalis being collected for commercial purposes. These deposit feeders play a significant role in cycling nutrients and maintaining the health of the seabed.
Notable Sea Turtles and Reptiles
The Mediterranean Sea serves as a feeding ground and nesting area for several species of marine reptiles, all of which are air-breathing and lay their eggs on land. The most frequently encountered species is the Loggerhead Turtle (Caretta caretta), which nests on beaches throughout the eastern and central Mediterranean, including sites in Greece, Turkey, and Libya. These turtles undertake long-distance migrations.
The Green Turtle (Chelonia mydas) also nests in the region, though its nesting sites are more concentrated in the warmer eastern Levantine basin, such as on the coasts of Cyprus and Turkey. A third, albeit rare, visitor is the Leatherback Turtle (Dermochelys coriacea), the largest of all living reptiles. These turtles feed primarily on jellyfish and typically enter the Mediterranean from the Atlantic on foraging expeditions.