The Mediterranean Sea, a basin nearly enclosed by landmasses, is a unique and diverse marine environment. Despite accounting for less than one percent of the world’s ocean surface, it harbors an estimated 10,000 to 17,000 different marine species. This makes it a biodiversity hotspot, supporting a high level of endemism where many species are found nowhere else.
Major Categories of Marine Life
The Mediterranean Sea supports a wide array of marine organisms, broadly categorized into fish, marine mammals, reptiles, invertebrates, and seabirds. Its waters contain various fish species, including commercially important ones like sea bass, gilthead sea bream, European hake, and sardines. Large pelagic fish such as bluefin tuna and various species of grouper also inhabit these waters.
Marine mammals are also present, with eight species of whales and dolphins frequently observed. These include large baleen whales like the fin whale and toothed whales such as the sperm whale, striped dolphin, and bottlenose dolphin. The critically endangered Mediterranean monk seal and harbor porpoises are also found.
Marine reptiles, primarily sea turtles, navigate the Mediterranean. Loggerhead and green sea turtles are commonly found. They utilize the sea for foraging and migration.
Invertebrates form a substantial part of the Mediterranean’s biodiversity. This group includes diverse molluscs like the common octopus, various crustaceans such as crabs, European spiny lobsters, and shrimp, and echinoderms like sea urchins and starfish. Cnidarians, which include jellyfish and various coral species, also contribute to the complex food webs and habitats within the sea.
Seabirds rely on the Mediterranean for feeding and breeding. Common species observed include Scopoli’s shearwater and various gulls such as Audouin’s gull and the yellow-legged gull. Other birds like the great cormorant and northern gannet are also part of the marine avian community.
Ecological Conditions Shaping Mediterranean Biodiversity
The Mediterranean Sea’s unique characteristics significantly influence its marine life. Its semi-enclosed nature, connected to the Atlantic Ocean through the narrow Strait of Gibraltar and to the Red Sea via the Suez Canal, creates distinct environmental conditions. Water exchange with the Atlantic brings in cooler, less saline water, which is then modified as it traverses the basin.
The Mediterranean exhibits higher salinity, ranging from about 38 to 40 parts per thousand. Surface water temperatures vary seasonally, from around 13°C in winter to over 27°C in summer. Deep waters maintain a relatively constant temperature. Recent studies indicate a trend of increasing temperature and salinity throughout the water column.
The sea’s circulation patterns are driven by temperature, salinity differences, and wind. These currents distribute nutrients and influence species habitats. The basin is generally oligotrophic, meaning it has low nutrient levels, with biological productivity decreasing from west to east.
Distinctive and Endemic Species
The Mediterranean Sea is notable for its high rate of endemism, with approximately 20% to 30% of its marine species found exclusively within its waters. These species have adapted to the basin’s specific environmental pressures.
One recognized endemic species is Posidonia oceanica, or Neptune grass. This marine flowering plant forms extensive underwater meadows that serve as habitats and nursery grounds for numerous other species. These meadows contribute significantly to the sea’s biodiversity and are important for carbon sequestration.
The Mediterranean monk seal is another emblematic endemic species. Once widespread, it is now one of the most endangered marine mammals globally. Its survival is closely tied to the health of the Mediterranean’s coastal and marine environments. The Yelkouan shearwater, an endemic seabird, nests on islands and rocky islets throughout the Mediterranean.
Lessepsian Migration: New Inhabitants
A significant phenomenon altering the Mediterranean’s marine life is Lessepsian migration, also known as the Erythrean invasion. This involves the movement of marine species from the Red Sea into the Mediterranean Sea through the Suez Canal, completed in 1869. The canal created an artificial pathway, connecting two previously separate marine ecosystems.
Named after Ferdinand de Lesseps, this migration has led to the establishment of numerous new inhabitants in the Mediterranean. Over 1,000 alien species have entered the Mediterranean, many originating from the Red Sea. These migrants are often well-adapted to warmer waters.
The influx of these new species can impact the existing ecosystem. They compete with native species for resources, sometimes outcompeting them and leading to shifts in species composition. The ongoing warming of the Mediterranean Sea further facilitates the survival of these tropical Red Sea species, contributing to a phenomenon sometimes termed “tropicalization” of the Mediterranean.