Mljet National Park (Nacionalni park Mljet) is located the northwestern part of the island of Mljet, spanning an area of 5,375 hectares of protected land and surrounding sea. Declared a national park on November 11, 1960, it represents the first institutionalized attempt to preserve the original ecosystem in the Adriatic.
The underwater environment of Mljet National Park is rich in various life forms, particularly endemic species. The Lake's seabed is notable for its abundance of shellfish, thriving under favorable conditions. Specific examples include the noble pen shell (Pinna nobilis), the largest shellfish in the region, and the fan mussel (Atrina fragilis).
The noble pen shell has two distinct shells connected by an elastic band resembling a joint. The upper shell is convex, and the lower one is flat, both folded with regular radiating ribs. It exhibits a light brown or reddish-brown color that changes based on its surroundings. Measuring between 8 to 15 cm, it moves across the seafloor by rapidly expelling water from its shell opening.
The fan mussel inhabits depths ranging from 2 to 30 meters, residing in sandy and sandy-muddy substrates. Its shell takes the form of a triangular spike, with the shortest side shaped like an arc. The shell's color is reddish-brown, turning yellowish-brown in younger individuals. The pointed part of the shell is embedded in the sand, anchored by byssal threads. Young fan mussels have numerous fragile leaflets on the surface of their shells. Older specimens have less pronounced leaflets, and their shells are covered with a variety of sponges, algae, polychaetes, bryozoans, and other animals. The largest fan mussel shells can reach up to 1 meter. The noble pen shell is a STRICTLY PROTECTED SPECIES, being the largest shellfish in the Adriatic and the Mediterranean. Small barnacles often live in symbiosis with the fan mussel, warning it of approaching danger and prompting it to close its shells. Occasionally, irregularly shaped pearls with no commercial value can be found inside the shells.
The date mussel is a cylindrical shellfish, reaching up to 12 cm in length. It lives in coastal rocks, cliffs, and larger solitary stones, creating its burrows, which expand as the mussel grows. It is very common in underwater caves. The date mussel has a lifespan of approximately 80 years and is a STRICTLY PROTECTED SPECIES.
The endemic jellyfish Aurelia relicta is found only in Mljet's lakes and cannot be found anywhere else in the world. Described in 2016, it occasionally surfaces but mostly stays in deeper sea layers, feeding on smaller plankton. Despite its large size, reaching up to 55 cm in diameter, it poses no threat to swimmers.
Due to specific conditions in this area, the Great Lake hosts the world's largest colony of the Cladocora caespitosa coral, a button-like stony coral. This coral lives in symbiosis with zooxanthellae, allowing colonies to be very large, developing into extensive reef formations. The Cladocora caespitosa reef in the Great Lake covers an area of 650 m² at depths ranging from 4 to 18 meters.
