Jellyfish Lake, Eil Malk Island, Palau, Pacific Ocean

Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of an ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish, Mastigias cf. papua etpisoni, and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons.
Lake stratification
Jellyfish Lake is stratified into two layers, an oxygenated upper layer (mixolimnion) and a lower anoxic layer (monimolimnion). The oxygen concentration in the lake declines from about 5 ppm at the surface to zero at 15 meters (at the chemocline). Stratification is persistent and seasonal mixing does not occur. The lake is one of about 200 saline meromictic lakes that have been identified in the world. However most of these lakes are of freshwater origin. Permanently stratified marine lakes are unusual, but on Eil Malk and on other nearby islands there are eleven other apparently permanent stratified marine lakes.