Although Jellyfish Lake is the most famous lake with jellyfish in Palau, it is not the only one. In more than 20 marine lakes (small saltwater lakes connected to the sea through limestone fissures) of Palau there is living jellyfish. Five of these lakes have been studied more - the mentioned Jellyfish Lake, Goby Lake, Uet era Ngermeuangel, Uet era Ongael and Clear Lake.
Testing ground for evolution
Each of the marine lakes of Palau has specific set of species occasionaly trapped in lakes by the receding sea 15 - 10 thousand years ago. Some lakes have steeper banks, some are elongated in east-west direction, some in south-north direction. Every such detail leads to different circumstances and consequently - different new species and subspecies of animals.
Some marine lakes of Palau are made even more exotic by the fact that they are meromictic - below the upper layer of marine water there is a layer of "dead", anoxic water separated by a layer of floating microorganisms, including a purple bacteria.
Such unusual monuments of nature serve as excellent "testing ground" for studies of evolution. Thus lately several scientists have made interesting treatises about these lakes. Lucky them (scientists!) - making science in beautiful Palau...
Jellyfish of president Salii
Clear Lake is only 3.9 ha large, maximum length of it is just approximately 330 meters, age of the lake - approximately 12,000 years. Depth of lake reaches 30 meters, purple-sulphur bacteria layer is located in the depth of 15 - 16 meters, below there is anoxic layer of water without life.
Although the lake is small, here lives endemic subspecies of jellyfish - Mastigias cf. papua salii, named after the third president of Palau Lazarus Salii.
Species compsition in Clear Lake is similar to species composition in Jellyfish Lake. Here also lives a predator of jellyfish - sea anemone Entamaea medusivora.
But there is important difference - while the Jellyfish Lake is elongated in east-west direction, Clear Lake is stretched in north-south direction. It seems that this seemingly slight difference has big influence on fauna of the lake. Border between the seawater and anoxic water in Clear Lake is more diffuse and oxygen level is falling already at 6 - 10 meters depth.
This is the most likely reason why jellyfish in Clear Lake appears sporadically, while in Jellyfish Lake there always (almost) is a lot of jellyfish.
Jellyfish of Clear Lake also has daily migration pattern, although less pronounced than in Jellyfish Lake, here jellyfish does not evade from shadow.
In order to survive sea anemone also had to elaborate different behaviour - besides the jellyfish it consumes mussels as well.