The Jellyfish Lake it is one of the most unusual places in Palau: an isolated marine basin, located on theEil Malk Islandwhere millions live golden jellyfish, whose sting is (almost) painless for humanseven if they have not completely lost the stinging cells that they use to hunt. It is a unique place in the world: a salt lake in the heart of the forest, separated from the sea, but still connected to it by tiny underground passages.
The lake is divided into two very clear layers: the oxygenated surface water, where they are concentrated jellyfish and plankton, and the deep one, anoxic and rich in sulphides. Basically, it’s like Jellyfish Lake is a kind of sandwich: da 0 to 15 meters water contains oxygen and allows life to thrive (jellyfish, plankton and bacteria); come on 15 to 30 meters a completely different kingdom begins, dark, oxygenless and full of hydrogen sulfide (H₂S).
Where is Jellyfish Lake: A lake split in two
We are at Palauin the Western Pacific Ocean. The island is Eil Malka piece of limestone raised from the sea millennia ago. The lake is closed inside a cavity, with steep walls that allow almost nothing to pass through. From the measurements of ecological and geochemical studies we know that the lake is approximately 400 – 460 meters and wide 150 – 160 meters. With a surface area of just over 5.7 hectaresgoes down to 30 meters of depth and is connected to the sea only by micro-cracks and tunnels digging into the limestone.
The water is salty, so it is not a “normal” lake. That’s what researchers call meromictic marine lake: means that the layers of water never mix completely.
Those who swim above it don’t see what happens below, but Jellyfish Lake is something of a sandwich. From 0 to 15 meters water contains oxygen and life thrives: jellyfish, plankton, bacteria. Come on 15 to 30 meters a completely different realm begins: dark, oxygenless and full of hydrogen sulfide (H₂S).
Between these two worlds there is something fascinating: one bacterial “lasagna”. purple, about thick 2 metersmade mainly by Chromatium, bacteria that carry out a particular photosynthesis using sulfide instead of oxygen. In the upper layer, however, life has literally exploded: a microbiological study from 1993 measures 4–8 million bacteria per milliliter of water at 13 meters deep: very high values compared to the surrounding barriers. From the bottom of the lake we know what sediments contain approximately 46% organic matter and that degradation processes are slow. This whole system is so stable and delicate that even scuba diving could upset it. And for this reason it is prohibited.
The golden jellyfish of Palau
Jellyfish Lake jellyfish belong to the subspecies Mastigias papua etpisoni and host microscopic algae, zooxanthellae, which photosynthesize. They are close relatives of those of the external lagoon, but this population is completely adapted to the lake. A 2018 study made one of the most comprehensive measurements using networks: they estimate themselves 7.1 ± 1.4 million jellyfish totals; if we exclude the tiny ones (bell <1 cm), the number drops to 2.6 ± 0.5 million. Furthermore, an estimate was also calculated using arobotcalled REMUS, which has a sonar that emits sound pulses upwards; When these signals encounter objects in the water column (in this case jellyfish), a part of the wave bounces back and is recorded; Jellyfish, being many and gelatinous, create a recognizable signal, and through this it has been estimated that they could be approximately 2.8 million.
Between 6 and 7 meters deep maximum density is reached, since light and temperature are more favorable to the algae that live in the jellyfish tissues. In fact, in addition to feeding on small prey called copepods, they rely heavily on the photosynthesis of the zooxanthellae they host. This is why every day they perform one horizontal migration of about 1 km to chase the sunlight. At night, however, they move vertically up to the chemocline (the limit of the transition layer of the various layers of the lake) to “take” nitrogen, which the algae need to grow.
A perfect system, until the climate breaks: during the El Niño climate phenomenon between 1997 and 1998, the jellyfish population collapsed to almost zero, while the polyps (which in the life of a jellyfish represent the benthic phasethat is, attached to rocks and sediments on the bottom of the lake or sea) have survived, allowing jellyfish to repopulate the lake.
Do jellyfish sting or not? A misunderstanding to be clarified
The tourist version says: “they are jellyfish that don’t sting”, but the scientific reality is more nuanced. These jellyfish still use stinging cells to capture zooplanktonespecially copepods. In fact, nematocysts they are still there and they work, but And unlikely to hear anything, as the sting is very mild and essentially harmless for humans. So the experience is perceived as “jellyfish that don’t sting”, even if from a biological point of view the stinging apparatus has not disappeared.
The comparison with Kakaban, where the sting is really lost
The most useful comparison comes from Lake Kakabanin Indonesia, where another population of Papuan Mastigias. A 2021 study compared jellyfish from Lake Kakaban with marine jellyfish and found something clear: in the sea there are three types of nematocysts, in the lake, however, alone a guysmaller and much reduced. Furthermore, in the lake jellyfish have short tentacles and very few stinging cells and contact with humans does not cause symptoms. It’s direct proof that in a closed environment, without predators, jellyfish really can lose the ability to sting.
