Natural Wonders: Jellyfish Lake

Eil Malk Island in Palau is a curious little island with some extraordinary residents. On the eastern edge of the island lies an isolated marine lake with thousands of bobbing golden jellyfish. 

Eil Malk Island and Jellyfish Lake. Photo: Shutterstock


Over 12,000 years ago, at the end of the last ice age, sea levels rose and filled Jellyfish Lake’s basin. When the waters receded, millions of jellyfish became trapped, cut off from the ocean. While minuscule cracks and extremely narrow tunnels still connect the lake to the ocean, maintaining salinity, only bacteria and microorganisms can pass through. While this may seem ultimately fatal to the jellyfish, it is actually a prime example of how well species adapt and evolve. 

The two main species of jellyfish here are the Golden (with a larger population) and Moon Jellyfish. Neither have a sting detectable to people. Separation from the ocean trapped their ancestors but also kept out predators. Their stinging defenses gradually weakened. This species of Golden jellyfish, in particular, is the only one on Earth that doesn’t pack a significant sting. 

Jellyfish by the thousands. Photo: Shutterstock


The jellyfish have thrived by forming a symbiotic relationship with the lake’s abundant algae, called zooxanthellae. The algae have been absorbed into the jellyfish, clustering on their tentacles and photosynthesizing sunlight. This energy sustains the algae but also allows the jellyfish to grow.

Jellyfish farmers

Scientists call this process “farming”. The jellyfish set out on a strict migration routine every day, chasing the sun and tending to the algae. They travel from west to east, east to west, then west to east again as the sun sets. As they travel, the jellyfish swim counterclockwise to give the algae full exposure to the sun. They repeat this process every day without fail. The Moon Jellyfish do not have such a pattern but are more active and search for food on the surface at night.

The lake itself has fascinated researchers. It is 30m deep and oddly stratified, with an upper oxygenated layer and a lower, toxic anoxic layer. The lake’s rock walls and thick barrier of trees block most of the wind and keep the two layers from mixing much.

The jellyfish remain in the oxygenated layer closest to the sunlight. Yet the water itself is murky, with visibility less than five metres. This murk is due to the presence of the anoxic layer 15m below. That layer, a mix of hydrogen sulfide, ammonia, phosphate, and purple sulfur bacteria is highly dangerous to both jellyfish and humans. 

Microorganism clusters on the tentacles. Photo: Shutterstock


A second Jellyfish Lake

A similar lake on Kakaban Island in Indonesia hosts four species of jellyfish: the Aurelia Aurita, Tripedalia Crystophora, Mastigias Papua, and Cassiopeia Ornata. Four species, thousands of individuals.

This lake bears a similar origin story. It formed during a period of upheaval 11,000 years ago, which separated it from the ocean. Jellyfish, sea cucumbers, sea anemones, sea snakes, crustaceans, and clams all became isolated in this brackish lake. Although their venom incapacitates their prey, the jellyfish sting has become too weak to bother humans. Tourists come to swim among the only jellyfish in the world, apart from those in Palau’s Jellyfish Lake, that are safe to touch.

Yet this Indonesian lake is under threat and suffers major population fluctuations. In 2005, 30 million jellyfish lived in the lake. Eleven years later, drought had almost completely wiped them out.

As of now, 700,000 jellyfish again live in the lake, but the ecosystem is extremely fragile. While snorkeling is allowed, diving is discouraged because it risks disturbing the water layers, which can kill the jellyfish. Nevertheless, several tourist operators promise a diving-with-the-jellyfish experience.