A Remote Lake Sealed Off Since the Ice Age Became Home to Millions of Jellyfish Found Nowhere Else on Earth

In the Rock Islands of Palau, a remote marine lake is home to one of the most unusual wildlife populations on Earth. Jellyfish Lake, located on the uninhabited island of Eil Malk, contains millions of jellyfish that have evolved in isolation for around 12,000 years, forming a subspecies found nowhere else on the planet.

Cut off from the ocean since the end of the last ice age, the lake developed as a closed ecosystem where life followed its own path. Today, it is filled with huge numbers of jellyfish moving together across the water, shaped by a fragile balance with their environment. This small, hidden lake has become one of the most unique natural spots in the Pacific.

A Lake Shaped By 12,000 Years Of Isolation

Known locally as “Fifth Lake,” this enclosed body of water is about 400 meters long and 30 meters deep. It formed when sea levels shifted at the end of the last ice age, cutting it off from nearby lagoons and the open ocean.

This separation created a self-contained ecosystem where species evolved under stable and isolated conditions. The inland sea is surrounded by mangrove trees, which support a variety of marine invertebrates. Over time, jellyfish became the most dominant and recognizable inhabitants of this environment.

A Jellyfish Found Nowhere Else

As explained in the coverage published by Coral Reef Research Foundation, the lake’s most notable species is the golden jellyfish (Mastigias papua etpisoni), a subspecies of spotted jellyfish named after former Palauan president Ngiratkel Etpison. After thousands of years without contact with outside populations, these jellyfish developed traits that distinguish them from their relatives in the surrounding ocean.

Their behavior is particularly distinctive. The jellyfish migrate across the bay in response to the Sun’s position. They begin the day by swimming toward the east at sunrise, stopping when they reach a shadow line.

Later, as the Sun moves, they shift toward the west. This synchronized movement creates dense aggregations that can appear like a drifting wall of jellyfish.

A Simple Partnership Driven By Sunlight

This daily migration is closely linked to a symbiotic relationship with dinoflagellates, microscopic algae that live within the jellyfish’s tissues. These algae rely on sunlight to produce energy through photosynthesis.

“The jellyfish and algae have a special and beneficial relationship. The jellyfish rotates and swims around the lake to ensure that the algae get enough sunlight for photosynthesis, and the algae gives the jellyfish some energy and nutrients in return.”

Each time the jellyfish glide through sunlit waters, they help the algae soak up light. In return, the algae feed their hosts, quietly sustaining both the jellyfish and the lake itself.

A Population Shaped By Climate Shifts

According to the same source, although the lake typically holds around 5 million golden jellyfish, the population has fluctuated significantly over time. In 2005, estimates suggested there were as many as 30 million individuals. By 2016, the number had dropped sharply, leading to the temporary closure of the lake to visitors for two years.

Researchers have linked this decline to El Niño, which raised water temperatures and disrupted algae growth. With fewer algae available, the jellyfish lost an important source of nutrients, contributing to the population drop. As environmental conditions improved, the ecosystem gradually recovered.

By 2019, the number of jellyfish had returned to the millions, allowing the lake to reopen. Alongside the golden jellyfish, moon jellyfish, also inhabit the lake, though they are far less numerous.

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