The lake where jellyfish evolved to live on light alone
In short: In a secluded saltwater pool in Palau, millions of jellyfish have bypassed the traditional food chain. By partnering with algae, these creatures have mastered photosynthesis, creating a self-sustaining ecosystem that thrives in a bizarre, three-layered world of oxygen and poison.
Jellyfish Lake, known locally as Ongeim’l Tketau on Eil Malk island, is a masterclass in evolutionary adaptation. Formed roughly 12,000 years ago during the end of the last ice age, this stratified saltwater pool is divided into three distinct zones. The top layer, extending down to about 43 feet, is home to millions of endemic golden jellyfish (Mastigias papua etpisoni). These jellies are unique because they have evolved to photosynthesize using algae, providing them with a nearly unlimited food supply independent of traditional hunting. The lake’s middle layer acts as a biological barrier. Between 43 and 50 feet deep, pink bacteria thrive in conditions that block light and oxygen from reaching the bottom. This creates a sharp divide between the life-sustaining surface and the deadly depths below.
The bottom layer, situated between 50 and 100 feet deep, is almost entirely devoid of oxygen due to this bacterial barrier. At the very bottom, decomposition releases poisonous hydrogen sulfide gas, creating a toxic environment where only specific microbes can survive. This isolation is key; while connected to the ocean by small cracks in limestone rock, the lake functions as an isolated ecosystem. The golden jellyfish likely descended from a few trapped spotted jellyfish (Mastigias papua) that were stranded when sea levels dropped 12,000 years ago. Over millennia, these survivors adapted to the unique chemistry of the pool, evolving into a subspecies named in honor of Palau's former president, Ngiratkel Etpison.
The population dynamics of the lake are staggering. While the Coral Reef Research Foundation (CRRF) typically estimates the population at around 5 million jellyfish, historical records show that in 2005, the numbers surged to over 30 million. This ability to flourish in such a specific niche highlights how extreme environmental pressures—like a toxic bottom and a lack of traditional prey—can drive rapid, specialized evolution.
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Sources
- Summary by Intellect · source: The Lake Filled With Over One Million Jellyfish | BBC Earth
- Also: Jellyfish Lake: Palau's saltwater pool with a toxic bottom and surface waters brimming with millions of jellyfish | Live Science
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