Jellyfish typically inhabit the ocean, but a rare biological phenomenon allows a population of these invertebrates to thrive in an inland lake. This unique environment fostered the evolution of jellyfish that have essentially lost their stinging capability, making them a global anomaly. This remarkable adaptation is the result of thousands of years of isolation within a stable ecosystem.
The Specific Location of the Non-Stinging Jellyfish
The country hosting this unusual natural wonder is Palau, an island nation located in Micronesia in the Western Pacific. This geological marvel is formally known as Jellyfish Lake, or Ongeim’l Tketau in Palauan. The lake is situated on Eil Malk Island within the Rock Islands Southern Lagoon, a UNESCO World Heritage Site.
Jellyfish Lake is one of approximately 70 marine lakes in the Palau archipelago, and the only one currently open to visitors. It is a landlocked saltwater body with brackish water, connected to the ocean through fissures and tunnels in the surrounding limestone. The lake is home to millions of golden jellyfish, Mastigias papua etpisoni, a unique subspecies found nowhere else.
Evolutionary Adaptation: Why the Stingers Disappeared
The lack of a noticeable sting is an evolutionary adaptation driven by a change in the golden jellyfish’s primary food source and the absence of certain predators. Unlike most ocean-dwelling jellyfish that sting prey, the golden jellyfish rely on a symbiotic relationship with zooxanthellae. These single-celled algae live within the jellyfish’s tissues and use sunlight for photosynthesis, providing a significant portion of the required energy.
To maximize this solar energy harvest, the jellyfish exhibit a distinct daily migration pattern across the lake, following the sun’s path. This constant quest for sunlight reduced the selective pressure to maintain a robust stinging mechanism for hunting. While the jellyfish still possess nematocysts (stinging cells), they are dramatically reduced in size and potency due to this dietary shift, making them functionally harmless to humans.
The Unique Environment of Jellyfish Lake
Jellyfish Lake is classified as a marine lake, formed roughly 12,000 to 15,000 years ago when rising sea levels filled a limestone depression. The lake is also meromictic, meaning layers of water with different densities do not mix. This stratification is important for the ecosystem’s stability and isolation.
The lake is divided into two main layers: an upper, oxygenated layer and a lower, anoxic layer. The top layer extends down to about 43 feet, where the golden jellyfish and other small organisms thrive. Below this is a denser, oxygen-deprived layer, starting between 43 and 50 feet deep, which contains high concentrations of toxic hydrogen sulfide gas.
This lower anoxic layer is inhospitable to most marine life, including potential predators. The stable barrier prevents mixing, maintaining a safe, isolated habitat for the golden jellyfish in the upper layer. Scuba diving is prohibited to prevent artificial disturbance that could mix these delicate water layers and harm the ecosystem.
Other Freshwater Jellyfish Around the World
The non-stinging golden jellyfish of Palau represent a specific adaptation within a marine lake, distinct from other jellyfish found in global freshwater systems. True freshwater jellyfish, such as Craspedacusta sowerbyi, exist in lakes and rivers across the world, including parts of North America, Europe, and Asia. This species originated in the Yangtze River basin in China and is a hydrozoan, a different class of cnidarian from the Palauan jellyfish.
Craspedacusta sowerbyi is a tiny jellyfish, usually the size of a coin, and its sting is also imperceptible to humans. Its presence is often sporadic and unpredictable, unlike the consistently blooming population in Palau. The key distinction is that the Palauan population evolved from a saltwater species trapped in a marine lake, while species like Craspedacusta sowerbyi are naturally adapted to freshwater environments.