An Evolutionary Shortcut
In the Togean Islands archipelago lies Mariona Lake, a marine lake that became separated from the surrounding sea. This isolation, which occurred over thousands of years due to geological activity, created a natural experiment. Marine organisms, including the golden jellyfish (Mastigias papua), were trapped in this new, predator-free environment. Without the threat of predation, the evolutionary pressure to maintain a potent defense mechanism vanished.
Over countless generations, the jellyfish's stinging cells, called nematocysts, devolved. While the jellyfish technically still possess venom, their stinging structures have atrophied to the point where they are unable to penetrate human skin. This process is regressive evolution, where a species loses a complex trait it no longer needs. The lake now contains a dense population of these harmless jellyfish, with some estimates in the millions. These jellyfish sustain themselves through a symbiotic relationship with algae known as zooxanthellae living within their tissues. The algae photosynthesize, providing energy for the jellyfish, which in turn migrate across the lake daily, following the path of the sun to maximize light exposure for their internal partners.
A Stratified World
Mariona Lake formed as a sinkhole, or doline, in the island's limestone bedrock. It remains connected to the ocean through small fissures and tunnels in the porous karst rock. These connections are too small for larger animals to pass through, maintaining the lake's isolation, but allow for the exchange of seawater. The mix of seawater with fresh rainwater creates a brackish environment.
The lake's ecosystem is highly stratified. The upper layer has oxygen and sunlight, supporting the jellyfish and various small fish and crustaceans that also found their way inside. The lower depths of the lake, however, are likely anoxic and inhospitable to most marine life. This layering is a common feature in such isolated marine lakes. The unique conditions have also led to the evolution of distinct sponges and algae that coat the underwater walls of the sinkhole. Mariona Lake is one of a few similar stingless jellyfish lakes known in the world, with other notable examples found in Palau and elsewhere in Indonesia.