Palau’s first marine lake formed just 12,000 to 15,000 years ago after the last ice age ended and sea levels rose. Palau’s rock islands were limestone peaks riddled with erosion-carved channels, fissures and depressions. Seawater seeping through the limestone transformed the largest depressions into marine lakes and swept in the larvae of spotted jellyfish and other sea creatures. In a mere moment of evolutionary time, the landlocked jellyfish radiated into five different subspecies, each attuned to its own isolated "island" of seawater. The jellies in the deepest lakes, which filled first and are therefore the oldest, diverged the most from their lagoon-living ancestor. The high jellyfish population was isolated and started to feed on quickly-reproducing algae.

The jellyfish has eight primitive eyes and algae that live within their cells. These algae are what the jellyfish live on. They have developed a symbiotic relationship with algae, called zooxanthellae, inside their bodies. The algae get energy from the sun, and the jellyfish in turn get energy from the algae - a perfect harmony...

The myth of Palau's "stingless" jellyfish has a certain beating-swords-into-plowshares charm:
The supposedly predatory ocean jellies, confined to the peaceful lake, became gentle algae farmers.

The facts seem to be that the lake jellyfish do have stingers, and they do use them to prey on zooplankton. Lake jellyfish actually get more of their energy from prey than lagoon jellyfish.

The jellyfish of Jellyfish Lake do have small stinging cells, or nematocysts. But because the stinging cells are so tiny, their sting is not detectable on most human tissue.