Jellyfish Lake in Pacific island of Palau, Micronesia is a very famous Snorkeling site. Jellyfish Lake, known to Palau’s natives as Ongeim’l Tketau, contains over ten million jellyfish that have descended and evolved from a common ancestor, the spotted jellyfish. They are non stinging, having lost their natural defense system because of the lack of major predators in the lake. So you may actually enjoy swimming with them much closer than would be possible anywhere else.
Millions of years ago, jellyfish were trapped in this lake after a submerged reef rose from the sea, creating a landlocked saltwater lake. In this lake, the jellyfish have adapted to the new conditions by losing their sting. They have eight primitive eyes and algae that live within their cells. These algae are what the jellyfish live on. Twice each day, the jellyfish in the lake swim from one side to the other. The jellyfish do this to get sunlight to their internal algae so that the algae can grow.
At night the jellyfish swim to a lower depth where the water is rich in nitrogen. The nitrogen also sustains the algae population. The jellyfish has only one main predator in this lake — the sea anemone. Jellyfish can be stung, trapped, and killed by these creatures.
Scuba diving is forbidden in Ongeim’l Tketau both because it damages the jellyfish and because the hydrogen sulfide in the bottom layer of the lake poses a serious risk to human life.
The marine lakes remain connected to the sea and, on a daily basis, the influence of sea level on the lakes can be witnessed as the tides rise and fall within the lakes in response to tidal changes in the surrounding lagoon.