Mastigias jellyfish feed on small animal plankton, killing them with stinging cells on their long tentacles. But the magic is that the sting is not the powerful sting of other ocean jelly fish so humans can swim along with them as they migrate across the lake and back during the day.
Mastigias have evolved a symbiotic relationship with single-celled marine algae. Inside the tissues of Mastigias' frilly arms live millions of zooxanthellae, which use solar energy to convert carbon dioxide into carbohydrates that the jellyfish use for energy. Swimming back and forward across the lake during the day gives their zooxanthellae plenty of light and avoids the shadows that form at the lake edges where their predators, the bottom-living sea anemones, live.
At night they stop moving horizontally and swim vertically in the lake (away from the edges) and scientists think this is to provide nutrients for their symbiots which are only available in deep water.
There are five jellyfish lakes in Palau and even though the jellyfish all belong to the species Mastigias, they look different from each other and from Mastigias in the nearby ocean. It's also been proposed that the swimming could play a part in mixing stratified layers of water. So if you don't mind hiking up to the steep rim then hiking down to the lake for the pleasure of getting up close and personal with the locals then this is the holiday for you.