photo by aSIMULAtor
The golden jellyfish (subspecies of Mastigias papua) is found in Jellyfish lake or Ongeim'l Tketau on the Pacific island of Palau. The site has become very popular with divers as the jellyfish have a mostly harmless sting and can be seen in the thousands. Divers do need to stay near the water's surface though as the bottom of the lake is heavy in poisonous hydrogen sulfide, the jellyfish descend into this hydrogen sulfide layer at night as part of their daily migration. They are a subgroup of the spotted jellyfish that has been isolated in a marine lake for around 12,000-15,000 years ago in this time they have changed a lot. Some of the ways the have changed are; losing the spots of the spotted jellyfish, strengthened their symbiotic relationship with algae, losing their terminal clubs, developing a complex daily migration pattern based on sun position, and having their sting reduced to something that is only really capable of killing very tiny crustaceans. There are nearby lakes (some of which have been isolated for less time) that also have isolated spotted jellyfish that have changed less or in slightly different ways, I've included an image at the bottom that shows some of the other varieties.