Swimming through countless jellyfish in Jellyfish Lake, Eil Malk island, Palau. Reflections of jellyfish are seen in the water surface. This lake contains millions of these golden jellyfish Mastigias cf. papua etpisoni, which contain symbiotic photosynthetic algae within their bodies. The algae produce sugars for the jellyfish to use as food. The jellyfish have a daily migration around the lake, swimming around so that they receive the best exposure to sunlight as the sun moves across the sky. Jellyfish Lake is connected to the nearby ocean by tunnels near its surface. These jellyfish are related to and descended from the spotted jellyfish Mastigias papua that live in the ocean. However, the lake has been essentially isolated from the ocean for some 12_15,000 years, allowing the evolution of very specialised forms. These jellyfish do have stinging cells, but they are not generally strong enough to affect humans, and snorkelling with the jellyfish in the lake is a popular tourist activity.