Jellyfish Lake, Palau
Several marine lakes in the Republic of Palau have been cut off from the sea for 10,000-15,000 years. The jellyfish (Mastigias species) in these lakes have evolved to become stingless. The jellyfish also host symbiotic algae which require sunlight to photosynthesize. In order to provide enough light for their algae, the jellyfish migrate across the lakes during the day, following the sun.