"Twelve thousand years ago these jellyfish became trapped in a natural basin on the island when the ocean receded," Jacob explains. "With no predators amongst them for thousands of years, they evolved into a new species that lost most of their stinging ability as they no longer had to protect themselves.
"These fascinating creatures survive by sharing a symbiotic relationship with algae that live inside of them. At night, the jellyfish go down to the depths of the lake where the algae feed on nutrients. During the day, the jellyfish come back to the surface and follow the sun across the lake in a massive migration. The algae convert the energy of the sun via photosynthesis into a sugar that feeds the jellyfish."
(via The Daily What)