Jellyfish Lake is a marine lake located on Eil Malk island in Palau. Eil Malk is part of the Rock Islands, a group of small, rocky, mostly uninhabited islands in Palau’s Southern Lagoon, between Koror and Peleliu. It is notable for the millions of golden jellyfish which migrate horizontally across the lake daily.

Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish, Mastigias cf. papua etpisoni, and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons.

Two species of scyphozoan jellyfish live in Jellyfish Lake, moon jellyfish and the golden jellyfish.

The golden jellyfish are most closely related to the spotted jellyfish that inhabit the nearby lagoons. They are similar to the spotted jellyfish in that they derive part of their nutrition from symbiotic algae (Zooxanthella) that live in their tissues and part of their nutrition from captured zooplankton.

The moon jellyfish were identified as Aurelia aurita by Hamner. Addition to the three species of Aurelia there are at least six other cryptic species in the Aurelia genus. Three of the cryptic species identified were from Palau. One of these cryptic species is common to four of Palau’s marine lakes with jellyfish populations including Jellyfish Lake.

The moon jellyfish were identified as Aurelia aurita by Hamner. Addition to the three species of Aurelia there are at least six other cryptic species in the Aurelia genus. Three of the cryptic species identified were from Palau. One of these cryptic species is common to four of Palau’s marine lakes with jellyfish populations including Jellyfish Lake.