Golden jellyfish!

(Photo from National Geographic -
(Photo from National Geographic –

I’m sure some of you have heard about the millions of golden jellyfish (genus¬†Mastigias) that inhabit landlocked marine lakes in Palau. I only heard about them a couple of days ago, and I think they’re really cool!

The different subspecies of golden jellyfish were effectively isolated from their ancestor (the lagoon-inhabiting jellyfish) and each other 5000 – 15 000 years ago when rising sea levels flooded dry land, creating marine lakes. These lakes are still connected to the ocean via small cracks or channels in the surrounding limestone, which limits the organisms that can pass through.

As you’d expect, the marine lake jellyfish are remarkably different from their closest relatives (lagoon jellyfish) as well as each other. The jellyfish live in symbiosis with intracellular photosynthetic algae (“zooxanthellae”/genus Symbiodinum), and rotate in the water for even sun exposure. Each day, the jellyfish “follow the sun” (migrating east in the morning, west in the afternoon). This ensures that they avoid the edge of the lake, as they always encounter a shadow before they reach the area inhabited by their predators (the white sea anemone).

Some detailed observations of the similarities and differences between the different marine lake populations and the “ancestral” lagoon lake population can be found in this paper.

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