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Ongeim’l Tketau – Palau’s Jellyfish Lake

When I think of jellyfish (Phylum: Cnidaria), I typically think of the ocean and not a lake.  Then again, nature loves to shake things up from time and time and Ongeim’l Tketau in Palau (aka “Jellyfish Lake”) serves as a delightful reminder of our biosphere’s affinity for the unusual.

Aerial view of Jellyfish Lake (2008)

Jellyfish lake is a marine lake; one of more than 50 saltwater-filled basins in Palau that formed between 5,000 – 15,000 years ago during a period of rising sea levels.  As the ocean rose and spilled over the land, a number of Papuan jellyfish (Mastigias papua) presumably left their typical habitat in the surrounding lagoons and entered at least 5 of these lakes.  When the ocean receded, these jellyfish become trapped within the lakes and established what are now isolated populations.  Though all of these lakes still retain at least some degree of communication with the ocean via networks of limestone tunnels, the isolation has remained sufficient enough to allow each of the 5 lakes to develop its own unique Mastigias papua subspecies (which have been named after 5 elected Presidents of Palau).

Mastigias papua in Palau. Source:
Mastigias papua in Palau. Source:

Of these lakes, Ongeim’l Tketau is unequivocally the most famous and home to over 10 million golden jellyfish (Mastigias papua etpisoni) that engage in a conspicuous daily migration in response to the movement of the sun.   Every morning, the fluther moves eastward toward the rising sun and then turns westward in the afternoon.  Why engage in this ~1 km daily shuffle?  The first reason is likely nutrition.  Like their lagoon ancestors, the lake jellyfish derive part of their daily energy requirement from the activity of photosynthetic endosymbionts called Zooxanthellae.  These unicellular algal protists live within the jellyfishes’ tissues and produce glucose, glycerol, and amino acids which are in turn used by the jellyfish in a way similar to coral.  (Incidentally, the rest of the jellyfishes’ nutrition comes from feeding on zooplankton within the lake).  By maximizing their exposure time to sunlight, the jellyfish bolster the photosynthetic productivity of their marvelous little Zooxanthellae and thus increase their own energy gains.

Anemones feeding on Mastigias jellyfish. Source:

The second reason for the migration is presumably to avoid the walls of the lake.  Attached to the walls is a patchwork of stark white sea anemones (Entacmea medusivora) with a penchant for devouring jellyfish much larger than themselves.  By moving eastward in the morning and westward in the afternoon, the jellyfish swarm ensures that it will always encounter a shadow before contacting the wall and becoming a meal for a hungry anemone.  This presumption seems to be validated by the fact that the response of these jellyfish to shadows is so dramatic that they will typically form a dense curtain right along the illuminated edge of the darkness, almost as though they struck an invisible wall.

Mastigias papua in Palau.
Mastigias papua in Palau. Source:

With this dramatic dance occurring every day right near the surface, simply observing the jellyfish from above can be spectacular in its own right, but if you’re looking for a more personal encounter, Ongeim’l Tketau also provides visitors with an opportunity to dive right into dense gelatinous flock.  Donning snorkels, masks, and fins, visitors can swim in what has been described by the World Wildlife Foundation as “a living lava lamp.”

Snorkeling among Palau’s lake jellyfish. Source:

I know what you’re thinking: “Don’t all those jellyfish sting?”  Fortunately, while the lake jellyfish have changed a great deal in relation to their lagoon ancestors, they have retained their very mild sting which is barely detectable by human skin.  Visitors often report only a mild tingling sensation near their lips, making for an absolutely surreal encounter in what is arguably one of the most unusual places on the planet.

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