Jellyfish lakes are very uncommon in the world, with only about 20 having been discovered. SeaTrek has the distinction of knowing the locations of four of them, which we take our guests to experience first hand to snorkel with amongst them in Raja Ampat and Sulawesi on our Eastern Cruises from October through April each year.


See what it's like to swim through a sea of stingless jellyfish as they pulse by in their thousands: Thanks to George Beccaloni for the footage


To find out more about them we sat down with marine biologist, Professor Joe Mueller of the College of Marin in California and resident expert on two of SeaTrek's end-of-year Jewels of Raja Ampat cruises, and asked him a few questions about these unique creatures and their habitats.

Where in the world are jellyfish lakes found? They are found mostly in Indonesia, Palau and Vietnam. The jellyfish are found in lakes and pools that have underground connections to the ocean and are confined to particular areas that are comprised mainly of limestone.

How many are there in the world? There are about 200 known marine lakes in the world, but we are finding more all the time, with 42 found in one recent study in 2018 in Indonesia alone. Of the 13 that were visited only four had jellyfish and it’s estimated that fewer than 20 worldwide are known to host jellyfish, so it's quite a rare and special occurrance.



What are the specific jellyfish species found in Indonesia's "jellyfish lakes"? The four main species are the golden jellyfish, moon jellyfish, upside-down jellyfish and the very small finger-tip size cube jellyfish. Some lakes have only one or two of these species, and the golden jelly (Mastigias papua ) and the moon jelly (Aurelia aurita) are the most common.

How commonplace are these species? Outside of marine lakes the more common marine subspecies of the golden jellyfish (the spotted jellyfish) is found throughout the Indian Ocean up through the China Sea to Japan down to Fiji and west to the Indo-West Pacific. The moon jellyfish is found throughout world’s oceans, both in inshore and offshore waters.



Why are the golden jellyfish so fascinating? Because this variety has been isolated from the sea for so long they have developed a way to obtain food symbiotically. Incorporated within their tissues are microalgae called zooxanthellae. These are the same species of algae found in corals that are responsible for assuring the correct chemistry is provided for coral polyps to make reefs out of calcium carbonate (the same that are expelled in the bleaching process that’s happening all over the world due to warming ocean waters).

Since the algae provide the jellies with food the jellies must maximise their intake of light. More light equals more food, so the jellies migrate daily. They migrate horizontally following the sun (up to 2 km a day) and they move vertically to the surface during the day for light and oxygen, and move down to the bottom for inorganic nutrients for their symbiotic algae at night (kind of like gathering soil for their garden of algae that live in their skin). The reproductive peak for the algae is at night, when there’s an abundance of nitrogen in the form of ammonia available to them. Then, during the day, they make lots of sugar and amino acids that spills out of their cells for their jelly host that makes sure they have maximum light for photosynthesis to make sugars. Sugar is provided for jellies, light and inorganic nutrients provided for algae. Symbiotic mutualism!

What do their limbs do (they seem to survive even if they lose a few)? The limbs you are referring to are termed oral arms. In jellyfish, in general, they are basically long lip-like extensions of the mouth. They usually have one to many grooves that facilitate the movement of food to the mouth. In many species, they’re lined with nematocysts (stinging cells) and pulsating hair-like cilia, stinging and moving dead food to the mouth. In the golden jelly their oral arms are all puffy and extended. I suspect this is to increase surface area to maximize light absorbing regions, much like many corals do with their bubbly light capturing tentacles. Kind of like adding an extra bin of soil for our gardens to provide more surface area to grow our green. Additionally, they have many small mouths that are feeding on microscopic zooplankton all along their oral arms. That’s what those frilly-like structures are that one can observe along their oral arms. So, in summary, they use their oral arms for gardening and food capture.



How old do they get and how big do they grow? They live for about four months and are between 3 - 10cm long and 2 - 7cm wide.

Why have these jellyfish lost their stings? It’s thought that they have lost their ability to sting due to being isolated so long in these landlocked lakes. They don’t have any predators in these lakes so they don’t need them for protection, and since they mostly grow their own food they don’t need them to dispatch and catch food as much. Also, some species like the moon and golden jellies, have stinging cells with venom that’s too weak to for humans to feel, even when they’re living in the sea or areas other than these marine lakes.



Any idea how long it has taken them to evolve this way? This is tough to say…since jellyfish are 95-98% water they don’t fossilize, so much of what we know about their past is determined genetically. I haven’t come across any studies that have investigated theses changes in marine lake populations. Many of the lakes they live in are estimated to be only about 7,000 - 12,000 years old so I would say they’ve adapted relatively fast compared to most evolutionary time scales. These lakes provide evolutionary biologists unique opportunities to study the evolutionary process since they’re so isolated and evolutionary change proceeds must faster than other ecosystems. They’re living laboratories of evolution! In the case of the golden jellyfish, scientists have been able to find a correlation between the age of the lake and the degree of adaptation to the environment. Speaking of living laboratories, these lakes are of particular interest to scientists since they are warmer and saltier than the oceans. These marine lakes allow us to develop models and inquiry methods as to how algae and animals are able to adapt to the warming oceans due to human induced climate change.

Any tips for snorkelling with the jellyfish? Watch how they move through the water and ponder how they are a beautiful microcosm of how to live life on Earth: they are ecological marvels, wondrous in their simplicity, masters at living in cooperation, and experts at mindfulness…all without a mind. An earthly perspective free of charge!





SeaTrek Sailing Adventures offers a number of cruises where you can experienceswimming with these fascinating creatures, both in Raja Ampat and Sulawesi. Click on the links below to find the cruise that suits you best and then get in touch with us directly. 

Jewels of Raja Ampat - October thru February

Papua's Whale Sharks and Birds of Paradise - October thru December

From the Spice islands to Sulawesi: Sailing the Undiscovered Frontier - February thru April







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