In the lake with jellyfishes (English version)*Recently, I traveled to a remote place in the Pacific where an incredible natural event flourished in a lake. The Jellyfish Lake.
How this lake emerged is quite interesting. During the Pleistocene parts of the ocean under tectonic pressure became entrapped inside some islands; these islands form today the Republic of Palau in the North Pacific. The island lakes are filled with seawater and some of them are still connected to the sea, while others are seawater fed by infiltration through limestones. In Palau, there are 6 lakes of this last category, each of them breeding an unique evolutionary story.
The emergence of a big cliff barrier shut down the direct connection with the ocean and further possibility of big animals to reach and live in the lake. Only larvae or small creatures were able to arrive there, and among them the larvae of the jellyfish Mastigias sp. Most of other invertebrates, though, were soon gone due to the poor nutrition status found in the lake, initiating "the jellyfish kingdom".
In the beggining, the jellyfishes in the lake were like the ones we find today in the ocean, with nematocysts - the toxin-containing cells bearing stingers. The nematocysts were an adaptation to ward away any predator that dare to approach the fragile animal. However, by becoming isolated, jellyfishes no longer shared their habitat with predators. The nematocysts were no longer necessary for this new environment: their presence became worthless. It became a waste of energy to produce cells and useless toxins. Therefore, evolving in a lake environment resulted in nematocyst loss. The Palauan Jellyfish Lake became like a natural amusement park, a place where we can swim among jellyfish that don't sting!
But how do the stingless jellyfish get its food in an almost dead lake? Among the few living creatures around them were zooxanthelae - the microscopic algae that we find in partnership with corals. At this point, an opportunity for symbiosis appeared: the algae started living and photosynthesizing inside the translucent jellyfish body taking advantage of the ride to the bright surface. A nice natural rythm is seen nowadays: during the day the jellyfishes move horizontally to shallow waters, giving the algae the sunlight they need. At night, they lurk in the lake's depth (~50m), where an anaerobic environment rich in nitrogen compounds is available for the algae's photosynthesis. The carbohydrate produced from photosynthesis is metabolized by the jellyfish, which developed a specific enzyme that allows its breakdown and digestion. It's a win-win situation for both species.
The Palauan Jellyfish Lake is one of the best known cases of geographical isolation studied on the planet. And one of the most psychedelic natural attractions as well. To arrive at the lake, one must climb up-and-down a cliff. The surrounding vegetation is pleasant. But it's only when you are in the water heading towards the center of the lake that you really start getting "the" weird feeling: it's impossible not to touch a jellyfish. There are thousands around, nearly impossible to avoid collision. At this point, one should only enjoy the quite unique swim in a gel-like pool.
When I was there, swimming in joy, I felt like I was inside a Buñuel movie or a Dalí painting. The water was not very clear, greenish, with those multiple moody jellyfish "dots" in all imaginable sizes pulsing close to the surface. Tentacles moved either slow for the big jellyfishes or very fast for the smaller ones. A very abstract picture for a straightforward scientist mindset. A strong extatic feeling took over, and I then realized: sometimes what we think is like a dream can actually be one of the most amazing real experiences of a lifetime.
*Welcome Spineless' lovers! Hope you enjoy the ride here, and if you read/understand Portuguese, please feel free to look around.
**Meus queridos leitores: Este post foi anteriormente publicado em português aqui, e foi livremente traduzido pro inglês para que eu pudesse participar do Circus of the Spineless, blogagem coletiva de ciência sobre invertebrados, organizada de tempos em tempos e que nesta edição está sendo hospedada no blog do PZMeyers. A quem interessar mais posts sobre invertebrados em inglês, vale a pena dar uma passada lá e conferir.