Did You Know? Jellyfish Lake

Ongeim’l Tketau (OTM), also known as Jellyfish Lake, is a 30 m deep basin filled with seawater indirectly connected by cracks and crevices to the lagoon. Typically the lake hosts 13 million golden jellyfish. While this is the only lake open to visitors, there are over 50 marine lakes in Palau, at least five of which contain golden jellyfish.

What are marine lakes? Marine lakes are isolated bodies of seawater, surrounded by land. All marine lakes retain connections to the ocean via channels through the encircling limestone rock. The number, size, and length of channels determine the degree to which water and organisms exchange between the lakes and the ocean. Each marine lake is unique in characteristics such as depth, size, shape and volume of the lake, number and positions of tunnels and vertical profiles of salinity, temperature, and dissolved oxygen. These and affect the types and amount of habitat, and therefore the numbers and kinds of marine species each lake supports. In fact, each lake is distinguished by a unique suite of habitats and species. Marine lakes occur in two main types: mixed and stratified. In mixed lakes, temperature, salinity, and the amount of dissolved oxygen do not change significantly with depth. The opposite is true in stratified lakes, and the deeper regions of these lakes lack oxygen and have high concentrations of hydrogen sulfide. Like most jellyfish lakes, OTM is stratified with the anoxic (oxygen-less) lower layer beginning at 12-14 m (40-45 ft). This transition is marked by a pink bacterial layer 1 meter thick.

Why call it Jellyfish Lake? Two species of jellyfish inhabit OTM: the golden jelly, Mastigias papua etpisoni, and the less common moon jelly, Aurelia. Fragile, transparent moon jellies are graceful swimmers that often hang suspended, unmoving in the water. These animals generally spend daylight hours 5m (15 ft) below the surface feeding on plankton. Golden jellies, in contrast, are a mutualistic union of jellyfish and microscopic algae. As in corals, the algae provide energy for themselves and the jellyfish by converting sunlight into sugars, some of which they share with their host. Golden jellies also acquire energy by capturing zooplankton with stinging cells located on their frilly oral arms. Like their ancestors in the lagoon (Mastigias papua), the sting of OTM’s Mastigias is mild and often undetectable. This has given rise to the myth that the jellies have lost their ability to sting due to isolation in a predator-free lake. However, you may feel their tingly sting if an oral arm comes in contact with sensitive skin like that around your mouth.

Is the lake really predator-free? The myth that the golden jellies lack predators is also discredited by the presence of the endemic white-sea anemone Entacmea medusivora (medusa-eating). Although immobile, these animals are capable of capturing and ingesting a passing jellyfish larger than themselves and can often be seen doing so on the rocky point 50 m from the dock. In fact, these anemones are likely responsible for the evolution of the remarkable, daily migration of the golden jellies

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