Jellyfish Lake

Jellyfish Lake is a unique marine lake located on Eil Malk Island in Palau. Eil Malk forms part of the Rock Islands, a group of small, rocky, and largely uninhabited islands situated in Palau’s Southern Lagoon between Koror and Peleliu. The lake is internationally renowned for its dense population of jellyfish and the daily horizontal migration undertaken by millions of individuals across its waters. Within the Rock Islands alone, there are approximately seventy marine lakes, making the region one of the world’s most significant natural laboratories for the study of isolated marine ecosystems.
Although Jellyfish Lake is connected to the surrounding ocean through fissures and tunnels in the limestone of an ancient Miocene reef, it remains sufficiently isolated to develop environmental conditions markedly different from the nearby lagoon. As a result, biodiversity within the lake is greatly reduced, and several species, most notably the golden jellyfish, have evolved characteristics distinct from their closest oceanic relatives.

Geographic and Environmental Setting

Jellyfish Lake occupies a limestone basin formed within uplifted reef rock. The surrounding rock walls and dense vegetation significantly shelter the lake from wind, limiting surface disturbance and vertical water mixing. Rainfall, surface runoff, and tidal exchange through near-surface tunnels constitute the primary sources of water input. These factors, combined with the tropical climate of Palau, contribute to the lake’s stable physical structure.
The lake experiences tidal exchange with the adjacent lagoon, but this exchange is dampened. Tidal fluctuations within the lake are roughly one-third the height of those in the surrounding sea and are delayed by over an hour. Because tidal water enters near the surface, deeper layers of the lake remain largely unaffected by this exchange.

Lake Stratification and Meromixis

Jellyfish Lake is a permanently stratified, saline meromictic lake, a rare phenomenon in marine environments. The lake is divided into two distinct layers that do not mix seasonally.
The upper layer, known as the mixolimnion, is oxygenated and extends from the surface to approximately 15 metres depth. Oxygen concentrations decline gradually from around 5 parts per million at the surface to zero at the chemocline. All organisms requiring oxygen, including jellyfish, a limited number of fish species, and copepods, inhabit this layer. The water in this zone is moderately turbid, resulting in limited visibility.
Below the chemocline lies the monimolimnion, a permanently anoxic layer extending from about 15 metres to the lake floor. Oxygen is entirely absent in this zone, while concentrations of hydrogen sulphide increase with depth, reaching very high levels near the bottom. The upper portion of this layer contains a dense band of photosynthetic bacteria, including purple sulphur bacteria. This bacterial layer absorbs incoming sunlight, leaving the deeper waters dark but unusually clear.
The anoxic layer also contains high concentrations of ammonia and phosphate, nutrients that are almost entirely absent from the upper layer. Direct exposure to this layer can be dangerous to humans, as toxic compounds may be absorbed through the skin. This risk is reduced by restrictions on diving depth and the prohibition of scuba diving in the lake.

Age and Geological History

Jellyfish Lake is estimated to be approximately 12,000 years old. This estimate is based on the depth of the lake basin, the thickness of accumulated sediments, and the timing of sea-level rise following the end of the last ice age. As global sea levels rose, seawater gradually flooded the limestone basin, creating the enclosed marine environment observed today.

Jellyfish Species

Two species of scyphozoan jellyfish inhabit Jellyfish Lake: the moon jellyfish (Aurelia sp.) and the golden jellyfish (Mastigias sp.).
The moon jellyfish belong to the genus Aurelia, which is now understood to consist of multiple cryptic species. Genetic research has shown that the moon jellyfish in Palau’s marine lakes represent distinct lineages that have been reproductively isolated for millions of years. Consequently, the most accurate designation for the moon jellyfish in Jellyfish Lake is Aurelia sp., rather than assignment to a single named species.

Golden Jellyfish and Evolutionary Adaptation

The golden jellyfish are most closely related to the spotted jellyfish (Mastigias papua) found in nearby Palauan lagoons. Like their relatives, they derive nutrition both from captured zooplankton and from symbiotic algae known as zooxanthellae, which live within their tissues and carry out photosynthesis.
Despite this relationship, the golden jellyfish of Jellyfish Lake are morphologically, physiologically, and behaviourally distinct. They have lost most of the characteristic white spots seen on lagoon-dwelling spotted jellyfish and exhibit a significant reduction of club-shaped appendages on their oral arms. These differences are interpreted as evolutionary responses to the lake’s unique environmental conditions.
Some marine biologists have proposed classifying the golden jellyfish as a subspecies, Mastigias cf. papua etpisoni, reflecting both their close relationship to lagoon populations and the uncertainty surrounding species boundaries within the Mastigias papua complex. Similar distinctive jellyfish populations have been identified in several other marine lakes in Palau.

Daily Migration Patterns

One of the most striking features of Jellyfish Lake is the daily horizontal migration of the golden jellyfish. This movement follows a consistent pattern shaped by sunlight, nutrient availability, and predator avoidance.
During the night, golden jellyfish undertake repeated vertical movements between the surface and the chemocline, possibly to obtain nutrients such as nitrogen for their symbiotic algae. In the early morning, they migrate from the western basin towards the eastern side of the lake. By early afternoon, the migration reverses, and the jellyfish move back towards the western end. At sunset, they briefly shift eastward again before returning to the western basin for the night.
While swimming near the surface, golden jellyfish rotate counterclockwise. This behaviour is thought to ensure even exposure of their symbiotic algae to sunlight, maximising photosynthetic efficiency.
The migration pattern in Jellyfish Lake is more sharply defined than in most other Palauan marine lakes. This difference is believed to be an evolutionary response to the presence of jellyfish-eating sea anemones in certain areas of the lake. By adjusting their timing and direction of movement, the jellyfish minimise exposure to these predators.
Moon jellyfish, in contrast, do not display an organised horizontal migration. Instead, they migrate vertically at night, moving towards the surface to feed on copepods, which also ascend during nighttime hours.

Population Fluctuations and Die-Off Events

Jellyfish Lake has experienced notable fluctuations in jellyfish population size. In the late 1990s, a dramatic decline in the golden jellyfish population was observed, culminating in the complete disappearance of medusae by the end of 1998. Subsequent research suggested that extreme climatic conditions associated with an El Niño event were the most likely cause, as changes in water temperature and stratification disrupted the jellyfish’s symbiotic algae.The population later recovered, illustrating both the vulnerability and resilience of the lake’s ecosystem.