Jellyfish Lake: Why is the Ongeim’l Tketau lake full of jellyfish?

jellyfish lake
Zooxanthellae within the jellyfish’s tissue harvest solar energy and photosynthesise

Found off the coast of Koror, Palau in the Philippine Sea, a rocky island uninhabited by humans is host to a colossal colony of jellyfish. The Ongeim’l Tketau marine lake, more commonly known as Jellyfish Lake, is just one of about five Palauan lakes inhabited by these glutinous beauties. The lake itself is an average of 30 metres (98 feet) deep and is connected to the sea by cracks and fissures in the rock.

The unusual chemical parameters of the lake mean it is highly stratified. The uppermost ‘layer’ of water is rich in oxygen, but around 15 metres (49 feet) beneath the surface the anoxic bottom layer begins – an oxygen-depleted zone high in hydrogen sulphide that makes the depths of the lake a no-go area for scuba divers.

However, swimming in the top layer is permitted – and recommended – as the whole water body is teeming with both golden jellyfish and moon jellyfish species. Neither species’ stings are dangerous to humans.

These fascinating animals are able to grow to such large numbers as the lake provides a safe, enclosed ecosystem with very few natural predators. However, there is one sea anemone species living in the lake that has a definite taste for a jellyfish supper.

Every day, an epic migration can be witnessed within the lake, as the jellyfish move to the eastern side of the lake in the morning and then back west again in the evening, tracking the progress of the Sun. It’s thought that this daily migratory behaviour helps the jellyfish avoid the shady shoreline areas where the jelly-hungry anemones can be found lurking in wait for an easy meal.

Jellyfish symbiosis

The golden jellyfish of Jellyfish Lake have an amazing mutually beneficial relationship with tiny microscopic algal organisms called zooxanthellae. The jellyfish provide the zooxanthellae with a safe place to live within their bodies, and in turn the zooxanthellae provide the jellyfish with food – a by-product of their photosynthesis. This makes maximum light exposure essential to the golden jellyfish’s survival, as the zooxanthellae harvest energy directly from sunlight. While the jellies are basking in the light, they use the pulsating movements of their bells and mild jet propulsion to gradually rotate themselves on the spot. That way, all their algae get an equal exposure to the sunlight. This provides another explanation as to why they follow the Sun across the lake every single day. The moon jellyfish species also follow the Sun because they eat phytoplankton in the water, which also utilises sunlight.

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