Sunday, 3 April 2011

Jellyfish Lake – Daily Migration of Millions

Every day millions of golden jellyfish migrate – no big surprise there.  However, you might not expect them to migrate horizontally across a lake.  Still, a visit to Jellyfish Lake on Eil Malk, an island in Palau, in the Pacific Ocean, will confirm just that.  Yet just how on earth did these jellyfish get to the lake in the first place?

To begin with, the body of water that is called Ongeim'l Tketau in Palauan is a marine lake.  It connects to the sea via tunnels and fissures in the limestone that encases it.  Even so, the lake is isolated – the golden jellyfish in the lake a different to those which swim in the sea just a stone’s throw away. As you descend under the water, a new and alien world presents itself.

There are so many jellyfish in the lake that their sheer mass is visible from the air. It is thought that the lake is about twelve thousand years old – this is an educated guess based on three factors.  First there is the depth of the lake, which is about thirty meters and the thickness of the sediment at its base –which is an astounding 20 meters thick.  The final element in the calculation of the age of the lake is the rising sea levels which, 12,000 years ago had risen to a sufficient height fill the basin which became the lake.

There are in fact two species of jellyfish living in Jellyfish Lake.  The first and most populous is the golden jellyfish with the moon jellyfish (not pictured here) being the lesser in number.  The golden jellyfish is related to the spotted jellyfish in the sea nearby but are distinct in a number of ways.  The most obvious is that they have lost the spots from which the spotted jellyfish gets its name.  The second is they have almost lost their clubs, which ordinarily would be attached to their oral arms.

Although they look huge in these pictures, perspective shows that many of them are small, often smaller than a human fist.

Rather than classifying them as a separate species altogether, marine biologists believe that the golden jellyfish should be classified as a subspecies of the spotted variety.  It is also thought that jellyfish in other marine lakes in the area could contain another four distinct subspecies.  12,000 years may not seem like a long time in evolutionary terms but it is enough time for one species to diverge a great deal from its original.

Unlike the moon, the golden jellyfish follow a distinct migratory pattern.  Each day they make journeys vertically down to a lower level of water (known as the chemocline) – it is thought they do this to gather nutrients.  Then they criss cross the lake throughout the day – from the western to eastern basin in the morning and back across in the afternoon. It is a horizontal migration that numbers in the millions.

As they swim to and fro the golden jellyfish also rotate their bodies in a counter clockwise direction.  It is thought that they are positioning themselves here in order to expose themselves to the greatest amount of sunlight possible for the symbiotic algae which live in their bodies to use to photosynthesize.

However, the jellyfish here are not at the top of the food chain - they do have a predator. It is also thought that the jellyfish have this strange migratory pattern so as best to avoid the anemones (see above) which prey on them. By avoiding shadows for the better part of the day it is thought that they also avoid the anemones which do not like strong sunlight.

In 1998 there was a precipitous fall in the numbers of golden jellyfish in the lake.  This was caused, so is thought by an El Nino event which made the water temperature in the lake rise.  The hotter water killed off the symbiotic alga in their bodies and by the winter of that year there were (apparently) no golden jellyfish left.  However, by 2000 the numbers were back up to normal.

As you can imagine, snorkelling is a popular tourist activity in Jellyfish Lake.  There are several tour visits a day but snorkelling is banned as the bubbles can potentially kill the jellyfish if they rise up and collect underneath their bells.  Also, deep diving is not encouraged because of the layer of sediment at the base which is toxic – to the point of being deadly - when absorbed through human skin.

Yet what about the infamous sting of the jellyfish?  Both species in the lake do possess stinging cells but they are not powerful enough to cause any harm to visitors.  Tourists are far more likely to be gobbled up by a local saltwater crocodile - like the one pictured in Palau above. Even then, only one instance of this has been reported in Palau in the last decade- and then it was not at Jellyfish Lake but elsewhere in the islands.

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