Clear Lake, Palau

Testing ground for evolution

Each of the marine lakes of Palau has a specific set of species occasionally trapped in lakes by the receding sea 15 – 10 thousand years ago. Some lakes have steeper banks, some are elongated in an east-west direction, some in a south-north direction. Every such detail leads to different circumstances and consequently – different new species and subspecies of animals.

Some marine lakes of Palau are made even more exotic by the fact that they are meromictic – below the upper layer of marine water there is a layer of "dead", anoxic water separated by a layer of floating microorganisms, including a purple bacteria.

Such unusual monuments of nature serve as excellent “testing ground” for studies of evolution. Thus lately several scientists have made interesting treatises about these lakes. Lucky them (scientists!) – making science in beautiful Palau…

Jellyfish of president Salii

Clear Lake is small (just 3.9 ha) and its maximum length is just some 330 meters, the age of the lake – approximately 12,000 years. Depth reaches 30 meters, purple-sulfur bacteria layer is located in the depth of 15 – 16 meters, below there is an anoxic layer of water without life.

Although the lake is small, here lives endemic subspecies of jellyfish – Mastigias cf. papua salii, named after the third president of Palau Lazarus Salii.

Species composition in Clear Lake is similar to species composition in Jellyfish Lake. Here also lives a predator of jellyfish – sea anemone Entamaea medusivora.

But there is an important difference – while the Jellyfish Lake is elongated in the east-west direction, Clear Lake is stretched in the north-south direction. It seems that this seemingly slight difference has a big influence on the fauna of the lake. The border between the seawater and anoxic water in Clear Lake is more diffuse and oxygen level is falling already at 6 – 10 meters depth.

This is the most likely reason why jellyfish in Clear Lake appears sporadically, while in Jellyfish Lake there always (almost) is a lot of jellyfish.

Jellyfish of Clear Lake also have a daily migration pattern, although less pronounced than in Jellyfish Lake, here jellyfish do not evade shadow.

In order to survive sea anemones also had to elaborate different behavior – besides the jellyfish it consumes mussels as well.

References

1. Coral Reef Research Foundation, Marine Lakes Research Accessed 08.02.10.
2. Jellyfish swarms, tourists, and the Christ-child, Mike N. Dawson, Laura E. Martin, Lolita K. Penland. Hydrobiologia 451: 131-144, 2001.
3. Five new subspecies of Mastigias (Scyphozoa: Rhizostomeae: Mastiigidae) from marine lakes, Palau, Micronesia, Michael N. Dawson. Journal of Marine Biological Association, U.K. 2005., 85., 679-694. Accessed 08.02.10.