Wednesday, March 08, 2006


Got the old iMac back last night and now we're back in business.


I found this tale about jellyfish on Mercherchar of the Palau Islands. Americans fought a pretty tough battle in those islands during the Second World War, I believe.

[Open quote] During the Second World War, sonar operators looking for submarines were puzzled by what seemed to be a false bottom of the sea that rose towards the surface every evening, and sank back down again the next morning. It turned out to be the bulk of the plankton, millions of tiny crustaceans and other creatures, rising to feed near the surface at night, then sinking at morning. Why should they do this? The best guess seems to be that during the hours of daylight they are vulnerable to visually hunting predators such as fish and squids, so they seek the dark safety of the depths by day. Why, then, come to the surface at night, for it is a long journey that must consume a lot of energy? One student of the plankton has compared it to a human daily walking 25 miles each way, just to get breakfast.

The reason for visiting the surface is that food ultimately comes from the sun, via plants. The surface layers of the sea are unbroken green prairies, with microscopic single-celled algae in the role of waving grass. The surface is where the food ultimately is, and that is where the grazers, and those that feed on the grazers, and those that in turn feed on them, must be. But if it is safe to be there only by night because of visually hunting predators, a diurnal migration is exactly what the grazers and their small predators must undertake. And apparently they do. The 'prairie' itself doesn't migrate. If there were any sense in doing so, it should swim against the animal tide, for its whole raison' d'ĂȘtre is to catch sunlight at the surface during the day, and avoid being eaten.

Whatever the reason, most of the animals in the plankton migrate down for the day and up for the night. The jellyfish, or many of them, follow the herds, like lions and hyenas tracking the wildebeest across the Mara and Serengeti plains. Although, unlike lions and hyenas, jellyfish don't target individual prey, even blindly trailing tentacles will benefit by following the herds, and this is one of the reasons jellyfish swim. Some species increase their catch rate by zigzagging about, again not individually targeting prey, but increasing the area swept by those tentacles with their batteries of lethal harpoons. Others just migrate up and down.

A different kind of migration has been described for the massed jellyfish of 'Jellyfish Lake' on Mercherchar, one of the Palau Islands (an American colony in the western Pacific). The lake, which communicates underground with the sea and is therefore salty, is named after its huge population of jellyfish. There are several kinds, but the dominant one is Mastigias, an estimated 20 million of them in a lake 2.5 kilometres long and 1.5 kilometres wide. All the jellyfish spend the night near the western end of the lake. When the sun rises in the east, they all swim straight towards it and therefore the eastern end of the lake. They stop before they reach the shore, for an interestingly simple reason. The trees fringing the shore cast a deep shadow, cutting off so much of the sun's light that the jellyfish's sun-seeking automatic pilot starts to drive them towards the now brighter west. As soon as they come out from the trees' shadow, however, they turn east again.

This internal conflict traps them around the line of the shadow, with the consequence (which I dare not think is more than coincidence) of keeping them a safe distance from the dangerously predatory sea anemones that line the shore itself. In the afternoon, the jellyfish follow the sun back to the western end of the lake, where the whole armada again becomes trapped at the shadow line of the trees. When it becomes dark, they swim vertically up and down at the western end of the lake, until the dawn sun lures their automatic guidance system back towards the east. I don't know what they might gain from this remarkable twice-daily migration The published explanation satisfies me too little to bear repetition For now, the lesson of the tale must be that the living world offers much that we don't yet understand, and that is exciting in itself. [Close quote] —Dawkins, THE ANCESTOR’S TALE, pp. 468-469

“Men never do evil so completely and cheerfully as when they do it from religious conviction.” —Blaise Pascal (See, it’s not only us modern atheists who think this is true.)

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