The Ancestors Tale by Richard Dawkins

Author:Richard Dawkins
Language: eng
Format: mobi
Tags: Non-fiction
Published: 2010-10-23T15:15:15+00:00

the number of molecules fixed and adjusting the pressure, the fish adjust the number of molecules. To sink, the fish absorbs some molecules of gas from its swim bladder into the blood, thereby reducing the volume. To rise, it does the reverse, releasing molecules of gas into the swim bladder.

In some teleosts, the swim bladder is also used to assist in hearing. The fish’s body being mostly water, sound waves propagate through it pretty much as they did through the water before they hit the fish. When they strike the wall of the swim bladder, however, they suddenly reach a different medium, gas. The swim bladder therefore acts as a kind of eardrum. In some species it lies right against the inner ear. In others it is connected to the inner ear by a series of small bones called the Weberian ossicles. These do a similar job to our own hammer, anvil and stirrup, but are completely different bones.

The swim bladder seems to have evolved — been ‘co-opted’ — from a primitive lung, and some surviving teleosts, such as bowfins, gars and bichirs, still use it for breathing. This perhaps comes as a little surprise to us, for whom breathing air seems like a significant ‘advance’ that went with leaving the water for the land. One might have supposed the lung to be a modified swim bladder. On the contrary, it seems that the primitive breathing lung forked in evolution and went two ways. On the one hand, it carried its old breathing function out onto the land, and we use it still. The other branch of the fork was the new and exciting one: the old lung became modified to form a genuine innovation — the swim bladder.

The Mudskipper’s Tale

On an evolutionary pilgrimage it is fitting that some of the tales, though told by surviving pilgrims, should deal with recent re-enactments of ancient evolutionary events. Teleost fish are so variable and so versatile, it is only to be expected that some of them might replay parts of the lobefins” history, and come out onto the land. The mudskipper is just such a fish out of water, and it lives to tell the tale.

A number of teleost fish species live in swampy water, poor in oxygen. Their gills cannot extract enough, and they need help from the air. Familiar aquarium fish from the swamps of South-East Asia, such as the Siamese fighting fish Betta splendens, frequently come to the surface to gulp air, but they still use their gills to extract the oxygen. I suppose, since the gills are wet, you could say the gulping is equivalent to locally oxygenating their gill water, as you might bubble air through your aquarium. It goes further than that, however, because the gill chamber is furnished with an auxiliary air space, richly supplied with blood vessels. This cavity is not a true lung. The true homologue of the lung in teleost fish is the swim bladder which, as the Pike’s Tale has shown, they use for keeping their buoyancy neutral.

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