Ricky R. Ervin, age 12, of Gastonia, North Carolina, for his question:
Does the lungfish really live in a cocoon?
Fishes must live in the water because they absorb dissolved oxygen through their gills. This is the general rule. But the remarkable lungfish is an exception. He has gills to extract oxygen from water and also simple lungs that can absorb oxygen from air. When his pond dries up, he builds himself a cocoon of mud and waits for the next rainy season.
Some 350 million years ago, lungfish were quite common throughout most of the world. Then, as now, they lived in swampy ponds, in sluggish streams and in rivers that tended to disappear during the dry season. Most fishes thrive in moving water, which dissolves plenty of oxygen from the air. The lungfishes thrive in stagnant water, where there is little oxygen.
Only six species survive in the modern world one in Australia, one is South America and four in Africa. These lungfishes estivate, which is the hot weather counterpart of winter hibernation.
The so called lungs are an adaptation of the fish's swim bladder. He takes in air, not through his nose, but through his mouth. Even when he finds himself in oxygen rich water, now and then he comes to the surface, swims with his mouth above the surface and swallows a breath of air. Some lungfish have only one lung. And though most of them are three or four feet long, some measure six feet or seven feet and weigh 100 pounds or so.
When the water level sinks as the dry season gets drier, the average lungfish prepares for a spell of estivation. The South American species merely tunnels down into the mud. When it becomes hard, he is encased in a solid cocoon that seals out the hot, dry summer air. During this resting period of estivation, the breathing and pulse rate slow down and the fish lives on his stored body fat.
An African lungfish may build a more elaborate cocoon. As the dry season approaches, he too tunnels into the mud. But before it sets hard, he gives off a sticky substance. When the mud dries, he is inside a form fitting tunnel with extra hard walls. At one end of the cocoon there is a small hole, which serves as a breathing tube.
The hard, sun baked cocoon protects the lungfish for several months during the dry season. And he can last much longer. Researchers have found that some can survive in their cocoons for as long as four years.
The big problems during this sort of estivation is the elimination of body wastes. One of these chemicals is urea, which is poisonous. In most animals, a concentration of ten parts per million is fatal. After a long period of estivating in his baked mud cocoon, a lungfish's concentration of urea may be 2,000 times more than the normally fatal dose. But somehow he survives and after a couple of hours back in the water, his system of chemical wastes returns to normal.