and, strangest of all, thickets of crimson-plumed tube worms anchored firmly to the seafloor and standing up to six feet [1.8 m] tall.

A Living Enigma

When biologists examined the tube worms, they found the animals to be a living enigma. They had no mouth and no digestive system. The question arose, How did they eat and assimilate food? Then came a startling discovery: The worms had red blood—not a bloodlike fluid but actual blood rich in hemoglobin—circulating through their body and featherlike plume.

The mysteries deepened when biologists opened up the flaccid sac of the tube worm’s body. Its tissues contained a bacterial culture composed of some 285 billion [10 billion] bacteria per ounce [gram] of tissue! In 1980 a biology student theorized that the tube worm lives by means of symbiosis—an arrangement where two species cooperate for mutual benefit. Research confirmed her hypothesis by showing that the tube worm, as host, feeds the bacteria, and the bacteria feed the worm.

Like gills, the plumes of the tube worm gather the ingredients, such as oxygen and carbon, that the bacteria need to manufacture food. The plumes do not wave directly in the searing vent water—that would be suicide—but in the region close to where near-freezing seawater and vent water mix. Of course, this food-manufacturing process requires energy. On the earth’s surface—and in the upper part of the ocean—sunlight energizes food production by causing vegetation to grow. But sunlight comes nowhere near the abyssal home of the tube worm.

In order to bind all the chemicals needed by the bacteria, tube worm blood is composed of hemoglobin molecules that are 30 times larger than hemoglobin molecules in humans. The blood transports these chemicals to the hungry bacteria, and the bacteria, in turn, manufacture food for the tube worm.