Early Ideas about Deep-sea Biology

In 1841 Edward Forbes was offered the chance to serve as naturalist aboard HMS Beacon, an English Royal Navy ship assigned to survey the Aegean Sea. For a year and a half the Beacon crisscrossed the Aegean waters. During that time Forbes was able to drag his small, triangular dredge—a tool with a leather net for capturing creatures along the sea bottom—at a hundred locations, at depths ranging from 6 to 1,380 feet. He collected hundreds of different species of animals, and he saw that they were distributed in eight different depth zones, each containing its own distinct assemblage of animal life, the way zones of elevation on the side of a mountain are populated by distinct sets of plants.

Forbes also thought he saw, as he later told the British Association, that “the number of species and individuals diminishes as we descend, pointing to a zero in the distribution of animal life as yet unvisited.” This zero, Forbes casually speculated—he simply extended a line on his graph of animal number versus depth—probably began at a depth of 1,800 feet. Below that was the final zone in Forbes’s scheme, zone nine, a zone that covered most of the ocean floor and thus most of the solid surface of Earth: Forbes called this the azoic zone, where no animal, to say nothing of plants, could survive.

Forbes’s azoic zone was entirely plausible at the time, and it was certainly far from the strangest idea that was then entertained about the deep sea. In the first decade of the nineteenth century, a French naturalist named François Péron had sailed around the world measuring the temperature of the ocean. He found that the deeper the water, the colder it got, and he concluded that the seafloor was covered with a thick layer of ice. Péron ignored the fact that water expands when it freezes and that ice therefore floats. A more popular belief at the time was that water at great depth would be compressed to such a density that nothing could sink through it. This ignored the fact that water is all but incompressible. But even the more sensible naturalists of the day were guilty of a similar misconception. They imagined the deep sea as being filled with an unmoving and undisturbable pool of cold, dense water. In reality the deep is always being refreshed by cold water sinking from above.

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