Habitats on land — rain forests, steppes, woodlands, deserts, alpine meadows, all well explored over the centuries — make up less than 1 percent of the planet’s biosphere. Why so little? The band of life is narrow. Fertile soil goes down only a few feet, and even the tallest trees stretch up only a few hundred feet. Birds can fly higher, but must return to the surface for nourishment.

Water, however, is a different story. It covers more than 70 percent of the earth’s surface and goes down miles. Scientists put the ocean’s share of the biosphere at more than 99 percent. Fishermen know its surface waters and explorers its depths. But in general, compared with land, the global ocean is unfamiliar.

Which helps explain why scientists have only recently come to realize that the bristlemouth — a fish of the middle depths that glows in the dark and can open its mouth extraordinarily wide, baring needlelike fangs — is the most numerous vertebrate on the earth.

“They’re everywhere,” Bruce H. Robison, a senior marine biologist at the Monterey Bay Aquarium Research Institute in California, said of the bony little fish. “Everybody agrees. It’s the most abundant on the planet.”