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.”
By human standards, the brute is tiny — smaller than a finger. But this strange little fish makes up for its diminutive size with staggering numbers, as well as a behavioral trick or two.
It turns out to be transgender, starting life as a male and, in some cases, switching to become a female. Scientists call it protandrous — that is, a male-first hermaphrodite — a phenomenon also seen in certain worms, limpets and butterflies.
John C. Avise, the author of “Hermaphroditism,” said the adult male bristlemouth tended to be smaller than the female and had a better developed sense of smell — apparently, he said, to find mates in the darkness.
“They occupy an environment that’s hard to access,” Dr. Avise said of the fish, so there is “precious little information” about their behavior.
A slightly repulsive means of adding to that information has been to inspect the stomach contents of larger fish. Predators of the bristlemouth turn out to include dragon fish and fangtooths, denizens of the abyss with daggerlike teeth.
Though the portrait of the bristlemouth is incomplete, scientists know enough to confidently assert that it far outstrips all other contenders for the title of most common vertebrate on the planet.
Noah Strycker, the author of “The Thing With Feathers,” a book about birds, recently told an interviewer that the domestic chicken “has more numbers” than any other vertebrate.
He put the planetary figure at 24 billion.
In contrast, ichthyologists put the likely figure for bristlemouths at hundreds of trillions — and perhaps quadrillions, or thousands of trillions.
“No other animal gets close,” said Peter C. Davison, a fish scientist at the Farallon Institute for Advanced Ecosystem Research, in Petaluma, Calif. “There are as many as a dozen per square meter of ocean.”
The bristlemouths are a rapacious family of deep-sea fishes that include the wildly successful genus Cyclothone — Greek for “circular,” in apparent reference to the creature’s gaping mouth. They are also known as roundmouths.
The genus has 13 species, such as the shadow bristlemouth. The main distinguishing features are subtle differences in the fins and luminous organs. All members wield bristlelike teeth. Over all, the fish are one to three inches in length, tan to black in color, and at times display a kind of ghostly translucence.
The first hints of the fish’s ubiquity came during the voyage of the H.M.S. Challenger, a British ship that sailed the globe from 1872 to 1876 and helped lay the foundations of oceanography. It lowered nets at dozens of research sites and hauled up the creatures from as deep as three miles.
The expedition’s thick reports described the tiny fishes as having rows of luminous organs, conspicuous jaws and sharp teeth. The studies noted different species but said little else. Just learning of the fish’s existence was hard enough.
The first scientist to view the animals in their dark habitat was William Beebe. In the early 1930s, Beebe, a senior explorer of what is now the Wildlife Conservation Society, plunged into the depths off Bermuda in a spherical submersible, gazed through its porthole — and saw aliens.
“Numberless little creatures” raced through his light beam, he wrote in his 1934 book, “Half Mile Down.” They turned out to be bristlemouths. A color plate in the book shows a group with jaws wide open while chasing a school of copepods, tiny crustaceans with long antennas.
Increasingly, bristlemouths won the abundance title as more and more nets and divers explored the deep. By 1954, N.B. Marshall, a distinguished marine biologist at the British Museum and author of “Aspects of Deep Sea Biology,” called them “the commonest fishes in the ocean.”
But a mystery proceeded to cloud the claim.
During the Cold War, the United States Navy puzzled over a global phenomenon known as the Deep Scattering Layer. It reflected sound waves back to the surface so effectively that, at times, it was mistaken for the seabed.
Biologists judged that it was composed of hordes of living things, because it migrated up near the surface at night and back down in daylight. The Navy wanted to better understand the layer to improve its tracking of enemy submarines, as well as hiding from them.
Research showed the region to be made of many creatures — krill, squids and long, gelatinous animals known as siphonophores. It also harbored many fish, but apparently few bristlemouths, which managed to avoid the nightly swim to the surface.
If the most common fish in fact had little to do with the teeming layer, had its abundance been overstated? Ocean textbooks from the 1970s to the 1990s said little about Cyclothone, the main genus of bristlemouths. Quietly, the king had been dethroned.
Then came a new wave of research, centering on careful trawls of the deep ocean with a new generation of nets in which the mesh was much finer. No matter how far the nets plunged, up came vast numbers of bristlemouths.
A team that trolled the Atlantic down to a depth of more than three miles reported in 2010 that the tiny fish “dominated the catches.”
Dr. Davison of the Farallon Institute, when he was a graduate student at the Scripps Institution of Oceanography, worked with colleagues there to plumb the Pacific off Southern California, doing so repeatedly from 2010 to 2012. Again, the little fish ruled.
Last year, oceanographers from Spain, Australia, Norway and Saudi Arabia reported on a research cruise that circumnavigated the globe to probe the life densities of the inky depths. It, too, reaffirmed the new wisdom, calling bristlemouths of the genus Cyclothone “the most abundant vertebrate on earth.”
For decades, Dr. Robison of the Monterey Bay Aquarium Research Institute has used robots to explore the Monterey Canyon, a deep gorge in California’s coastal seabed that goes down more than a mile. Unlike most ocean scientists, he has closely observed swarms of the little creatures in their native habitat.
“They have a swimming pattern that’s really unfishlike,” he said in an interview. “In smaller ones, the whole body gets involved, wiggling and wriggling through the water. It’s not a typical swimming behavior with the fin.”
Dr. Robison added that bristlemouths have very small eyes that in the dim habitat seemed to play little or no role in finding prey. Instead, like many aquatic vertebrates, the fish apparently relies on its lateral line — a system of sense organs that can detect movement and vibration in the surrounding water. The organs run lengthwise down both sides of a fish’s body from the gill region to the tail.
And the rows of glowing dots on the bristlemouth’s abdomen? Dr. Robison said they appeared to be camouflage that helped the creature hide from predators.
In the ocean’s twilight zone, where little sunlight falls, it is hard for predators to see downward but, in daytime, possible for them to observe things above, which appear as silhouettes. To avoid giving away their presence, some prey species use bioluminescent spots to blend in with the surrounding light, a strategy known as counter-illumination.
Dr. Robison said bristlemouths appeared to “counter-illuminate so their shadows won’t be seen.” He said scientists had shown experimentally that ocean creatures could make their silhouettes disappear, and that the intensity and wavelength from their glowing organs could shift to exactly match the surrounding light.
“But nobody,” Dr. Robison said, has yet been able to prove that the ruse of counter-illumination “can fool a predator.”
It has taken roughly a century and a half, but science has finally come to know the bristlemouth and its ranks of trillions fairly well, even if questions remain. Not so other creatures of the deep. If the tortuous route to identifying the dominant fish is any indication, it will take longer still for science to learn about the uncommon forms of life that roam the sunless depths — the planet’s main biosphere.
“We keep seeing lots of different critters we haven’t seen before,” Dr. Robison said of voyages in the Monterey Canyon and beyond. He added, “The deeper you go, the stranger things get.”
The current tally of animal species on the planet runs to about two million, including the bristlemouths. Dr. Robison said the global ocean might harbor a million more species unknown to science.
“It’s at least a million,” he said. “That’s because there’s so many places we haven’t looked.”