A fanfin seadevil, a type of deep sea anglerfish found in the Atlantic Ocean. There are 168 species of deep sea anglerfish.CreditCreditDavid Shale/Minden Pictures
Few wonders of the sunless depths appear quite so ghoulish or improbable as anglerfish, creatures that dangle bioluminescent lures in front of needlelike teeth. They are fish that fish.
Typically, the rod of flesh extending from the forehead glows at the tip. Anglerfish can wiggle the lure to better mimic living bait. Most species can open their mouths wide enough to devour prey whole, using their fangs not only as daggers but as bars of a cage. Some can open their jaws and stomachs so wide as to trap victims much larger than themselves.
(Note: this portrayal applies only to female anglerfish. The males, with rare exceptions, are puny.)
Anglerfish came to the attention of science in 1833, when a specimen of the bizarre fish — a female — was found on the shores of Greenland. Since then, scientists have learned most of what they know by pulling dead or dying specimens from nets. Lifestyle clues have been sparse.
“It was amazing,” Theodore W. Pietsch, an emeritus professor at the University of Washington in Seattle and a world authority on anglerfishes, said of the video. “They’re glorious, wonderful things that need our attention, and our protection.”
In 2014, Bruce H. Robison, a senior marine biologist at the Monterey Bay Aquarium Research Institute in California, caught sight of an anglerfish known as the black seadevil while exploring the deep bay, and managed to record minutes of its enigmatic swimming.
“Instead of examining dead fish, we’re now doing behavioral studies,” he said in an interview. “It’s a significant transition.”
Many kinds of anglerfish inhabit the ocean. But most attention goes to deep-sea variety. So far, scientists have identified 168 species of the strange, elusive fish.
The new videos add otherworldly drama and insights to a sparse but fascinating body of existing knowledge. In his 1964 book “Abyss,” Clarence P. Idyll, a fisheries biologist at the University of Miami, said the rod tips could glow in yellows, yellow-greens, blue-greens and oranges tinged with purple.
“Deep-sea creatures must find these colored lights irresistible as they flicker and flash faintly in the dark waters,” he wrote.
Speciation has produced a great diversity of protruding lights and rods. Some anglerfish have a long barbell extending from the lower jaw as well as a rod above. One species, Lasiognathus saccostoma, bears not only a movable rod but extending from it a line, a float, a lighted bait and three hooks. The hooks, Dr. Idyll wrote, “are, alas, not actually for catching prey” but simply ornamental.
Anglerfish, he noted, are “rarely as large as a man’s fist.” But one specimen, from a depth of 2.2 miles off West Africa, was a foot and a half long. It was also unusual in having its glowing bait conveniently located inside its enormous mouth.
The largest known deep anglers are the warty seadevils. The females typically run about two-and-a-half feet long, and free-swimming males less than a half inch.
The examination of stomach contents has revealed that anglers eat shrimplike animals, squids, worms and lanternfish, a common type of deep-sea fish with large eyes and a highly developed visual system that apparently can detect colors.
When an anglerfish suddenly opens its giant mouth, Dr. Idyll wrote, the resulting suction pulls in the luckless victim. After the jaw slams shut, small teeth on the floor of the mouth and throat deliver the meal to the fish’s belly.
The first undersea video recordings of the creatures were made in 1999, and caught a surprise. Scientists from the Woods Hole Oceanographic Institution on Cape Cod, Mass., had set up an undersea observatory in the North Pacific between California and Hawaii. It lay more than three miles down.
A seven-foot-long tethered robot named Jason was lowered to survey the surrounding area. Soon, its operators were startled to see a fish drifting in the bottom current upside-down, with its extremely long rod hanging downward in a graceful, forward-arching curve. Unexpectedly, they found two other fish similarly upended.
Jon A. Moore, a fisheries biologist at Florida Atlantic University, identified the creatures as whipnose anglerfish, although of an unknown species. In a 2002 paper, he wrote that they apparently were looking for prey by trolling over the muddy seabed with glowing bait. Visible just below the fish, he noted, were “numerous small burrows.”
In an interview, Dr. Moore said the video represented “the first time anyone had seen” any kind of whipnose in its own dark habitat. He added that, despite the intervening years, the question of what the fish were pursuing on the Pacific floor remains a mystery.
The Monterey research institute — in Moss Landing, Calif., at the midpoint of the bay shoreline — was established in 1987 by David Packard, the billionaire co-founder of Hewlett-Packard and a creator of Silicon Valley. It has built generations of increasingly smart, fast robots that probe the nearby waters.
In 2005, nearly a mile deep in the waters off Monterey, institute scientists were flying a tethered robot when they tracked an angler for a record 24 minutes. The resulting paper, by Dr. Pietsch and another University of Washington scientist, detailed a series of behaviors, from swimming bursts to long bouts of drifting.
Overall, they wrote, their observations supported the theory that “these animals are lethargic, lie-and-wait predators.”
The range of known behaviors grew larger when institute scientists probed seamount chains west of the Monterey Canyon. Expeditions in 2002 and 2010 videotaped odd anglers with a bulbous body, a shaggy lure and fins that the fish used to walk along the rocky seabed. The scientists speculated that walking disturbs the seawater less than swimming does, reducing the chances of startling nearby prey.
The newest video to go public was made off the Azores by a research team from the Rebikoff-Niggeler Foundation, based on the island of Horta. In 2016, a half-mile down, Kirsten and Joachim Jakobsen were returning to the surface in their submersible when they spotted a female angler “resplendent with bioluminescent lights,” as Science magazine described the fish. It was later identified as a fanfin seadevil, a ghoul of the deep with a bushy lure.
The team also videotaped a dwarf male fused to her underside — a permanent sperm donor. Males of that species had never before been seen by humans.
Young male anglerfish face the challenge of finding a mate in the ocean’s vastness. They have large olfactory organs, which suggests that suitors follow a trail of pheromones. If courtship is successful, the male fuses permanently to the female, and their tissues and circulatory systems commingle.
In the case of the Azores discovery, “the size of her belly indicates that she was gravid,” or full of offspring, Kirsten Jakobsen said in an email.
The foundation team was able to track the pair for 25 minutes; what mesmerized was not only the procreative union but the halo of filaments that radiated outward from the female’s body, shimmering with points of light.
Dr. Pietsch, of the University of Washington, said the rays contain nerves and may act like sensory antennae, alerting the angler to nearby prey. “We’ve hypothesized that they pick up vibrations, like the whiskers of a cat,” he said.
He and a colleague in Germany are trying to determine whether the shimmering lights in the rays are bioluminescent or were merely reflecting light from the submersible. If the rays are glowing, he said, “it would be really important.”
The new videos make clear — more so than the old sketches and portraits — that anglerfish look truly demonic. Why the nightmarish appearance?
Dr. Robison noted that the exotic features of anglerfish make perfect sense as evolutionary adaptations to an icy, dark world in which meals are few and survival depends on cunning.
“Part of what appeals to us about other fish is that they’re sleek and streamlined and built for speed,” he said. “That’s attractive. But most anglerfish aren’t built for speed. Their predatory approach is ambush. They draw things in. To aid that approach, they need to be stable in the water column, to hold themselves in position.”
In the desert of the deep sea, he said, “they have to take advantage of every prey opportunity that comes by. That’s why they have such huge mouths and distensible stomachs: to take in a meal that might have to last for months.”
“The big teeth may appeal to the 12-year-old in all of us,” he added. “But those are really useful, too, in not only grasping prey but trapping it in that maw.”
Most exciting, Dr. Robison said, is that much about the realm of the anglerfish remains ripe for discovery. Monterey Bay may be “the best studied patch of ocean in the world,” but it still produces surprises about life in the abyss.
Water covers more than 70 percent of the Earth’s surface and goes down miles; all told, the global sea accounts for 99 percent of the terrestrial biosphere.
“There’s a whole world of ocean out there,” Dr. Robison said. “And most of it is unexplored.”