A Fish That Fishes for Other Fish Lives Its Life Upside Down

Usually, a belly-up fish isn’t long for this world. But video evidence from the deep ocean suggests that some species of anglerfish — the nightmarish deep-sea fish with bioluminescent lures — live their whole lives upside down.

“Just when you think they couldn’t get any weirder, anglerfish outdo themselves,” said Pamela Hart, an associate professor at the University of Alabama who researches fish that live in extreme conditions.

The behavior, documented earlier this month in the Journal of Fish Biology, is “beyond anyone’s wildest imagination,” said Elizabeth Miller, who studied the evolution of deep-sea fish as a postdoctoral fellow at the University of Oklahoma. (Neither Dr. Miller nor Dr. Hart was involved in the discovery.)

Whipnose anglerfish are small sea monsters with a fishing rod-like appendage on their faces. While a whipnose’s body is no bigger than that of a house cat, it has a long, floppy spine that sprouts from its nose and stretches up to four times its body length. The fish tempt prey with bioluminescent bacteria that live in the tip of the lure. (This applies to female whipnoses, said Andrew Stewart, curator of fishes at the Museum of New Zealand and an author of the study. The males of the species are “sad little tadpole things” a fraction of the size of the females, and without the lure.)

For nearly a century, scientists assumed whipnose anglerfish would dangle their lures in front of their faces, as many anglerfish with shorter lures do. But now, videos from underwater missions in the Atlantic, Pacific and Indian Oceans suggest that whipnoses spend their lightless days upside down, with their long lures hanging toward the seafloor.

The videos are confirmation of a tantalizing observation from more than 20 years ago, Dr. Stewart said.

In 1999, a remotely operated vehicle, or R.O.V., caught glimpses of whipnose anglerfish floating motionless, and, notably, upside down, about midway between Hawaii and California. Researchers suspected that they were targeting prey on the seafloor, but scientists couldn’t discard the possibility that it was just one goofy fish behaving abnormally, Dr. Hart explained — a hazard of animal behavior studies.

If that whipnose was a goof, then they’re all goofs, based on evidence from the footage that has been captured by remote subs and crewed vehicles. In a video filmed near the Izu-Ogasawara Trench off Japan, a whipnose drifts with the current, her body parallel to the seafloor, mouth agape and hundreds of tiny teeth glistening in the light.

Suddenly, she bursts into motion, using her powerful tail to swim in a tight circle, still inverted. Eventually she calms and begins drifting again, only to bump into the R.O.V.’s light apparatus — probably a shock for a creature used to living in the featureless deep sea. Then she uses the tiny fins at her side to backpedal into the darkness.

In other videos, “the propellers and power of the submersible tumbled the anglerfish so it was right side up,” Dr. Stewart said. But the whipnoses weren’t having any of it, “they very quickly reverted to being upside down again,” he said.

While humans may find it hard to take a belly-up predator seriously, swimming upside down may make the whipnose more lethal. Researchers suspect that, by keeping their lures farther from their mouths, whipnose anglerfish could take down larger and faster prey without accidentally biting themselves. Dr. Stewart said that one dissected whipnose specimenhad a gonatid squid in its belly — a real prize.

“Squid are very much the Ferrari of the deep ocean,” he said, adding that whipnose anglerfish “must be extremely fast and efficient for them to have nailed a gonatid.”

This new insight into the whipnoses’ behavior underscores how revolutionary R.O.V. footage has been for deep-sea biology, Dr. Stewart said. Before this technology, scientists relied on dead specimens hoisted from the deep by trawling nets and pickled to preserve their delicate tissues, which are often damaged by the drastic change in pressure. There was nothing in the whipnose anglerfish’s anatomy to suggest their bizarre behavior.

“These videos are really precious,” Dr. Miller said. “Even a short, one-minute video tells us so much about how the anglerfish is living its life that we can’t otherwise get.”