Marine Biologists Find Fish That Spend Whole Lives Swimming Upside Down

A species of anglerfish spends their whole lives swimming upside down, scientists have found.

An international team of researchers published these findings in a new study in the Journal of Fish Biology, after witnessing the strange findings on some remotely operated undersea vehicles.

This species of anglerfish, also called Gigantactis, can be found in all oceans across the world at depths of 3,000 to 8,000 feet. They are perhaps best known for their bioluminescent lure, which stems off the center of its face, prompting the nickname "lantern fish." Because they reside in such deep waters, they are rarely seen by humans, meaning we know very little about their behavior.

The fish have been observed swimming upside down before, back in 1999—however, until now, it was assumed to be a one-off occurrence.

This species of anglerfish is distinguishable by long lures that hang from their faces. The researchers first caught sight of the species swimming upside down, off the Izu-Ogasawara Trench off Japan.

Confused by the sighting, the scientists continued to look into the strange behavior.They found eight examples of other anglerfish displaying this behavior in their deep sea habitat.

It became clear that this was no short-term occurrence. They were in fact doing it full time.

The pilot of the submersible used for this research, Auke Prins, described the first time they saw the behavior in the study.

"For most of the time the fish was swimming next to our beam of light, and we only saw the silhouette," he said. "When first approached, the fish was hanging horizontally in the water, and after about two minutes it swam off upwards/towards the surface with its illicium bent backwards."

More observation needs to be done to confirm why the fish are behaving in this way, but the researchers believe that it evolved as a result of its long lure.

The study hypothesized that the fish find it easier to manipulate the lure when in an upside-down position.

"Half the observations were of these anglerfish swimming near the sea floor relatively close to the bottom compared to total water depth," the study reads.

The scientists noticed that the long lures were reaching down towards plants on the seabed.

This suggests that the species may find this swimming position beneficial for catching prey. The study suggested that the fish use the lure to move plants around on the sea bed, positioning themselves perfectly for catching prey when it emerges.