With a name like coffinfish, it’s no surprise these odd-looking creatures have evolved to thrive in the dark ocean bottom.
Scientists already knew that the deep-sea dwellers—sometimes called sea toads—have special fins for “walking” on the seafloor. But now, a new study has revealed another coffinfish adaptation—massive, inflatable gill chambers that expand the animal’s body with seawater, allowing them to take up more oxygen and hold their breath for up to four minutes.
This behavior—the first ever discovered in a fish—could be a way to save energy in a food-scarce environment. (Read how walking fish made the leap from water to land.)
“It’s cool—it’s like a different method of inflation that no other fish uses,” says study co-author Stacy Farina, an assistant biology professor at Howard University. Pufferfish, for instance, ingest large amounts of seawater to expand their highly elastic stomachs.
Studying such deep-sea adaptations help biologists improve their knowledge of the different ways that creatures have evolved to live in extreme environments.
There are more than 20 species of coffinfish, actually a type of anglerfish, found across the world at depths of up to 8,200 feet.
“They’ve completely adapted to be a seafloor animal. They hardly ever swim,” adds co-author Nick Long, who conducted the research as an undergraduate biology student at Dickinson College in Pennsylvania.
“Some people call them lazy.”
Life in the slow lane
For the study, Farina and Long dissected and CT-scanned coffinfish specimens at the Museum of Comparative Zoology at Harvard University, where Farina was a postdoctoral fellow. They also studied video footage of several species of live coffinfish captured by drones operated by the National Oceanic and Atmospheric Administration’s Okeanos Explorer vessel.
According to the research, published recently in the Journal of Fish Biology, sea toads have inflatable gill chambers that can increase their body volume by up to 30 percent. In terms of a person, this would mean inflating your lungs to the size of your whole abdomen, Farina says.
The team was especially intrigued by the fish’s unique ability to hold its breath, which the video suggested is part of its normal breathing pattern. Such behavior is usually seen in animals with lungs, although catfish will sporadically hold their breath in low-oxygen situations, she says.
The scientists suspect the coffinfish inflate their bodies to conserve energy—breathing, after all, requires some effort. (See pictures of strange-looking sea creatures.)
Even though coffinfish will eat anything that fits in their mouths—from fish to octopus to worms—“it’s pretty unlikely that prey will appear on any given day,” Long says.
John Caruso, senior professor emeritus at Tulane University who was not involved in the study, called it “excellent.”
His only concern, Caruso says, is that the coffinfish on the drone footage may have been holding their breath because they were annoyed by the bright lights of the underwater drone. More observations are needed to confirm that the practice is part of its normal breathing behavior, he says. (Watch a strange “walking” fish that has experts stumped.)
On the defensive?
In addition to saving energy, the coffinfish expansion could be a defense against predators, similar to a pufferfish, the study suggests.
Hsuan-Ching Ho, an associate professor at the Institute of Marine Biology at National Dong Hwa University in Taiwan who described three new species of coffinfish in 2016, doubts this is true, however.
That’s because pufferfish can lock seawater inside their guts in order to keep their shape if squeezed or bitten, while the gill chambers of coffinfish are essentially open, meaning the water would just leak out if they were bitten.
Caruso, however, says that the defense mechanism is a “plausible hypothesis.”