These Fish Have Legs—and They Can Use Them to Taste Prey

David Kingsley was walking through a small Massachusetts aquarium one day when he spotted an animal that stopped him in his tracks: an unusual-looking fish that had six leg-like appendages and feathery fins protruding from its body. Called a sea robin, the fish reminded Kingsley of a centaur, the mythical creature that’s half-horse, half-man.

"The sea robins on display completely spun my head around because they had the body of a fish, the wings of a bird and multiple legs like a crab,” Kingsley, a developmental biologist at Stanford University, tells CNN’s Ashley Strickland.

“I’d never seen a fish that looked like it was made of body parts from many different types of animals,” he adds.

He was so intrigued that he decided to spend some time investigating these strange creatures. Why do sea robins have legs? How do they use them? And how did the fish evolve these adaptations?

Kingsley and his collaborators have come up with some possible answers to these and other questions, which they describe this week in two new studies in the journal Current Biology.

Sea robins use their leg-like appendages to move around on the ocean floor, alternating between walking and swimming based on whatever is most efficient, the researchers find. They can move each leg individually, which sets them apart from other fish species that use their fins to walk or perch.

Some species of sea robins, such as Prionotus carolinus, also use their legs to dig up prey buried in the sand. (They’re so good at hunting that other fish are known to opportunistically follow them around.)

When the researchers took a closer look, they saw that the legs of these digging fish were covered in sensory bumps called papillae, similar to human taste buds. They also learned that the papillae have taste receptor molecules, similar to those found in the human digestive tract.

In laboratory experiments, the team confirmed that the digging fish were using their legs to sense buried prey’s chemicals—they weren’t just using them to blindly feel around for something to eat.

These fish use legs to taste the seafloor (link in description)

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This discovery wasn’t necessarily a surprise, as other types of fish, such as goatfish and catfish, have taste receptors on the barbels that protrude from their heads.

“As mammals, we’re biased to think that taste buds lie only in the mouth,” says Neil Shubin, an evolutionary biologist at the University of Chicago who was not involved with the new research, to the New York Times’ Sofia Quaglia. “What would our world be like if we could taste with our hands? Getting an ice cream cone, or eating a slice of pizza, would be a whole new experience.”

By sequencing the fish’s genome and using CRISPR gene editing techniques, researchers were also able to identify the gene that’s responsible for their legs, tbx3a. This gene has been around for hundreds of millions of years; a variation of it is also responsible for producing limbs in humans.

“It’s normally expressed in a particular local domain of fin and limb buds in a whole range of animals from fish to mammals,” Kingsley tells NewScientist’s James Woodford. “This is an excellent example of making new body parts by modifying old, shared tools.”

Since only some species of sea robins use their legs to dig and taste food, this suggests that the legs probably evolved for locomotion first. Then, most likely, a few species continued evolving and developed the ability to also use their limbs to find and perceive food. This split likely occurred sometime within the last 10 to 20 million years, which is fairly recent in evolutionary timescales, per CNN.

“This is a fish that grew legs using the same genes that contribute to the development of our limbs and then repurposed these legs to find prey using the same genes our tongues use to taste food—pretty wild,” says Nicholas Bellono, a molecular and cellular biologist at Harvard University who co-authored the studies, in a statement.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.