"Remarkable Fish Uses Its Legs for More Than Just Walking"

It’s a bird! It’s a crab! No, it’s a fish that can taste with its legs.

Some sea robins, a group of fishes with two winglike fins and six crablike legs, use their legs to dig in sand and find buried prey with a sense much like taste, researchers report in two papers published September 26 in Current Biology. Most sea robins seem to use their legs only for walking. But an ancient gene important for the formation of limbs in humans and other animals, as well as a gene involved in building taste buds, helped a few species develop legs that taste.

“New things came from old parts,” says David Kingsley, a developmental biologist at Stanford University. A walking fish with taste organs on its limbs may look “really new and cool and different, but when you dig in… the new things have come by taking a tool kit of preexisting genes and deploying them in new ways.”

Northern sea robins (Prionotus carolinus) are skilled at finding buried crabs or shrimp to snag a meal. So skilled, in fact, that other fish seem to follow along to take advantage of any spoils. Previous studies suggested that northern sea robin legs could pick up chemical cues, but it was unclear if the limbs could sense anything as they dug into the seafloor.

Fish have taste buds in their mouths, and some fish even have taste buds on the outside of their bodies, says Peihua Jiang, a neurobiologist at the Monell Chemical Senses Center in Philadelphia who was not involved in the work. So while it doesn’t surprise him that northern sea robins have an exterior sense of taste, it’s “actually quite amazing” that their legs have become sensory organs, he says. “That’s completely a new feature [for fish], not what we typically see for how you detect or process taste information.”

It’s hard to define what taste is for many water-living creatures in part because some chemicals diffuse easily in water, but there are other aquatic animals that also use appendages to sense food, says Nicholas Bellono, a physiologist at Harvard University. Octopuses, for instance, use their arms to detect greasy molecules sticking to the shells of prey (SN: 4/18/23).

For both studies, Kingsley, Bellono and colleagues used a diverse mix of experiments to explore how northern sea robins excavate food. Behavioral tests showed that fish swimming in tanks home in on extract from store-bought mussels as they sift through sand with legs that end in a shovel-like structure. Close-up images revealed small sensory mounds called papillae that cover the legs, like the taste buds that cover tongues.

At first, the team wasn’t sure whether the papillae might be sensing chemicals in a manner like smelling or tasting, or whether the fish were doing something new, says Corey Allard, a biologist at Harvard University. “We found it’s a combination. [A sea robin leg] uses a lot of the same receptors that taste does, but repurposed and arranged in a way that’s very different” from the taste buds in the mouth.

Additional genetic and physiological experiments found that the papillae have touch-sensitive nerve cells and taste sensors that help these sea robins figure out w...