This fish has legs — and it uses them for more than just walking

It’s a bird! It’s a crab! No, it’s a fish which may 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 mode a lot like taste, researchers report in two papers published September 26 in Current Biology. Most sea robins appear to use their legs best for walking. But an ancient gene important for the formation of limbs in humans and other animals, as well as to a gene excited about building taste buds, helped some 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 even as 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 truth, that other fish appear to follow along to make essentially the most of any spoils. Previous studies suggested that northern sea robin legs may p.c. up chemical cues, alternatively it became unclear if the limbs may 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 on the Monell Chemical Senses Center in Philadelphia who became now not excited about the work. So while it doesn’t surprise him that northern sea robins have an exterior sense of taste, it’s “really quite amazing” that their legs changed into sensory organs, he says. “That’s completely a new feature [for fish], now not what we in most cases see for the way you detect or process taste information.”

It’s challenging to define what taste is for lots of water-living creatures partly because some chemicals diffuse without problems in water, but there are other aquatic animals that also use appendages to sense food, says Nicholas Bellono, a physiologist at Harvard University. Octopuses, as an instance, use their arms to detect greasy molecules sticking to the shells of prey (SN: four/18/23).

For both studies, Kingsley, Bellono and colleagues used a a spread mixture 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 result in a shovel-like structure. Close-up images revealed small sensory mounds also is named papillae that cover the legs, a twin of the taste buds that cover tongues.

In the foundation, the team wasn’t sure whether the papillae will likely be sensing chemicals in a way 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 spread the identical receptors that taste does, but repurposed and arranged in a strategy 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 determine where to dig. The researchers detected high levels of activity from a gene also is named t1r3 — which provides the instructions to make a receptor found in mammalian sweet-detecting taste buds — on the tips of each leg. What’s more, an ancient gene also is named tbx3a that drives limb development in many animals, including people and chickens, became a significant for now not best sea robin leg formation but also in forming the papillae and driving the digging behavior.

Most sea robins, including a non-digging species also is named the striped sea robin (P. evolans), have smooth, rod-shaped legs that more than likely can’t taste, the team figured out. That difference implies that northern sea robins and another digging relative, the leopard sea robin (P. scitulus), are among some species benefitting from an “evolutionary innovation,” says Amy Herbert, a developmental biologist at Stanford University.

Though the legs aren’t technically legs, Herbert notes. While the fishes do use the appendages for moving around — which prompted the team to call them legs — their position on a sea robin’s body is more kind of like using arms to walk.

Whether arms or legs, Jiang wonders whether the limbs’ papillae can sense bitter compounds that don’t signal food but in its place tell sea robins after they’ve found something that they steer clear of. He also desires to grasp if the taste buds in sea robin mouths detect the identical tastes that their legs do. “After they truly find food, what’s the next move?”