These Fish Transformed Their Dorsal Fins Into Taste Buds

When ichthyologist Paulo Presti, a graduate student at the University of São Paulo in Brazil, found a jar containing a preserved Remo flounder inside, he was stumped. “When I turned the jar to look at the specimen, I saw a strange little thing on its head on its blind side,” he says. “I thought, ‘Oh, my God, what is that?’”

Presti’s graduate research project is to examine and dissect examples of all of the families of flat fishes, like the Remo flounder (Oncopterus darwinii), which is native to the southwestern Atlantic Ocean along South America. The Remo flounder is very flat, and during its maturation process, its two sides begin looking different. A small stalk forms underneath the head, and the left eye migrates to what becomes the top of the fish. In addition, the bottom, or blind, side loses its pigmentation.

A few decades after naturalist Charles Darwin wrote the first record of this bottom feeder and its appendage in the 1830s, zoologist, ichthyologist and herpetologist Franz Steindachner concluded that the “strange little thing” was part of the flounder’s dorsal fin, says Presti.

The iconic image of a dorsal fin sticking out of the water conjures up memories of a certain monstrous movie shark, but in real life, scientists have discovered several examples of these fins not looking or functioning like fins at all.

The typical dorsal fin is a triangular-shaped piece of skin wrapped around muscles and bony spines or rays that sits on a fish’s back, behind its head. As a fish swims through water, the dorsal fin stabilizes it and keeps it from rolling over. But over time, many fish have evolved different uses and appearances for these skin flaps.

Some dorsal fins look like a skinny finger extending from a fish’s head that searches and tastes the seafloor for yummy treats, as seen on the underside of the Remo flounder. Others resemble a bioluminescent fishing rod, like the one on top of the anglerfish, that lures prey toward its gaping mouth. Or they look like the large ribbed suction cup on the top of the remora, or suckerfish, that attaches to an unsuspecting shark—or diver—to hitch a ride.

But are these fascinating appendages really dorsal fins? Generally, it was assumed that these structures were modifications of this fin, and now researchers have begun to use science to prove it. A recent study from Presti and colleagues finds that the Remo flounder modified the front part of its dorsal fin into an appendage loaded with taste buds that it can move through the sand on the ocean floor.

He explains that with a careful dissection, it’s clear the appendage is derived from that fin, but researchers in the 1800s didn’t have the technology to definitively prove it. “Also, the literature said that it may be sensory, but the researchers didn’t go any further with this,” he says.

For his May paper in Scientific Reports, Presti performed a thorough dissection of two preserved Remo flounders and meticulously analyzed their bones, muscle and nerves with X-ray computed tomography and studied their skin with scanning electron microscopy (SEM).

From the outside, the appendage—which the team dubbed the “gustatory stalk”—looks like a thin finger bent at the first joint, and it is recessed in a deep groove in the head on the blind side of the flounder when it is not in use. Tiny projections stick out from the stalk, and the crooked tip has structures called a conical protuberance, a globular head and a rugged flap.

When Presti dissected the nerves, he found one that was thicker than the others. By tracing its path, he could tell that the nerve connected to taste buds, but the buds weren’t in the mouth. Instead, SEM images of the stalk revealed taste buds along its central part, and on the projections and rugged flap.

Analyzing the muscles, Presti could see how the stalk would move. When the stalk is not in use, it lies against the head. “When the stalk is activated, it kind of dives into the sand,” he says. “The Remo flounder feeds mostly on small crustaceans that live buried in the sand, and when they protrude their stalk and dive it into the sand, they can taste chemical particles from those small crustaceans.”

Some flat fishes are already known to have taste buds on their dorsal fins and along their bottom sides, says Matt Girard, a zoologist at the Smithsonian’s National Museum of Natural History who was not involved in the study. “But this species is really cool because it has basically an appendage, it’s a fin ray that sticks into the sand, presumably,” he says. “It’s like being at the beach—you can sift through the sand, but if you have a metal detector, you’re going to find a lot more things.”

Girard says that the team did a “great job” demonstrating aspects of the fish that weren’t verified yet. “The technology we have today allows us to really get in and see those taste buds, so we can show everybody that this is exactly what’s happening,” he says. Girard also notes that there’s still much more to do, as “this work just opens up another opportunity to learn more about what’s going on in our ocean.”

From the Remo flounder to the anglerfish to the suckerfish, dorsal fin modifications provide insight into how fish evolve to fit their needs.

“So, you have all of these different fishes using a fin that you would think was for swimming,” says Girard. “We have some that are suckering onto sharks with the dorsal fin, or we have others with bioluminescence and fishes are using them to attract prey, and now we have another species that’s using it to taste and find prey.”

Fish are finding new ways to leverage their bodies to get things done. “Each discovery surprises us every time,” he says.

Katie Cottingham | READ MORE

Katie Cottingham is a writer covering biology, medicine, chemistry and materials science. Her work has been featured in Scientific American, Science, Chemical & Engineering News, INFORM, EpiGenie and more.