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The sea robin has fascinated scientists for decades. It has the body of a fish, the wings of a bird and the legs of a crab.
“Legs on a fish sound like, um, well, that’s one of the weirdest things I’ve ever seen,” said David Kingsley, a developmental biologist at Stanford University. He compared sea robins to aquatic centaurs.
If that wasn’t special enough, two studies published Thursday in the journal Current Biology by Dr. Kingsley and other researchers show that sea robins use their spindly legs to taste stuff, too.
Dr. Kingsley’s team was interested in identifying what spurred the creation of three pairs of bendable appendages with leg-like bones, muscles and nervous connections.
First, Amy Louise Herbert, a researcher in Dr. Kingsley’s lab, worked to make sea robins reproduce so that the team could observe the development of their embryos. The legs dropped out of the fins, they settled down on the seafloor, and the fish could start trodding around.
Then the team sequenced the fish’s genome and found that a gene called tbx3a — a variation of tbx3, which helps make limbs in humans too — plays a crucial role. After using CRISPR gene editing to tweak tbx3a, they found that some fish hatched with smaller protuberances more similar to fins than legs. Others had just one pair of legs or a whopping five pairs instead.
“New things are built from old friends,” Dr. Kingsley said. Tbx3a, he added, “has been around in the lower part of fins and limbs for literally hundreds of millions of years. There’s a bunch of hints that at the molecular level, evolution may be more predictable than people thought.”
But the sea robin’s boots aren’t just made for walking.
Prior research suggested their legs are also used to sense food buried in the seafloor. So the scientists hid mollusks under the sand and tested different sea robin species’ skill at finding them at varying depths. Then, they hid small capsules with amino acids and odors from the mollusks to confirm the fish were sensing the potential prey’s chemicals, and not merely their shapes and sizes.
One species of sea robin, Prionotus carolinus, was brilliant at finding buried prey.
When the researchers analyzed the molecules responsible for these abilities, they found the sea robin’s shovel-shaped legs were covered in little protrusions similar to the papillae on your tongue. It’s as if they taste their prey from above the sand before digging it up.
The molecules aren’t arranged exactly like taste buds, though, said Thomas Finger, who researches chemical sensory development at the University of Colorado and was not involved in the studies. “These are a whole different chemical sense.”
He notes we have similar cells throughout our digestive tract that use taste receptor molecules to detect taste chemicals, but we don’t sense it as taste in our bodies.
Nicholas Bellono, a molecular biologist at Harvard University and a study author, said it was “conceptually quite similar” to how, for instance, octopuses taste their food with their eight arms, but the sensory receptors and how they’re connected to the nervous system, “is all wildly different.”
“It’s kind of like evolution has reused some of these pieces that already exist,” he added, “but combined them in new and novel ways.” Sea robins also have brain segments that may have evolved to interpret the sensory information coming from what used to just be fins, and now are legs with taste receptors.
Crucially, when compared with other sea robin species, only P. carolinus and another closely related species had taste cells in the legs. This suggests that legs evolved as only legs first, and then a select few species further evolved sensory hardware to exploit a new avenue for food, said Corey Allard, a cell biologist in Dr. Bellono’s laboratory.
But Dr. Allard highlighted another mystery in need of more research: Other sea robin species can sense some chemicals with their legs without any taste-receptor molecules or papillae-like protrusions.
In the grand scheme of fish evolution, it makes sense that sea robins would evolve legs that can taste, said Neil Shubin, an evolutionary biologist at the University of Chicago who was not involved in the studies. Catfish and goatfish, similarly, have taste receptors on whisker-like barbels that stick out of their head.
“As mammals, we’re biased to think that taste buds lie only in the mouth,” Dr. Shubin said. “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.”
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