In the murky depths of the ocean, an unusual fish scurries across the seafloor, uncovering prey with six leg-like appendages. These creatures, known as sea robins, have captivated scientists with their ability to “walk” and even taste the sand beneath them. Two new studies published in Current Biology reveal surprising insights into how these bottom-dwelling fish use their specialized fins to sense, dig, and capture food.
The sea robin, with the body of a fish, the “wings” of a bird, and “legs” akin to a crab, is a truly unique creature. And it uses its appendages for more than just movement. According to David Kingsley, a developmental biologist at Stanford University, the fish’s shovel-shaped limbs are covered in tiny protrusions, similar to taste buds, which allow them to detect prey buried beneath the sand.
“This is a fish that grew legs using the same genes that contribute to the development of our limbs,” Kingsley told CNN. “Then, it repurposed these legs to find prey using the same genes our tongues use to taste food — pretty wild.”
Walking, Digging, and Tasting the Seafloor
Sea robins represent an intriguing case study of evolution’s ability to repurpose old structures for new uses. One species, Prionotus carolinus, is especially adept at digging for prey with its papillae-covered legs. Meanwhile, another species, Prionotus evolans, uses its legs mainly for walking without the same sensory abilities.
The two studies that uncovered these findings began when researchers, including Corey Allard from Harvard University, observed sea robins alternately swimming and walking in lab tanks. The fish demonstrated remarkable skill at digging up hidden prey, even without visual cues. But a mix-up occurred when a new shipment of sea robins arrived. The new fish looked nearly identical but lacked the digging ability. This led to the serendipitous discovery that the two species were genetically distinct.
So, Kingsley and his colleagues used gene-editing techniques, such as CRISPR, to tweak the sea robins’ genetic code. They focused on the tbx3a gene, with a role in forming limbs in both fish and humans. By manipulating this gene, they were able to produce fish with varying numbers of leg-like appendages, shedding light on how these unique traits evolved.
The sea robin’s legs are like a blueprint for understanding how new organs develop, the researchers say. Their findings suggest that sea robins adapted their pectoral fins over millions of years. Eventually, they became sensory tools that provide a competitive edge in environments where food is scarce.
“We settled on the term ‘legs’ because of the striking walking function of these appendages,” study coauthor Amy Herbert, a postdoctoral scholar in Kingsley’s lab at Stanford, told CNN in an email. “However, they do not have the same structure as human ‘legs’ nor are they in the same position.”
A Clue to Life’s Mysteries
The discovery of the sea robin’s sensory legs raises broader questions about the role of genetic factors in shaping evolutionary adaptations. Neil Shubin, an evolutionary biologist at the University of Chicago, who was not involved in the studies, noted, “In the grand scheme of fish evolution, it makes sense that sea robins would evolve legs that can taste,” adding that catfish and goatfish 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 told the NY Times. “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.”
Indeed, some sea robin species developed these abilities as recently as 10 to 20 million years ago, allowing them to thrive in the sandy, shallow waters of New England and the Atlantic seaboard. These adaptations, the researchers believe, offer a glimpse into how natural selection can create highly specialized traits from existing genetic material.
The sea robin’s remarkable legs are just one example of nature’s ingenuity. “New things are built from old friends,” Kingsley said, pointing out that the tbx3 gene has played a role in fin and limb development for hundreds of millions of years. “There’s a bunch of hints that at the molecular level, evolution may be more predictable than people thought.”
What Comes Next?
Moving forward, the researchers aim to uncover more details about the sea robin’s sensory mechanisms. Allard, who is now starting his own lab at Harvard, hopes to further investigate these fish. There is more to learn on how these specialized legs developed and how they are connected to the fish’s nervous system.
Understanding the sea robin’s adaptations may provide broader insights into the evolution of sensory organs in other species, as well as the evolution of our trusty legs. Bipedalism is a defining feature of our species, and we only know so much about how, when, and why that change occurred.
For now, the sea robin’s legs serve as a reminder of evolution’s boundless creativity in shaping the natural world.
The findings appeared in two separate studies published in the journal Current Biology (one and two).
