Sea robins, a type of ocean-dwelling fish, are uniquely adapted to life on the seafloor. Their six leg-like structures allow them to move, burrow, and hunt with such skill that other fish often gather nearby to steal their leftovers. These unusual, “legged” fish have now become the focus of a detailed scientific investigation.
Corey Allard, a postdoc fellow at Harvard University’s Department of Molecular and Cellular Biology, and Stanford researchers are examining the fish’s genetic development. Their research offers detailed insights into how sea robins use their legs, which genes govern the formation of these limbs, and how the species might serve as a model for studying evolutionary adaptations.
“Sea robins are an example of a species with a very unusual, very novel trait,” Allard said. “We wanted to use them as a model to ask, ‘How do you make a new organ?’”
Some fish dig, some walk
The “legs” of sea robins are actually extensions of their pectoral fins, with three on each side. Allard aimed to investigate whether these appendages function as genuine sensory organs, a hypothesis that had been suggested but never validated. He conducted experiments with captive sea robins while they hunted for food, observing their alternating behaviors of swimming and “walking.”
The fish would also scratch the sand to uncover hidden prey, such as mussels and other shellfish, relying on their senses rather than sight. The research team discovered that these legs are responsive to both mechanical and chemical stimuli; in one experiment, the fish successfully located capsules containing single chemicals buried in the sand.
A surprising twist occurred during the study when they received a new shipment of fish that closely resembled the original sea robins. However, these newcomers did not exhibit the digging behavior of their predecessors.
It was later revealed that they had acquired a different species altogether. The researchers ended up characterizing two species: Prionotus carolinus , which can dig to locate buried food and exhibit high sensitivity to touch and chemical signals, and P. evolans , which lacks these sensory abilities and primarily uses its legs for movement and exploration rather than for digging.
Insights into bipedalism
Upon examining the leg structures of the two sea robin species, researchers discovered that the digging variety had shovel-shaped legs adorned with protrusions known as papillae, which resemble taste buds.
In contrast, the legs of the non-digging species were rod-shaped and devoid of papillae. This distinction led the scientists to conclude that papillae represent evolutionary sub-specializations tailored for specific functions.
Studying the legs of sea robins extended beyond the fascination with these unusual creatures. The walking fish present a valuable model for comparing specialized traits and enhancing our understanding of how evolution enables organisms to adapt to particular environments.
Approximately six million years ago, humans developed the ability to walk upright, marking a significant divergence from our primate ancestors. Bipedalism is a defining characteristic of humanity, yet much remains unknown about how, when, and why this transition occurred.
The adaptations of sea robins to their ocean floor habitat might provide insights. For instance, genetic transcription factors involved in the development of sea robins’ legs are also present in the limbs of other species, including humans.
“Although many traits look new, they are usually built from genes and modules that have existed for a long time,” David Kingsley of Stanford University said in the press release . “That’s how evolution works: by tinkering with old pieces to build new things.”
The study has been published in the journal Current Biology .
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