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Forget crab legs. When it comes to taste, the gams on sea robin fishes have crustaceans beat by a mile.
The strange creatures are notable for being fish that use their six leg-like appendages to walk on the seafloor. In actuality, the legs are oddly shaped pectoral fins. While fishers have observed sea robins in the western Atlantic Ocean digging up buried prey, biologists had not examined what, exactly, was going on. Now, the mystery has been put to rest: The fish evolved the ability to taste using their legs.
“This is a fish that grew legs using the same genes that contribute to the development of our limbs and then repurposed these legs to find prey using the same genes our tongues use to taste food—pretty wild,” Nicholas Bellono, a professor of molecular biology at Harvard University, said in a press release.
Bellono and his colleagues were able to figure this out through a simple experiment. They took sea robins and placed them in tanks, where they had buried mussels. As a control, they also placed capsules containing nothing but sea water. Sure enough, the fish dug up the mussels, but not the capsules. To ensure this behavior was based on taste, rather than touch, they replicated the experiment with capsules containing mussel extract, selected for containing chemicals known to stimulate fish taste buds. Once again, the sea robins were observed digging up what their legs touched.
Where do new animal traits come from? We studied walking fish with “legs” to understand the evolution of novel organs and behavior.@CoreyAHAllard, Amy Herbert, David Kingsley, @MCB_Harvard, @StanfordMed, @HHMINews, @MBLScience
– https://t.co/ih2g8Zvva3
– https://t.co/8x0jADddbU pic.twitter.com/PuPpcalZRU
— Nicholas Bellono (@NBellono) October 19, 2023
Even more surprisingly, the fish didn’t need to actually touch the potential prey to know it was a tasty morsel. The taste receptors were so sensitive that the fish could detect chemicals diffusing through the sand. There was a limitation, however; the food had to be within four inches (100 millimeters) for the fish to sense it.
The findings were so weird that the marine biologists needed to publish two papers to sum them up. Both were published in the journal Current Biology. In the first, they summarized their experiments and described the complex sensory systems in the sea robins’ legs. The fish’s limbs are covered in small bumps called papillae, which contain cells sensitive to both taste and touch (you can feel your own papillae in the form of the rough texture of your tongue).
“We were originally struck by the legs that are shared by all sea robins and make them different from most other fish,” said David Kingsley, a professor at Stanford who also worked on the studies. “We were surprised to see how much sea robins differ from each other in sensory structures found on the legs. The system thus displays multiple levels of evolutionary innovation from differences between sea robins and most other fish, differences between sea robin species, and differences in everything from structure and sensory organs to behavior.”
To a human, it’s difficult to imagine sensing taste with any body part other than a mouth. After all, that’s where the food goes. Yet the presence of taste receptors in strange places isn’t uncommon. Flies taste with their feet, and other insects can have cells involved in taste on their antennae, or even their wings. So scientists know it’s possible to evolve novel ways for animals to taste their food, but it wasn’t clear how that process developed in sea robins. That was the question they set out to answer in the second study.
To do so, they dove into the genes of sea robins. They found a protein called tbx3a, which is involved in how DNA gets transcribed into messenger RNA. Tbx3 proteins have previously been found to play a key role in the development of hind legs in vertebrates, including humans, and sea robins appear to be no different. But the protein had an extra role in sea robins; by editing genes in some experimental fish embryos, the scientists were able to discover tbx3a was also important in the development of the papillae, as well as the fish’s habit of digging up what they taste.
“Although many traits look new, they are usually built from genes and modules that have existed for a long time,” said Kingsley. “That’s how evolution works: by tinkering with old pieces to build new things.”
In their papers, the scientists said they hope to conduct further studies to better understand the evolutionary process that resulted in such a weird, little animal. If you want to conduct your own sea robin experiment, apparently they ironically taste pretty fantastic, particularly when encrusted in parmesan.
