Like a creature from a Doctor Seuss book, the sea robin is an unusual looking fish. It’s built with the body of a typical fish, the “wings” of a bird, and legs like a crustacean. Those six crab-like legs make it particularly well-suited for scurrying, digging, and finding food on the bottom of the ocean. However, its legs also aren’t just used for getting around. Instead, they are full sensory organs that sea robins use to find buried prey while digging in the sand. The findings are detailed in two studies published September 26 in the Cell Press journal Current Biology.
‘How do you make a new organ?’
Sea robins are long fish with bony heads that are found in temperate seas around the world. Their signature “legs” are extensions of their pectoral fins, of which they have three on each side. Sea robins are so good at finding food that other fish sometimes stay around them to get some of the leftovers.
In 2019, study co-author and Harvard University postdoctoral fellow Corey Allard observed these legged fish at Cape Cod’s Marine Biological Laboratory and had to know more.
“We saw they had some sea robins in a tank, and they showed them to us, because they know we like weird animals,” Allard said in a statement. “Sea robins are an example of a species with a very unusual, very novel trait. We wanted to use them as a model to ask, ‘How do you make a new organ?’”
[Related: Boo! New species of ghost shark uncovered in New Zealand.]
Allard collaborated with researchers from Stanford University that were studying the sea robin’s developmental genetics. They first sought to determine if the legs are sensory organs, which had been suspected, but not confirmed.
Allard ran experiments using captive sea robins hunting prey. While hunting they switch between these short bursts of swimming and walking around on their legs. They will also scratch at the surface of the sand from time to time to find mussels, shellfish, or other buried prey without needing to see it.
The team realized that the legs were sensitive to chemical and mechanical stimuli during the experiments. Even when they buried capsules that contained only one chemical, the fish could locate them.
VIDEO CREDIT: Current Biology Allard Herbert Krueger et al.
Fresh fish
In the middle of the study, the team received additional shipment of fish. While they looked like the original fish, they didn’t dig and find buried capsules or prey like the others.
“I thought they were just some duds, or maybe the setup didn’t work,” study co-author and Harvard University molecular biologist Nicholas Bellono said in a statement.
The team had actually acquired a different species of sea robins. They ended up categorizing both in these studies. Prionotus carolinus dig to find buried prey and are highly sensitive to touch and chemical signals. Prionotus evolans do not have these sensory capabilities. Instead, they use their legs for probing and movement, but not for digging.
When they examined the differences in the legs of both species, they noticed some key differences. Prionotus carolinus–thes ones who dig–had more shovel-shaped legs covered in some protrusions called papillae. These are similar to the taste buds on our tongue. The non-digging fish’s legs were rod-shaped and didn’t have papillae. The team believes that the papillae are evolutionary sub-specializations.
Figuring out how these special appendages evolved could help scientists understand how evolution allows for adaptations to very specific environments. Roughly six million years ago, our own species learned to walk upright. Bipedalism separated us from our primate ancestors, but we only know a small bit about how, when, and why that switch occurred. Sea robins and their adaptations to living at the bottom of the ocean could have some clues. The genetic transcription factors that handle the development of sea robins’ legs are also found in other animals’ limbs–including our own.
Genes made for walking
The second study focused on genetics and included David Kingsley and Amy Herbert from Stanford, Italian physicist Agnese Seminara, biologist Maude Baldwin from the Max Planck Institute in Germany. The team examined the genetic underpinnings of this unusual walking trait.
“Although many traits look new, they are usually built from genes and modules that have existed for a long time,” study co-author David Kingsley said in a statement. “That’s how evolution works: by tinkering with old pieces to build new things.”
[Related: This eyeless cavefish grows extra taste buds on its head.]
In that study, they used techniques including transcriptomic and genomic editing to pinpoint which gene transcription factors are used in leg formation and function. They also created hybrids between two sea robin species that have distinct leg shapes to explore the genetic basis for these differences.
“Amy and Corey did a lot to describe this animal, and I think it’s pretty rare to go from the description of the behavior, to the description of the molecules, to the description of an evolutionary hypothesis,” Bellono said. “I think this is a nice blueprint for how one poses a scientific question and rigorous follows it with a curious and open mind.”