Sea robin as an evolutionary model for trait development
Robins are marine fish that are particularly suited to a lifestyle on the seabed: six leg-like appendages make them particularly adept at wandering, digging and finding prey so that other fish tend to hang out with them and steal their prey. A chance encounter in 2019 with these strange legged fish at the Cape Cod Marine Biology Laboratory was more than enough to inspire Corey Allard to want to study them.
"We saw that they were keeping robins in a tank, and they showed them to us, because they know we like strange animals," said Allard, a postdoctoral fellow in the lab of Nicholas Bellono, a professor in the Department of Molecular and Cellular Biology. Bellono's lab studies the sensory biology and cellular physiology of many marine animals, including octopuses, jellyfish and sea snails.
"Robins are an example of a species with a very unusual and novel trait," Allard continued. "We wanted to use them as a model to ask, 'How do you create a new organ?'"
Allard's deep dive into the biology of the sea robins led to a collaboration with researchers at Stanford University studying the developmental genetics of the fish and culminated in two consecutive papers in Biology Current, featuring Luno, Amy Herbert and David Kingsley of Stanford University and others. The studies provide the most comprehensive understanding to date of how the robins use their legs, which genes control the appearance of the legs and how they can be used as a conceptual basis for evolutionary adaptations.
Robins' "legs" are actually extensions of their pectoral fins, with three on each side. Allard first wanted to determine whether the legs were true sensory organs, something scientists had suspected but never confirmed. He conducted experiments to observe captive sea robins hunting prey, in which they alternate between short swims and "walking". They even occasionally scrape the surface to find buried prey such as clams and shrimp, without visual cues. The researchers realized that the feet were sensitive to both mechanical and chemical stimuli. They even buried capsules with just a few chemicals, and the fish could easily find them.
A chance event led to another surprising discovery. They received a new shipment of fish midway through the study that looked like the originals, but the new fish, Allard said, didn't dig and find buried prey or capsules like the originals could. "I thought they were just bad fish, or maybe the setup wasn't working," Bellono recalled, laughing.
In the end it turned out that the researchers had acquired a different species of robins. In their studies, they ended up characterizing both - Prionotus carolinus, which dig to find buried prey and are very sensitive to touch and chemicals, and P. evolans, which lack these sensory abilities and use their legs for locomotion and probing but not for digging.
Examining the differences in the legs between the two species, they discovered that the digging species' legs were spoon-shaped and decorated with bumps called papillae, similar to our taste buds. The legs of the non-digging fish were stick-shaped and lacked papillae. Based on these differences, the researchers concluded that the papillae are evolutionary subspecialties.
Allard's paper describing the evolution of new sense organs in robins includes an analysis of robin specimens from the Museum of Comparative Zoology to examine leg morphologies between species and across time. He found that burrowing species are restricted to only a few locations, suggesting a relatively recent evolution of this trait.
The purpose of studying robin's legs wasn't just to hang out with strange animals (although that was fun too). The walking fish are a model organism with great potential for comparing specialized traits, and to teach us about how evolution enables adaptation to very specific environments.
About 6 million years ago, humans developed the ability to walk upright, and split from their primate ancestors. Bipedalism is a defining characteristic of our species, and we know little about how, when, and why this change occurred. Sea robins and their adaptations to life on the ocean floor can offer clues. For example, there are genetic transcription factors that prevent the development of the legs of sea lions that are also found in the limbs of other animals, including humans.
The other research that focused on genetics involved Kingsley's lab at Stanford; Italian physicist Agza Seminara; and biologist Moda Baldwin from the Max Planck Institute in Germany; and comprehensively examined the genetic basis of the unusual feature of the walking fish. The researchers used techniques including transcriptomic and genomic editing to identify which genetic transcription factors are used in the formation of legs and their function in the robins. They also produced hybrids between two robin species with different leg shapes to investigate the genetic basis for these differences.
"Amy and Corey worked a lot to describe this animal, and I think it's quite rare to go from describing the behavior, to describing the molecules, and to describing the evolutionary hypothesis," Bellono said. "I think this is a beautiful recipe for how to pose a scientific question and follow it rigorously with a curious and open mind."
