Some types of sea thrush, a peculiar species of fish that lives on the ocean floor, use legs covered in taste buds to detect and dig up prey on the sea floor, according to new research published Thursday in the journal Current Biology.
Sea thrushes are so skilled at finding prey as they walk along the ocean floor with their six leg-like appendages that other fish follow them in hopes of snatching up some freshly unearthed prey, said the authors of the two new studies.
David Kingsley, co-author of both studies, first encountered the fish in the summer of 2016 after giving a seminar at the Marine Biological Laboratory in Woods Hole, Massachusetts. Kingsley is the Rudy J. and Daphne Donohue Munzer Professor in the department of developmental biology at Stanford University School of Medicine.
Before leaving to catch a flight, Kingsley stopped at a small public aquarium, where he observed the sea thrushes and their delicate fins, which resemble bird wings, and their leg-like appendages.
“The robins on display completely amazed me because they had the body of a fish, the wings of a bird, and multiple legs like a crab,” says Kingsley in an email. “I had never seen a fish that looked like it was made from the body parts of many different types of animals.”
Kingsley and his colleagues decided to study sea thrushes in a laboratory setting, revealing a number of surprises, including the differences between sea thrush species and the genetics responsible for their unusual features, such as fish-shaped flippers. legs that evolved to function largely as sensory organs.
The results of the new research show how evolution leads to complex adaptations in specific environments, such as the ability of sea thrushes to “sense” their prey using their sensitive and fast appendages.
A highly unusual animal
The robins’ distinctive extremities are actually extensions of their pectoral fins, according to study co-author Amy Herbert, a postdoctoral researcher in Kingsley’s lab at Stanford.
“We adopted the term ‘legs’ because of the remarkable walking function of these appendages,” says Herbert in an email. “However, they do not have the same structure as human ‘legs’ nor are they in the same position.”
Other fish species have modifications to their pectoral or pelvic fins that allow them to walk or balance, but sea thrushes can move their legs individually, which makes them more skilled at walking and digging, Herbert said.
“Robins are an example of a species with a very unusual and very new trait,” says Corey Allard, lead author of the study, in a statement. “We wanted to use them as a model to ask, ‘How do you create a new organ?’” Allard is a postdoctoral fellow in the department of molecular and cellular biology at Harvard University, where he works in the laboratory of Nick Bellono, co-author of the study and professor at Harvard.
The researchers took some sea thrushes to Bellono’s lab for study and to see if they could unearth buried prey. The team observed the fish alternating between short periods of swimming and walking. They were also seen scratching the sandy surface at the bottom of the tanks without any visual sign to indicate where prey might be buried.
“To our surprise, they were very, very good at this and could even dig up ground, filtered mussel extract and isolated amino acids,” says Bellono.
To continue their research, the study authors asked for more sea thrushes, only to discover that they represented a completely different species with varying characteristics.
Genetically distinct walking fish
The two groups of sea thrushes looked the same, but the newly arrived fish did not dig or find buried prey.
“This time, the new sea thrushes found nothing, despite readily eating prey at the surface,” says Bellono in an email. “We thought maybe we were doing something wrong, but it turns out we accidentally got a different species.”
The confusion allowed for some serendipitous discoveries for researchers. The highly sensitive fish they initially studied belonged to the species known as the northern sea thrush, or Prionotus carolinus. And the fish that did not have sensory capabilities and used their legs mainly for walking were the striped sea thrush, or Prionotus evolans.
The burrowing robins had shovel-shaped legs covered in protrusions called papillae, which are similar to the taste buds on our tongues. Meanwhile, non-burrowing robins had rod-shaped legs without papillae.
As scientists studied fish at the genetic level and compared how their legs developed over time, they realized that species that dig are found in only a few places, such as the shallow, sandy waters of New England and the northeast Atlantic coast. , which suggests that fish evolved this trait recently.
“We believe that the species that dig and those that don’t are separated by about 10 to 20 million years, which means that papillae must have emerged some time after that,” said Allard.
Although all robin species have leg-like appendages, only a few have the macroscopic sensory organs that allow them to sense their environment, Kingsley said.
The study authors’ research revealed that burrowing robins rely on a regulatory gene called tbx3a not only to develop their specialized fin adaptations, but also to form the papillae that prompt them to dig. The tbx3 gene also plays a role in limb development in humans, mice, chickens and other fish species, according to the study authors.
“This is a fish that grew legs using the same genes that contribute to the development of our limbs and then reused those legs to find prey using the same genes that our tongues use to taste food — pretty crazy,” says Bellono.
But why have only some sea thrushes developed this sensory ability? Researchers have a few hypotheses.
“One is that using their legs to dig up buried prey” allows them a new way to forage for food that they couldn’t before, according to Herbert. “Another is that walking rather than swimming in some environments may be more energy efficient for sea thrushes.”
Sea thrushes stand out among other walking fish because their pectoral fins, also called walking fin rays, are highly articulated, and their skeletal and muscular anatomy show unique modifications that allow sea thrushes to walk , according to Jason Ramsay, assistant professor in the biology department at Rhode Island College. But fish also have adaptations in their nervous system related to their legs, suggesting their sensory function, according to Ramsay. He did not participate in the new studies.
“A common question is: did these walking rays evolve due to selective (adaptive) pressures that support a walking function, a sensory function, or a combination of both,” says Ramsay via email. “These new studies provide further evidence suggesting it was likely the latter scenario.”
Allard is starting his own lab at Harvard, while Herbert is starting a lab at the University of Chicago. Both researchers said they were eager to discover the exact mechanisms behind the evolution of robins’ sensory appendages.
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This content was originally published in Discover the fish that has “bird wings” and “crab legs” on the CNN Brasil website.
Source: CNN Brasil
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