Sea urchins are fish that use their legs to taste the sea floor.

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Some species of seabirds, strange oceanic fish that live on the sea floor, use feet covered in taste buds to sense and dig prey along the sea floor, according to new research.

Seabirds are so adept at snatching up prey as they walk along the ocean floor on their six-legged appendages that other fish follow them in hopes of catching some of the newly discovered prey, the authors of two studies say. New studies Published Thursday in the magazine Current Biology.

David Kingsley, co-author of both studies, first discovered 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 board the plane, Kingsley stopped at a small public aquarium, where he saw sea birds with their delicate fins, which resembled the feathery wings of birds, as well as their leg-like appendages.

“I was amazed by the seabirds on display because they had the bodies of a fish, the wings of a bird, and multiple legs like a crab,” Kingsley said in an email.
“I have never seen a fish that looks like it is made from body parts from so many different species of animals.”

Kingsley and his colleagues decided to study seabirds in a laboratory setting, and they discovered a host of surprises, including differences between seabird species and the genes responsible for their unusual features, such as leg-like fins that have evolved to function largely as sensory organs.

The results of the new research conducted by the study team show how evolution leads to complex adaptations in specific environments, such as the ability of seabirds to “taste” prey using their highly sensitive, fast-moving limbs.

The distinctive limbs of seabirds are actually extensions of their pectoral fins, said study co-author Amy Herbert, a postdoctoral researcher in Stanford’s Kingsley Lab.

“We settled on the term ‘legs’ because of the amazing walking function of these appendages,” Herbert said in an email. “However, they do not have the same structure as human ‘legs’ and do not assume the same posture.”

Other fish have modifications to their pectoral or pelvic fins that allow them to walk or stand, but seabirds can move their legs individually, making them better able to walk and dig, Herbert said.

“Seabirds are an example of a species with a very unusual and very novel trait,” said the study’s lead author, Corey Allard, in a statement. “We wanted to use them as a model to ask the question, ‘How do you make a new organ?’” Allard is a postdoctoral fellow in the Department of Molecular and Cell Biology at Harvard University, where he works in the lab of study co-author Nick Belluno, a professor at Harvard.

The researchers brought some seabirds back to Belluno’s lab to study and see if they were able to detect buried prey. The team observed the fish alternate between short periods of swimming and walking. They were also seen scratching the sandy surface that covered the bottom of the tanks, with no visual cues to tell them where the prey was buried.

“We were surprised that they were so good at this, and were even able to detect the ground, filtered mussel extract, and the individual amino acids,” Belluno said.

To continue their research, the study authors sent more seabirds to the lab — only to discover that they represented a completely different species with different characteristics.

The two groups of seabird specimens looked similar, but the newly delivered fish did not dig or find buried prey.

“This time, the new seabirds didn’t find anything, even though they readily eat prey on the surface,” Belluno said by email. “We thought we might be doing something wrong, but it turns out we accidentally got a different species.”

This mix-up has led to some unexpected discoveries. The highly sensitive fish they initially studied belonged to a species known as the northern seabird, or Prionotus carolineus. The fish that lacks sensory abilities and uses its legs primarily for walking is the striped seabird, or Prionotus evolanes.

Burrowing sea nightingales had shovel-shaped feet covered with protrusions called papillae, which are similar to the taste buds on our tongues. Non-burrowing sea nightingales had rod-shaped feet without any papillae.

As scientists studied the fish at the genetic level and compared how their legs evolved over time, they realized that species that dig are found only in a few locations, such as the sandy shoals of New England and the upper East Coast of the Atlantic Ocean, suggesting that the fish evolved this trait only recently.

“We think the fossil and non-fossil species are separated by about 10 to 20 million years, which means the papillae had to have appeared sometime after that,” Allard said.

While all seabird species have leg-like appendages, only some have microscopic sensory organs that allow them to taste the environment, Kingsley said.

The study authors’ research revealed that burrowing seabirds rely on a regulatory gene called tbx3a not only to develop their specialized fin adaptations but also to form the papillae that allow them to burrow. Tbx3 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 has grown legs using the same genes that contribute to our limbs and then reused those legs to find prey using the same genes that our tongues use to taste food — which is very wild,” Belluno said.

But why did some seabirds acquire this sensory ability? Researchers have put forward two hypotheses.

One is to use the legs to uncover buried prey. This allows them a new way to forage for food than they had before, Herbert said. “Another reason is that walking instead of swimming in some environments may be more energy efficient for seabirds,” he added.

The squid are unique from other walking fish because their pectoral fins, also called movable fin rays, are highly articulated, and their skeletal and muscular anatomy show unique adaptations that enable the squid to walk, said Jason Ramsey, an assistant professor in the Department of Biology at Rhode Island College. But the fish also have adaptations in their nervous system that are linked to their legs, suggesting a sensory function, Ramsey said. He was not involved in the new studies.

“A common question is whether these rays evolved due to selective (adaptive) pressures supporting a walking function, a sensory function, or a combination of the two,” Ramsay said by email. “These new studies provide more evidence that the latter scenario is more likely.”

Allard began setting up his own laboratory at Harvard, while Herbert began setting up a laboratory at the University of Chicago. The two researchers said they They are keen to discover the precise mechanisms behind the development of seabird sensory appendages.