A fish with legs uses them to "taste" the seabed for prey, reveals new research. Sea robins have the body of a fish, wings of a bird, and six legs similar to a crab.
Two new studies have revealed that the legs aren't just used for walking on the seafloor. They are sensory organs used to detect buried prey - such as mussels and other shellfish - while digging, according to the research published in the journal Current Biology.
American scientists explained that the legs are actually extensions of their pectoral fins, of which they have three on each side.
Professor Nicholas Bellono, of Harvard University, Massachusetts, said: "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."
Prof Bellono, along with Prof David Kingsley of Stanford University and their colleagues, didn't set out to study sea robins at all. They came across the unusual species during a trip to the Marine Biological Laboratory in Woods Hole, Massachusetts.
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After learning that other fish follow the sea robins, apparently due to their ability to uncover buried prey, the researchers became intrigued and took some sea robins back to the lab to find out more.
They confirmed that the sea robins could indeed detect and uncover ground-up and filtered mussel extract and even single amino acids. The team found that sea robins' legs are covered in sensory papillae, each receiving dense innervation from touch-sensitive neurons.
They explained that the papillae also have taste receptors and show chemical sensitivity that drives the sea robins to dig. Prof Kingsely said: "We were originally struck by the legs that are shared by all sea robins and make them different from most other fish.
"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 behaviour."
The research team confirmed that the papillae are a key evolutionary innovation that has allowed the sea robins to succeed on the seafloor in ways other species can't. The second study looked deeper into the genetic basis of the sea robin's unique legs.
They used state of the art genome sequencing, transcriptional profiling, and study of hybrid species to understand the molecular and developmental basis for leg formation.
Their analyses identified an ancient and conserved transcription factor, called tbx3a, as a major determinant of the sea robins' sensory leg development.
Genome editing confirmed that they depend on the regulatory gene to develop their legs normally. The researchers explained that the same gene also plays a "critical" role in the formation of sea robins' sensory papillae and their digging skills.
Prof Kingsley added: "Although many traits look new, they are usually built from genes and modules that have existed for a long time.
"That's how evolution works: by tinkering with old pieces to build new things."
