Imagine wading in the shallows at the beach and tasting mussels and clams hidden under the sand — with your feet.
That's exactly how two species of North American fish with wing-like fins, crabby-looking legs and croaky calls evolved to hunt prey.
A pair of studies published today in Current Biology show genes that help develop legs and tongues in humans are also found in fish called leopard sea robins (Prionotus scitulus) and northern sea robins (Prionotus carolinus).
One study examined how a gene — known to have a role in limb development for several vertebrates including people, chickens, mice and other fish — splits part of its largest fins into six "legs" for several sea robin species.
And the second study went a step further and showed leopard and northern sea robin legs had tiny papillae, like the little bumps found on your tongue that contain taste buds, which acted as sensory organs for finding buried food.
Those fish use their legs to dig for prey whereas other species simply use their limbs to walk along the sea bed.
Further research has potential to provide insights into human developmental conditions, as well as general evolution within the sea robin family.
There are more than 100 species of sea robins, also known as gurnards, found the world over.
They've evolved a spectacular array of characteristics, from armour and venom to dramatic skin patterns, and the ability to bellow or squeak.
[map]A case of curiosity
The two studies came about when researchers from Stanford and Harvard universities independently encountered the bizarre-looking fish at Woods Hole, north-east of New York City, at a local aquarium or the town's Marine Biological Laboratory.
"The tank of fish on display were the coolest, weirdest fish I'd ever seen," study co-author David Kingsley, a developmental biologist at Stanford University, said.
"They had the body of fish, wings of a bird, legs of a crab. The fish are actually walking along the bottom of the tank."
When study co-author Nicholas Bellono, a cellular biologist at Harvard University, first saw a sea robin, it was like someone had cobbled together an "animal made up of a bunch of another animals".
"It really is a great model of evolution," he said.
"It's a fish with legs."
The two groups, led by Amy Herbert at Stanford and Corey Allard at Harvard, eventually teamed up to study the fish's evolutionary quirks.
The researchers found a gene, called tbx3a, was activated during the fish's leg development phase.
Using the gene editing tool CRISPR, the team made changes to the tbx3a gene in sea robin embryos and documented their growth.
Some of the gene-edited fish developed smaller legs with fewer bones.
And in some instances, fish grew more or fewer legs.
Professor Kingsley said if the team were able to breed the gene-altered fish with each other, they could study the effects of inheriting one or two edited tbx3a gene copies on their babies' development.
Discovering how the tbx3a gene worked in fish could help research into human conditions such as ulnar-mammary syndrome, a rare inherited disorder which affects, among other things, limb development, he added.
But the action of the tbx3a gene was not the only curious leg discovery mystery the researchers found while studying the sea robin.
Is that the same fish?
While observing northern sea robins in tanks, the researchers noticed each fish scouring the sandy tank with its legs.
When the researchers hid tiny traces of prey in the sand, the fish found them.
Professor Bellono said they were not really sure how the fish managed it.
"[Then] we accidentally brought back a different sea robin species [the striped sea robin]," he said.
"And this sea robin could not find the buried prey."
This striped sea robin would eat mussels or other snacks placed right in front of it, but could not find food under the sand.
Professor Bellono said that's when they compared the legs of the two species.
"It was super obvious that the legs of the digging sea robin were specialised in many ways," he said.
"They almost looked like shovels … and on the ends of those shovels were these little bumps called the papillae that look a lot like our taste buds on our tongue.
"And those were full of the touch sensors."
The finding is just one of many insights into small-scale evolution that has already generated interest from other universities keen to take a look at sea robins.
Professor Kingsley said there were also species which could be studied in Australia.
"You've got a spectacular species there along the southern coast of Australia, these spiny gurnards [Lepidotrigla papilio] and they have legs … but they've got these fantastic colour patterns," he said.
"I think that's a really interesting example of pigmentation traits and innovation that have evolved again in that sea robin, as opposed to the sea robins that are found in the Atlantic Coast around Massachusetts where we started."
