There is a cultural trope of ‘fish tales’, fantastical over-exaggerations told by fishermen about the impossible size or features of their catch. Northern sea robins (Prionotus carolinus) are a real-life species that seem like such an imaginary fish tale. At first glance they look like normal fish, but they have six legs growing out of their torso – slender strands of tissue which separate from a nearby fin and allegedly grow long and firm enough to let them walk and possibly even dig up food. This sounds fake, even ridiculous, and certainly wasn't assured, but as a team of researchers led by David Kingsley (University of Stanford, USA), Nicolas Bellono (University of Harvard, USA) and colleagues report, these fish are even more remarkable than the stories that preceded them. Through a series of clever experiments, they show that these legs not only walk and dig but also come equipped with delicate senses of touch and smell.
The scientists began by testing the fishing community's unverified rumour that northern sea robins could find and dig up buried prey. The researchers filled a tank with a thick layer of sand at the bottom, hid mussels and capsules filled with food in it, and then video-taped the fish's movements. The sea robins walked along the tank's sandy bottom, probing and scratching at the sand here and there with their strange legs; within 30 min, the sea robins had found every bit of food. To make this even more impressive, they ignored capsules of seawater. The fish could accurately locate food even when it was invisibly buried under the sand. Were they tasting the water to know when food was nearby?
There is a simple way to test this: when animals taste or feel, nerves send that information to the brain. The researchers recorded electrical signals from nerves in the legs with tiny probes while they gently touched the appendages and applied tasty chemicals, registering electrical signals on both occasions. This settled the matter: these legs were capable of taste and touch, a lot like tongues. In fact, just like the human tongue is textured with little bumps (papillae) loaded with taste buds, northern sea robins have papillae coating their legs.
But there is more than one species of sea robin. While catching northern sea robins from the wild, the researchers accidentally caught another species, the striped sea robin (Prionotus evolans). Kingsley, Bellono and colleagues repeated the previous tests with this second species, checking whether the fish could also find buried food or taste with their novel limbs, but they failed to do so. The striped sea robins didn't dig or detect buried mussels, and there was no evidence of taste or tongue-like papillae on their legs. While both species used their legs for walking and feeling around, only northern sea robins could taste and dig. The scientists went one step further by investigating 13 species of sea robin, including animals preserved in museums. Only two of these species had bumpy papillae and a sense of taste, and they were closely related to each other. So this looked a lot like a newly evolved trait; they found – fittingly – that fish legs that can taste are rare.
In their research paper, the scientists also mention anecdotes about northern sea robins being so successful that less gifted fish follow them to scavenge scraps of unearthed meals. So for once, this fish tale was not an exaggeration and the truth was even more incredible, with the scientific discovery of appendages that taste growing out of their chest. Just imagine – what would your life be like if you could smell and taste with your fingertips?
