Mudskippers are funny little googly-eyed fish that come up on land to feed on worms, bugs, and small crustaceans. But before surfacing, they fill their mouth cavities with water to create a “hydrodynamic tongue” that helps them catch and swallow their prey. The findings, published this week in Proceedings of the Royal Society B, suggests that the use of “water tongues” by amphibious fish may have led to the evolution of the more familiar fleshy, pink tongues we see on land.
Vertebrates invaded the terrestrial environment around 400 million to 350 million years ago, and there are many studies that focus on shifts in breathing and locomotion. But what about feeding mechanisms? Suction feeding works underwater but not so much in the air. To capture and swallow prey, modern land vertebrates such as frogs, lizards, and turtles rely on a sticky tongue that’s supported by parts of the skeleton like the horseshoe-shaped hyoid bone in the throat. Our aquatic ancestors used their hyoid bone to help suck food into their mouths from the surrounding water.
To understand the terrestrialization of fish-feeding systems, a University of Antwerp team led by Krijn Michel obtained five adult Atlantic mudskippers (Periophthalmus barbarous) from Nigeria. They performed CT scans on one of them, and they recorded high-speed videos and X-ray videos of the other four eating brown shrimp on a Plexiglas pane.
“First it spews out the water, then very rapidly… it’s sucking the water back up again. They’re using the water that is in their mouth as a substitute for a tongue,” Michel tells Nature. Specifically, mudskippers use their mouth cavity filled with water as a protruding and retracting hydrodynamic tongue. And they’re able to manipulate their mouthfuls of water by moving their hyoid bone upwards with motions that resemble those seen in newts. Fish who eat underwater move their hyoid in the opposite direction.
Step by step, it goes something like this: After the mouth opens up, water begins to protrude outwards, and just before the jaws are placed around the prey, the water spreads around the prey, surrounding it. As the jaws close and the prey is engulfed, part of the expelled water is sucked back into the cavities in the mouth. You can watch some very cool slow-motion footage here.
“They’re very good at feeding on land. We put the food there and within a fraction of a second it’s gone,” Michel adds. “They’re remarkably bad at feeding underwater. They miss the food completely sometimes.” Using their hydrodynamic tongue, mudskippers don’t need to return to the water to swallow their prey. Unless, however, the shrimp was placed on top of sanitary pads and all the water they spit out gets absorbed.
Hydrodynamic tongue usage, the authors say, may be an intermediate stage in the evolution of the vertebrate tongue. "This could be an in-between, from which a fleshy tongue could have evolved,” Michel tells New Scientist. “This has never been considered a possibility, until now.”