That’s how shrimpfish roll

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Shrimpfish swim forward standing on their heads. And that puts a rare spin on fishy turns.

Long, skinny Aeoliscus punctulatus mostly hang tails-up in the water with their mouths probing the seabed. But they aren’t just holding a pose, as upended trumpet fish do when hiding among sea grasses. The shrimpfish do most of their swimming horizontally in their vertical stance, sliding along the sand like pens in invisible hands.

“They’re absolutely bizarre fish,” says Frank Fish of West Chester University in Pennsylvania, who studies animals’ aquatic motions. A tank of shrimpfish caught his eye at a public aquarium in Eilat, Israel, near where he was on sabbatical in 2014. He and Roi Holzman of Tel Aviv University set up high-powered lights to take detailed video of the fish moves, becoming themselves objects of tourists’ curiosity.

Most other fishes in cross section approximate a circle or an oval, Fish says. But a shrimpfish cross section looks more like a slice of an airplane wing. The fish’s back, its leading edge as it glides along, is wider and its sides taper toward the belly like a wing’s drag-reducing taper. And the usual fin on a fish back has moved over evolutionary time to join the shrimpfish tail and anal fins in a wavering multitool cluster.

As befits a mostly vertical fish, the center of buoyancy has shifted tailward of the center of gravity, Fish and Holzman found. (Imagine swimming with a beach ball — it would be much more stable above than under the mass of a beachgoer.) Because of this alignment, the shrimpfish hangs in the water and turns easily, Fish says. In general, fish don’t roll along their long axis, but a turning shrimpfish does. “They do full pirouettes on their heads,” Fish says. “I’d like to see a prima ballerina do that.”

ROLL OUT  A shrimpfish in the wild, after feeding nose down, starts swimming — also nose down. Courtesy of Frank Fish.