The yellow boxfish doesn’t look nimble. Squat and rectangular, it resembles a plastic storage bin with fins. Even its coloring suggests clumsiness — juveniles are hard hat yellow with black spots, as if to say “coming through!”
But on a coral reef, you’ll find these cuboid creatures darting in and out of tight spaces, snatching shrimp from crevices and cornering like BMX champs. The combination of their body plan and swimming style “really boggles the mind,” said Pim Boute, a doctoral candidate at Wageningen University in the Netherlands.
It has also inspired decades of research into how, exactly, they manage to move with such agility. The latest foray, carried out by Mr. Boute and colleagues when he was a master’s student at the University of Groningen and published in Royal Society Open Science last week, lays out the role of one understudied element: the fish’s tail fin.
Most fish, from minnows to sharks, have pliant bodies, which they undulate to move through the water. But boxfish sport a set of hard, bony plates, called a carapace. The carapace acts like a suit of armor — protecting them against predators, but restricting their flexibility. So if they want to move, “they can only use their fins,” Mr. Boute said. It also gives them their strange shapes: other boxfish species look like purses, Frisbees or ottomans.
In 2015, a group of researchers, including Mr. Boute’s two co-authors, published a study suggesting that these carapaces make the bodies of some boxfish species inherently unstable in the water. (Other studies have come to the opposite conclusion, saying that ridges on the carapace actually help with stability.)
If that’s the case, the fins not only propel and steer the fish but steady it, too, Mr. Boute said. Based on previous studies, as well as his own underwater observations, he figured the tail fin was “quite important” for modulating yaw — swerving motions that occur in the horizontal plane. (When a car hits black ice and fishtails, for example, it’s experiencing yaw.)
To test this theory, Mr. Boute and colleagues used three-dimensional plastic models of finless yellow boxfish. (Such stand-ins are common in this type of study, he said, because it’s difficult to measure forces acting on a live fish.) They placed each model in a tank, on a rod that kept it in place, and sent water rushing past it — as though it were swimming — while a sensor measured the rotational force the fake fish experienced.
They did this repeatedly, each time changing the angle of the boxfish to the water flow. They then put the models through the same tests, but added a tail fin. They tried the fin in both open and closed positions — when a yellow boxfish tail fans out, it more than doubles in size — and in a series of postures, from sticking straight out to kinked right or left.
The researchers found that without a tail fin, the boxfish was at the mercy of the water flow: had it not been attached to a rod, it would have been nudged left or right. But an open tail fin stabilized the fish, no matter how the body was angled. A closed tail fin had a subtler effect, counteracting the influence of the oncoming water to varying degrees depending on the fish’s position. The force measurements also indicated that when the tail turned, the fish would turn, too.
This suggests that by opening, closing and turning its tail fin, the boxfish can “control the unstable system that is the body” — leaning into some turns and course-correcting others, depending on where it wants to go, Mr. Boute said.
It also reinforces the viewpoint that the boxfish’s shape is inherently wobbly — which Mr. Boute expected, because the fish’s body flits around so much, he said.
Malcolm Gordon, a retired professor of ecology and evolutionary biology at the University of California, Los Angeles, called the new results “scientifically sound refinements” in the decades-long quest to figure out how boxfish work. But he remains on the opposite side of the stability debate, and thinks the carapace helps to stabilize the fish.
The boxfish, however, keep doing what they do best: driving scientists to distraction with their unlikely moves. As soon as you watch one treat a reef like an X-Games course, “it’s obvious that they use the tail for steering,” Mr. Boute said.
“But to quantify this — yeah, it was quite some work.”