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.