Meet the yellow boxfish! Even though its shape is not sleek, this fish can easily steer its bulky body underwater. Scientists in the Netherlands recently studied how the fish moves.

The researchers tested plastic models of boxfish. Some models had the fish’s boxy body but no fins. Other models sported a tail fin that folded and fanned out in different ways. The researchers placed the models in a tank of flowing water to mimic the fish swimming. The scientists discovered that the finless models were wobbly. The finned ones were much more stable as they “swam.” 

That suggests that the boxfish uses its tail fin like a ship’s rudder “to steer wherever it wants to go,” says study author E. J. Stamhuis.