When I first learned about the buoyancy of nautiluses, I assumed that they actively pumped gas into their shells, like a balloonist filling up for a flight. Had I lived in the nineteenth century, this notion would have been in line with all the available scientific speculation. However, decades of careful observation in the twentieth century revealed that the nautilus concerns itself only with pumping water out of its shell. Where the water used to be, gas slowly seeps in, but this is simply an accident that illustrates the difficulty of maintaining a vacuum.
I also assumed this. TIL
