Some 30 years ago or so I was in Florida in one of those shops that sells shells and other ocean artifacts. They had the shells of the chambered nautilus and next to them shells with the same shape as the nautilus but thin, almost like paper, and translucent. They were called the paper nautilis. I assumed it was another species related to the chambered nautilus that we're familiar with.
It sat on my shelf for years, and one day I looked at it and it had disintegrated, probably eaten by roaches. A few days ago I stumbled upon an article online that showed a picture of the exact shell, but it wasn't a nautilus at all, it was a shell of a pelagic (open ocean) female octopus called an argonaut.
The females of these octopuses construct these shells and then live in them in the open ocean at optimum depths 30 feet or so, below the motion of the surface currents. There are no chambers in the shell unlike the nautilus. The octopus bobs to the surface trapping air in the shell then submerges to a depth of neutral buoyancy, and there she lives.
Argonaut octopus and her shell washed up on a beach. She was found by a person and returned to the ocean.