"Females of this unusual octopus species sequester themselves in thin, translucent shells with which they drift across the open seas. Paper nautiluses, also called argonauts, secrete the shells to serve as cases for their eggs—but it has recently been discovered that they also function as air-trapping ballast tanks, which allow the cephalopods to hang effortlessly in the water column without sinking. Clever octopodes!"
The common octopus is the most studied of all octopus species. Its range in the eastern Atlantic extends from the Mediterranean Sea and the southern coast of England to at least Senegal in Africa. [wikipedia]