The Argonaut, An Evolutionary Defying Open Water Octopus With a Shell

Bizarre Beasts explained how the female argonaut is a evolutionary defying octopus that has a shell and lives in the open sea. This shell isn't made of calcium or aragonite as in other mollusks, but rather is a hood that is secreted by the female of the species for breeding and buoyancy, so they can traverse the pelagic column of the ocean.

Argonauts are the only known type of octopus that is pelagic,which means that they live in the openocean rather than on the seafloor ...that makes an external shell,though their shell isn't quite like that of other mollusks - more on that in a minute. Because the curious thing about modern cephalopods is that they allevolved from bottom-dwelling mollusks with shells.