Meet the Argonaut

The argonaut Argonauta sp. is a pelagic octopus and like most octopuses, they have a rounded body, eight arms and no fins. However, unlike most octopuses, argonauts live close to the sea surface rather than on the seabed. 
Females argonauts grow to be up to 5 times the size of the males. Argonauts are also called paper nautiluses because of the paper-thin egg-case that the females secrete. This is used as a brood chamber for the developing eggs and also contains trapped surface air to maintain buoyancy. 
The photograph shows one such egg-case, picked up on a beach in Oman.