The argonaut or paper nautilus lives in pelagic habitats of the subtropics and tropics. Pelagic habitats are those in the middle of the ocean away from coastlines and the bottom. The argonaut's shell is unique amongst molluscs as it is only built by the female as protection for her eggs which are contained inside on long threads. The female argonaut lives in the front of the shell and guards her eggs.
Argonauts show sexual dimorphism, where the male is much smaller than the female. Females reach a size of up to 10 cm, with the shells being up to 45 cm across, while the male barely reaches 2 cm across. The male argonaut has a third tentacle, the hectocotylus; as in all other male cephalopods, this has been adapted for reproduction, and becomes detached from the body after mating. No argonauts with a regrown hectocotylus have been found and it is assumed that they die soon after fertilisation.
Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2004-5