Greater argonaut

A. argo is the largest species in the genus and also produces the largest eggcase. Live animals have a characteristic blue sheen on the first arm pair and around the eyes. The eggcase is characterised by two rows of small, sharp tubercles running along a narrow keel, smooth ribs across the walls of the shell, and a thickening along the shell aperture, which forms distinct protrusions or 'horns' on either side. Argonauta cygnus Monterosato, 1889 was described based on a shell which lacked these protrusions, although it is now considered a junior synonym of A. argo. The greatest recorded size of an A. argo eggcase is 300 mm. A damaged beak of a female A. argo (ML = 40.0 mm; caught at 20°56′N 175°33′W / 20.933°N 175.550°W / 20.933; -175.550), measuring 4.3 mm in hood length and 7.8 mm in crest length, is mentioned in A Handbook for the Identification of Cephalopod Beaks.