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), 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.

A. argo is cosmopolitan, occurring in tropical and subtropical waters worldwide. A dwarf form exists in the Mediterranean Sea, which was described as Argonauta argo mediterranea Monterosato, 1914, although this taxon is now regarded as invalid.

In Japanese, this species is called "Aoi-gai" (kanji: 葵貝, Katakana: アオイガイ), which means "Shell of Aoi" (not Malva but Asarum caulescens). Japanese have compared two eggcases of argonauts lapped each other to a leaf of Asarum caulescens. Another name is "Kaidako" (kanji: 貝蛸 or 貝鮹, Katakana: カイダコ), which means "Shell Octopus".