Argonauta argo

Argonauta argo, also known as the greater argonaut, is a species of pelagic octopus belonging to the genus Argonauta. The Chinese name for this species translates as 'white sea-horse's nest'. A. argo was the first argonaut species to be described and is consequently the type species of the genus. The type specimen of A. argo was collected in the Mediterranean Sea and is deposited at the Linnean Society of London. 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. 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. The female of the species, like all argonauts, creates a paper-thin eggcase that coils around the octopus much like the way a nautilus lives in its shell, hence the name paper nautilus. A. argo is thought to feed primarily on pelagic molluscs. The species is preyed on by numerous predators. It has been reported in the stomach contents of Alepisaurus ferox from the south-western Pacific. Males of this species reach sexual maturity at a mantle length (ML) of 8 mm. Females mature at about double the size of Argonauta bottgeri and Argonauta hians. They begin to secrete an eggcase at 6.5–7.0 mm ML. Eggs are usually laid when females reach 14–15 mm ML, although the size at which this takes place differs across the animal's range. A small A. argo residing in an 88 mm long eggcase was estimated to be carrying 48,800 embryos. Females grow to 100 mm ML, while males do not exceed 20 mm ML.