Greater Argonaut

The greater argonaut is the largest of the argonauts, an open-water octopus whose female makes a paper-thin eggcase that coils around her body like a shell, which is why it is also called the paper nautilus.

It lives in open tropical and subtropical seas worldwide, keeping to deeper water by day and rising towards the surface at night to feed. Despite the name it is not a nautilus at all, and the delicate case is not a true shell but something the female makes herself to hold her eggs.

Appearance

Live animals show a blue sheen around the eyes and on the first pair of arms. The sexes differ enormously, with females reaching about 10cm in mantle length and males no more than 2cm. The eggcase is the largest in the genus, and can be told by the two rows of small sharp bumps along its narrow keel and the horn-like point on each side of the opening. A dwarf form lives in the Mediterranean Sea.

Behaviour

Females ride on jellyfish in the open ocean, gripping the upper surface with their side and lower arms. People had noticed this for a long time before anyone understood it. One female examined in detail was holding on to a jellyfish whose upper surface had been about half eaten away, with two apparent bite holes leading into its gut, so the host may serve as both meal and cover.

Mass strandings happen now and then, notably on South African and southern Australian coasts. They are seasonal, usually falling between April and August as spawning comes to an end.

Feeding

Open-water molluscs are thought to be the main food. Argonauts do not seem to chase prey. Captive females have been seen spreading the webs of their upper arms, then shooting out an arm to grab anything that touched them, which suggests they catch whatever bumps into them at sea. A female on a jellyfish can apparently also suck food particles out of its gut through the channels she has bitten.

Life cycle

Males mature when their mantle is just 8mm long. Females mature at about twice the size seen in Argonauta bottgeri and Argonauta hians, and usually start laying at 14 to 15mm, although this varies across the range. Even a small female can hold tens of thousands of embryos in her case.

Threats

Many predators eat it, and Alepisaurus ferox caught in the south-western Pacific have had it in their stomachs.

Relationship with people

In parts of Guangdong, China, it has a local name that translates as white sea-horse's nest. The sea horse here has nothing to do with the fish of that name: in the regional speech the word means a sand crab, such as a ghost crab.

In Taizhou, older people use a related pair of names, calling the shell a sea-horse shell and the soft animal an octopus.