Greater Argonaut

• The female can be as much as 600 times heavier than the tiny male.
• Only the female secretes the translucent case the species is known for.
• It can release over 85,000 eggs in batches of 2,000-4,000.
• Only the female has a shell, and it is not a true shell: she secretes it with two modified, membrane-like arms, and uses it as a brood chamber for her eggs.
• And as a float. A study of live animals showed that the female rises to the surface, traps air in the shell, seals it with her arms and carries it down with her: that air exactly offsets her weight at the depth where she lives, leaving her neutrally buoyant.
• Sexual dimorphism here is among the most extreme in the animal kingdom: the female's body can measure more than 10 cm while the male's is barely 2, with no shell.
• The male has a modified arm, the hectocotylus, which detaches inside the female during mating and stays there. When it was discovered it was taken for a parasitic worm and given its own species name.
• The male mates only once and dies; the female can mate several times and store the sperm.
• The empty shell floats and washes up on beaches, where for centuries it was mistaken for that of the nautilus — hence the name paper nautilus.

Skeleton type

Invertebrate - no internal skeleton, with an external egg case secreted by the female alone (cephalopod)

Habitat and environment

Open pelagic waters of tropical and subtropical oceans worldwide

Birth weight

A tiny egg laid in the paper casing the female secretes

Size at birth

Tiny larva at hatching

Sexual maturity

Male: 8 mm of mantle length | Female: roughly twice that, though she starts secreting the shell from 6.5-7 mm

Gestation / incubation

The egg develops inside the shell over a period the sources do not put a figure on

Litter / clutch size

A potential fecundity of at least 85,000 eggs, released in batches of 2,000-4,000

Sources consulted: Life in Water Wiki · ResearchGate (Andaman Sea paper nautilus study) · Animalia.bio · PMC (argonaut genome study) · WWF · OctoNation · The Cephalopod Page · National Geographic