Argonauts And Paper Nautiluses

The Argonautidae are a family of pelagic cephalopods that inhabit tropical and temperate oceans of the world. The family encompasses the modern paper nautiluses of the genus Argonauta along with several extinct genera of shelled octopods. Though argonauts are derived from benthic octopuses, they have evolved to depart the sea floor and live their life-cycle in the open seas.

The family is characterised by brittle white shells constructed by the females, but which the dwarf male argonauts lack. These shells are primarily egg-cases, and are not attached to the body of the female. Paper nautiluses are often found washed up on beaches and are valued for their delicate beauty. The shell also plays the role of a buoyancy device, which the female controls by varying the amount of gulped air. All eggcases possessing nodes, ribs, and a double keel, with the exception of those of fossil Kapal batavus, have been assigned to Argonauta. Eggcases lacking these morphological features have been placed in the other Argonautidae genera.

Fossil Argonautidae eggcases have been described from Japan, New Zealand, Sumatra, Europe, and California.

The argonauts or paper nautiluses (a misnamer, use of which must be discouraged) are very abundant in tropical to warm-temperate waters of the world. A large number of nominal species exists, but the group needs revision to verify the species, perhaps 6 to 8 in all. The family is monotypic. The "shell", actually in incubation chamber for the eggs, is popular among collectors because of its beauty, coloration, sculpture and fragility. The largest species, Argonauta argo, attains a maximum size of nearly 30 cm shell diameter, it enters fish markets in India and Japan when fortuitous oceanographic conditions cause mass aggregations so that large numbers can be captured. Normally it is non-schooling, solitary group. Sexual dimorphism very marked, with adult females relatively large, up to 10 to 15 times larger than adult males; hectocotylus of males autotomous (self-amputating) into the egg mass that is attached inside a large, external, calcium carbonate egg case ("paper nautilus shell") in which the female also resides, holding on to the case with extremely broad webs on the dorsal arms (l); suckers biserial; web weakly developed; no water pores; no shell vestige.