Argonauta

***Argonauta***, the argonauts, is the only living genus in the **Argonautidae** family. They are a group of pelagic octopods.

They are also called **paper nautiluses**, because of the paper-thin eggcase that females make. This structure does not have the gas-filled chambers of nautilus shells, and is not a true cephalopod shell. It is an evolutionary innovation unique to the genus *Argonauta*.

Argonauts are found in tropical and subtropical waters worldwide: they live in open ocean. Like most octopods, they have a rounded body, eight arms and no fins. However, unlike octopuses, argonauts live close to the sea surface rather than on the seabed. *Argonauta* species have very large eyes and small distal webs. They have a mantle-funnel locking apparatus, which is a way to identify the genus. It consists of knob-like cartilages in the mantle and corresponding depressions in the funnel. Unlike the closely related genera *Ocythoe* and *Tremoctopus*, *Argonauta* species lack water pores.

## Physical description
### Sexual dimorphism and reproduction
In Argonauts, males and females are quite different in size and lifespan. Females grow up to 10&nbsp;cm and make shells up to 30&nbsp;cm, while males rarely grow larger than 2&nbsp;cm. The males only mate once in their short lifetime, but the females can get pregnant repeatedly. The females have been known since ancient times while the males were only described in the late 19th Century. The males use a modified arm, the hectocotylus, to transfer sperm to the female. For fertilization, the arm is inserted into the female's pallial cavity, then is detached from the male.

The hectocotylus was originally described as a parasitic worm. The hectocotyl arm and its correct function were described by Aristotle. His account was disbelieved for over 2000 years, but was rediscovered in the early 19th century.

Image:Argonauta nodosa lithograph.jpg|Mature female *A. nodosa*
Image:Argonauta hians female.jpg|Juvenile female *A. hians*
Image:Argonauta hians male.jpg|Immature male *A. hians*

### Eggcase 
Female argonauts make a laterally compressed, calcareous eggcase in which they live. The eggcase curiously resembles the shells of extinct ammonites. It is secreted by the tips of the female's two greatly expanded dorsal tentacles (third left arms) before egg laying. After she deposits her eggs in the floating eggcase, the female herself takes shelter in it, often along with the male's detached hectocotylus. She is usually found with her head and tentacles protruding from the opening, but she retreats deeper inside if disturbed.

These ornate, curved white eggcases are occasionally found floating on the sea surface, sometimes with the female argonaut still clinging to it. It is made of calcite, with a 3-layered structure The argonauts may have evolved to use ammonite shells for their egg laying, eventually becoming able to mend the shells and perhaps even make their own.

The egg case also contains a bubble of gas used for buoyancy similar to shelled cephalopods, although it does not have chambers as seen in other shelled cephalopods. The smallest species is *Argonauta bottgeri*, with a maximum recorded size of 67&nbsp;mm.

Image:Argonauta nodosa with eggcase lithograph.jpg|Female *A. nodosa* with its eggcase
Image:Argonauta-argo-001.jpg|The eggcase of *A. argo*
Image:Argonauta-nodosa-001.jpg|The eggcase of *A. nodosa*
Image:Argonauta-hians-001.jpg|The eggcase of *A. hians*

### Beak
The beaks of *Argonauta* species are distinctive. They  are characterised by a very small rostrum and a fold that runs to the lower edge or near the free corner. The rostrum is 'pinched in' at the sides. This makes it much narrower than in other octopuses, with the exception of the closely allied monotypic genera *Ocythoe* and *Vitreledonella*. Argonaut beaks are most similar to those of *Ocythoe tuberculata* and *Vitreledonella richardi*. They differ in 'leaning back' to a greater degree than the former and because they have a more curved jaw angle than the latter.

## Classification
The genus *Argonauta* contains up to seven existing species. Several extinct species are also known.

The extinct species *Obinautilus awaensis* was originally assigned to *Argonauta*, but has since been transferred to the genus *Obinautilus*.

## Other websites 
*Tree of Life web project: *Argonauta* 

## References