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Invertebrate of the Week #19 – Argonauts (Genus: Argonauta)

Meet an unusual group of octopuses in which the females make paper-thin shells out of arm-gland secretions and the males can’t mate without losing one of their limbs.

Despite the nickname of ‘paper nautilus’, these creatures aren’t nautiluses at all, but rather octopuses. Yet unlike the ‘traditional’ octopuses, which are usually associated with the sea floor or in reefs and rocky substrates, argonauts are pelagic (i.e. they go about their lives swimming in the water column.)

A female argonaut (aka ‘paper nautilus’). Genus: Argonauta. (Image by Valerie Taylor/Caters News)

Females are the most-commonly encountered members of the genus, ostensibly due both to their larger size relative to the males (males are only about 10% the size of females) and the fact that it is the females which are capable of forming the unusual paper-thin shell unique to this group of animals.

The shell originates from two web-like pads on the front-most pair of arms which are held dorsally as the shell forms from secretions from glands within the webbing.

The shell is not integrated into the argonaut’s body and it is therefore able to exit the structure and swim freely unlike a true nautilus.

The early Greek civilizations that encountered argonauts postulated that the webbing on the arms was used for navigation. They theorized that the argonauts they observed had killed some other organism and taken up residence inside the shell of its former prey. It then used the webs on its forearms as sails to navigate the seas like ships. This belief is the origin of the animals’ name.

Argonaut means ‘sailor on the Argo‘; being a combination of ‘argo’ and ‘nautilus’ (derived from the Greek ναυτίλος nautílos, meaning ‘sailor’)

An argonaut partially out of its shell. (Image: Marevision/Getty Images)

It is now known, of course, that argonauts create their own shell and that the single-chambered structure is a wholly unique evolutionary development and is not a true cephalopod shell (like the shell of a true nautilus or a cuttlebone).

The shell serves at least two functions: as a brood chamber for developing eggs and as a ballast tank.

Argonauts capture surface air and use that reservoir to help maintain neutral buoyancy while cruising between the surface and depths of 750 meters in the tropical and temperate oceans where they are found.

This is in contrast to other cephalopods like squids which lack such an instrument and must expend considerable amounts of energy in order to maintain their position between the ocean surface and ocean floor.

Argonaut diving mechanics. (Image: Julian Finn)

Males of the genus are far less conspicuous and lack shells entirely. While the females are iteroparous (able to produce young multiple times throughout their lives), the males are believed to only live long enough to breed once.

As with other cephalopod males, they possess a modified right forelimb called a hectocotylus. This functions as the male intromittent organ and contains a single large spermatophore that is transferred to the female when the entire structure detaches and is left intact in the female’s mantle.

In other words, sex as an argonaut essentially involves the male loading one of his arms with a pouch of sperm and driving that arm into the female where it is broken off and remains there for future use in fertilizing eggs.

Male argonaut and hectocotylus. (Image)

 

As far as the argonaut diet, Museums Victoria reports that the remains of pteropods, heteropods, octopods, and crustaceans have all been recovered from the stomachs of female argonauts and Argonauta hians has been observed to feed on comb jellies.

Speaking of jellyfish, argonauts are often found attached to jellyfish in what appears to be a parasitic association.

The argonaut attaches to the top of the jellyfish’s bell (aboral surface), bores holes into its gastric cavity, and then (presumably) feeds on the contents. It has also been postulated that the argonaut may enjoy defensive benefits from this association; deceiving potential predators by appearing to be part of one large jellyfish and presumably benefiting from the deterrence that the jellyfish’s nematocysts provide.

Further Reading and References
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