Life Science SERIES: Invertebrate of the Week

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 ‘paper nautilus’, argonauts aren’t nautiluses at all. Argonauts are pelagic octopuses and spend their lives moving about the water column.

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

Female Argonauta are encountered far more often than males. That’s not surprising. For one thing, female argonauts are larger than males (males are only about 10% the size of females). Females are also the sex capable of forming the unusual paper-thin shell unique to this group of animals.

A female argonaut’s shell originates from two web-like pads on the front-most pair of arms. She holds those arms dorsally as the shell forms from secretions from glands within the webbing. Since the shell is not integrated into the female argonaut’s body, she is able to exit the structure and swim freely unlike a true nautilus.

Early Greek civilizations that encountered argonauts therefore postulated that the webbing on the arms was used for navigation. They theorized that the argonauts they observed had simply 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.

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

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

That shell serves too important functions. One is as a brood store. The second is 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 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 (semelparous).

As with other cephalopod males, male argonauts possess a modified right forelimb called a hectocotylus. The hectocotylus is the male intromittent organ (e.g. penis) and contains a single large spermatophore. The entire structure detaches and is transferred, intact, to the female’s mantle.

In other words, mating essentially involves the male loading one of his arms with a pouch of sperm and driving that arm into the female. It then breaks off and remains there for future use in fertilizing eggs.

Male argonaut and hectocotylus. (Image)

As far as diet is concerned, Museums Victoria reports that the remains of pteropods, heteropods, octopods, and crustaceans have all been recovered from the stomachs of female argonauts. 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 into thinking the pairing is one large jellyfish.

Further Reading and References