Greater Argonaut

The Greater Argonaut, which is also known internationally as the Paper Nautilus, is a pelagic octopus and perhaps the most unique cephalopod of the Mediterranean Sea. Its name is derived from the Argonauts of Greek mythology because ancient people believed that it used its shell like a boat and its arms like oars or sails to travel across the water.

This species exhibits one of the most extreme cases of sexual dimorphism in the animal kingdom, where the female and the male are completely different in their size and anatomy.

The female is large and possesses eight arms. Her two upper arms are wide and have a specialized gland that secretes a calcite substance. With this substance, she builds a beautiful, pure white, and extremely thin eggcase that resembles parchment or paper. This eggcase is not physically attached to her body, but she holds it tightly with her tentacles.

The male is a tiny octopus that is completely naked and does not possess an eggcase, while being hundreds of times smaller than the female.

It should not be confused with the True Nautilus, which lives in the Indian Ocean and possesses a thick permanent shell with chambers, while belonging to a completely different taxonomic order.

In contrast to the Common Octopus, which lives on the seabed of the Mediterranean, the Greater Argonaut is a purely epipelagic species. It lives freely in the open sea and usually stays in the surface waters of tropical and subtropical regions while allowing itself to be carried by warm marine currents and winds. Many times after major storms, especially during the winter months, people can find their empty white eggcases washed up on the beaches of the Aegean and Ionian seas.

It feeds on whatever it can capture in the open sea. Its diet includes primarily pelagic molluscs, small planktonic crustaceans, and small fish. It is also known to attach itself to jellyfish not only to feed on them but also to use them for protection and transport through the water column.

The reproduction process of this species is unique in nature. The tiny male possesses a specialized arm called a hectocotylus, where he stores his spermatophore. When he encounters the giant female, this arm detaches from the body of the male and swims autonomously to enter the mantle of the female to fertilize the eggs. The male dies shortly after this event or is occasionally consumed by the female to provide additional energy for egg development.

The female lays her eggs inside the paper eggcase, which functions as a safe brood chamber that she carries with her until the young hatch.

The Greater Argonaut has absolutely no commercial or culinary value as a catch and does not represent a target for fishers. It is sometimes caught accidentally as bycatch in pelagic drift nets. However, its wonderful white eggcase is highly sought after by shell collectors around the world and is considered one of the most beautiful and lucky findings that someone can discover while walking along the beach.

Historically, scientists mistakenly believed that the Greater Argonaut stole its shell from another animal in the same way that a hermit crab does until research proved that the female secretes it herself.

Hydrostatic Buoyancy Control: A landmark study demonstrated that the female uses her eggcase as an active buoyancy device. She rises to the sea surface to gulp a measured volume of air and seals it with her specialized dorsal arms. She then uses powerful water jets to descend where the trapped air bubble is compressed by water pressure. This allows her to achieve perfect neutral buoyancy and hover or swim effortlessly in the midwater layers without sinking.

Reproductive Mechanics of the Hectocotylus: A 2021 study in the Strait of Messina revealed surprising details about the reproductive arm of the male. The penile filament of the hectocotylus is protected in a folded state inside a transparent membranous pocket or sac and is only everted during copulation. The detached arm can actively explore the eggcase, and the shell apex actually functions as a secondary storage and protective unit for the hectocotyli received from multiple males until the female eggs reach complete maturity.

Genomic Breakthroughs: In 2022, researchers completed the first draft genome of the Greater Argonaut, which revealed highly surprising results. Its genome is less than half the size of other octopuses, representing the smallest cephalopod genome ever mapped. The study found that its Hox gene cluster is nearly intact, which contradicts the fragmented genome architecture found in other octopuses. Additionally, scientists identified specialized clusters of reflectin and tyrosinase genes. Reflections allow the female to use her dorsal arms to reflect light like an iridescent mirror to camouflage her eggcase against the open water, while tyrosinases are associated with the evolutionary innovation of creating her unique calcified eggcase from ancient shell genes.