Head-Footed Argonaut

Thursday, January 2, 2025

Head-Footed Argonaut

Today’s special animal friends are the argonauts – not the Argonauts, although they’d be an interesting, if unsafe, group with which to share a few New Year’s Eve drinks. Argonauts, the animal friends, are four (probably) species of octopus in the genus Argonauta of the family Argonautidae. An argonaut of any species is also known as a “paper nautilus”; they are distantly related, at the Class Cephalopoda level, to actual nautiluses. The “paper” appellation refers to a fragile, white shell made by female argonauts, mainly for use as an egg case.

https://sciencephotogallery.com/featured/1-argonaut-egg-case-pascal-goetgheluckscience-photo-library.html

Unlike other octopi, argonauts are pelagic, which means they live in the open ocean. A female argonaut builds her shell with calcite secreted by two long, specialized tentacles. Then she gulps air into the shell. She seals the shell by wrapping these webbed tentacles over it. The weight of the shell vs. the buoyancy of the air causes her to float just below the surface of the water. She puts up to 40,000 eggs into the case. At this point, scientists say, things get weird.

The female argonaut can have a mantle five inches long, while the shell may be 12 inches across. Male argonauts are about ¾ inches long. Mating occurs when a male detaches a specialized “arm” which he grew in a pouch under his left eye. This protuberance is called a hectocotylus, and it has grooves all over it holding sperm. He gives the hectocotylus to a female – scientists don’t know how they even find one another in the ocean – and she puts it in the case with the eggs. Scientists think the male dies after this, but they aren’t sure: a male argonaut has never been observed out in the ocean.

Like other octopi and squids, argonauts are carnivorous predators. They eat crustaceans, mollusks, jellyfish, and salps (“sea squirts”). After grasping the prey with their tentacles, they bite it with their hard beak and inject venom. If the prey is in a shell, the argonaut drills through the shell with a radula, an abrasive tongue-like organ. Predators of argonauts include tuna, billfish, dolphins, and the very scary lancetfish, Alepisaurus ferox.

For protection, argonauts can change color and release ink; these are typical defenses of octopi and squid. Male argonauts have been found inside the bodies of aggregate salps (Pegea socia); scientists don’t know what’s up with that.

All four (probably) argonaut species are rated Least Concern by IUCN. Noury’s argonaut, Argonauta nouryi, is believed to be the least common. It is found in the Pacific Ocean, from the coast of Mexico and Central America to Polynesia and the Coral Sea. A. nodusus, the knobby argonaut, produces a shell covered with bumps. It is found only the Southern Hemisphere, especially around New Zealand, Australia, and South Africa.

Argonauta hians is known to cling to floating objects, including animals such as jellyfish, for transportation and concealment. Sometimes dozens will hold onto one another in a long chain.

A. argo, the largest species, is found in the Mediterranean Sea as well as tropical and subtropical oceans around the world. It has been known since antiquity and features in Minoan and Mycenean art.

Argonauts are difficult to keep in aquariums.

None of the articles mentioned their lifespan, but it is known that some females will produce multiple clutches of eggs, with can be fertilized by either a single hectocotylus or more than one. All the species are occasionally victims of mass stranding events, causation unclear. As we often observe here at TSAF, the ocean is incredibly weird.