Scientists Think They've Cracked The Mystery of This Cute Octopus And Its 'Shell'

After removing their genetic code for a tough coat of armor, a seafaring octopus has reinvented a recipe for making a shell.

A recent genetic analysis of the paper nautilus or greater argonaut has shown that it doesn't have the same shell as its closest relatives.

Most mollusks have done away with their tough outer garment in favor of wearing their shells on the inside. Many have lost their shells or have only vestigial remnants left.

Cuttlefish and the ram's horn squid have shells on their insides. The ram's horn squid has a spiral shell that is similar to a skeleton. It's found washed up on the beach.

The nautilus has an external shell that it uses to regulate its weight as it floats through the ocean. The shell and ancestors of the now extinct cephalopods are composed of minerals such as aragonite and calcite.

The ancestors of all modern cephalopods are thought to have had protective structures at some point in the past.

In spite of being commonly referred to as paper nautili, argonauts are actually a different kind of animal. In this group, only the females have the ability to make a protective spiral casing. The shapes of these shells are almost the same, but this one has a completely different structure.

Instead of being attached to their mantle, argonauts hold their homes with their arms.

Researchers have wondered how and why a single group reclaimed a shell after losing their external shells.

Working with a team of researchers from across Japan, Masa-aki Yoshida from Shimane University was able to sequence the DNA of an animal. The argonauts' genome was compared to that of other mollusks.

The team states in their paper that most of the proteins are not shared with the shell matrix proteins ofcephalopods and relatives.

Some of the genes and the proteins they express have been found in other shell mollusks. The argonaut cobbled together their protective shell using unrelated to ancestral shell formation.

argonauts do not live near the sea floor or other structures. They have taken on the life of drifters, floating in the tropical and subtropical open seas. The lifestyle of the nautilus is similar to this one.

Easy floating was necessary to achieve this. Their shell doesn't have the complex internal structure of air chambers but it can still trap some air.

The argonaut's egg case is the reason why only females develop them. The females brood their eggs within the shell's protection, which means they don't have to hide their eggs on the sea floor.

A remarkable example of convergent evolution can be seen in the fact that the shell has been completely reinvented to help it transition from a dweller to a drifter.

The research was published in a scientific journal.