Scientists believe they have cracked the mystery of this cute octopus and its “shell”.

Long after its ancestors erased their genetic code for sturdy armor, a seafaring octopus reinvented a recipe for making a seashell.

A recent genetic analysis of the paper nautilus, or larger argonaut (Argonauta argo) has revealed a surprising origin for its protective shell, which does not resemble the shell of its closest relatives.

Instead of wearing their shells on the outside like sensible snails, most cephalopods (which are also mollusks) have done away with their robust outer shell. Many, like octopuses and squid, have either lost their shells entirely or have only remnants left.

Others, such as cuttlefish and ram’s horn cuttlefish (spirula spirula), wear their shells on the inside. The ram’s horn squid has an inner chambered spiral envelope it looks like some kind of skeleton. Buoyant and surprisingly durable, it is often found washed up on beaches.

A rare exception among cephalopods is the nautilus (Nautilus belauensis) that still has an outer shell—complete with air chambers that it uses to regulate its buoyancy as it swims the open oceans. Its shell, and that of the now-extinct cephalopod ancestors, is made of proteins containing minerals such as aragonite and calcite in intricate microscopic structures.

Forming sometime in the Ordovician period at least 440 million years ago, the ancestors of all modern cephalopods are said to have had these protective structures.

Although commonly referred to as paper nautili, argonauts are actually a genus of octopus. In this unusual group, only the females produce a protective spiral wrap by secreting calcifying proteins from their arms. Argonauts wear these shells outwardly like a nautilus, and their shapes are almost identical, but this shell has a completely different microscopic structure.

Also, Argonauts don’t hold on to their mantle, instead holding on to their shell houses with multiple arms.

Since most octopuses lost their outer shells long ago, researchers have wondered how and why a single group reclaimed a shell.

Working with a team of researchers from across Japan, Shimane University marine biologist Masa-aki Yoshida sequenced the DNA of Argonauta argo. They compared the Argonauts’ genome to other mollusks, including the California two-spotted octopus (Octopus bimaculoides) and the Nautilus.

“Consistent with previous results, most proteins do not appear to be shared with the coat matrix proteins [cephalopods and relatives]including those from Nautilus,” writes the team in their article.

However, some of the genes and the proteins they express have been identified in other shelled mollusks such as the limpet (Lottia gigantea) and Japanese pearl oyster (Pinctada fucata). Other sequences have been found in shellless squid — suggesting the Argonauts cobbled together their protective shell using proteins unrelated to ancestral shell formation.

Unlike other squid, argonauts are not benthic animals — they don’t live near the sea floor or other structures. Instead, they have adopted the lives of drifters and have spent their entire lives drifting amidst the tropical and subtropical open seas. This is the same pelagic lifestyle shared by the Nautilus.

To achieve this, the Argonauts needed techniques to enable even simple levitation, Yoshida and his team explain. While its shell lacks the more complicated internal structure of the Nautilus’ air chambers, it can still trap some air.

This shell is also known as the Argonaut egg shell, which would explain why only females develop it. The females incubate their eggs within the protection of the shell, eliminating the need to hide their eggs on a substrate like the sea floor as most other squid do.

Argonauts appear to have reinvented the clam from scratch to ease its transition from substrate-dweller to aquatic-drifter, mimicking the nautilus in a remarkable example of convergent evolution.

This study was published in Genome Biology and Evolution.