It looks like a shell, but it has an octopus and 40,000 eggs in it

Argonauta Argo is not your typical octopus. When a female partakes, she first holds her partner’s detachable, sperm-filled limb inside her. Then she starts making something like a handbag.

She uses the tips of two of her blue-tinted arms to secrete a mineral formula, which she transforms into a wafer-thin basket in the shape of a shell. The structure can grow to nearly 12 inches (30 cm) in length and become home to more than 40,000 embryos. The argonaut octopus crawls into its shell-like bag, traps some air bubbles and uses its buoyancy to float just below the surface of the water in warm oceans around the world.

This egg holder bears such an uncanny resemblance to the hard shells of the Nautiloids, the octopus’s distant relatives, that scientists gave the argonaut the “Paper Nautilus.” But now genetic sequence data reveals that the octopus independently evolved the genes to make its intricate embryonic armor, rather than reusing the DNA it inherited from its shelled ancestors.

These findings set aside some misconceptions among scientists about how cephalopods evolved, he said: Davin Setiamargaa researcher at the National Institute of Technology, Wakayama College in Japan, who shared the new data with colleagues last month in the news Genome biology and evolution.

The last common ancestor of most cephalopods probably had a pearly shell with chambers, not unlike the iconic one carried by the nautilus, a shelled cephalopod that survives to this day. But over millions of years of evolution, gentle cephalopods such as octopuses, cuttlefish and cuttlefish evolved to internalize and shrink that outer shell while adapting to their individual habitats. That’s why when you think of octopus, you think squishy (although there are some exceptions, like ram’s horn squid).

Because the argonaut still carries around a nautilus shell-like structure, it has fueled scientific debate about whether and how an animal can lose such structure over the course of evolution and then regain it. Other researchers initially speculated that argonauts reactivated archaic genes from the mollusk era to form their egg carton. But after sequencing the genome of A. argo from samples collected in the Sea of ​​Japan, the data suggested otherwise. Like their nautilus relatives, scientists found that argonauts have protein-coding genes needed to build what scientists call “true shells,” the kind you find around an oyster. But they use very different genes than the nautilus to make these formations. It means that the shell-like egg carton did not evolve from the ancestral shell, but it is the argonauts’ own evolutionary innovation for a new purpose.

“By looking at the genomes, we see that there are many different ways for animals to make biomineralized structures,” said Caroline Albert’s, a researcher at the Marine Biological Laboratory in Massachusetts, who was not involved in this study. “It tells us that evolution can take many different paths to make similar things.”

The findings also spark a debate about whether the argonaut’s egg carton should really be called a shell.

“I mean, look at them,” said Dr. Setiamarga as he held the two structures in front of his face during a video call, highlighting the argonaut’s ship. “Of course they look the same, but it’s very brittle. This is like your crackers, you know, it’s like crackers you put some cheese on.”

Michael Vecchione, a zoologist at the Smithsonian National Museum of Natural History who was not involved in the study, has long argued that it shouldn’t be called a shell “because there’s a big difference in how it’s built.” Other mollusks make their shells with mantle tissue secreted by a gland, said Dr. Vecchione, while the arm tips of A. argo secrete the shell material from the argonaut.

He hopes these new findings will finally convince people to stop calling it a shell and start a rebrand. “It really drives me crazy that people call them ‘paper nautilus,'” said Dr. Vecchione.

Aside from the shell debate, Dr. Setiamarga and colleagues are helping scientists understand more about how argonauts evolved to be pelagic, or to live in open waters, and not bottom dwellers like other octopuses that prefer the depths.

It also has some long-term implications for questions about cephalopodic evolution as a whole, as it fills in some gaps between how the evolution went from the Nautiloids to the modern octopus, according to Masa-aki Yoshidadirector of the Oki Marine Biological Station at Shimane University in Japan, and another author of this study.

dr. Yoshida and Dr. Setiamarga are already working on more research. By mapping this rearrangement in the argonaut’s evolutionary history, “we can say that the octopus is not an alien,” said Dr. Yoshida.