It Looks Like a Shell, but an Octopus and 40,000 Eggs Live Inside

It Looks Like a Shell, but an Octopus and 40,000 Eggs Live Inside

Argonauta Argo is not a typical octopus. When a female mates, first she keeps her partner’s detachable, sperm-filled limb inside of her. Then she begins making something like a handbag.

She uses the tips of two of her blue-sheened arms to secrete a mineral formula, crafting it into a paper-thin basket shaped like a shell. The construction can grow to nearly a foot in length, becoming the home of more than 40,000 embryos. The argonaut octopus crawls inside its shell-like purse, traps some air bubbles inside, then uses its buoyancy to bob just beneath the surface of water in warm oceans around the world.

This egg holder has such an uncanny resemblance to the hard shells of the Nautiloids, the octopus’s distant relatives, that scientists nicknamed the argonaut the “Paper Nautilus.” But now, genetic sequencing data reveals the octopus independently evolved the genes to make its intricate embryo armor, instead of repurposing DNA it inherited from its shelled-ancestors.

These findings upend some misconceptions among scientists about how cephalopods evolved, said Davin Setiamarga, a researcher at the National Institute of Technology, Wakayama College in Japan, who detailed the new data last month with colleagues in the journal Genome Biology and Evolution.

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

Because the argonaut still carries around a nautilus-shell-like construction, it has fueled the scientific debate about whether, and how, an animal can lose such a structure during the course of evolution, then get it back. Other researchers initially speculated that argonauts reactivated archaic genes from the mollusk era to form their egg case. 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 entirely different genes than the nautilus do to make these formations. It means the shell-like egg case didn’t evolve from the ancestral shell, but it is the argonauts’ own evolutionary innovation for a new purpose.

“By looking at the genomes, we’re seeing that there are many different ways for animals to make biomineralized structures,” said Caroline Albertin, 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 sorts of things.”

The findings also feed into a debate over whether the argonaut’s egg case should really be called a shell.

“I mean, look at them,” Dr. Setiamarga said while holding up the two structures in front of his face during a video call, highlighting the argonaut’s vessel. “They look the same, sure, but it’s very brittle. This is just like your crackers, you know, it’s like crackers that 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 been arguing it shouldn’t be called a shell “because there’s a major difference in how it’s constructed.” Other mollusks make their shells with mantle tissue secreted by a gland, Dr. Vecchione said, whereas the arm tips of A. argo secrete the argonaut’s shell material.

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

Beyond the shell debate, Dr. Setiamarga and colleagues’ new genome sequence may help scientists understand more about how argonauts evolved to be pelagic, or living in open waters, and not benthic, like other octopuses who prefer the depths.

It also has some long-term implications for questions about cephalopodic evolution as a whole because it fills in some of the gaps between how evolution went from the Nautiloids to the modern octopus, according to Masa-aki Yoshida, director 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 out this rearrangement in the argonaut’s evolutionary history, “we can say the octopus is not an alien,” Dr. Yoshida said.

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