Argonaut ‘shells’ evolved independently, study finds

The eight-armed, pink-spotted Argonauts lead lives that seem mysterious. Unlike other octopuses, they spend their days floating near the surface of tropical waters, detaching their arms to have sex, and curling up in a milky white sheath. And that’s just what we’ve seen in the wild so far. But by studying the genomes of argonauts, scientists hope to reveal much more about these puzzling cephalopods.

For researchers in Japan, the latest of the Argonaut mysteries concerns its “shell,” which is actually a self-made, paper-thin egg case that surrounds the females. This case – which is why the Argonauts are commonly called the paper nautilus (despite their status as octopuses) – has been of particular interest to scientists and philosophers for thousands of years, even piqued the curiosity of Aristotle.

Argonauts are distant relatives of hard-shelled cephalopods called nautiluses, leading scientists to question whether shared genetic information could lead to similar shell formation. Researchers already knew that certain proteins used to build nautilus shells were not present in argonaut cases, meaning that the argonaut “shell” was not the same in composition as that of the nautilus. What they didn’t know was whether the Argonauts still harbored in their genomes the genetic information used to build those nautilus shells.

In a study published today in Genomic Biology and Evolutiona team of biologists sequenced genomes of the species Argonauta argo, the greater Argonaut, to understand the origin of the Argonaut shells. According to Devin Setiamarga, a molecular biologist at Wakayama College’s National Institute of Technology and lead researcher on the genome project, although argonauts have the genes needed to build shells like their nautilus relatives, they use completely different genes to make their egg cases .

This came as a surprise, says Setiamarga. “We thought there was a possibility that the Argonauts had simply reactivated some of these old genes to form the shell of the mollusc,” he says. “But we found that’s not actually the case. They use a different set of genes.

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The scientists looked at the genetic information in the cells of argonauts to determine whether genes needed to build shells in other cephalopod species, such as nautiluses, were reassigned to form an egg in these animals.

Argonauts are notoriously difficult to keep in captivity, so to provide genetic information, Setiamarga and his team collected samples of larger argonauts with the help of local fishermen in the Sea of ​​Japan near the Oki Islands. The researchers then sequenced the extracted Argonaute DNA to understand its functions. By comparing this information with that of related molluscs, the scientists were able to determine that the proteins needed to form eggs were not used to build shells in their relatives, suggesting that eggs evolved independently. Still, the authors note that more research is needed to better understand why the argonauts retained the genetic information their relatives used for their own shells, and whether those genes might serve another purpose for the argonauts.

“Although you can find the genes used to form the shell of a mollusc in the genome of the Argonauts, they do not use them to form their [egg cases]. So we don’t know what else they’re doing in the genome. That will be another matter,” says Setiamarga.

Although this research suggests that the eggshell evolved separately from the nautilus shell, these structures still share a key function: buoyancy. Unlike many octopuses, which live in the benthic zone on the ocean floor, argonauts are pelagic: they flounder in the open sea. Without the seabed anchor, buoyancy control is vital. “Inside the shell, the female tends to have an air pocket,” said Janet Voight, associate curator of invertebrates at Chicago’s Field Museum, who was not part of the research team. “And then she goes down into the water column, that air pocket gets compressed, and that gives her some buoyancy. So it compensates for the weight of her and her developing eggs.” Nautiluses similarly use their shells to regulate their buoyancy, relying on osmosis to control the movement of water and gas inside.

However, both male and female nautiluses are born with shells and add more chambers as they age, while male argonauts, which are about the size of the tip of a human thumb, never make egg cases. Female argonauts, on the other hand – or eight – make their own “shells” after mating, literally with their own hands, by separating a mineral called calcite with two specialized hands.

This study is a small step toward better understanding these elusive creatures, Setiamarga says. “If you want to know the details of how certain traits evolve, at the end of the day you still have to look back at the genome,” he says. About ninety-eight percent of the animals on the planet are, like the Argonauts, invertebrates, he notes. “And we don’t have enough genomic information about them. If you care about conservation, we don’t have enough information to develop scientifically accurate policies.

Sequencing the argonaut genome not only adds to our understanding of invertebrates—it also improves what we know about marine life. “This is a group of species that are found in almost every ocean, and we don’t know the basics about them. So doesn’t it look like it should?” Voight says. “The oceans are so important to us, and as climate change worsens, we’re going to find that out.” If we don’t learn about them now, we might not make it.”

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