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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 1 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 recently 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. But over millions of years of evolution, soft-bodied cephalopods like octopuses, squids and cuttlefish evolved to internalise that outer shell and shrink it while adapting to their individual habitats.

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. — NYT

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