Argonauta Argo is not a typical octopus. When a female mates, she first keeps her partner’s tip filled with separate sperm inside her. Then you start making something like a handbag.
She uses the two ends of her shiny blue arms to pop a metallic composition, turning it into a paper-thin shell-shaped basket. The masonry can reach nearly a foot in length, becoming home to more than 40,000 fetuses. The argonaut octopus crawls inside its shell-like pouch, traps some air bubbles inside, then uses its buoyancy to roam underwater in warm oceans around the world.
This egg carrier has such a curious resemblance to the hard shells of octopuses, distant relatives of the octopus, that scientists have dubbed the legged forerunner “Paper Nautilus.” But now, genetic sequencing data reveals that octopuses independently evolved their genes to make complex embryonic armor, rather than reusing the DNA they inherited from their scaly ancestors.
These findings overturn some misconceptions among scientists about how cephalopods evolved Davin Setiamarjaa researcher at the National Institute of Technology, Wakayama College in Japan, who detailed the new data last month with colleagues in the magazine Genomic biology and evolution.
It is likely that the last common ancestor of most cephalopods had a pearly chamber, unlike the icon worn by the nautilus, the projectile cephalopods that have survived to this day. But over millions of years of evolution, soft-bodied cephalopods such as octopus, squid, and cuttlefish have evolved to accommodate and shrink that outer shell while adapting to their individual habitats. That’s why when you think of an octopus, you think it’s squishy (although there are some exceptions, like ram’s horn squid).
Since the legged forerunner still had a nautilus-shell-like structure, it sparked scientific debate about whether an animal could lose such a structure during evolution, and how it could lose it, and then regain it again. Other researchers initially speculated that argonauts reactivated ancient genes from the age of mollusks to form the shell of their eggs. But after sequencing the A. argo genome from samples collected in the Sea of Japan, the data indicated otherwise. Scientists, like their nautilus relatives, have found that argonauts have protein-coding genes needed to build what scientists call “true shells,” the kind you find around oysters. But they use genes completely different from those used by nautilus to make these formations. This means that the shell-like egg shell did not evolve from the ancestral shell, but rather the evolutionary innovation of argon pioneers for a new purpose.
“By looking at the genome, we see that there are many different ways in which animals can make biomineral structures” Caroline Albertine, a researcher at the Marine Biology Laboratory in Massachusetts, who was not involved in this study. “It tells us that evolution can take many different paths to make similar types of things.”
The findings also fuel a debate about whether the shell of an argonaute egg should really be called chance.
“I mean, look at them,” said Dr. Setiamarga, holding the two hulls in front of his face during a video call, highlighting the legged pioneer ship. “They look alike, sure, but they’re very crunchy. That’s just like a biscuit, you know, it’s like a biscuit you put some cheese on.”
Michael VikionD., a zoologist at the Smithsonian National Museum of Natural History who was not involved in the study, has long been arguing that it shouldn’t be called a shell “because there’s so much difference in how it’s built.” Other mollusks make their shells with mantle tissue secreted by the gland, Dr. Viccione said, while the tips of the arms of A. argo secrete argonaute shell material.
He hopes these new findings will finally convince people to stop prescribing it as chance and start rebranding. It actually drives me crazy that people call ‘paper nautilus,’ Dr. Viccione said.
Beyond the shell controversy, sequencing the new genome of Dr. Setiamarga and his colleagues may help scientists understand more about how the legs evolved to the sea surface, or live in open water, rather than the bottom, like other octopuses that prefer depths.
It also has some long-term ramifications for questions about vertical evolution as a whole because it fills in some of the gaps between how evolution passed from Nautiloids to modern octopuses, according to Masa Aki YoshidaD., director of the Aoki Marine Biological Station at Shimane University, Japan, and another author of the study.
Dr. Yoshida and Dr. Setiamarja are already working on more research. “We can say that the octopus is not alien,” said Dr. Yoshida.
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