The relationship between octopuses and shellfish

A new paper notes that an octopus appears to have independently evolved something like a shell, even though its ancestors and relatives lost the genetic code
that produced the true shell.

Argonauta argo is an octopus that lives in the high seas
of the tropics and subtropics.
The female has a protective, spiral-shaped shell that protects the eggs
inside.
Researchers have long wondered the origin
of this egg case.
It looks very much like the shell of the well-known pearl nautilus (a distant relative of Argonaut), which has a really hard shell and lives on the bottom of the sea, but this may just be a coincidence
.
Although the eggshell of Argonauta argo and the shell of the nautilus are formed through the secretion of proteins, they are reported to form differently and look different
under the microscope.
Did the eggshell evolve, or did it develop independently?

A team of researchers from Japan, led by Masa-aki Yoshida and Davin Setiamarga, tried to shed light on Argonauta argo's genomic background by sequencing a draft genome of the species and show how the species adapted to the open ocean and obtained shell-like egg shells
.
Previously, scientists had avoided Argonauta argo because it was difficult to keep them in
aquariums.
However, authors here have access to a location
in the Sea of Japan that is perfect for getting fresh samples.

The new genomic data found here provide several features
related to shell evolution and eggshell formation.
The researchers found eggshell protein-coding genes in Argonauta argo and found that most of these genes could not be used to form shells
in distantly related species, including nautilus.
This suggests that although octopuses' distant ancestors likely had shells, their shells did not evolve
further.

Yoshida and Setiamarga said: "Argonaut's genome is particularly interesting because it shows that the isotropic breaks reported in the octopus genome are not known to be a common feature
of this population.
We have shown that, contrary to popular belief, cephalopods do not necessarily exhibit unique genomic evolution
.
We anticipate that our findings will further study the genomic evolution of metazoans, molluscs and cephalopods, which remain largely unexplored
.
”

Gene recruitments and dismissals in the argonaut genome provide insights into pelagic lifestyle adaptation and shell-like egg case reacquisition