According to a new study published in Genome Biology and Evolution, a type of octopus appears to have evolved independently to develop something resembling a shell, despite having lost the genetic code that produced actual shells in its ancestors and relatives.
|The shell-like egg case of Argonauta argo. Credit: Genome Biology and Evolution
Argonauta argo is an octopus found in tropical and subtropical open seas. Female argonauts have a spiral, shell-like egg case that protects the eggs within. The origin of this egg case has long puzzled scientists. It resembles the shell of the well-known pearly nautilus (a distant relative of the argonaut), which has a true hard shell and lives on the ocean floor, but this could simply be a coincidence.
While both the argonaut's egg case and the nautilus' shell are formed through protein secretion, they are reportedly formed differently and appear dissimilar under the microscope. Did the egg case develop independently or as a result of the shell?
A team of Japanese researchers led by Masa-aki Yoshida and Davin Setiamarga attempted to reveal the genomic background of argonauts and show how the species adapted to the open ocean and acquired its shell-like egg case by sequencing the species' draft genome. Previously, scientists avoided targeting argonauts because it was difficult to keep the animals in aquaria for research purposes. The authors of this paper, on the other hand, had access to a location in the Sea of Japan that was ideal for obtaining fresh samples.
The newly discovered genome data sheds light on several aspects of shell evolution and egg case formation. The researchers discovered the egg case protein-coding genes in argonauts and discovered that the majority of these genes are not used to form shells in distantly related species such as the Nautilus. This suggests that while argonaut octopuses' distant ancestors most likely had shells, the shells did not evolve into egg cases.
"The argonaut genome is especially intriguing because it demonstrates that the break in synteny reported in the known octopus genome is not a general trait of this group," Yoshida and Setiamarga explained. "We have shown that, contrary to popular belief, cephalopods do not always have distinct genome evolution. We anticipate that our findings will contribute to the study of metazoan, mollusk, and cephalopod genome evolution, which has so far been largely unexplored."