Long after its ancestors erased their genetic code for a tough armor, a marine octopus rediscovered a recipe for making a shell.
A recent genetic analysis of the paper nautilus or greater argonaut (Argonauts slow) revealed a surprising origin for its protective shell, which is unlike the shell of its closest relatives.
Instead of wearing their shells on the outside like logical snails, most cephalopods (which are also molluscs) have shed their hard outer clothing. Many, such as octopus and squid, have either lost their shells entirely or have only remnants left.
Others, such as cuttlefish and ram squid (Spirula spirula), wear their shells inside. The ram’s horn squid has an inner spiral shell with chambers which acts as a skeleton. Uncomfortable and surprisingly durable, it is they are often found washed up on beaches.
A rare exception among cephalopods is the nautilus (Nautilus belauensis), which still has an outer shell – filled with air chambers that it uses to regulate its buoyancy as it floats in the open oceans. Its shell, and those of now-extinct cephalopod ancestors, is composed of proteins that incorporate minerals such as aragonite and calcite into intricate microscopic structures.
Having appeared sometime in the Ordovician period, at least 440 million years ago, the ancestors of all modern cephalopods are thought to have had these protective structures.
Although commonly referred to as paper nautilus, argonauts are actually a genus of octopus. In this unusual group, only females produce a protective spiral sheath, secreting calcifying proteins from their arms. Argonauts wear these shells on the outside like a nautilus, and their shapes are almost identical, yet this shell has a completely different microscopic structure.
Additionally, instead of clinging to their mantle, argonauts grasp their shell houses with several of their arms.
As most octopuses lost their outer shells long ago, researchers wondered how and why a single group would recover a shell.
Working with a team of researchers from across Japan, marine biologist Masa-aki Yoshida from Shimane University sequenced the DNA of Argonauts slow. They compared the argonaut genome to other molluscs, including the California two-spot octopus (Octopus bimaculoides) and the nautilus.
“According to previous results, most of the proteins are apparently not shared with the shell matrix proteins of [cephalopods and relatives]including those of Nautilus,” the team wrote in their paper.
Some of the genes and proteins they express have, however, been found in other molluscs such as the butterflyfish (Lottia gigantea) and Japanese pearl oyster (Pinctada fucata). Other sequences were found in shellless octopuses—suggesting that the argonaut put together their protective shell using proteins unrelated to ancestral shell formation.
Unlike other octopuses, argonauts are not benthic – they do not live near the sea floor or other structures. Instead, they have taken up the life of drifters, floating between the tropical and subtropical open seas all their lives. This is the same pelagic lifestyle shared by the nautilus.
To achieve this, the Argonauts needed techniques that would also allow for easy levitation, Yoshida and team explain. While their shell does not have the more complex internal structure of the nautilus’ air chambers, it can still trap some air.
This shell is also known to be the egg case of the argonaut, which would explain why only females develop them. Females lay their eggs within the protection of the shell, eliminating the need to hide their eggs in a substrate such as the sea floor, as most other octopuses do.
The Argonauts appear to have completely reinvented the shell from scratch to aid its transition from substrate dweller to water drifter, mimicking the nautilus in a remarkable example of convergent evolution.
This research was published in Genome Biology and Evolution.
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