Long after its ancestors suppressed their genetic code for tough armor, a sea octopus has reinvented a recipe for making a shell.
A recent genetic analysis of the paper nautilus or greater argonaut (argonaut argo) revealed a surprising origin for its protective shell, which bears no resemblance to the shell of its closest relatives.
Instead of wearing their shells on the outside like sentient snails, most cephalopods (which are also molluscs) have removed their tough outer clothing. Many, like octopuses and squids, have either completely lost their shells or have only vestiges left.
Others, such as cuttlefish and rams horn squid (spirula spirula), carry their shells inside. The rams horn squid has a spiral chambered inner shell which acts as a kind of skeleton. Buoyant and surprisingly durable, it is often found washed up on beaches.
A rare exception among the cephalopods is the nautilus (Nautilus belauensis), which still has an outer hull – complete with air chambers which it uses to regulate its buoyancy as it floats through open oceans. Its shell, and those of now-extinct cephalopod ancestors, include proteins incorporating minerals such as aragonite and calcite into complex microscopic structures.
Originating in the Ordovician period, at least 440 million years ago, the ancestors of all modern cephalopods would have had these protective structures.
Although commonly referred to as paper nautiluses, argonauts are actually a type of octopus. In this unusual group, only the females produce a protective spiral envelope, by secreting calcifying proteins from their arms. Argonauts carry these shells externally like a nautilus does, and their shapes are almost identical, but this shell has a completely different microscopic structure.
Additionally, instead of being tethered to their mantle, Argonauts cling to their shell houses with several of their arms.
Since most octopuses lost their outer shell a long time ago, researchers wondered how and why only one group recovered a shell.
Working with a team of researchers from across Japan, marine biologist Masa-aki Yoshida of Shimane University sequenced the DNA of argonaut argo. They compared the argonaut genome to other molluscs, including the California two-spotted octopus (bimaculoid octopus) and the nautilus.
“Consistent with previous results, most of the proteins are apparently not shared with shell matrix proteins of [cephalopods and relatives]including those of Nautilus,” the team wrote in its post.
However, some genes and the proteins they express have been found in other shelled molluscs such as the limpet (Lottia gigantea) and the Japanese pearl oyster (pinctada fucata). Other sequences have been found in shellless octopuses – suggesting that the argonaut cobbled together their protective envelope using proteins unrelated to ancestral shell formation.
Unlike other octopuses, argonauts are not benthic – they do not live near the seabed or other structures. Instead, they took on the life of dinghies, floating amid tropical and subtropical high seas their entire lives. This is the same pelagic lifestyle shared by the nautilus.
To achieve this, the argonauts also needed techniques that allowed them to float easily, explain Yoshida and his team. Although their shell lacks the more complicated internal structure of nautilus air chambers, it can still trap air.
This shell is also known to be the casket of the argonaut, which would explain why only females develop them. The females brood their eggs within the protection of the shell, eliminating the need to hide their eggs on a substrate such as the seabed as most other octopuses do.
The argonauts appear to have completely reinvented the shell from the ground up to facilitate its transition from substrate dweller to water drifter, mimicking the nautilus in a remarkable example of convergent evolution.
This research was published in Biology and evolution of the genome.
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