Sunday Spinelessness – The other mollusc shell

By David Winter 01/07/2012

Here’s a really cool animal, a female argonaut (sometimes called a paper nautilus):

It may not be immediately obvious from the photo, but argonauts are octopuses. Strange octopuses, because the seven species that make up the family Argonautidea are among a handful of octopuses that are capable of swimming through the water column (rather than hanging out on the ocean floor) and they are the only octopuses that fashion themselves a shell. 
The argonaut shell has been a topic of consideration, confusion and conjecture for biologists for a long time. Only females produce the shell. Male argonauts are tiny (about a tenth of the size of the female) and only really serve as sperm donors (in fact, they donate an entire sperm-transferring organ, called the hectocotylus). Once mated, a female argonaut starts producing her shell and lays her eggs in its base. This behaviour has lead some biologists to conclude the shell’s primary function is to act as an egg case.  We now know that shell is also used to help the argonaut maintain its position in the water column. By propelling herself to the surface and rocking back and forth an argonaut can introduce an air bubble into her shell. While she’s near the surface that air bubble will make her buoyant, but by diving downwards she can reach a point where the increasing water pressure (which compresses the air bubble, decreasing its buoyancy) cancels out the buoyant effect, letter her float in the water colum. At that point she’s free to swim about in two dimensions without having to maintain her vertical position.

You can watch this remarkable behaviour  here:


I don’t want to talk too much more about the purpose of the argonaut shell, partly because it has already been well covered. Ed Yong wrote a predicably clear and interesting post on the research which uncovered it (which also produced an interesting comments thread) and the lead researcher, Julian Finn from Museum Victoria in Australia, also discussed his work in a really great video.

Instead, I want to talk about the origin of the argonaut shell. Octopuses are molluscs, part of a group of soft-bodied animals that includes clams and mussels and snails. Most molluscs have shells. In fact, despite being arugably the most morphologically diverse of the 35 animal phyla, only a few small groups of molluscs don’t contain at least some species that produce shells. The easiest way to explain the presence of shells in so many different molluscan groups is to hypothesize that the last common ancestor of all molluscs had a shell, and most of that ur-mollsuc’s descendants have retained this organ. 

In evolutionary biology we call traits that are shared between organisms as a result of their shared evolutionary history “homologies”. Homologous traits are often compared with “analgous” ones, parts of organisms that are similar as the result of independent innovations in different evolutionary lineages. We can illustrate the concept using a bat’s wing as an example. The forelimbs of bats and whale are made up of the same bones, despite the fact that whales swim and bats fly. That’s because bats and whales are both mammals, and they inherited their forelimb bones from a common ancestor before each group radically repurposed their limbs. On the other hand, despite the fact that both bats and stoneflies fly, the insect wing and the bat wing are separate evolutionary inventions and not something the two groups share as a result of shared evolutionary history:

The protective shells of snails and clams are homologous to each other, and to the internatilized shells that some squids use to stay afloat. But the argonaut shell is something entirely different. The argonaut shell is made of calcite, where most molluscan shells are argonite. Moreover, the minerals that make up the argonaut shell are extruded from the octopuses tentacles, where other molluscs have an organ called the mantle that they use to produce their shell.
The fact the argonaut shell is made of different stuff than other molluscan shells, and with the aid of  a different organ, suggests it is a unique evolutionary innovation. So how did shells evolve twice within the molluscs? I can’t provide you with a definitive answer, but I do like one (only slightly crazy) speculation. Earth’s oceans used to be dominated by another group of shelled molluscs called ammonites. Adolf Naef pointed out that argonaut shells are very similar to some ammonite shells, and suggested the ancestors of ammonites might have laid their eggs in discarded ammonite shells (some modern octopuses certainly spend time hanging out in mollusc shells). Naef suggested ancestral argonauts might then have acquired the ability to repair broken shells (developing the mineral secreting organs on their tenticles) and finally to create their own.
It’s a pretty out-there sort of an idea, and I don’t know how you could actually test it. But wouldn’t it be cool if the ammonite shell was still being dutifully copied every day, 65 million years after the last ammonite died?