The beautiful spiral

By Mark Carnall

At this year’s Oxford Festival of Nature I ran a spotlight session on cephalopods, the group of molluscs that includes squids, octopuses, cuttlefish, nautiluses and ammonites. While many visitors recognised the distinctive shells of nautiluses, they often weren’t too sure about the animals that made them.

This is not surprising because, confusingly, there are three different animals often referred to as ‘nautiluses’ and which all create strikingly similar shells or shell-like structures. This is deeply mysterious because there is no direct biological relationship between either the animals or the structures they make…

To helpful clarify just what’s going, here’s a quick guide to glassy nautiluses, chambered nautiluses and paper nautiluses, and the beautiful spiral structures they create.

Glassy nautilus

The glassy nautilus is the outsider of the ‘nautiluses’. It is actually a free-swimming gastropod – the group of molluscs that includes snails, slugs and limpets. The glassy nautilus creates extremely fragile transparent, glass-like shells, but unlike many other shelled gastropods, it can’t retract into its shell, which only covers a small portion of the body.

These fragile shells are understandably quite rare and are said to be worth their weight in gold; unfortunately that wouldn’t be very much as they are extremely light.

Chambered nautilus

Perhaps the most familiar of the three creatures here are the chambered nautilus,  cephalopods belonging to a very old group that first appeared nearly 500 million years ago. Despite being known and collected for a long time – examples of polished Nautilus shells mounted in gold and silver from the 16th century can be seen at the Ashmolean Museum – the living animals weren’t actually scientifically described until the 19th century.

‘Chambered’ refers to the internal walls of the shell which form chambers as the animals grow. The living nautilus occupies the most recently grown and largest chamber. A structure called a siphuncle runs throughout the chambers, adjusting the gas and fluid in each to aid in buoyancy.

A nautilus shell cut in half, or sectioned, is often used as a symbol to demonstrate the mathematical beauty of nature, and you’ll see it in logos worldwide. Unfortunately, as with most biology, these chambers aren’t formed with mathematical regularity; growth rates are affected by environment and diet.

It was thought that measuring the chambers in fossil nautiloids, if they were laid down regularly, could tell us how far the moon has been from Earth in the past. Disappointingly, this is not the case.

Argonauta, or paper nautilus

The last of our ‘nautiluses’ is the argonaut, or paper nautilus, which is a type of octopus. The structure it creates looks superficially similar to the shells of the chambered nautilus and glassy nautilus, and not surprisingly it was thought to be a paper thin shell with some affinity to the chambered nautiluses. In fact, paper nautiluses ‘shells’ are not true shells at all, but are structures secreted by female argonauts as a brood chamber for eggs.

Argonaut shells are arguably better known than the animals that make them. But unlike other kinds of mollusc shells, which can be reliably used to delineate different species, argonaut shells take a diverse array of forms across individuals thought to be of the same species. Female argonauts can also repair and replace these cases, adding to variation in their forms.

A strange similarity
What’s striking about chambered nautilus and argonaut shells is their superficial similarity, despite the animals being in two distantly-related cephalopod groups. Both argonauts and nautiloids use their shells to remain buoyant in the water column but there are a myriad of different biological solutions to solving this problem, so why so similar?

It’s tempting, though not scientific, to suppose that argonauts are somehow tapping into their deep evolutionary history of chambered shelled relatives; however, superficial resemblance aside, the shells of argonauts are chemically, mechanically, structurally and physiologically completely different to those of the chambered nautilus.

So how and when did argonauts evolve this egg case-making behaviour? Fossil examples provide little evidence of how it happened and don’t reveal whether case-making is the ancestral state that has subsequently been lost in related free-swimming cephalopods that brood their young differently.

So the strange similarity between these three structures – the shell of the chambered nautilus, that of the glassy nautilus (not a nautilus really, but a gastropod), and the egg case of the argonaut – remains a beautiful and intriguing mystery.