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.

Top: Chambered nautilus (Image: Manuae) Middle: Glassy nautilus (Image: Johan Jacob Tesch) Bottom: Paper nautilus, or argonaut (Image: Comingio Merculiano)

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

Shell of a ‘glassy nautilus’ Carinaria lamarckii.

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

Bisected young Nautilus shell showing the internal chambers. The small tubes along the middle of the chamber walls is where the siphuncle runs, a structure that moves fluid and gas in the chambers.
Bisected young Nautilus shell showing the internal chambers. The small tubes along the middle of the chamber walls are where a structure called the siphuncle runs; this moves fluid and gas in the chambers.

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 fragile ‘paper nautilus’ the egg case and brooding chamber of an argonaut, Argonauta.
The fragile ‘paper nautilus’: the egg case and brooding chamber of an argonaut, Argonauta.

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.

Preparation showing series of argonaut egg cases of varying sizes.
Preparation showing series of argonaut egg cases of varying sizes.

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?

The three different kinds of ‘nautilus shells’ from left to right chambered nautilus Nautilus, glassy nautilus Carinaria and paper nautilus Argonauta.
The three different kinds of ‘nautilus shells’ from left to right chambered nautilus Nautilus, glassy nautilus Carinaria and paper nautilus Argonauta.

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.

Darwin, dolphins and a ‘Monkeyana’

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The Museum is home to a vast collection of natural history specimens but is perhaps less well-known for its substantial art and object collection. This material became the focus of Charlie Baker and Imogen Stead, two of our summer interns, as they spent six weeks researching, organising and curating it for the Museum.

The range and amount of material was formidable: numerous prints, non-scientific objects, paintings, photographs and sculptures from across the Museum, all coming together into a single organised collection for the first time. Here, Charlie and Imo unearth just a small sample of some of the items they catalogued during their time at the Museum:

Nautilus Imperialis
This beautiful print shows a fossil of the Nautilus Imperialis. It is one of the largest prints the Museum holds: measuring 48cm x 43 cm, it’s too large for the scanner! It has a small pamphlet of text stuck to it, just visible in the picture, and we speculate that this may have been promotional material for James Sowerby’s Mineral Conchology of Great Britain, the first volume of which was published in 1812, the same year as this print.

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Nautilus Imperialis print

Plate 60 from The Animal Kingdom
This is a plate from Henry MacMurtrie’s translation of Georges Cuvier’s Le Règne Animal, showing a few species of the genus Delphinus, or Common Dolphins. The Museums has 32 plates from this book in the collection. The publication demonstrates the intellectual collaboration between countries and the international appeal of Cuvier’s famous work. In fact, the collections holds plates from the original French and two different English translations.

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Plate 60 from ‘Animal Kingdom‘ (Le Régne Animal) by Georges Cuvier, 1829

Slide cutter
This tool was found in a chest of drawers in the Hope Library at the Museum. It initially baffled us, but staff identified it as a slide cutting tool. The circular blade scores the glass, and the notches are used to carefully break off the piece of glass. When cataloguing and storing it, we discovered the blade is still sharp enough to cut through a sheet of paper!

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Slide cutter tool

Photo of Charles Darwin
This framed photo of the great Victorian scientist is one of 22 pieces of art hanging in the Museum’s Hope Library. What makes this copy of the photo special, however, is the caption beneath it: “I like this photograph very much better than any other which has been taken of me.”

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Photograph of Charles Darwin with his annotations

Monkeyana Cartoon
One of the most bizarre items we came across was this 1828 satirical cartoon about lawyers. The collection has eight ‘Monkeyana’ cartoons, all by Thomas Landseer.

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Monkeyana Cartoon by Thomas Landseer, 1828