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.

Gemstones, fairy boats and the Orchid Mantis

This year the Museum is playing host to three poets in residence as part of our Visions of Nature year. The poets, John Barnie, Steven Matthews, and Kelley Swain, have been working alongside staff in our collections and out in the Museum itself to gain inspiration for their writing over the past six months. In the autumn, they will take part in a number of events and activities to present their work, and will be publishing a small anthology at the end of the year.

Here Kelley Swain reveals what has inspired her poems during one of her recent visits to the Museum.

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In May, I had the opportunity to meet with several curators who had previously introduced me to their collections. This time, with the orientation they’d provided, I had more specific questions:

Kelley2For Monica in Minerology, I wanted to know more about what people historically considered ‘sympathies’ of gemstones. She directed me to Nichols’ Faithfull Lapidary of 1652, the oldest book in English about the properties of gemstones. The Museum has a copy of the book on loan to the Bodleian, so I’m heading in the direction of that most famous library to get to see the book, perhaps on my next visit.

With Amo in Entomology, I was eager to pay another visit to the Orchid Mantis whom I’d named ‘Daphne’ on my last visit. In the interim, the Mantis had moulted to her final life stage – she has wings and is now fully adult, but we don’t know how long she’ll live. She might not be there when I next visit. (Amo said this is the reason she stopped naming the insects – she became too attached.)

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Daphne the Orchid Mantis in March 2016

It was especially wondrous to see that Daphne’s colouring had changed from that of a bright pink-and-white orchid blossom to something darker, that looks just like a dying orchid blossom – and she seemed much less alert than before – almost sleepy. I know a bit about cryptography and camouflage, but that the lifespan of the Orchid Mantis directly correlates with the lifespan of the orchid blossoms on which it hunts is more than I might have imagined. I’ve been working on a poem about Daphne, which, by the time of our poetry-residency anthology is published, is likely to be ‘in memoriam’.

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Daphne the Orchid Mantis in May after moulting

And thanks to Mark, Curator of Life Collections, I was able to finally see something that I’ve wanted to see for a very long time: not only one, but a large collection of fragile, mysterious Argonauta brood chambers. Cepalopods are a particular interest of mine, and while many people have collected marvellous ammonite fossils in Lyme Regis or admired the highly-polished specimen of a nautilus shell, Argonauta brood chambers are often mistaken for similar shells, when they are something rather different.

They are secreted by the female of the species (which is a type of octopus). Though this case is not attached to her body, she lives in it with her tentacles dangling out – she can spin it and turn it, but will never completely let go of it. This is the egg sac or brood chamber for her young: she lays her eggs in it, and when they hatch, they are in a safe, contained chamber in the sea. It’s likely that, as with many cephalopods, senescence sets in around the time the young are about to hatch – that is, the adults deteriorate and seem to have a kind of dementia, until they die and their bodies fall apart, just as the young hatch and need food.

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An Argonauta argo specimen from the Museum’s collections

The thing about the Argonauta that most intrigues me is that, though this papery shell is not attached to the female, she will die without it. Studies have revealed that other species of octopus, placed in cages, would squeeze out of the bars of the cage to reach food; the Argonaut, on the other hand, would not let go of its shell, remained in the cage, and died.

Argonauts can direct their buoyancy and swimming, but seem to drift with tides, and there are a few videos online of scuba divers happening upon them: they seem quite directionless and cumbersome. Despite this, to me they are, without doubt, one of the most beautiful, mysterious, and poignant creatures in the ocean, and it was a great gift to be able to see so many Argonauta cases in the Museum stores.

The Argonaut is often called the ‘paper nautilus,’ though their beautiful remnants were also called ‘fairy boats’. The best chapter I’ve read so far on these creatures is ‘Flight of the Argonauts’ by Helen Scales in her book Spirals in Time. I’m working on a poem about these marvellous creatures for the Museum residency.