Nature studies | How spiral shells are made

We’ve all picked up a spiralled shell on the beach and marvelled at how nature can produce something so intricate and mathematically beautiful, but how are they actually formed?

The shape of spiral shells, such as nautilus shells, are known by mathematicians as ‘logarithmic spirals’. These are shapes that follow the Fibonacci sequence and there are many examples in nature, from sunflower hearts to entire galaxies: for any rotation angle in these shapes, the distance from the origin of the spiral increases by a fixed amount. 

In nautilus shells like the Necklace Shell, above, there are many chambers, which the mollusc inside inhabits. As the creature grows, it needs bigger and bigger chambers to live in, so it creates these via a process called biomineralisation. 

During biomineralisation, the mollusc’s mantle (a soft organ that secretes a substance high in calcium carbonate) gets to work, making a deposit at the opening of the shell, following the ‘expand, rotate, twist’ rule to create the spiral. Here’s how that works:

1. Expand: it must deposit more material than it did when creating the previous part of the spiral.

2. Rotate: by depositing more material on one side of the opening, it creates a full rotation of the aperture, making a doughnut shape.

3. Twist: It rotates the point of deposition each time. The twist part is what gives a helicospiral shell, rather than a planospiral shell like that of the nautilus. So it’s not always necessary but it’s rather cool and clever, so we’ve included it here. 

Nearly all shells rotate to the right and are known as dextral shells. It’s very rare to see a sinistral shell, where the spirals rotate to the left. But what’s fascinating is that the fossil record shows this was not always the case and the direction of spiral shells changes over the centuries. Why? Nobody really knows. Perhaps, as a species, they got giddy…

The shell images used here are taken from The Shell Spotter’s Guide by Helen Scales with illustrations by Ella Sienna (National Trust Books). We have an extract and a short guide to shell spotting in our August issue, which is on sale now.
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Augur shell by Ella Sienna

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September | Things to Appreciate

A few things to see outdoors this month and a thing to do, too…

As well as being a bountiful time for your nature table. September seems like a good month to appreciate the coast now it’s quieter and to make some memories to see you through winter, too. Here’s an idea for a simple make that does both those things - Scented Seashell Candles.

You’ll need: a bain-marie; a selection of seashells*; beeswax and tealight wicks (both available from craft suppliers); essential oil of your choice – we like sweet orange and ylang ylang.

1 Clean and dry your shells, then place them in a plastic container of sand to help them stay still.

2 Set up your bain-marie with a small metal bowl inside a larger pan of boiling water.

3 Add the beeswax to the smaller bowl and wait for it to melt. Once it’s liquid, add in a few drops of your essential oil and stir.

4 Pour some of the beeswax into the empty shells and then add a tealight wick to each one. Wait for them to set before lighting, and enjoying the relaxing scent and a reminder of happy days by the sea.

*Only take empty shells from the beach and only a few at a time. Have a look round your house and garden, too – you may well find a few seashells from beach days gone by.

This kitchen table project was featured on our Almanac Pages, where each month we collate a few seasonal things to note and notice, plan and do. The nature table image above was taken by Alice Tatham of The Wildwood Moth who takes a photograph for our back cover each month, featuring things to appreciate in nature. She also runs workshops on seasonal photography and publishes seasonal journal stories from her home in Dorset.

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