Patterns in Space and Time

How seashells get their colour

Bronwen Scott

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Three very glossy shells on a light background. The shells are in various shades of grey, pink and cream. One has very fine grey and white lines, another has cream bands edged with pink with slanted grey and white lines between, and the third has one pale pink band on a background of dark grey speckled with white.
Vittina waigiensis (Neritidae), a pseudopolymorphic species, having bets each way with their colour patterns. Philippines, Indonesia, New Guinea and the Solomon Islands. Own collection. © Bronwen Scott.

I gathered shells upon the sand,
Each shell a little perfect thing,
So frail, yet potent to withstand
The mountain-waves’ wild buffeting.

Edith Nesbit, Sea-Shells

No 18th century cabinet of curiosity was complete without a collection of shells. The finest came from tropical oceans: cones patterned with flags and dashes, cowries as smooth as glass, and volutes covered in mysterious writing. Some were rare — and there were tales of collectors buying and then crushing specimens to preserve the value of the ones already in their collections — but all were beautiful and strange.

Humans have always prized shells. About 75,000 years ago, in Blombos Cave, South Africa, people collected Tick Shell (Nassarius kraussianus, Nassariidae) and strung them together on sinews to make necklaces. The millennia-old tradition continues across the world. In lutruwita/Tasmania, tiny shells are collected from kelp beds and threaded on cotton.

But how are those marvellous colour patterns formed?

Dot by dot along the growing edge of the shell, building up the pattern over time.

The shell is made up of three layers. From the outside in, they are the periostracum (a protein coat), ostracum (the main part of the shell) and the inner nacreous layer. All parts are formed by the mantle, an anatomical structure unique to molluscs. The mantle multi-tasks — it assembles proteins, it sequesters minerals, and it secretes and shapes the shell layers from the materials it has gathered. From its cells, the mantle produces the whorls and sculpture, the colours and gloss. It creates the shell in all its glorious forms.

The process by which those colour patterns form is both straightforward and complex. Because <extreme Gerard Butler voice> This! Is! Nature!

A pointed shell with a glossy surface and a pattern of fine black and white zigzags.
Vittina turrita (Neritidae) with mesmerising zigzags. Indo-West Pacific. Own collection. © Bronwen Scott.

Noting that the same general patterns — dots, dashes, dapples and stripes — occurred in completely unrelated animals, British mathematician Alan Turing developed a mathematical model to describe the way in which they developed. His model looked…

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Bronwen Scott

Zoologist, writer, artist, museum fan, enjoying life in the tropical rainforest of Far North Queensland. She/her. Website: bronwenscott.com