How to grow a shell
And protect New York from hurricanes
These shells belong to a group of organisms known as molluscs.
What is a mollusc?
Molluscs include some familiar animals such as snails, slugs, mussels, squid and octopus, and also many more less familiar groups. Not all molluscs produce shells, but amongst those that do there are many variations on the basic shell structure.
Despite their diversity of colours and forms, mollusc shells are largely made of the same mineral – calcium carbonate – combined with small amounts of protein. They offer their inhabitants protection, and may even be adapted into tools for burrowing, or as flotation devices.
With Mark Carnall
How do molluscs grow their shells?
Complex patterns in nature can form according to simple mathematical models.
Molluscs produce their shells using an organ known as the mantle. Cells in the mantle release proteins and minerals that harden to form the shell. New material is added to the shell at its opening, resulting in spiral growth. As more material is added, the degree of 'twist' in the spiral remains constant. But with new material added to the shell at an accelerating rate, the open end of the shell widens continuously. The result is a shell that grows in a logarithmic spiral.
Diversity can arise from just a few variations on this simple model.
Some shells have a tightly coiled, snake-like appearance. This happens when the width of the shell's open end does not increase much as it grows. When the width of the opening increases significantly, from the outside the shell looks barely coiled at all.
The height of a shell's spire varies in different species of snail. The further the shell’s open end extends from the tip of the spire as it grows, the taller the spire becomes. Increasing the number of spirals can also increase height of the shell.
Small changes in the growth of the shell at different points can create a huge diversity of ornamentation. Ridges, flutes and spines emerge where part of the shell grows more quickly or in a different direction to the rest.
The shape of a mollusc's shell can tell us about its lifestyle. Burrowing bivalves may use the ridges on their shells to help them embed themselves in the sediment. In other species, the development of spines may help bivalves defend themselves from attacks by predatory fish.
How could bivalve shells protect us from hurricanes?
Dr Leanne Melbourne discusses her research into bivalves and shares how they help protect coastal populations from extreme weather events.
With Dr Leanne Melbourne
Visit the Natural Forms display
Opened between 2022 and 2024, our new displays on biodiversity showcase the variety of life on Earth and consider important questions about preserving this diversity for future generations.
The Natural forms display explores the diversity that can arise from just a few variations on the simple model of a logarithmic spiral, using specimens from the Museum's collections.
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