Works in Progress - Autumn 2010

How the Seashell Got Its Stripes

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By Elyse Graham


 

Scientists have long suspected that the stripes, zigzags, and spirals on mollusk shells have a neurological origin, but the exact cause of the colorations has remained elusive. The problem is so intriguing that even mathematicians have weighed in with equations to explain shell construction. Alan Turing, for instance, proposed in 1984 that the invertebrates’ cells, as they grow, release chemicals that diffuse through the mantle–the organ that deposits the shell. Another possibility, now being pursued by researchers headed by George Oster of the University of California-Berkeley and Bard Ermentrout at the University of Pittsburgh, is that the patterns are a residue of conversation among neurons.

They have developed both a mathematical model based on the properties of neural nets and a computer program to simulate shell secretion and pattern based on the model. The advantage over Turing’s chemical hypothesis is that they do not require a different model for every shell type: by varying the parameters of a single model, researchers can simulate the growth of nature’s abundant shell shapes and patterns.

According to the model, mollusks build their shells using a brainless form of memory. Neurons in the shell’s mantle “taste” the newest layer of shell, sense dark or light pigment, and then gossip through synapses, which pass the information to the cells that secrete new layers. The most important parts of the output are data on width and shape, since the shape of the shell helps the critter to burrow.

Oster and Ermentrout plan to apply these insights to a more advanced mollusk, the cuttlefish, which has a mantle that produces pigment patterns using an analogous neural system. Shellfish create their patterns over weeks or months, but cuttlefish create and change their patterns almost instantly, flowing miraculously from seaweed stripes to mottled stone to a sandy grain–”the most spectacular camouflage mechanism in the animal kingdom,” says Roger Hanlon, an expert on these animals. Cuttlefish start out with color swatches in their memory banks, but when they find themselves against a new backdrop, they can recombine those memories to construct a statistical approximation of the local views. Oster, Ermentrout, and Hanlon are working out how the cuttlefish looks at the background and changes its skin to match it.

Elyse Graham is pursuing a Ph.D. in English Literature at Yale University.


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