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How do sea molluscs create the consistent patterns on their shells? How do they know when and how to change the pigment?
Mike Follows
Sutton Coldfield, West Midlands, UK
Sea mollusc shells are patterned not through conscious control, but by chemistry acting at the shell’s growing edge. A thin strip of tissue known as the mantle secretes both the shell material and its pigments as the animal grows. Pigment-producing cells along the mantle communicate only with their immediate neighbours through chemical signals, switching pigment production on or off locally rather than following any overall plan.
This local signalling closely matches Alan Turing’s reaction-diffusion model, first described in a paper he published in 1952. Of course, Turing is more famous for breaking the German Enigma code.
In his model, interacting molecules act as an activator or inhibitor. Activator molecules promote their own production through an autocatalytic process and also stimulate the production of inhibitor molecules. Pigment appears when the concentration of the activator reaches a critical threshold, but the inhibitor diffuses more rapidly, suppressing the pigment formation in surrounding areas.
The activator behaves like ink seeping into paper from scattered points, while the inhibitor spreads more quickly, absorbing the ink and preventing it from reaching a critical concentration elsewhere.
The precise locations at which activator concentrations cross the threshold arise from tiny, random fluctuations at the outset of the process, ensuring that each shell pattern is unique. This sensitive dependence on initial conditions – the defining feature of chaos theory – doesn’t lead to unbounded disorder. Instead, the outcome is tightly constrained by genetics, which fixes the system’s underlying parameters and ensures patterns stay recognisably consistent within a species.
The same mechanism produces a tiger’s stripes and a leopard’s spots, for example, yet each species can be recognised solely by the pattern of its coat.
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