Heart-shaped mollusc has windows that work like fibre optics

A heart-shaped mollusc has evolved tiny windows that work like fibre-optic cables, the first known example in nature.

Heart cockles (Corculum cardissa) are bivalve molluscs a bit like clams that have a symbiotic relationship with photosynthetic algae that live inside them. The algae have a safe home, get light to photosynthesise and provide nutrients for their hosts.

Unlike other bivalves, heart cockles don’t open their shells up wide, yet they somehow funnel light to their interior even while staying shut.

Now, Dakota McCoy at the University of Chicago and her colleagues have found that there are transparent calcium carbonate crystal structures in the heart cockle shells that function like fibre-optic bundles, letting light inside to bathe the algae. “If you don’t have to open and can just have a transparent window, that’s a very safe way to irradiate your algae,” says McCoy.

The researchers examined fragments of different heart cockle shells and the transparent structures within them, as well as the intensity and colour of light that gets through. They found that the windows were made from long, thin fibres of a mineral called aragonite – a form of calcium carbonate – which lets twice as much of the photosynthetically useful light through as it does harmful ultraviolet light. “We put on sunblock because UV causes mutations and cancer. The heart cockles are using these windows as a sunblock,” says McCoy.

While the aragonite threads look similar to manufactured fibre optics, they lack a protective, insulating sheath, called cladding, yet transmit light just as effectively. This could serve as an inspiration for cladding-free fibre-optic cables, which would be cheaper to manufacture.

The natural, UV-blocking properties of the shells could also be used to help protect corals, which, like the cockles, host photosynthetic algae inside them, but are more susceptible to environmental stresses like light and heat, says McCoy.