To Build Better Fiber Optic Cables, Ask Clam

The findings were published Nov. 19 in the journal Nature Communications.

Found in the warm, equatorial waters of the Indo-Pacific, heart cockles have a mutually beneficial relationship with microscopic algae that live inside their tissues. But algae need light to thrive.

While the algae get shelter and a safe place to live and grow, the clams benefit by feeding on the sugars the algae produce through photosynthesis.

To maintain this close bond, heart cockles have mastered the art of indoor gardening, directing light to their otherwise dark interiors.

They have evolved natural skylights to fuel the growth of their algal companions without opening their shells and exposing themselves to the beaks and claws of potential predators.

"They essentially evolved translucent windows in their shells," said first author Dakota McCoy , who started this work as an NSF PRFB Fellow advised by Sönke Johnsen at Duke. McCoy has since become an assistant professor at the University of Chicago.

Using a laser scanning microscope to study the 3D geometry of heart cockle shells, the researchers discovered that beneath each window, tiny translucent bumps smaller than a grain of sand function as lenses, concentrating sunlight into a beam that penetrates into the clam's interior where the algae reside.

"I imagine it like some organic cathedral with stained glass windows, with the light falling on the parishioners inside," said Johnsen, the senior author and a biology professor at Duke.

The researchers got another surprise when they looked at the shells under a scanning electron microscope.

Heart cockles and many other marine animals use a special form of calcium carbonate called aragonite to make their shells. Under a microscope, most of the heart cockle's shell has a layered structure, with thin plates of aragonite stacked in different orientations, "kind of like fancy brickwork," McCoy said.

But within each window, the material of the shell forms tightly packed, hairlike fibers rather than plates, all lined up in the direction of incoming light.

"It looks just completely different than what you'd expect," McCoy said.

Computer simulations showed that the size, shape and orientation of the fibers transmit more light into the heart cockles' interior than other possible designs the creatures could have hypothetically come up with.

In particular, they let in light within the blue and red ranges -- the optimal wavelengths for photosynthesis -- but appear to block ultraviolet radiation from penetrating into their shells where it might otherwise damage their DNA.

"Together, the fibers and the lenses create a system for filtering out bad wavelengths, channeling in the good wavelengths, and focusing so that they go far enough into the shell, so that the algal symbionts get the best lighting environment possible," Johnsen said.

The researchers also found that, because the bundled fibers in their shells are so tiny and packed together, if you shine a light through them, a high-resolution image of whatever is beneath appears on the other end, almost like a TV screen.

The team said more work is needed to understand what, if anything, the heart cockles are doing with this image projection superpower.

One day the clams could offer inspiration for new ways of designing fiber optic cables that allow light to travel great distances, even around curves, without escaping and losing signal along the way, Johnsen said.

"The shells do a very cool feat," McCoy said.

This research was supported by grants from the National Science Foundation (2109465, 1933624 and ECCS-2026822).