“Fiber Optic Cables” in Clams

Like plants, some animals harness the power of the sun for survival, albeit indirectly through symbiotic partnerships. Photosynthetic bivalves like giant clams and heart cockles require the products of photosynthesis from microscopic algae living in their soft tissues.

Now, researchers in the United States have discovered that heart cockles have natural nanophotonics in their shell windows that screen out UV radiation but still transmit ample sunlight for photosynthesis (Nat. Commun., doi: 10.1038/s41467-024-53110-x). The findings, which appear to be the first example of bundled fiber optic cables in a living creature, may lead to improved designs of synthetic nanophotonics.

Fiber optic skylights

While giant clams gape open to let sunlight in, heart cockles evolved transparent windows in their otherwise opaque shells. They can keep their heart-shaped shells closed while still letting light irradiate their soft tissues and the photosynthetic algae within. A heart cockle placed in the shade will move to the sun, and it will use its foot to clean sand or mud on the sun-facing side of the shell.

“Previous researchers have hypothesized that heart cockles have interesting nanophotonics technology in their shells,” said study author Cody McCoy, an assistant professor of ecology and evolution at the University of Chicago. “We wanted to learn more to see whether we could inspire new technologies based on nature.”

McCoy and her colleagues combined methods from evolutionary biology and materials science to demonstrate that heart cockles have fiber optic “skylights” in their shells. Using Raman spectroscopy, they confirmed that the shells, both the windows and the opaque regions, were composed of aragonite, a crystalline form of calcium carbonate. Then, with Fourier transform infrared radiation spectroscopy, the researchers identified the crystallographic orientation of the aragonite in the shell windows compared with the opaque regions of the shell.

The mineral microstructure forms fibrous prismatic crystals oriented roughly perpendicular to the shell’s surface that transmit more light into the heart cockles’ interior than other possible designs, according to optical simulations.

Differing crystal orientations

While opaque regions have aragonite that is planar and crossed in orientation, aragonite in shell windows forms bundled fiber optic cables that can not only transmit light but also project high-resolution images. The mineral microstructure forms fibrous prismatic crystals oriented roughly perpendicular to the shell’s surface that transmit more light into the heart cockles’ interior than other possible designs, according to optical simulations.

Some specimens even have small transparent bumps on the interior of their shells, located exclusively beneath each window, which act as simple condensing lenses. In addition, the windows transmit more than twice as much photosynthetically active radiation (400 to700 nm) as UV radiation (300 to 400 nm).

“We now know that heart cockles, unlike many corals, seem to protect themselves and their algae symbionts from UV radiation. Light stress can play a significant role in coral bleaching,” said McCoy. “Also, the heart cockles' fiber-optic-cable bundles seem partially self cladding, which could inspire new technologies.”