The humble heart cockle: Nature's fiber optics

The name heart cockle, despite being wonderfully sophomoric, is apt for the Indo-Pacific native clam. The moniker goes beyond the mollusk’s heart-like shape: It just can’t help but fall head over shell for a certain microscopic alga in its native waters, with which it forms a symbiotic relationship. In return for a home that’s safe from most predators, the algae feed the host heart cockle with a sugary byproduct produced via photosynthesis.

Biologists and other experts in the field might take umbrage with this proposition, as photosynthesis generally needs the sun to make the arrangement work. The heart cockle presumably realized this too somewhere down its evolutionary ladder and so has evolved, developing natural skylights in their shells in a more reminiscent pattern than you might have guessed.

Using a laser scanning microscope to study the 3D geometry of heart cockle shells, researchers at Duke University discovered tiny translucent bumps smaller than a grain of sand under each window that function as lenses. These bumps allow sunlight to enter the normally dark space without the need for the clam to open. But when given a closer look, researchers realized that within the bumps, the shell material resembles something familiar: tiny optical fibers.

Underwater image courtesy of iStock.com/gionnixxx and seashell image courtesy of iStock.com/violetastock.

Most of the heart cockle’s shell is made up of a special form of calcium carbonate called aragonite, which has a layered structure, with thin plates of the material stacked in different orientations. And 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. The fibers specifically filter in blue and red light and appear to block ultraviolet radiation, with the former being most optimal for photosynthesis.

And while this is great for plant life, as the researchers shined light down on the bundled fibers, the shell’s cramped fiber arrangement allowed them to see a high-resolution image of whatever appeared on the other side, likening the phenomenon to a TV screen.

The researchers said more work will need to be completed to understand the reason heart cockles have this ability to project images, although it may be safe to assume that it’s not to show predators the meat inside. And while the jury is still out on the clams’ imaging feat, computer simulations on the layered fibers found that the size, shape, and orientation of the fibers transmit more light into the heart cockles’ interior than other possible designs that the creatures could have hypothetically come up with. With this knowledge, researchers believe that they might inspire new designs for fiber optic cables that allow light to travel great distances, even around curves, without escaping and losing signal along the way.

In any case, whether nature’s cardiac-shaped mollusk will lead us to improve our communication technologies, the promise of advancements like this would leave anybody happy as a … well … you know.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-53110-x).