Heart cockles have windows in their shells to let in light for symbiotic algae

A team of marine biologists, ecologists and evolutionary specialists from the University of Chicago, Stanford University and Duke University has found that heart cockles have windows in their shells to allow in light needed by the algae that live inside the shells.

In their paper published in the journal Nature Communications, the group describes their study of the mollusks and how their findings could be applied to fiber cable technology.

Heart cockles are bivalve mollusks that resemble clams. And like clams, they have a symbiotic relationship with the algae that live inside their shells. In the relationship, the algae get food from the seawater that passes in and out of the shell, and light, which is needed for photosynthesis. In return, the cockles get food when they eat the algae.

Prior research has shown that most mollusk-dwelling algae get sunlight when the host opens wide, allowing in sunlight. But heart cockles do not open—instead, as the researchers in this new effort have found, they allow sunlight to pass through their shells courtesy of tiny windows.

The researchers found that heart cockle shells are made from aragonite, which is a crystalline form of calcium carbonate. To form the windows, an organic matrix that controls growth forms the aragonite into elongated fibrous crystals. By contrast, in the shells’ opaque regions, the aragonite is planar and crossed in orientation. Beneath the windows, the aragonite forms into bundled fiber optic cables that act as condensing lenses to focus light. Testing showed the structures allowed twice as much light to pass through as would be the case if they were just simple windows.

The researchers also found that the natural fibers block harmful UV rays that would kill the algae. They state that the natural fiber looked very much like artificial fiber cables, only without the cladding covering, which manufacturers add to communications fibers to protect them. They suggest the natural design used by the cockles could perhaps serve as inspiration for a cheaper way to make less-expensive artificial fiber cables.

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