Heart cockle shells transmit sunlight for photosynthesis using bundled fiber optic cables and condensing lenses


McCoy, D. E., Burns, D. H., Klopfer, E., Herndon, L. K., Ogunlade, B., Dionne, J. A., . . . Johnsen, S. (2022). Heart cockle shells transmit sunlight for photosynthesis using bundled fiber optic cables and condensing lenses. bioRxiv, 2022.10.28.514291.


  • Photosymbiosis
  • Symbiodinium
  • photonics, optics
  • symbiosis, bivalves
  • Corculum cardissa
  • FDTD
  • FEM
  • transmission
  • transparency



Heart cockle shells transmit sunlight to their photosynthetic symbionts and project high-resolution images using fiber optic cables, while condensing lenses focus the light and the shell structure screens out UV radiation.


Heart cockles (Corculum cardissa and spp.) are bivalves that have evolved transparent windows in their shells to allow light to reach their photosynthetic symbionts. These windows are arranged in patterns like radial stripes, mosaics, or spots. The windows are made of aragonite, a form of calcium carbonate.

The aragonite in the windows forms elongated, fibrous crystals, unlike the planar, crossed aragonite found in opaque shell regions. These fibrous crystals are optically co-oriented along the aragonite's c-axis, which has the highest refractive index. This arrangement, similar to human-made fiber optic cables, transmits light efficiently. These aragonite structures are the first known instance of fiber optic cable bundles in an organism.

The shell windows act as a natural fiber optic system, transmitting up to 40% of visible light (400-700nm) while filtering out a significant portion of harmful UV radiation (300-400nm). The observed size of the aragonite fibers (around 1µm wide) and their c-axis orientation contribute to optimal light transmission.

Additionally, some heart cockles possess small, transparent bumps on the inner shell surface beneath each window, acting as condensing lenses. These lenses focus sunlight, potentially directing it towards symbiont-rich tissues deeper within the shell.

These adaptations represent an intricate biophotonic solution to the challenge of balancing light requirements for photosynthesis with protection from predation and UV damage. This system may inspire new optical technologies in the future.

Origin: https://www.biorxiv.org/content/biorxiv/early/2022/10/31/2022.10.28.514291.full.pdf

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