Department of Ecology and Evolution, The University of Chicago, Chicago, IL, USA.
Marine Biological Laboratory, Woods Hole, MA, USA.
Nat Commun. 2024 Nov 19;15(1):9445. doi: 10.1038/s41467-024-53110-x.
Many animals convergently evolved photosynthetic symbioses. In bivalves, giant clams (Cardiidae: Tridacninae) gape open to irradiate their symbionts, but heart cockles (Cardiidae: Fraginae) stay closed because sunlight passes through transparent windows in their shells. Here, we show that heart cockles (Corculum cardissa and spp.) use biophotonic adaptations to transmit sunlight for photosynthesis. Heart cockles transmit 11-62% of photosynthetically active radiation (mean = 31%) but only 5-28% of potentially harmful UV radiation (mean = 14%) to their symbionts. Beneath each window, microlenses condense light to penetrate more deeply into the symbiont-rich tissue. Within each window, aragonite forms narrow fibrous prisms perpendicular to the surface. These bundled "fiber optic cables" project images through the shell with a resolution of >100 lines/mm. Parameter sweeps show that the aragonite fibers' size (~1 µm diameter), morphology (long fibers rather than plates), and orientation (along the optical c-axis) transmit more light than many other possible designs. Heart cockle shell windows are thus: (i) the first instance of fiber optic cable bundles in an organism to our knowledge; (ii) a second evolution, with epidermal cells in angiosperm plants, of condensing lenses for photosynthesis; and (iii) a photonic system that efficiently transmits useful light while protecting photosymbionts from UV radiation.
许多动物趋同进化出了光合作用共生关系。在双壳类动物中，巨蛤（扇贝科：砗磲蛤属）张开贝壳以照射共生藻，但心蛤（扇贝科：心蛤属）则保持关闭状态，因为阳光可以穿过它们贝壳上的透明窗。在这里，我们发现心蛤（C. cardissa 和 spp.）利用生物光子学适应来传输阳光以进行光合作用。心蛤向共生藻传输了 11-62%的光合有效辐射（平均值为 31%），但只传输了 5-28%的潜在有害紫外线辐射（平均值为 14%）。在每个窗口下方，微透镜将光线聚集在一起，以便更深地穿透富含共生藻的组织。在每个窗口内，霰石形成与表面垂直的狭窄纤维状棱柱。这些捆绑在一起的“光纤电缆”以大于 100 线/mm 的分辨率将图像投射穿过贝壳。参数扫描表明，与许多其他可能的设计相比，霰石纤维的尺寸（~1 μm 直径）、形态（长纤维而不是薄片）和取向（沿光轴）传输更多的光。因此，心蛤贝壳窗口是：（i）我们所知的生物体中首次出现光纤电缆束；（ii）与开花植物表皮细胞的第二次进化，用于光合作用的聚光透镜；以及（iii）一种高效传输有用光同时保护光合共生体免受紫外线辐射的光子系统。
