A clam: made of light and all the while afraid of the dark

This very heavy animal is made of almost weightless light: the clam gives light to the zooxanthellae, which produce sugars and proteins, which the clam uses to make its calcium carbonate shell. Light does have weight: a box of light is heavier than an empty box. Darkness weighs nothing. And the clam is afraid of the dark.
— Read on www.theguardian.com/environment/commentisfree/article/2024/aug/20/a-clam-made-of-light-and-all-the-while-afraid-of-the-dark