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

A giant clam has hundreds, if not thousands, of eyes, which lie on the exposed flesh that lines the shell, and work like pinhole cameras. “They respond by withdrawing the mantle to movements of dark objects, even if these cast no shadow on the animal as a whole,” reads one description. They also tell the clam to close when the light dims or changes direction. I read this and imagine the dark shape of a scuba diver scanning her torch across the curves of the clam’s mouth. Depending on how deep or shallow these curves are, the clam looks happier or angrier.

There are many species of giant clam. The Latin name of one kind, commonly found in the waters around Indonesia, Palau and Vanuatu, is Hippopus hippopus . Giant clams are about as wide as a short woman is tall. Their muscle, or flesh (the part of them that is alive), is only a fraction of their weight. Dead, the largest giant clam recorded weighed only 20kg less than it weighed alive: 250kg. The heaviest weighed 340kg alive, and 333kg dead. (By comparison, the human skeleton weighs between 1.5kg and 3.5kg.) They are so heavy they don’t need to attach themselves to the sea bed. Their pearls don’t shine – there is no spare light.

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Giant clams are heavy because their shells are very strong. In Palau, clam shells were used to make adzes (for hacking, cutting and planing), hooks and tools for pounding roots. Palauan creation myths describe an empty sea in which a giant clam came to life. The clam grew and grew, until it gave birth to Latmikaik, “the mother of human children” – like Venus in a scallop shell. She gave birth to Palauans “ with the help of storms and ocean currents ”.

The most common species, Tridacna gigas , features glowing tissue in blues, greens and yellows. The glimmering cells are called iridocytes, as in iridescence: they absorb sunlight, then emit it as light waves that stimulate the photosynthetic algae, known as zooxanthellae, which live in the fleshy body of the clam.

Tridacna comes from the Greek words for “three” and “to bite”. “Some spendthrift and gourmand” came up with it, Pliny the Elder wrote some time in the first century AD, “wishing it to be understood thereby, that they are so large as to require three bites in eating them”. The giant clam can’t eat or trap a person: that’s a myth.

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.

• Helen Sullivan is a Guardian journalist. She is writing a book for Scribner Australia

• Do you have an animal, insect or other subject you’d like to see profiled by this columnist? Email helen.sullivan@theguardian.com