A moving shadow over the stony corals and an unexpected spurt of water! A blue crescent from the outside which is nestled snug inside the coral. Meet the clever clam of the reefs – the iridescent scallop, Pedum spondyloideum.
Settling on a living coral as a young clam, it gets a tough armour as the coral grows around it. While completely surrounded, what is left behind is only the iridescent blue mantle with its edge lined by hundreds of tiny mirror-like eyes. Using stacks of guanine crystals as mirrors to reflect blue-green light and dual retinas in each eye to detect motion, this tiny bivalve has adapted a sophisticated apparatus to ward off an approaching predator. Living in mutual harmony, when the scallop senses the shadow of a threat approaching its armour provider (the coral), it quickly snaps its shells shut, jutting out a sharp force of water aimed at it.
So next time you go snorkeling and spot these blue diamonds tucked inside the live reefs, do not hover over it long or you may just be greeted by a splosh of water across your face!
Text by Jhinook Mitra.