If you find yourself walking along the beaches of the English Channel, you might come across a mat of green goo, as if someone had tipped a jar of mint sauce onto the beach. But if you get down on your hands and knees, and stare at the goo with a magnifying glass, you’ll see its true nature.

Worms.

Little green worms, each just a few millimetres long, writhing together in their millions.

Their formal name is Symsagittifera roscoffensis, but Channel Islanders call them mint sauce worms for obvious reasons. Their green colour comes from the algae in their bodies, which provide them with nutrients by harnessing the sun’s energy. The algae also gave the creatures their other name, favoured by early 20th