Do not adjust your monitor. This seaweed is deep red—but happens to appear a bright shimmering blue on a sunny day because of a quirk in its surface properties.
Unlike that dress this seaweed does actually appear in two different colours. When covered in water and illuminated by the sun, the deep red fronds of the Irish moss, Chondrus crispus, reflect some blue light too, providing an iridescent sheen. It’s the result of a series of thin transparent protective layers that happen to be the right thickness to reflect the blue light, points out New Scientist. It’s unclear why the trait may have evolved, but the researchers speculate that it may protect the seaweed from UV light.
Image by Juliet Brodie/The Trustees of the Natural History Museum