Why do some animal sponges have bright colors in shallow water?

The bright colors observed in many shallow-water sponges are primarily attributed to their symbiotic relationships with algae. These sponges harbor various types of algae, such as green algae or blue-green algae, which live within their tissues and provide them with vibrant hues ranging from red, yellow, and orange to violet.1

This coloration is directly dependent on the availability of light, which is abundant in shallow-water environments. The algae utilize these pigments for photosynthesis; consequently, if these sponges are moved to dark environments where photosynthesis cannot occur, the algal pigments are no longer produced, and the sponges may turn white. Additionally, in some species, the intensity of the color is directly related to the number of symbionts present, with specimens in strongly illuminated zones displaying more intense coloration than those in shaded or cave-dwelling environments. Beyond their aesthetic role, these symbiotic algae can also act as a protective screen against sunlight for the sponge.123