These pink stony crusts are found in sunlit parts of the seafloor from the poles to the tropics, and collectively are called crustose coralline algae. Crustose coralline algae play a variety of important roles in many marine ecosystems, particularly coral reefs. We call them coral reefs because their three-dimensional structures are built by stony coral animals, which produce limestone as they grow ever upwards towards the sun. Corals may create the major structures—the bricks of the reef, if you will—but no brick wall will stand strong without cement. Enter our unassuming friends the crustose coralline algae. They grow as a crust over and between the fragments and gaps in coral reefs and essentially cement the coral bricks together. You might think of these pink crusts as the silent masons of coral reefs, the underdogs hidden in the shadow of the more charismatic stony corals.

Close examination of the structure of crustose coralline algae (using a scanning electron microscope) reveals an internal structure that looks like a honeycomb. Limestone lines the walls of individual cells of the algae, which stack up to form the hard crust. As layer upon layer of algae grow over one another and over the surface of the porous reef, the foundation of the coral reef grows stronger. These crusts grow slowly—often just 0.4 to 1.2 inches (10 to 30 mm) per year—and depend on grazing fish to clear them of rapidly growing fleshy seaweeds, which threaten to smother both the coralline algae and the coral. This cleaning is not only important for the living crusts themselves—it also helps baby corals, which often prefer to settle on the clean pink surfaces. In places with rough waves, too rough for corals to survive, crustose coralline algae are the primary reef builders.