Marine scientists once thought the pink or reddish purple blotches lining tidepools were corals, tiny colonial creatures. In fact, these blotches are seaweeds in the group called red algae. Specifically, they are Encrusting Coralline Algae. These tiny plants absorb calcium carbonate from sea water into their cell walls, hardening them to resist waves and grazing animals.
In tidepools you can see how successful this unusual plant is. The walls, rocks, even some of the living shells may be covered with pink. As tough as these crusts are, some creatures eat them. The dunce-cap limpet, a little conical gastropod, can do this and often has coralline algae growing on itself. Sea urchins graze the crusts getting the lime they need to maintain their own skeletons.
Though coralline algae have to resist excessive grazing to survive, a little benefits them. Other algae grow on the crusts threatening to block sunlight needed for photosynthesis. The shellfish called abalone are particularly good at cleaning off this layer as they feed. Coralline algae seem to give off a chemical that attracts abalone larvae to settle on them and grow up to provide this service.
When the crusts get old and thick enough they provide tiny nooks and crannies where juvenile sea urchins, limpets, and chitons (pronounced as "ki-tons") can hide from predatory fish.
Durable as they seem, we can easily damage these colorful and crusty algae and the creatures living on or in them when we disturb tide pools. Observe the pools without wading in them. Using a small stick to point things out to others will prevent oils from your hands affecting tidepool dwellers.