Rocky coasts are areas of active erosion, where pounding waves and abrasive particles held in the water work away at the bedrock foundations of continents and islands. Life on these shores has evolved ways to tolerate the force of the waves and to cling to the rocks to avoid being swept away. Small size is one adaptation, larger forms being more easily dislodged by moving water. A thin film of slow-moving water—the boundary layer—coats wetted rock, so small, flat organisms such as isopods, sea stars, and chitons can avoid the full force of the wave action and thrive on rocky shores. Crabs simply squeeze into cracks and crevices to find shelter from crashing waves.space
Colonies of sessile, or attached, forms trap fine sediments and create habitat for motile invertebrates such as polychaetes, gastropods, and crustaceans.
Hard surfaces, living and nonliving, are coated with a microbial film of bacteria, cyanobacteria, diatoms, and protozoans—important food sources for grazing invertebrates.
Waves carry oxygen and dissolved nutrients, plankton, and organic debris to sustain attached organisms and carry away their wastes. Filter-feeders hence dominate on more exposed coasts. Barnacles are widespread on the upper shore of such habitats; mussels are common. Motile limpets graze on encrusted red algae and the biofilm of cyanobacteria.