The California mussel is one of the most common creatures on California's rocky shores and in tide pools. Mussels attach themselves to rocks very tightly. After doing this several times, these threads of cement hold the mussel to the rock, sometimes permanently. Once a mussel has found a home, it opens little valves on its sides slightly to let in seawater and food particles. Much like a fish, it filters the food from the water by using its gills.
Mussels like to eat mostly fine organic material and plankton.
Colonies of mussels are often hurt badly by big waves on the open coast. Starfish eat mussels and pry some of them off the rocks. Holes in the colonies can cause the waves to break even more of them off the rocks - much like soil erosion when trees are cut down. A rock face that has been completely cleared of mussels takes about 2.5 years to be repopulated. So when you find a mussel, it's a good idea to leave it on the rock.