Layers of interwoven mussel shells look lifeless when exposed to air, but under water they come alive. The shells open slightly and tiny hairs, or cilia, beat rhythmically to draw in water carrying tiny particles of food. Where waves pound the rocks, mussels out-compete other animals and plants for space. But mussels can't take over completely—other predators, such as lobsters, crabs and sea stars, eat them in areas where waves don't pound as hard. Algae, barnacles and others use the cleared living space.
The California mussel attaches to rocks by fibers called byssal threads. These threads are produced in liquid form by the byssal gland. The liquid runs down a groove formed by the foot. When the foot pulls back, exposing the liquid to sea water, the liquid solidifies into a thread. A large mussel moves by breaking old threads, then attaching new ones to another spot; a small mussel creeps around on its foot.
Though it may look rugged, the rocky shore habitat is fragile. Rocky shore creatures are at risk from coastal development and pollution (including waste oil and agricultural runoff). And some tide pools are in danger of being "loved to death" by visitors. Tread lightly as you explore tide pools to avoid crushing plants and animals, and never take creatures from their habitat.
To collect enough food to survive, a mussel filters two to three quarts (about two to three liters) of water an hour!
A California mussel grows to full size in about three years.