Physical Description

  • Bivalved (two-shelled) mollusks with thick blue-black shells.
  • Shells are thick and rounded on one end, narrow and pointed on other end.
  • Strong, radial (extending from a center point) ribs and irregular growth lines on shells.
  • Inner shell is shiny, iridescent blue-grey in coloration.
  • Attach to rocks and other mussels by their fibers called byssal threads.
  • Can grow up to 5 inches in length.


  • Alaska to Baja California, Mexico.


  • Live in rocky shores and tide pools.
  • Found at depths of 0-79 feet.


  • Spawns all year, spawning peaks July to December in California.
  • Reproduce by broadcast spawning, releasing eggs and sperm into the water.
  • Mussels are sexually mature at three years.


  • Eat via tiny hairs, or cilia, beating to draw particles out of the water.
  • Fine organic detritus and living plankton filtered out of the water.


  • Shiner surfperch eat larval mussels.
  • Adults eaten by American oystercatcher, surfbird, shore crabs, humans, sea otter, dogwinkle snail, rock snails, giant sea star, lobsters, crabs.

Interesting Facts

  • When exposed to air, California mussels close up completely to avoid drying out. Closing also protects them from predators. Tread lightly at the tide pools, and try not to step on these closed mussels! 
  • Mussels are often called ecosystem engineers because over 300 species can live in between mussels, using mussel beds as refuge from predators. 
  • To collect enough food to eat, mussels filter 2-3 quarts of water per hour.
  • Mussels can sometimes grow in groups of one million or more! 

Sources: Monterey Bay Aquarium;; Walla Walla University

Photo: Beth Besom