California Mussel
Mytilus californianus

Physical Description

  • Bivalve (two-shelled) mollusks with thick blue-black shells
  • 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
  • Attach to rocks and other mussels by their fibers called byssal threads
  • Up to 5 inches in length


  • Alaska to Baja California, Mexico


  • Rocky shores, tidepools
  • Depths of 0-79 feet


  • Fine organic detritus and living plankton filtered out of the water


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

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 tidepools, and try not to step on these closed mussels! 
  • Mussels are often called ecosystem engineers because over 300 species can live in between mussels 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;

Look at what other animals live within a clump of these ecosystem engineers on a Full Moon Pier Walk, hosted by Birch Aquarium!

Photo: Beth Besom

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