Mytilus californianus or the California mussel is the keystone species pictured below:
California Mussel
Wikipedia – California Mussel
The muscle can be used to determine if there are toxic substance in an estuarine environment. The mussel has been transplanted in areas along the California coast where dischargers could be expelling toxic substances. The mussel will die if trace metals or synthetic organic chemicals exceed limits tolerable to the species. Mussels were chosen as an indicator organism for their ability to concentrate the toxics above ambient sea water levels (becuase their tissues will absorb the toxics & not release then), ability to provide a composite sample (time averaged), and the fact that they are stationary allows scientists to ensure the impacted water is coming from a specific source.
The mussels are extracted in areas of Special Biological Significance and tested for the SOCs and trace metals applicable to the monitoring requirements of the Mussel Watch Program (MWP). The results of the MWP are published annually and have so since 1978.
The MWP is one of three programs in the state of California started to adhere to the Porter-Cologne Water Quality Control Act (Section 13163) to establish a coordinated Primary Water Quality Monitoring Network. The results of the MWP along with the Toxic Substance Monitoring and Bay Protection and Toxic Cleanup Programs are compiled into reports to satisfy requirements of the Federal Clean Water Act (sections 104, 106, 208, 301, 303, 304, 305, 307, 308, 314, & 402). San Diego Region Basin Plan