Summer 2018

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


The two species of bay mussel present on the Pacific coast of North America, Mytilus trossulus and M. galloprovincialis, are morphologically very similar and typically difficult to distinguish by external characters. Mytilus trossulus is native to the eastern Pacific, and occurs in bays, estuaries and the outer coast from central California to Alaska. Its introduced counterpart, M. galloprovincialis, has replaced M. trossulus in southern California, and is found as far north as Humboldt Bay. Previous studies have shown that these sibling species co-occur and form genetic hybrids in central California between Monterey Bay and Humboldt Bay, though the exact hybrid zone is not well understood. Additionally, large numbers of adult M. galloprovincialis have drifted with tsunami debris from Japan to the US since 2012. The goal of this project is to further clarify the region of overlap in the species ranges of these mussels in the eastern Pacific. Mussel populations in harbors and marinas between San Diego, CA and Newport, OR were sampled between 2013 – 2015, and tsunami debris was sampled between 2012 – 2014. Mussels were identified using a PCR assay for a nuclear marker which varies in size for each species. Mytilus galloprovincialis is now present in higher abundances in northern California, and M. trossulus is diminishing in abundance in its southern range. Mytilus galloprovincialis was the most abundant mussel on tsunami debris, and the Asian mussel Mytilus coruscus and a bivalve-inhabiting hydrozoan (Eutima japonica) were found on debris items. Tsunami debris has not resulted in detectable populations of M. galloprovincialis in Oregon.