Mytilus galloprovincialis

Invasion History

First Non-native North American Tidal Record: 1987
First Non-native West Coast Tidal Record: 1987
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

In the past three decades, morphological and genetic surveys have shown that many of the mussel populations formerly recognized as ‘Mytilus edulis’ in the Northern and Southern Hemispheres, are other closely related species, M. trossulus and M. galloprovincialis (Mediterranean Mussel). These had been recognized either as species or subspecies, based on subtle morphological features. Additional species, in the Southern Hemisphere had been recognized either as species or subspecies, based on subtle morphological features, and later synonymized, and sometimes resurrected. These include M. chilensis in the Southeast Pacific, M. platensis in the Southwest Atlantic, and M. planulatus in the southwest Pacific (Zwabicka et al. 2019). Molecular methods have identified many mixed populations in transitional zones, in invaded regions, or at natural ecological boundaries, including hybrids of two or more species. Mytilus galloprovincialis, native to the Mediterranean and the Atlantic coast of southern Europe was shown to be introduced and widespread on the West Coast of North America, Asia, South Africa, and possibly elsewhere (McDonald and Koehn 1988; McDonald et al. 1991; Geller et al. 1994; Suchanek et al. 1997; Geller 1999; Rawson et al. 1999; Westfall and Gardner 2010). Mytilus edulis, in its current sense, is native to the North Atlantic, from Cape Hatteras, North Carolina and the Bay of Biscay to Newfoundland, Iceland, northern Norway, and the mouth of the Baltic. The native 'blue mussel' of the northern Northwest Atlantic (Maine-Labrador), Baltic Sea, and northern North Pacific (California and Japan to the Bering Sea) is M. trossulus (Bay Mussel) (McDonald et al. 1991; Hilbish et al. 2000).

Mytilus galloprovincialis was first described from the Mediterranean Sea, where it occurs mainly on the northern shore, and appears to be rare or absent from most of the Mediterranean, North African coast, and the Levantine basin. Its range extends through the Black Sea to the Sea of Azov (Hilbish et al. 2000; Shurova 2001; Smietanka et al. 2004). In the Atlantic, purebred populations are found from Morocco to the southern Bay of Biscay, and zones of co-occurrence and hybridization occur northward along the entire west coast of the British Isles (McDonald et al. 1991; Hilbish et al. 2000; Smietanka et al. 2004). 

The origins of Mytilus sp. in the southern hemisphere have been more difficult to interpret. Those in South Africa have been easily recognized as recently introduced M. galloprovincialis (Griffiths et al. 1992). However, populations in Australia, New Zealand, and southern South America, appear to have some genetic differences from Northern Hemisphere populations (Hilbish et al. 2000). Shells of Mytilus sp. are known from subfossil shell middens in Australia and New Zealand, predating European colonization. Genetic analysis suggests a prehistoric origin of Australasian Mytilus, from a ‘proto-galloprovincialis’ ancestor, with introgression of M. edulis genes (Daguin and Borsa 2000; Hilbish et al. 2000). Daguin and Borsa (2000) suggest that Australasian populations be recognized as a distinct subspecies of M. galloprovincialis, native to that region. The southern lineage in Australia is now recognized as a separate species, M. planulatus (Popovic et al. 2019; Zbawicka et al. 2019). Oyarzun et al. (2016) refer to the southern South American form as M. chilensis, and the South Atlantic form as M. platensis. Molecular surveys (Westfall and Gardner 2010; Colgan and Middlefart 2011) have indicated the presence of both a Southern Hemisphere M. galloprovincialis, and purebred Northern Hemisphere M. galloprovincialis as well as hybrids between northern M. galloprovincialis and M. edulis in Australia, New Zealand, and Chile. In a study of Mytilus in the Strait of Magellan, Oyarzun et al. (2016) found hybridization between Northern and Southern Hemisphere M. galloprovincialis, and both forms with M. edulis. Also, at one location, there was hybridization with Southern M. galloprovincialis with Northern Hemisphere M. trossulus (Oyarzun et al. 2016).

Please note: In many parts of its introduced range, including the Pacific coast of North America, M. galloprovincialis has invaded other Mytilus communities, and was not detected until these regions were surveyed using biochemical techniques. Thus, dates of invasion are unknown in most cases, and the dates given on our maps are the dates of molecular analyses.

North American Invasion History:

Invasion History on the West Coast:

On the West Coast of North America, M. galloprovincialis has invaded communities formerly dominated by the similar M. trossulus, so the date of invasion is unknown. However, in southern California in the 1940s, a dramatic increase in abundance of 'Bay' or 'Blue' mussels was noticed, causing some naturalists to suspect either an invasion or a resurgence of a native species (Coe 1946, cited by Carlton 1979). The occurrence of M. galloprovincialis was first discovered in California (San Diego, Los Angeles Harbor, Port San Luis, San Francisco Bay, Muir Beach, and Tomales Bay) by amino-acid electrophoresis of samples collected in 1987 (McDonald and Koehn 1988). The invasion appears to have begun somewhere between Point Conception and San Diego, where the native M. trossulus is now rare or absent. Shipping during World War II is a possible vector. Mytilus galloprovincialis now occurs at least as far south as Punta La Mina del Fraile, Baja California, Mexico (31.2?N, Curiel-Ramirez and Caceres-Martinez 2004), and ranges north to the Strait of Georgia, British Columbia (~50?N) (Wonham 2004; Shields et al. 2010). The abundance of Mytilus galloprovincialis north of Monterey Bay appears to be fluctuating with oceanographic cycles, increasing rapidly in the 1990s, but now contracting during a cold phase of the Pacific Decadal Oscillation (Hilbish et al. 2010). The Mediterranean Mussel is physiologically adapted to be more competitive in warm waters than the native M. trossulus, and tends to outgrow and outbreed the native, under suitable conditions (Shinen and Morgan 2008; Braby and Somero 2006; Lockwood and Somero 2011a).

Currently, Mytilus galloprovincialis is dominant south of Monterey Bay, but in the region between Monterey Bay and Humboldt Bay their abundance has fluctuated. Mytilus galloprovincialis and its hybrids dominated many estuaries from 1990 to 1995 (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999), comprising 40-75% of the mussels sampled, but dropped sharply after 2000, being only ~2% of the mussels sampled in 2005-2007 (Braby and Somero 2006; Hilbish et al. 2010). North of Humboldt Bay, to Cape Flattery, Washington reported frequencies of M. galloprovincialis are low and its establishment is uncertain (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999).

Aquaculture of M. galloprovincialis in open waters in California is being carried out at several locations, including Agua Hedionda Lagoon, Santa Barbara Channel, Tomales Bay, and until recently, Humboldt Bay (Conte 1992; California Department of Fish and Game- Marine Region 2010). Aquaculture has probably played a role in expanding the range and maintaining the abundance of M. galloprovincialis in less favorable conditions. In Puget Sound and the Strait of Georgia, British Columbia, M. galloprovincialis and hybrids are locally common near ports, marinas and aquaculture operations, where introductions occur regularly, and where warmer, shallow, and confined waters provide better growing conditions. Extensive aquaculture of M. galloprovincialis began in the 1990s, and is carried out in many locations in Puget Sound and British Columbia (Heath et al. 1995; Anderson et al. 2002; Wonham 2004; Richoux et al. 2006; Elliott et al. 2008; Shields et al. 2008; Shields et al. 2010). In a few locations in the Strait of Georgia-Puget Sound region, M. galloprovincialis exceeded 25% of the mussels sampled (4 of 70 sites sampled, Wonham et al. 2004; 6 of 29 sites, Elliott et al. 2008, Puget Sound only). The most northern site where M. galloprovincialis is reported is Sayward, Vancouver Island (50° 23’ 31 N) (Shields et al. 2010).

Invasion History on the East Coast:

Mytilus galloprovincialis was observed in hull fouling in Halifax Harbor, Nova Scotia in 2007-2009 (Sylvester et al. 2011). It has likely been carried to many East coast ports. Detection of established populations would require molecular surveys.

Invasion History in Hawaii:

Mytilus galloprovincialis has been observed in Pearl Harbor, Oahu (in 1998, on the hull of USS Missouri, spawning in the harbor). Larvae were also detected in a US submarine ballast tank (Apte et al. 2000).

Invasion History Elsewhere in the World:

Mytilus galloprovincialis may have been first reported in the northwest Pacific in Kobe, Japan in 1934 (Kanamura 1935, cited by Wilkins 1983). Prior to that time, Mytilus mussels (M. trossulus) were known in Japan only from Hokkaido (Wilkins et al 1983). Extensive settlements of M. galloprovincialis occurred in south and central Japan (Wilkins et al. 1979; Inoue et al. 1997; Kurihara et al. 2009). Mytilus galloprovincialis colonized the Sea of Japan by 1941 (Iwasaki 2006) and Hong Kong by 1983 (Lee and Morton 1985). At the northern end of Japan, and the southern Pacific coast of Russia, they have formed a hybrid zone with M. trossulus (Skurikhina et al. 2004; Brannock et al. 2010). However, from 1978 to 2006, abundance of M. galloprovincialis decreased sharply on the eastern shore of Japan, possibly due to increasing water temperatures, while native mussels were less affected (Kurihara et al. 2009).

In southeastern Australia, New Zealand, and southern South America, the situation is complicated by the presence of apparently native Southern Hemisphere genotypes of M. galloprovincialis, resulting from a prehistoric natural migration (Daguin and Borsa 2000; Hilbish et al. 2000), and at the same time, introduced Northern Hemisphere genotypes, presumably introduced by shipping and aquaculture (Westfall and Gardner 2010; Colgan and Middlefart 2011; Gardner and Westfall 2012). One recent estimate is 100,000 to 600,000 for European M. galloprovincialis and the Australian form, now known as M. planulatus (Popovic et al. 2019). At the same time, introduced Northern Hemisphere genotypes, presumably introduced by shipping and aquaculture have become established and hybridize with native forms (Westfall and Gardner 2010; Colgan and Middlefart 2011; Gardner and Westfall 2012; Oyarzun et al. 2016). Northern genotypes have been found near ports and aquaculture operations, but also at some sites on open coastlines (Westfall and Gardner 2010; Colgan and Middlefart 2011). Multiple invasions have probably occurred in these regions, but the dates of introductions cannot be determined. However, one invasion in the remote Auckland Islands off New Zealand, now a restricted biosphere preserve, most likely occurred during 19th century seal-hunting operations (Westfall and Gardner 2010).

 

The extent of invasion of exotic Northern Hemisphere M. galloprovincialis in the Southern Hemisphere is contentious. Multiple studies have reported different degrees of invasion and hybridization in South America, Australia, New Zealand, and Sub-Antarctic Islands (Westfall and Gardner 2010; Colgan and Middlefart 2011; Oyarzun et al. 2016; Gardner and Westfall 2012; Zbawicka et al. 2019). Some the differences may depend on the degree of focus on ports and aquaculture versus less-disturbed habitats.

Popovic et al. (2019) found evidence for two separate invasions of Australia by European M. galloprovincialis, by populations from the Atlantic coast of Europe, and from the Mediterranean. Populations from Bateman's Bay in southhern New South Wales had a stronger contribution from Atlantic populations, while those from Sydney Harbour were closer to Mediterranean populations. Both populations had repeated hybridization with native M. planulatus (Popovic et al. 2019).  Zbawicka at al. al (2022) found M. galloprovincialis × M. planulatus on the south coast of Astralia from Western Australia to New South Waltes, varing in frequency from 30% (Port Arthur, Tasmania) to 97% (South Australia).  Hybrids were more dominant in mussel-farming operations. Of the 10 wild and farmed sites sampled, only one wild population was 100% M. planulatus (Zbawicka et al. 2022). Zbawicka et al. (2022) suggest aqaculture of M. planulatus to preserve the native species.

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Description

Mytilus galloprovincialis has a smooth shell roughly shaped like an elongate triangle, with the beak forming the apex. The anterior margin is straight, while the posterior margin is broadly rounded. The surface is marked by concentric growth lines. The exterior color is black to bluish-black, or brownish, while the interior is white, with a violet margin, and a distinct muscle scar. The shell can reach a length (height) of 150 mm (Abbott 1974; Coan et al. 2000; Coan and Valentich-Scott, in Carlton 2007). Like other mussels, M. edulis clings to hard surfaces (rocks, logs, structures) by a cluster of byssus treads.

Mytilus galloprovincialis is part of the M. edulis species complex, which includes M. galloprovincialis, M. trossulus, and possibly other species (e.g. M. chilensis, M. desolationis, M. planulatus. M. platensis), whose distinctness is disputed (Westfall and Gardner 2010; Oyarzun et al. 2016; Zbawicka et al. 2022). Mytilus edulis tends to have a longer hinge plate and adductor muscle scar than either M. galloprovincialis or M. trossulus. However, overlap is considerable. The species are best distinguished morphologically by statistical analysis of multiple measurements (McDonald et al. 1991) or by molecular methods (McDonald et al. 1991; Heath et al. 1995; Shields et al. 2008; Westfall and Gardner 2010).

Larvae of M. galloprovincialis are described by Semenikhina et al. (2008). Late veligers are roughly egg-shaped, with a prominent eye-spot. Settlement occurs at 330-350 µm in Pacific Russia (Semenikhina et al. 2008). To our knowledge, larvae of M. edulis cannot be distinguished visually from those of M. galloprovincialis or M. trossulus.

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Taxonomy

Ecology

General:

Mytilus galloprovincialis has separate sexes and individuals mature at one year of age or less. This species has a prolonged spawning season, and in some regions, spawns year round (Seed 1969). Fertilized eggs develop into a planktonic trochophore larva, then into a shelled veliger. Female M. galloprovincialis, fed different phytoplankton diets, produce means of 1.5-3.5 million eggs. The larvae settle at 330-350 µm, after about 2-4 weeks in the plankton (Satuito et al. 1994; Caceres-Martinez and Figueras 1998; Semenikhina et al. 2008; Lopez-Duarte et al. 2012). On the coast of San Diego County, California larvae dispersed over a mean distance of 35 km determined by genetic and trace-metal analysis, though a large proportion settled more locally, while a few (1.5%) dispersed for more than 125 km. There was extensive genetic exchange between bays and between open-coast and bay populations (Lopez-Duarte et al. 2012).

Larvae of M. galloprovincialis can settle and metamorphose on a wide variety of surfaces, including rock, wood and vegetation. Initially, juveniles can be quite mobile, using their byssus threads to move up and down, and attaching to drifting substrates such as vegetation. As they grow, they are attracted to other mussels. Extensive beds develop on rocky surfaces, but also on soft sediments, in which mussels are connected to each other and the substrate by byssus threads, creating a complex habitat (Robinson et al. 2007). Mussels are strong-filter feeders, and create substantial currents as they pump in water ingesting phytoplankton and other suspended material. They deposit the uneaten material as pseudofeces, creating deposits of silt around and within the mussel bed (Bertness 1999; Buschbaum et al. 2009).

Mediterranean Mussels are characteristic of shallow subtidal and intertidal zones, and can be subject to sharp changes in temperature when exposed to the air, and changes in salinity due to rainfall and river flow. Adult M. galloprovincialis can tolerate salinities of 10 PSU, but require at least 20 PSU for successful larval development (His et al. 1989). Populations in the Black Sea (~18 PSU) may have lower salinity tolerances.

Food:

Phytoplankton

Competitors:

Mytilus trossulus; Mytilus edulis; Perna perna

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Coarse Woody Debris	None
General Habitat	Marinas & Docks	None
General Habitat	Rocky	None
General Habitat	Vessel Hull	None
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Tidal Range	Mid Intertidal	None
Vertical Habitat	Epibenthic	None
Vertical Habitat	Littoral	None

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Life History

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Tolerances and Life History Parameters

Maximum Temperature (ºC)	31	Experimental, ranges of 27.5-31 reported by authors cited by Hicks and McMahon (2002) and Schneider (2008).
Minimum Salinity (‰)	10	Experimental, mussels from Black Sea (Shurova 2001)
Maximum Salinity (‰)	38	Mediterranean salinity- this species probably tolerates higher salinities.
Minimum pH	7.6	Gestoso et al. 2015, 40-60% mortality, over 22 days (Gestoso et al. 2015)
Minimum Reproductive Temperature	15	Mussel larvae from Arcachon, France (Bay of Biscay) developed well at 15-25 C (His et al. 1989)
Maximum Reproductive Temperature	25	Mussel larvae from Arcachon, France (Bay of Biscay) developed well at 15-25 C, but (His et al. 1989)
Maximum Reproductive Salinity	38	Based on occurrence in Mediterranean Sea. Mussel larvae from Arcachon, France (Bay of Biscay) developed well at 20-35 PSU (His et al. 1989)
Minimum Duration	26	Japan, laboratory rearing, 20 C, Satuito et al. 1994
Maximum Duration	40	NW Spain, unspecified temperature- Caceres-Martinez and Figueras 1998
Minimum Length (mm)	14	Japan, Wilkins 1983
Maximum Length (mm)	100	Abbott 1974; Coan and Valentich-Scott, in Carlton 2007
Broad Temperature Range	None	Cold temperate-Warm temperate
Broad Salinity Range	None	Polyhaline-Euhaline

General Impacts

Mytilus galloprovincialis has been listed by the Invasive Species Specialist Group of the International Union for Conservation of Nature (IUCN) as one of the '100 worst invasive species.' However, its economic and ecological effects are complex. On the West Coast of the US, Japan, Chile, Australia, and South Africa, it has become an important aquaculture species (Inoue et al. 1997; Robinson et al. 2005; Elliott et al. 2008; Colgan and Middlefart 2011), but it has also interfered with aquaculture of other species, such as oysters (Chavanich et al. 2010). It can foul ships’ hulls and industrial-water systems (Chavanich et al. 2010). While it has hybridized and competed with native mussels and other shore invertebrates, its beds have created complex habitats and food supplies for some native organisms (Robinson et al. 2005).

Economic Impacts

Shipping- Mussels of the genus Mytilus are abundant and troublesome fouling organisms on ships, docks, jetties, pipes, and industrial water systems (Woods Hole 1952). They are rarely distinguished to species, but in Japan, where M. galloprovincialis invaded in the 20th century, they led to greatly increased ship fouling and the use of toxic fouling paints, including TBT, with negative effects on native mollusks (Chavanich et al. 2010).

Fisheries- Mytilus galloprovincialis is considered a highly desirable seafood species, and is widely gathered, fished, cultured, and eaten in its native and invaded ranges. Culture is usually done on racks, sometimes in the intertidal zone, or on ropes suspended from rafts or piers (Conte 1992). In North America, Mediterranean Mussels are cultured from California to British Columbia (Anderson 2002; Conte 1992; Wonham 2004; Elliott et al. 2008; California Department of Fish and Game 2010; BC Shellfish growers Association http://bcsga.ca/about/industry-encyclopedia/mussels). Mytilus galloprovincialis is cultured in Japan, Chile, Australia, and South Africa, and doubtless, in other regions as well (Inoue et al. 1997; Robinson et al. 2005; Colgan and Middlefart 2011). The ecological impacts of mass off-bottom culture of native or introduced mussels are great, and include alteration of water flow, increased sedimentation, alteration of nutrient fluxes, and provision of new habitat for native and non-indigenous fouling organisms, such as tunicates (McKindsey et al. 2011).

Industry- Mytilus galloprovincialis is considered to be the most expensive fouling organism of power plants in Japanese waters, causing major expenses for damage and cleaning (Iwasaki 2006). Its impacts in invaded areas may be difficult to recognize where it has replaced M. trossulus or the Southern Hemisphere form of M. galloprovincialis (Woods Hole Oceanographic Institution 1952).

Ecological Impacts

Competition- Many of the invasions of Northern Hemisphere M. galloprovincialis were initially undetected, because this mussel replaced other morphologically very similar members of the Mytilus galloprovincialis complex, primarily M. trossulus (Bay Mussel) in the North Pacific, but also the native Southern Hemisphere genotype of this species, in Australia, New Zealand and Chile (McDonald et al. 1991; Suchanek et al. 1997; Geller 1999; Hilbish et al. 2000; Westfall and Gardner 2010). In warm-temperate environments and full marine salinities, M. galloprovincialis outperforms M. trossulus (Braby and Somero 2006; Lockwood and Somero 2011), outgrowing and out-reproducing M. trossulus. It also engages in interference competition, by moving, out filtering, and smothering M. trossulus (Shinen and Morgan 2009). Examination of museum specimens and shells from Indian middens indicate that M. trossulus was the sole mussel species south of Point Concepcion, California, and has been completely replaced by M. galloprovincialis (Geller 1999). However, north of Monterey Bay, since 2000, changing ocean conditions and cooler temperatures associated with the Pacific Decadal Oscillation have shifted the competitive balance back to favoring M. trossulus. Long-term cyclic changes in the distribution of the two mussels are likely (Braby and Somero 2006; Hilbish et al. 2010; Lockwood and Somero 2011a, Lockwood and Somero 2011b). In shallow estuaries with warmer summer temperatures and variable salinities, such as San Francisco Bay, Puget Sound, and Georgia Strait, complex interactions between M. galloprovincialis and M. trossulus occur (Braby and Somero 2006). Extensive replacement of M. trossulus has also been seen in northern Japan (Daguin and Borsa 2000; Brannock et al. 2005). Interactions between the Northern Hemisphere and Southern Hemisphere genotypes of M. galloprovincialis have not yet been studied.

In several parts of the world, M. galloprovincialis has been observed to compete with other genera of mussels, and with other bivalves. In South Africa, it grows faster and has greater reproductive output than three species of native mussels (Aulacomya ater, Choromytilus meridionalis, and Perna perna; Branch and Stefanni 2004). Some of its advantage there may come from its virtual lack of parasites (Calvo-Ugarteburu and McQuaid 1998). It has largely replaced the smaller native mussel Aulacomya ater in the intertidal zone on the West Coast of South Africa (Branch and Steffani 2004; Griffiths et al. 2005; Branch et al. 2008). On the south coast of South Africa, it has had a stand-off, resulting in coexistence with the native P. perna, with the more desiccation-resistant and prolific, but less firmly attached M. galloprovincialis predominating in the upper intertidal, while P. perna resists waves and post-recruitment mortality better than the invader (Bownes and McQuaid 2006; Zardi et al. 2007; Nicastro et al. 2008). In Japan, Septifer virgatus (a native mussel) and Crassostrea gigas (Pacific Oyster) have been reportedly replaced in many habitats (Chavanich et al. 2010).

Mytilus galloprovincialis, like other mussels, is a major ecosystem engineer on rocky shores, so it competes not just with other attached bivalves, but competes for space with such shore species as limpets, anemones, and seaweeds. In southern California, dense settlements of mussels in the 1940s (probably M. galloprovincialis) were reportedly 'smothering' local fauna (Carlton 1979). In South Africa, M. galloprovincialis affects two species of native limpets (Scutellastra granularis, S. argenvillei) by depriving them of space for grazing on rocky shores (Branch and Steffani 2004; Griffiths et al. 2005; Branch et al. 2008). In Japan, M. galloprovincialis is reported to displace the barnacle Chthamalus challengeri, and the seaweed Sargassum fusiformis (Chavanich et al. 2010).

Habitat Change- Mytilus galloprovincialis is a major ecosystem engineer of shore habitats, occupying large spaces in the intertidal zone, but also creating a complex 3-dimensional environment, with much hard surface for attachment by other organisms, and sheltered niches where mobile organisms can hide. Where M. galloprovincialis has replaced M. trossulus, these impacts may have been small, but where M. galloprovincialis invasions have greatly increased the biomass, structure, and intertidal range of mussel beds, the impacts are significant. On the west coast of South Africa, while M. galloprovincialis has largely replaced the native mussel Aulacomya ater, small specimens of A. ater extend higher up in the intertidal zone, because of the shelter offered by the dense beds of M. galloprovincialis. Similarly, while large specimens of the limpet Scutellastra granularis are deprived of space for grazing, smaller specimens can utilize mussel shells as substrate, reach high densities, and increase larval output, in spite of being unable to reach full adult size. Another limpet, S. argenvillei, is displaced from the mussel beds because it cannot reach reproductive size there (Branch and Stefanni 2004; Branch et al. 2008). Overall, in this region, spreading beds of M. galloprovincialis increased the complexity of the habitat and also decreased the patchiness of the intertidal habitat (Robinson et al. 2007). In central and southern Japan, where no native Mytilus species were present, the invasion of M. galloprovincialis has resulted in the mass deposition of pseudofeces in the sediment, creating hypoxic conditions (Chavanich et al. 2010).

Food/Prey- The presence of M. galloprovincialis may increase food availability for predators in many invaded areas, since it tends to be larger than M. trossulus, and because it apparently reaches higher densities than many native mussels. One case is the threatened African Black Oystercatcher, Haematopus moquini, on the west coast of South Africa, which switched its diet to M. galloprovincialis and increased its brood size (Griffiths et al. 1992; Hockey and Van Erkom Schurink 1992).

Hybridization- Mytilus galloprovincialis has extensively hybridized with the northern Bay Mussel M. trossulus in the northeast and northwest Pacific, sometimes at very high frequencies (up to 70%). On the west coast of the US, hybrids are rare south of Monterey Bay, where M. trossulus is now virtually absent. The frequency of hybridization decreased from the 1990s to the 2000s, apparently due to cyclic ocean temperature changes (El Nino and the Pacific Decadal Oscillation) (McDonald et al. 1991; Sarver and Foltz 1993; Rawson et al. 1999; Braby and Somero 2006; Hilbish et al. 2010). North of California, hybridization was rarer, and more frequent near ports and mussel farms (Elliott et al. 2008; Shields et al. 2010). The hybrid zone in the northwest Pacific runs from the Vladivostok area, Russia, to northern Japan (Suchanek et al. 1997; Matsumasa et al. 1999; Skurikhina et al. 2004; Brannock and Hilbish 2010). Although rates of F1 (first-generation hybrids) are often high, the frequency of F2 hybrids is small, indicating that little genetic exchange is occurring between the two species (Rawson et al. 1999; Brannock and Hilbish 2010). This result could indicate poor survival or reduced fertility of hybrids, due to genetic incompatibility (Rawson et al. 1999).

In a recent survey of Mytilus populations in British Columbia, Crego-Prieto et al. (2015) found that M. galloprovincialis X M. trossulus hybrids were most abundant near mussel farms, and on open coasts, compared to estuaries. They suggest hybridization with M. trossulus should be limited by not pemitting farming of M. galloprovincialis north of its present range in the province, and by restricting farming to sheltered, estuarine waters (Crego-Prieto et al. 2015). Hybridization has also been observed between native Southern Hemisphere M. galloprovincialis and introduced Northeast Atlantic lineages at several locations near ports in New Zealand. This introgression is considered a threat to the genetic biodiversity native mussel populations, particularly on remote southern ocean islands (Gardner et al. 2016).

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Regional Impacts

NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Competition
In this bioregion, the native M. trossulus once was the sole mussel species. Based on field (Bodega Marine Laboratory, open coast) and lab experiments, Mytilus galloprovincialis displaces M. trossulus (Bay Mussel) and M. californianus (Sea Mussel) through interference competition (limiting movement, outfiltering, and overgrowing) with the native mussels as well as simply outgrowing them (Shinen and Morgan 2009). Physiological observations indicate that M. galloprovincialis is better adapted metabolically to warmer temperatures than M. trossulus, while M. trossulus is favored by cooler temperatures (Braby and Somero 2006; Lockwood and Somero 2011a). Mytilus trossulus is better adapted to low salinities (Lockwood and Somero 2011a). Consequently, climatic cycles such as the Pacific Decadal Oscillation and El Nino can cause cyclic fluctuations in the range, abundance, and hybridization of the two species (Braby and Somero 2006; Hilbish et al. 2010; Lockwood and Somero 2011a b).
P112	_CDA_P112 (Bodega Bay)		Ecological Impact	Competition
Based on field (Bodega Marine Laboratory, open coast) and lab experiments, Mytilus galloprovincialis displaces M. trossulus and M. californianus through interference competition (limiting movement, outfiltering, overgrowing) with the native mussels as well as simply outgrowing them (Shinen and Morgan 2009). In 1990-1991in Bodega Bay, purebred M. galloprovincialis comprised ~10% of total mussels sampled (Sarver and Foltz 1993), but ~60% in a 1994 survey (Rawson et al. 1999) and in 2002, at 2 sites, ~30-85% (Braby and Somero 2006).
P090	San Francisco Bay		Ecological Impact	Hybridization
Mytilus galloprovincialis was first detected in molecular surveys, begun in 1985 (McDonald et al. 1991). This bioregion is a hybrid zone for Mytilus galloprovincialis and M. trossulus. Since molecular studies began in the late 1980s and early 1990s, both species and hybrids have coexisted but have varied in abundance with changes and gradients of salinity and temperature. At different sites in the Bay, the frequency of M. galloprovincialis X M. trossulus hybrids has varied from ~0-40% (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999; Braby and Somero 2006), apparently changing both along environmental gradients, and fluctuating with cyclic changes in temperature and river flow.
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Hybridization
This species was first detected in molecular surveys which began in 1985 (McDonald et al. 1991). Central California is a hybrid zone for Mytilus galloprovincialis and M. trossulus. Since molecular studies began in the late 1980s and early 1990s, both species and their hybrids have coexisted but have varied in abundance with cyclic changes and gradients of salinity and temperature, with M. galloprovincialis prevailing at higher salinities and higher temperatures, while M. trossulus predominated at lower salinities, and at lower temperatures (Braby and Somero 2006). The fact that most detected hybrids are first-generation (F1) suggests little genetic exchange between populations and possible genetic incompatibility (Rawson et al. 1999). In earlier surveys, the hybrid zone extended north of Cape Mendocino (McDonald et al. 1991; Sarver and Foltz 1993; Rawson et al. 1999). In 2005-2007, the frequency of M. galloprovinicalis and hybrids, north of 39°N declined from ~15-70% to about 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
P080	Monterey Bay		Ecological Impact	Hybridization
In 1990-1994, 10-30% of mussels examined in Monterey Bay were hybrids (M. trossulus X M. galloprovincialis) outnumbering purebred M. trossulus (0-15%) (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999). In 2005-2007, the frequency of hybrids was about 20-35%, but the abundance of purebred M. galloprovincialis had declined, due to changing oceanographic conditions, resulting in lower water temperatures, and a southward shift in the hybrid zone (Braby and Somero 2006; Hilbish et al. 2010). In 2010, at Moss Landing, introgression of M galloprovincialis into M. trossulus exceeded gene flow in the reverse direction (Saarman and Pgson 2015).
P086	_CDA_P086 (San Francisco Coastal South)		Ecological Impact	Hybridization
~12% of mussels examined at Half Moon Bay were hybrids (M. trossulus X M. galloprovincialis), compared to 24% purebred M. trossulus (Rawson et al. 1999).
P095	_CDA_P095 (Tomales-Drakes Bay)		Ecological Impact	Hybridization
In 1994-1995, ~16% of mussels examined at Muir Beach were hybrids (M. trossulus X M. galloprovincialis) compared with 56% purebred M. trossulus (Rawson et al. 1999). Changing environmental conditions (declining water temperatures) due to climate cycles, have probably reduced the frequency of M. galloprovincialis (Braby and Somero 2006; Hilbish et al. 2010).
NEP-IV	Puget Sound to Northern California		Economic Impact	Fisheries
Mussel aquaculture was conducted in Humboldt Bay starting in 1992, but had ceased by 2008 (Conte 1992; California Department of Fish and Game 2010).
NEP-VI	Pt. Conception to Southern Baja California		Economic Impact	Fisheries
Mussel aquaculture has been conducted in San Diego Bay, Agua Hedionda Lagoon and the Santa Barbara Channel, consisting of growing mussels on ropes from former oil platforms (Conte 1992; California Department of Fish and Game 2010). Websites indicate that the Santa Barbara Channel and Agua Hedionda operations continue (http://www.sbmariculture.com/shellfish.html www.ediblecommunities.com/sandiego/pages/.../afishionados.pdf).
P065	_CDA_P065 (Santa Barbara Channel)		Economic Impact	Fisheries
Mussel (M. galloprovincialis) aquaculture has been conducted in San Diego Bay and the Santa Barbara Channel, involving growing mussels on ropes from former oil platforms (Conte 1992; California Department of Fish and Game 2010). Websites indicate that the Santa Barbara Channel operations continue (http://www.sbmariculture.com/shellfish.html).
NEP-V	Northern California to Mid Channel Islands		Economic Impact	Fisheries
Mussel aquaculture has been conducted in Tomales Bay (Conte 1992; California Department of Fish and Game 2010). Websites indicate that these operations continue (http://tomalesbayoysters.com/).
P110	Tomales Bay		Economic Impact	Fisheries
Mussel aquaculture has been conducted in Tomales Bay (Conte 1992; California Department of Fish and Game 2010). Websites indicate that these operations continue (http://tomalesbayoysters.com/).
NEP-VI	Pt. Conception to Southern Baja California		Ecological Impact	Competition
Mytilus trossulus (Bay Mussel) was present in southern California as indicated by the presence of mussels in prehistoric middens, and DNA testing of museum specimens (Carlton 1979; Geller 1999). In southern California, a rapid increase in mussel abundance in the 1940s (Coe 1946, cited by Carlton 1979), may have marked the invasion of M. galloprovincialis and the replacement of M. trossulus. Local observers cited dense settlements of mussels 'smothering' local fauna (Carlton 1979). In recent studies, M. trossulus and M. trossulus X M. galloprovincialis hybrids have been rare south of Point Conception (Sarver and Foltz 1993; Suchanek et al. 1997; Geller 1999; Rawson et al. 1999; Braby and Somero 2006).
NEP-VI	Pt. Conception to Southern Baja California		Ecological Impact	Hybridization
In recent sampling, hybrids between native M. trossulus and M. galloprovincialis are rare, but probably were much more abundant in initial stages of invasion. The fact that most detected hybrids are first-generation (F1) suggests little genetic exchange between populations and possible genetic incompatibility (Rawson et al. 1999). Genetic incompatibility between the two species could have sped up the decline of M. trossulus, once M. galloprovincialis became more abundant, due to wastage of gametes.
P020	San Diego Bay		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in San Diego Bay, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). See the discussion of the bioregion for details.
P020	San Diego Bay		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details.
P030	Mission Bay		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in Mission Bay, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). See the discussion of the bioregion for details.
P030	Mission Bay		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion of the bioregion for details.
P022	_CDA_P022 (San Diego)		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). At this time, very heavy settlement of Mytilus 'edulis' occurred at the La Jolla pier, and was accompanied by a decline in recruitment of M. californianus, the native Sea Mussel (Coe 1946, cited by Carlton 1979).
P022	_CDA_P022 (San Diego)		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion of the bioregion for details.
P023	_CDA_P023 (San Louis Rey-Escondido)		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997).
P023	_CDA_P023 (San Louis Rey-Escondido)		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion of the bioregion for details.
P040	Newport Bay		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). At this time, during a very heavy settlement of Mytilus 'edulis' in Newport Bay 'everything is smothered' (Roestler, in Burch 1946, cited by Carlton 1979).
P040	Newport Bay		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details.
P050	San Pedro Bay		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997).
P050	San Pedro Bay		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details.
P060	Santa Monica Bay		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997).
P060	Santa Monica Bay		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details.
P062	_CDA_P062 (Calleguas)		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997).
P062	_CDA_P062 (Calleguas)		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details.
P064	_CDA_P064 (Ventura)		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997).
P064	_CDA_P064 (Ventura)		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details.
P065	_CDA_P065 (Santa Barbara Channel)		Ecological Impact	Competition
Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997).
P065	_CDA_P065 (Santa Barbara Channel)		Ecological Impact	Hybridization
Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. In 2010, the Santata Barbara population was found to be pure M. galloprovincialis (Saarman and Pogson 2015).
P090	San Francisco Bay		Ecological Impact	Competition
Mytilus galloprovincialis was first detected in molecular surveys, begun in 1985 (McDonald et al. 1991). This bioregion is a hybrid zone for Mytilus galloprovincialis and M. trossulus. Since molecular studies began in the late 1980s and early 1990s, both species and hybrids have coexisted but have varied in abundance with changes and gradients of salinity and temperature with M. galloprovincialis prevailing at higher salinities and higher temperatures, while M. trossulus predominated at lower salinities, and at lower temperatures. These interactions were complex, so that M. trossulus outnumbered M. galloprovincialis at low salinity sites in San Francisco Bay, even though these sites had higher temperatures (Braby and Somero 2006).
P069	_CDA_P069 (Central Coastal)		Ecological Impact	Competition
Mytilus galloprovincialis continues to compete with and dominate native M. trossulus south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010). See the bioregion discussion for details.
P069	_CDA_P069 (Central Coastal)		Ecological Impact	Hybridization
Extensive hybridization between Mytilus galloprovincialis with native M. trossulus continues to occur south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010).
P073	_CDA_P073 (Central Coastal)		Ecological Impact	Competition
Mytilus galloprovincialis continues to compete with and dominate native M. trossulus south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010). See the bioregion discussion for details.
P073	_CDA_P073 (Central Coastal)		Ecological Impact	Hybridization
Extensive hybridization between Mytilus galloprovincialis with native M. trossulus continues to occur south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010).
P080	Monterey Bay		Ecological Impact	Competition
In 1990-1994, purebred M. galloprovincialis comprised 45-95% of mussels in Monterey Bay samples, while purebred M. trossulus were 0-20%, and hybrids were 10-30% (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999). However, by 2005-2007, purebred M. galloprovincialis declined to about 10-70% (Braby and Somero 2006; Hilbush et al. 2010), indicating that changing environmental conditions (declining water temperatures) due to climate cycles, were not favorable to M. galloprovincialis (Braby and Somero 2006; Hilbish et al. 2010).
P070	Morro Bay		Ecological Impact	Competition
Mytilus galloprovincialis continues to compete with and dominate native M. trossulus south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010). See the bioregion discussion for details.
P070	Morro Bay		Ecological Impact	Hybridization
Extensive hybridization between Mytilus galloprovincialis with native M. trossulus continues to occur south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010).
P086	_CDA_P086 (San Francisco Coastal South)		Ecological Impact	Competition
In 1994-95, 64% of mussels at Half Moon Bay were purebred M. galloprovincialis, while 24% were M. trossulus (Rawson et al. 1999). Changing environmental conditions (declining water temperatures) due to climate cycles, have reduced the frequency of M. galloprovincialis to ~35% (Hilbish et al. 2010).
P095	_CDA_P095 (Tomales-Drakes Bay)		Ecological Impact	Competition
In 1994-1995, about 32% of mussels examined at Muir Beach were purebred M. galloprovincialis, while 52% were M. trossulus (Rawson et al. 1999).
P110	Tomales Bay		Ecological Impact	Competition
In 1990-1991, ~90% of mussels sampled in Tomales Bay were M. galloprovincialis (Sarver and Foltz 1993; Suchanek et al. 1997), while only ~5-10% were M. trossulus. In 2002, 60% were M. galloprovincialis (Braby and Somero 2006). The continued dominance of the introduced mussel here, while declining in nearby estuaries (Hilbish et al. 2010), may have been a result of ongoing aquaculture operations.
P112	_CDA_P112 (Bodega Bay)		Ecological Impact	Hybridization
Mytilus galloprovincialis X M. trossulus hybrids comprised ~0-5% of total mussels sampled (Sarver and Foltz 1993), about 15% in a 1994 survey (Rawson et al. 1999) and in 2002, at 2 sites, ~10-15% (Braby and Somero 2006).
P114	_CDA_P114 (Gualala-Salmon)		Ecological Impact	Competition
In 1994-1995, at Gualala Point and Bodega Head, purebred M. galloprovinialis comprised 42-45% of the mussels sampled, compared to 40-44% of native M. trossulus (Rawson et al. 1999). Changing environmental conditions (declining water temperatures) due to climate cycles, have probably reduced the frequency of M. galloprovincialis (Braby and Somero 2006; Hilbish et al. 2010).
P114	_CDA_P114 (Gualala-Salmon)		Ecological Impact	Hybridization
Mytilus galloprovincialis X M. trossulus hybrids comprised 14% of total mussels sampled in a 1994-1995 survey (Rawson et al. 1999).
P116	_CDA_P116 (Big Navaro-Garcia)		Ecological Impact	Competition
In 1994-1995, purebred M. galloprovincialis comprised ~50% of the mussels sampled at Shelter Cove, compared to about ~30% for purebred M. trossulus (Rawson et al. 1999). However, the percentage decline to ~5% by 2005-2007, apparently because of lower temperatures resulting from the Pacific Decadal Oscillation, made M. galloprovincialis less competitive (Hilbish et al. 2010).
P116	_CDA_P116 (Big Navaro-Garcia)		Ecological Impact	Hybridization
In 1994-1995, M. galloprovincialis X M. trossulus hybrids comprised 12-20% of the mussels at Shelter Cove. In 2005-2007, the frequency of hybrids declined sharply, to about 1-2% (Hilbish et al. 2010), apparently due to lower temperatures resulting from the Pacific Decadal Oscillation, making M. galloprovincialis less competitive (Hilbish et al. 2010).
P117	_CDA_P117 (Mattole)		Ecological Impact	Competition
In 1994-1995, purebred M. galloprovincialis comprised ~74% of mussels sampled at Cape Mendocino, while purebred M. trossulus were 18% (Rawson et al. 1999). In 2005-2007, the frequency of M. galloprovincialis and hybrids declined from ~20-40% to about 5%. This change was attributed to lower temperatures resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
NEP-IV	Puget Sound to Northern California		Ecological Impact	Competition
In this bioregion, the native M. trossulus was once the sole mussel species. Sampling in Humboldt Bay in 2002 indicated a greatly increased frequency of M. galloprovincialis and hybrids (~30 + 30%) over that seen in the 1990s (0-8%) (Braby and Somero 2006). However, in 2005-2007, the frequency of M. galloprovincialis and hybrids north of 39°N declined to less than 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
NEP-IV	Puget Sound to Northern California		Ecological Impact	Hybridization
In surveys done in the 1990s, the frequency of M. galloprovincialis X M. trossulus hybrids was low (1-8%) (Sarver and Foltz 1993; Rawson et al. 1999), but in samples taken in 2002, hybrids were more common (~20%) (Braby and Somero 2006). However, in 2005-2007, the frequency of hybrids, north of 39°N declined to less than 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
P130	Humboldt Bay		Economic Impact	Fisheries
Mussel aquaculture has been conducted in Humboldt Bay, starting in 1992, but had ceased by 2008 (Conte 1992; California Department of Fish and Game 2010).
P130	Humboldt Bay		Ecological Impact	Competition
Sampling in Humboldt Bay in 2002 indicated a greatly increased frequency of M. galloprovincialis and hybrids (~30 + 30%) over that seen in the 1990s (0-8%) (Braby and Somero 2006). However, in 2005-2007, the frequency of M. galloprovincialis and hybrids north of 39°N declined to about 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
P130	Humboldt Bay		Ecological Impact	Hybridization
In surveys done in the 1990s, the frequency of M. galloprovincialis X M. trossulus hybrids was low (1-8%) (Sarver and Foltz 1993; Rawson et al. 1999), but in samples taken in 2002, hybrids were more common (~20%) (Braby and Somero 2006). However, in 2005-2007, the frequency of hybrids north of 39°N declined to less than 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
P117	_CDA_P117 (Mattole)		Ecological Impact	Hybridization
In 1994-1995, purebred M. galloprovincialis comprised ~74% of mussels sampled at Cape Mendicino, while purebred M. trossulus were 18% (Rawson et al. 1999). In 2005-2007, the frequency of M. galloprovincialis and hybrids declined from ~20-40% to about 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
CA	California		Ecological Impact	Competition
In this bioregion, the native M. trossulus once was the sole mussel species. Based on field (Bodega Marine Laboratory, open coast) and lab experiments, Mytilus galloprovincialis displaces M. trossulus (Bay Mussel) and M. californianus (Sea Mussel) through interference competition (limiting movement, outfiltering, and overgrowing) with the native mussels as well as simply outgrowing them (Shinen and Morgan 2009). Physiological observations indicate that M. galloprovincialis is better adapted metabolically to warmer temperatures than M. trossulus, while M. trossulus is favored by cooler temperatures (Braby and Somero 2006; Lockwood and Somero 2011a). Mytilus trossulus is better adapted to low salinities (Lockwood and Somero 2011a). Consequently, climatic cycles such as the Pacific Decadal Oscillation and El Nino can cause cyclic fluctuations in the range, abundance, and hybridization of the two species (Braby and Somero 2006; Hilbish et al. 2010; Lockwood and Somero 2011a b)., Competition with, and nearly complete replacement of, native M. trossulus occurred in San Diego Bay, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). See the discussion of the bioregion for details., Competition with, and nearly complete replacement of, native M. trossulus occurred in Mission Bay, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). See the discussion of the bioregion for details., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). At this time, very heavy settlement of Mytilus 'edulis' occurred at the La Jolla pier, and was accompanied by a decline in recruitment of M. californianus, the native Sea Mussel (Coe 1946, cited by Carlton 1979)., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997)., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997)., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997)., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997)., Mytilus galloprovincialis continues to compete with and dominate native M. trossulus south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010). See the bioregion discussion for details., Mytilus galloprovincialis continues to compete with and dominate native M. trossulus south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010). See the bioregion discussion for details., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997). At this time, during a very heavy settlement of Mytilus 'edulis' in Newport Bay 'everything is smothered' (Roestler, in Burch 1946, cited by Carlton 1979)., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997)., Competition with, and nearly complete replacement of, native M. trossulus occurred in southern California waters, possibly in the 1940s (Carlton 1979; McDonald et al. 1991; Suchanek et al. 1997)., In 1990-1994, purebred M. galloprovincialis comprised 45-95% of mussels in Monterey Bay samples, while purebred M. trossulus were 0-20%, and hybrids were 10-30% (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999). However, by 2005-2007, purebred M. galloprovincialis declined to about 10-70% (Braby and Somero 2006; Hilbush et al. 2010), indicating that changing environmental conditions (declining water temperatures) due to climate cycles, were not favorable to M. galloprovincialis (Braby and Somero 2006; Hilbish et al. 2010)., Mytilus galloprovincialis continues to compete with and dominate native M. trossulus south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010). See the bioregion discussion for details., In 1994-95, 64% of mussels at Half Moon Bay were purebred M. galloprovincialis, while 24% were M. trossulus (Rawson et al. 1999). Changing environmental conditions (declining water temperatures) due to climate cycles, have reduced the frequency of M. galloprovincialis to ~35% (Hilbish et al. 2010)., Mytilus galloprovincialis was first detected in molecular surveys, begun in 1985 (McDonald et al. 1991). This bioregion is a hybrid zone for Mytilus galloprovincialis and M. trossulus. Since molecular studies began in the late 1980s and early 1990s, both species and hybrids have coexisted but have varied in abundance with changes and gradients of salinity and temperature with M. galloprovincialis prevailing at higher salinities and higher temperatures, while M. trossulus predominated at lower salinities, and at lower temperatures. These interactions were complex, so that M. trossulus outnumbered M. galloprovincialis at low salinity sites in San Francisco Bay, even though these sites had higher temperatures (Braby and Somero 2006)., In 1994-1995, about 32% of mussels examined at Muir Beach were purebred M. galloprovincialis, while 52% were M. trossulus (Rawson et al. 1999)., In 1990-1991, ~90% of mussels sampled in Tomales Bay were M. galloprovincialis (Sarver and Foltz 1993; Suchanek et al. 1997), while only ~5-10% were M. trossulus. In 2002, 60% were M. galloprovincialis (Braby and Somero 2006). The continued dominance of the introduced mussel here, while declining in nearby estuaries (Hilbish et al. 2010), may have been a result of ongoing aquaculture operations. , Based on field (Bodega Marine Laboratory, open coast) and lab experiments, Mytilus galloprovincialis displaces M. trossulus and M. californianus through interference competition (limiting movement, outfiltering, overgrowing) with the native mussels as well as simply outgrowing them (Shinen and Morgan 2009). In 1990-1991in Bodega Bay, purebred M. galloprovincialis comprised ~10% of total mussels sampled (Sarver and Foltz 1993), but ~60% in a 1994 survey (Rawson et al. 1999) and in 2002, at 2 sites, ~30-85% (Braby and Somero 2006)., In 1994-1995, at Gualala Point and Bodega Head, purebred M. galloprovinialis comprised 42-45% of the mussels sampled, compared to 40-44% of native M. trossulus (Rawson et al. 1999). Changing environmental conditions (declining water temperatures) due to climate cycles, have probably reduced the frequency of M. galloprovincialis (Braby and Somero 2006; Hilbish et al. 2010)., In 1994-1995, purebred M. galloprovincialis comprised ~50% of the mussels sampled at Shelter Cove, compared to about ~30% for purebred M. trossulus (Rawson et al. 1999). However, the percentage decline to ~5% by 2005-2007, apparently because of lower temperatures resulting from the Pacific Decadal Oscillation, made M. galloprovincialis less competitive (Hilbish et al. 2010)., In 1994-1995, purebred M. galloprovincialis comprised ~74% of mussels sampled at Cape Mendocino, while purebred M. trossulus were 18% (Rawson et al. 1999). In 2005-2007, the frequency of M. galloprovincialis and hybrids declined from ~20-40% to about 5%. This change was attributed to lower temperatures resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010)., Sampling in Humboldt Bay in 2002 indicated a greatly increased frequency of M. galloprovincialis and hybrids (~30 + 30%) over that seen in the 1990s (0-8%) (Braby and Somero 2006). However, in 2005-2007, the frequency of M. galloprovincialis and hybrids north of 39°N declined to about 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
CA	California		Ecological Impact	Hybridization
This species was first detected in molecular surveys which began in 1985 (McDonald et al. 1991). Central California is a hybrid zone for Mytilus galloprovincialis and M. trossulus. Since molecular studies began in the late 1980s and early 1990s, both species and their hybrids have coexisted but have varied in abundance with cyclic changes and gradients of salinity and temperature, with M. galloprovincialis prevailing at higher salinities and higher temperatures, while M. trossulus predominated at lower salinities, and at lower temperatures (Braby and Somero 2006). The fact that most detected hybrids are first-generation (F1) suggests little genetic exchange between populations and possible genetic incompatibility (Rawson et al. 1999). In earlier surveys, the hybrid zone extended north of Cape Mendocino (McDonald et al. 1991; Sarver and Foltz 1993; Rawson et al. 1999). In 2005-2007, the frequency of M. galloprovinicalis and hybrids, north of 39°N declined from ~15-70% to about 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010)., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion of the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion of the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion of the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details., Extensive hybridization between Mytilus galloprovincialis with native M. trossulus continues to occur south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010)., Extensive hybridization between Mytilus galloprovincialis with native M. trossulus continues to occur south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010)., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. See the discussion for the bioregion for details., Hybridization between M. galloprovincialis and M. trossulus was probably widespread during the early invasion and replacement of M. trossulus. In 2010, the Santata Barbara population was found to be pure M. galloprovincialis (Saarman and Pogson 2015)., In 1990-1994, 10-30% of mussels examined in Monterey Bay were hybrids (M. trossulus X M. galloprovincialis) outnumbering purebred M. trossulus (0-15%) (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999). In 2005-2007, the frequency of hybrids was about 20-35%, but the abundance of purebred M. galloprovincialis had declined, due to changing oceanographic conditions, resulting in lower water temperatures, and a southward shift in the hybrid zone (Braby and Somero 2006; Hilbish et al. 2010). In 2010, at Moss Landing, introgression of M galloprovincialis into M. trossulus exceeded gene flow in the reverse direction (Saarman and Pgson 2015)., Extensive hybridization between Mytilus galloprovincialis with native M. trossulus continues to occur south of Monterey Bay (McDonald et al. 1991; Hilbish et al. 2010)., ~12% of mussels examined at Half Moon Bay were hybrids (M. trossulus X M. galloprovincialis), compared to 24% purebred M. trossulus (Rawson et al. 1999)., Mytilus galloprovincialis was first detected in molecular surveys, begun in 1985 (McDonald et al. 1991). This bioregion is a hybrid zone for Mytilus galloprovincialis and M. trossulus. Since molecular studies began in the late 1980s and early 1990s, both species and hybrids have coexisted but have varied in abundance with changes and gradients of salinity and temperature. At different sites in the Bay, the frequency of M. galloprovincialis X M. trossulus hybrids has varied from ~0-40% (Sarver and Foltz 1993; Suchanek et al. 1997; Rawson et al. 1999; Braby and Somero 2006), apparently changing both along environmental gradients, and fluctuating with cyclic changes in temperature and river flow., In 1994-1995, ~16% of mussels examined at Muir Beach were hybrids (M. trossulus X M. galloprovincialis) compared with 56% purebred M. trossulus (Rawson et al. 1999). Changing environmental conditions (declining water temperatures) due to climate cycles, have probably reduced the frequency of M. galloprovincialis (Braby and Somero 2006; Hilbish et al. 2010)., Mytilus galloprovincialis X M. trossulus hybrids comprised ~0-5% of total mussels sampled (Sarver and Foltz 1993), about 15% in a 1994 survey (Rawson et al. 1999) and in 2002, at 2 sites, ~10-15% (Braby and Somero 2006)., Mytilus galloprovincialis X M. trossulus hybrids comprised 14% of total mussels sampled in a 1994-1995 survey (Rawson et al. 1999)., In 1994-1995, M. galloprovincialis X M. trossulus hybrids comprised 12-20% of the mussels at Shelter Cove. In 2005-2007, the frequency of hybrids declined sharply, to about 1-2% (Hilbish et al. 2010), apparently due to lower temperatures resulting from the Pacific Decadal Oscillation, making M. galloprovincialis less competitive (Hilbish et al. 2010)., In 1994-1995, purebred M. galloprovincialis comprised ~74% of mussels sampled at Cape Mendicino, while purebred M. trossulus were 18% (Rawson et al. 1999). In 2005-2007, the frequency of M. galloprovincialis and hybrids declined from ~20-40% to about 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010)., In surveys done in the 1990s, the frequency of M. galloprovincialis X M. trossulus hybrids was low (1-8%) (Sarver and Foltz 1993; Rawson et al. 1999), but in samples taken in 2002, hybrids were more common (~20%) (Braby and Somero 2006). However, in 2005-2007, the frequency of hybrids north of 39°N declined to less than 5%. This change was attributed to lower temperatures, resulting from the Pacific Decadal Oscillation (Hilbish et al. 2010).
CA	California		Economic Impact	Fisheries
Mussel aquaculture has been conducted in Tomales Bay (Conte 1992; California Department of Fish and Game 2010). Websites indicate that these operations continue (http://tomalesbayoysters.com/)., Mussel (M. galloprovincialis) aquaculture has been conducted in San Diego Bay and the Santa Barbara Channel, involving growing mussels on ropes from former oil platforms (Conte 1992; California Department of Fish and Game 2010). Websites indicate that the Santa Barbara Channel operations continue (http://www.sbmariculture.com/shellfish.html)., Mussel aquaculture has been conducted in Tomales Bay (Conte 1992; California Department of Fish and Game 2010). Websites indicate that these operations continue (http://tomalesbayoysters.com/)., Mussel aquaculture has been conducted in Humboldt Bay, starting in 1992, but had ceased by 2008 (Conte 1992; California Department of Fish and Game 2010).

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