Mytilus edulis

Invasion History

First Non-native North American Tidal Record: 1992
First Non-native West Coast Tidal Record: 1992
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Mytilus edulis has long been treated as a single species, naturally distributed through the temperate regions of the Northern and Southern Hemispheres. However, taxonomists have identified geographical subspecies or full species, based on subtle morphological features (McDonald et al. 1991). The use of molecular techniques, initially amino-acid electrophoresis, and later DNA assays, has enabled biologists to define the ranges of the three major species of the complex, M. edulis, described by Linneaus from Sweden; M. galloprovincialis (Lamarck 1819), described from the Mediterranean; and M. trossulus (Gould 1850), described from Oregon. These techniques have also enabled researchers to document cryptic invasions and hybridization among members of the M. edulis complex (McDonald et al. 1991; Heath et al. 1995; Hilbish et al. 2000; Rawson et al. 2001; Shields et al. 2008; Shields et al. 2010).

Mytilus edulis, as currently defined, is native to the North Atlantic from Newfoundland, Iceland, northern Norway, and the White Sea, south to Cape Hatteras, North Carolina (NC) and the Bay of Biscay, at the French-Spanish border (Hutchins 1947; McDonald et al. 1991; Hilbush et al. 2000; Jones et al. 2008). In 2004, a small, isolated population was discovered in Svalbard (78N), apparently recently transported by the West Spitsbergen Current. This mussel had been absent on Svalbard for at least 1000 years (Berge et al. 2005). At the northern and Baltic borders, M. edulis co-occurs and hybridizes with M. trossulus, the high-latitude Atlantic-Pacific-Baltic species (McDonald et al. 1991; Hilbush et al. 2000; Rawson et al. 2001; Smietanka et al. 2004.). Where the two species co-occur, M. trossulus ranges further into brackish water, apparently based on greater larval tolerance to low salinity (Qiu et al. 2002).  In the east Atlantic, at its sorthern border in Portugal, it hybridizes with M. galloprovincialis (McDonald et al. 1991; Hilbush et al. 2000). In the west Atlantic, the traditional limit for Mytilus edulis has been Cape Hatteras, a major biogeographical boundary (Hutchins 1947), but occasional settlement has been seen as far south as Beaufort, NC, facilitated by upwellings of cool water (Wells and Gray 1960). Jones et al. (2009) did temperature tolerance experiments that suggest that survival at Beaufort and Oregon Inlet would be poor, and predict that the southern limit of M. edulis is shifting northward. Mytilus edulis has been introduced to British Columbia, in aquaculture operations (Heath et al. 1995; Shields et al. 2008; Shields et al. 2010), and to Chile, Australia, and New Zealand by ballast water, fouling, or aquaculture (Westfall and Gardner 2010). Most specimens identified during genetic surveys have been hybrids, either with native M. trossulus, with the presumably native Southern Hemisphere genotype of M. galloprovincialis (in Chile, Australia, and New Zealand), or the introduced Northern Hemisphere form of M. galloprovincialis (Heath et al. 1995; Shields et al. 2008; Shields et al. 2010; Westfall and Gardner 2010). However, a small percentage of Mytilus surveyed in British Columbia were identified as purebred M. edulis (Shields et al. 2010). Purebred M. edulis have been identified from the Atlantic coast of Argentina, the Falkland Islands, and southern Chile (McDonald et al. 1991; Oyarzun et al. 2016). Oyarzun et al. (2016) consider the southwest Atlantic populations as native, but we treat them as cryptogenic.

North American Invasion History:

Invasion History on the West Coast:

Early (pre-1990s) references to 'M. edulis' on the West Coast mostly refer to the native M. trossulus, although introductions of varieties or subspecies of the widely defined species were frequently suspected (Carlton 1979). Attempts at cultivating M. trossulus in Canada have been unsuccessful, because of its low meat yield, fragile shells, and in British Columbia high, mortality in culture (Heath et al. 1995; Rawson 2001). Importation of M. edulis was suggested as a solution to these problems (Jamieson 1989, cited by Heath et al. 1995). Mytilus edulis was imported from Prince Edward Island in 1992, raised for one generation in closed systems, and the F2 generation was stocked in a mussel farm in Genoa Bay, Vancouver Island. Mussels derived from this stock are/were being raised in at least four aquaculture operations in the Strait of Georgia (Centre for Shellfish Research, Vancouver Island University, http://www.viu.ca/csr/healthandhusbandry/Genetics.asp). Genotypes of 'alien mussels' and hybrids with M. trossulus were detected in British Columbia waters by Heath et al. (1995), although they could not distinguish between M. edulis and M. galloprovincialis. Shields et al. (2008) found that 8 of 292 animals sampled at Ladysmith, Vancouver Island, in 2006 were M. galloprovincialis X M. edulis hybrids, although no purebred animals were found. Using a different locus, on mussels sampled in 2006 at Ladysmith, 2% of animals at Ladysmith were purebred M. edulis, 2% were M. edulis X M. trossulus, and 4% were M. edulis X M. galloprovincialis (Shields et al. 2010). Additional M. trossulus X M. edulis hybrids were found on floats in Burrard Inlet, Vancouver Harbour in 2004 (Richoux et al. 2006). While these numbers are quite small, they suggest that M. edulis genotypes are likely to persist in the population.

Invasion History Elsewhere in the World:

Mytilus edulis has been introduced to several locations in the Southern Hemisphere, where they have hybridized with a presumably native form of M. galloprovincialis, which probably colonized the region in prehistoric times (Hilbish et al. 2000; Westfall et al. 2010), and with the more recently introduced M. galloprovincialis, bearing Northern Hemisphere genotypes (Westfall et al. 2010). Purebred M. edulis have been found in the southwest Atlantic, in Argentina, the Falkland Islands, and southern Chile (McDonald et al. 1991; Oyarzun et al. 2016). A high frequency (15-95%) of M. edulis was found in the eastern portion of the Strait of Magellan (Oyarzun et al. 2016). Purebred M. edulis have not yet been found in the Southern Hemisphere, but hybrid mussels with M. edulis markers have been found in bays in Chile, Australia (Port Phillip Bay and Tasmania), and at several sites in New Zealand, including the relatively isolated Auckland Islands, located southeast of the main islands. These mussels were sampled in 1994-2010 (Westfall et al. 2010), but the dates of introduction of M. edulis to these regions are not known. It is likely that shipping (both fouling and ballast water) and aquaculture have transported M. edulis to the Southern Hemisphere. In Chile, mussel aquaculture is especially important, while in the case of the Auckland Islands, 19th century whaling ships are a likely vector, since travel to these islands (now a conservation reserve) has been restricted in recent decades (Westfall et al. 2010). However, a recent analysis using Single Nucleotide Polymorphisms (SNPs) indicates that the 'edulis-like' genotypes represent native cryptic species, M. platensis in the Southwest Atlantic, M. planulatus, and M. aoteanus in islands south of New Zealand. Mytilus spp. in the Falkland Islands and Kerguelen were a mixture of M. platensis and M. chilensis. In this view, invasions of Northern Hemisphere Mytilus are much less extensive than reported in earlier papers (Zbawicka et al. 2019).

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Description

Mytilus edulis has a smooth shell which is 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 brown, while the interior is white, with a violet margin, and a distinct muscle scar. The shell can reach a length (height) of 100 mm (Abbott 1974; Morris 1975; Gosner 1978). Like other mussels, M. edulis clings to hard surfaces (rocks, logs, structures) by a cluster of byssus threads.

Mytilus edulis is part of the M. edulis species complex, which includes M. galloprovincialis, M. trossulus, and possibly other species (e.g. M. chilensis, M. desolationis) whose distinctness is disputed (Westfall and Gardner 2010). 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 201; Zbawicka et al. 2019).

Larvae of M. edulis are described by Chanley and Andrews (1971). Late veligers are roughly egg-shaped, with a prominent eye-spot. Settlement occurs at 215-305 μm (Chanley and Andrews 1971). 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 edulis has separate sexes and individuals mature at one year of age or less. This species has a prolonged spawning season, and in some habitats spawns year round (Seed 1969). Fertilized eggs develop into a planktonic trochophore larva, then into a shelled veliger. The larvae settle at 215-305 μm (Chanley and Andrews 1971). Larvae of Mytilus edulis 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 by byssus threads, creating a complex habitat (Bertness 1999; Buschbaum et al. 2009). Mussels are strong-filter feeders, and create substantial currents as they pump in water to ingest 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).

Blue Mussels are characteristic of the shallow subtidal and intertidal zone, 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. edulis can tolerate salinities of 10 PSU, but require at least 20 PSU for successful larval development (Hutchins 1947; Qiu et al. 2002; Jones et al. 2009). In experiments simulating trasport in ship fouling, 60-70% survived exposure to freshwater, for up to 14 days.  Freshwater survival was much, o.5 to 60% at 15-25 °C (Riley et ql. 2022).

Food:

phytoplankton, detritus

Consumers:

crabs, snails, fishes, birds, humans

Competitors:

barnacles, tunicates, bryozoans, seaweeds

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Grass Bed	None
General Habitat	Coarse Woody Debris	None
General Habitat	Oyster Reef	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
Tidal Range	High Intertidal	None
Vertical Habitat	Epibenthic	None

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Life History

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	-1.8	Based on geographical range, occurrence in areas with sea ice
Maximum Temperature (ºC)	35	Water temperature, experimental, Jones et al. 2009.
Minimum Salinity (‰)	10	Experimental, Qiu et al. 2002.. In simulated voyages, using animals from the Netherlands, M. edulis had a high tolerance at 10.5-36 PSU (van der Gaag et al. 2016), with 1052 days for 100% mortality.
Maximum Salinity (‰)	35	Typical Atlantic Ocean salinity, probably tolerates higher salinities.
Minimum Reproductive Temperature	16	Onset of spawning, Gulf of St. Lawrence, Myrand et al. 2000
Maximum Reproductive Temperature	12	Larval development was unsuccessful at 6 C, but was completed at 12 C (Sprung 1984)
Minimum Reproductive Salinity	20	Experimental, development of eggs and larvae, Qiu et al. 2002
Maximum Reproductive Salinity	35	Typical Atlantic Ocean salinity, probably tolerates higher salinities
Minimum Duration	20	Lab, 18 C, Sprung 1984
Maximum Duration	30	Lab, 12 C, Sprung 1984
Minimum Length (mm)	9	based on field growth data, England, and spawning at 1 year of age (Seed 1969)
Maximum Length (mm)	100	Maximum adult length, Lippson and Lippson 1997
Broad Temperature Range	None	Cold temperate-Warm temperate
Broad Salinity Range	None	Mesohaline-Euhaline

General Impacts

In its native range, Mytilus edulis is ecologically and economically important, as a major ecosystem engineer, a major component of coastal food webs, and an important food resource (Bertness 1999; Buschbaum et al. 2009; Shields et al. 2010; Centre for Shellfish Research, Vancouver Island University, undated). However, as far as we know, M. edulis has become established in wild habitats, mostly as a hybrid with a low genotypic frequency, with native mussels (M. trossulus and the Southern Hemisphere genotype of M. galloprovincialis), or with introduced M. galloprovincialis (Shields et al. 2008; Shields et al. 2010; Westfall and Gardner 2010).

Economic Impacts

Fisheries- Mytilus edulis are being extensively cultured in open waters in The Strait of Georgia, British Columbia, where they are considered superior to the native M. trossulus, having higher meat yields, less fragile shells, and reduced likelihood of summer mortality (Rawson 2001; Centre for Shellfish Research, Vancouver Island University, undated). So far, purebred M. edulis have been found in the wild only at one location, and the frequency of hybrids is low, so the effect on 'natural' mussel populations appears to be small (Richoux et al. 2006; Shields et al. 2008; Shields et al. 2010).

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Regional Impacts

NEP-III	Alaskan panhandle to N. of Puget Sound		Economic Impact	Fisheries
Mytilus edulis is actively cultured in open water on a large scale in British Columbia waters (Shields et al. 2008; Shields et al. 2010).

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