Mytilus edulis

Researched ByLizzie TylerData Supplied ByUniversity of Sheffield
Refereed byThis information is not refereed.
Taxonomy
Scientific nameMytilus edulisCommon nameCommon mussel
MCS CodeW1695Recent SynonymsNone
PhylumMolluscaSubphylum
SuperclassClassPelecypoda
SubclassOrderMytiloida
SuborderFamilyMytilidae
GenusMytilusSpeciesedulis
Subspecies  
Additional InformationMytilus edulis and Mytilus galloprovincialis often occur in the same location in the northern range of Mytilus galloprovincialis. As they both show great variation in shell shape due to environmental conditions (Seed, 1968, 1992), they are often difficult to distinguish. In addition, they may hybridize. However, in Mytilus galloprovincialis:
  • the umbones turn down, giving the basal line of the shell a concave appearance;
  • the valves are higher and less angular;
  • the mantle edges are darker, becoming blue or purple, and
  • Mytilus galloprovincialis tends to grow larger (Tebble, 1976).
Note no single morphological characteristic can be used to separate Mytilus species (Gosling, 1992c; Seed, 1992, 1995). Recent evidence suggests that there are only three lineages of the genus, Mytilus edulis, Mytilus galloprovincialis and Mytilus trossulus, although some authorities suggest that all of the smooth shelled mussels belong to the same species (for discussion see Seed, 1992).
Taxonomy References Fish & Fish, 1996, Hayward & Ryland, 1990, Hayward et al., 1996, Tebble, 1976, Gosling, 1992(c), Bayne, 1976b, Clay, 1967(d), Gosling, 1992(a), Seed, 1992, Seed, 1995, Seed, 1968
General Biology
Growth formBivalvedFeeding methodPassive suspension feeder, Active suspension feeder
Mobility/MovementTemporary attachment, Permanent attachmentEnvironmental positionEpilithic
Typical food typesBacteria, phytoplankton, detritus, and dissolved organic matter (DOM).HabitAttached
BioturbatorNot relevantFlexibilityNone (< 10 degrees)
FragilityIntermediateSizeMedium(11-20 cm)
HeightInsufficient informationGrowth RateSee additional information.
Adult dispersal potential<10mDependencyIndependent
SociabilityGregarious
Toxic/Poisonous?No
Additional InformationMytilus edulis is one of the most extensively studied marine organisms. Therefore, this review is based on comprehensive reviews by Gosling (ed.) (1992a), Bayne, (1976b), Newell (1989), and Holt et al. (1998). Where appropriate the original source references in these reviews are given.

Mytilus edulis is gregarious, and at high densities forms dense beds of one or more (up to 5 or 6) layers, with individuals bound together by byssus threads. Young mussels colonize spaces within the bed increasing the spatial complexity, and the bed provides numerous niches for other organisms (see importance). Overcrowding results in mortality as underlying mussels are starved or suffocated by the accumulation of silt, faeces and pseudofaeces, especially in rapidly growing populations (Richardson & Seed, 1990). Death of underlying individuals may detach the mussel bed from the substratum, leaving the bed vulnerable to tidal scour and wave action (Seed & Suchanek, 1992).

Growth rates
Growth rates in Mytilus spp. are highly variable. Part of this variation is explained by genotype and multilocus heterozygosity (Gosling, 1992b) but the majority of variation is probably environmentally determined. The following factors affect growth rates in Mytilus spp. Several factors may work together, depending on location and environmental conditions (Seed & Suchanek, 1992) or the presence of contaminants (see sensitivity, e.g. Thompson et al., 2000):

  • temperature;
  • salinity;
  • food availability;
  • tidal exposure;
  • intraspecific competition for space and food, and
  • parasitism.
For example, in optimal conditions Mytilus edulis can grow to 60 -80mm in length within 2 years but in the high intertidal growth is significantly lower, and mussels may take 15 -20 years to reach 20 -30mm in length (Seed & Suchanek, 1992). Bayne et al. (1976) demonstrated that between 10-20 °C water temperature had little effect on scope for growth. Latitudinal variation in temperature influences shell structure in Mytilus species (Carter & Seed, 1998).

Predation and mortality
Several factors contribute to mortality and the dynamics of Mytilus edulis populations, including temperature, desiccation, storms and wave action, siltation and biodeposits, intra- and interspecific competition, and predation. But predation is the single most important source of mortality.
Many predators target specific sizes of mussels and, therefore, influence population size structure. The vulnerability of mussels decreases as they grow, since they can grow larger than their predators preferred size. Mytilus sp. may be preyed upon by neogastropods such as Nucella lapillus, starfish such as Asterias rubens, the sea urchin Strongylocentrotus droebachiensis, crabs such as Carcinus maenas and Cancer pagurus, fish such as Platichthys flesus (plaice), Pleuronectes platessa (flounder) and Limanda limanda (dab), and birds such as oystercatcher, eider, scooter, sandpiper, knot, turnstone, gulls and crows (Seed & Suchanek, 1992; Seed, 1993). Important predators are listed below.
  1. Dogwhelks (Nucella lapillus) feed on mussels on the mid to lower rocky shore primarily in spring and summer, and are capable of removing 0.1-0.6 mussels/ whelk/ day. Dogwhelk predation is curtailed by periods of strong wave action or desiccation. Mytilus edulis can defend itself from predatory gastropods, several mussels working together to immobilise the gastropod with byssus threads (Seed & Suchanek, 1992).
  2. Flounders were found to be important predators in Morecambe Bay and Liverpool Docks, as were plaice and dabs in Morecambe Bay (Dare, 1976; Holt et al., 1998).
  3. Asterias rubens usually feeds at low densities in the lower shore or sublittoral in northern Europe preferring large, up to 70mm, mussels. Asterias rubens may periodically, and unpredictably, rise dramatically in number forming swarms in the lower and middle shore, denuding the extensive areas of Mytilus sp. (Seed, 1969). For example, Dare (1976; 1982b) recorded a swarm of Asterias rubens in Morecambe Bay consisting of 450 starfish /m² that covered up to 2.25ha and consumed 4000 tonnes of first year mussels.
  4. Crab predation is most intense on the lower shore and sublittoral, with crabs selecting mussels up to around 70mm. Small mussels are especially vulnerable since they can be crushed by all sizes of crabs. Vulnerability to crab predation decreases with increasing mussel size (Seed & Suchanek, 1992).
  5. Oystercatchers and eider duck consume large numbers of mussels, primarily over winter. Raffaelli et al. (1990) recorded the removal of 4500 mussels /m² (within the preferred size of 10-25mm) within 60 days by a flock of 500 eider in the Ythan estuary. Eider remove mussels in clumps, which they shake to remove the target mussel. This results in additional mortality for those mussels removed from suitable substratum in the clump and leaves bare patches in the mussel beds, which may increase the risk of the loss of further mussels by water movement. Eider may, therefore, significantly affect the structure of the mussel bed (Seed & Suchanek, 1992; Holt et al., 1998). Mussels are often the primary food for oystercatchers on sedimentary shores and mussel density may limit oystercatcher numbers in certain areas (Craeymeersch et al., 1986). In enclosure experiments clumps of mussels only established in protected enclosures, suggesting that bird predation significantly reduced juvenile recruitment (Marsh, 1986).
  6. Bird predation has a significant effect on mussel productivity (Holt et al., 1998). For example, in the Ythan estuary, bird predation (eider, oystercatcher and herring gull) accounted for 72% of the annual Mytilus edulis production (Raffaelli et al., 1990), and in the Wadden Sea, oystercatchers consumed 40% of the annual mussel production (Meire & Ervynck, 1986; Holt et al., 1998).
Epifauna and epiflora
Fouling organisms, e.g. barnacles and seaweeds, may also increase mussel mortality by increasing weight and drag, resulting in an increased risk of removal by wave action and tidal scour. Fouling organisms may also restrict feeding currents and lower the fitness of individual mussels. However, Mytilus edulis is able to sweep its prehensile foot over the dorsal part of the shell (Thiesen, 1972, Seed & Suchanek, 1992). Fouling by ascidians may be a problem in rope-cultured mussels (Seed & Suchanek, 1992).

Diseases and parasites
The polychaete Polydora ciliata may burrow into the shell of Mytilus edulis, which weakens the shell leaving individuals more susceptible to predation by birds and shore crabs resulting in significant mortality, especially in mussels >6 cm (Holt et al., 1998).
Bower (1992), concluded that, although most parasites did not cause significant mortality, several species of parasite found in mussels could also infect and cause mortality in other shellfish. This suggested that mussel populations may be reservoirs of disease for other shellfish (see sensitivity or reviews by Bower, 1992; Bower & McGladdery, 1996).

Public heath
Mytilus edulis is a filter feeding organism, which collects algae, detritus and organic material for food but also filters out other contaminants in the process. Shumway (1992) noted that mussels are likely to serve as vectors for any water-borne disease or contaminant. Mussels have been reported to accumulate faecal and pathogenic bacteria and viruses, and toxins from toxic algal blooms (see Shumway, 1992 for review). Bacteria may be removed or significantly reduced by depuration (removing contaminated mussels into clean water), although outbreaks of diseases have resulted from poor depuration and viruses may not be removed by depuration. Recent improvements in waste water treatment and shellfish water quality regulations may reduce the risk of bacterial and viral contamination. Shellfish should also be thoroughly cooked, not 'quick steamed', to ensure destruction of viruses (Shumway, 1992). The accumulation of toxins from toxic algal blooms may result in paralytic shellfish poisoning (PSP), diarrhetic shellfish poisoning (DSP) or amnesic shellfish poisoning (ASP). These toxins are not destroyed by cooking. Shumway (1992) suggested that mussels should only be collected from areas routinely monitored by public health agencies, or obtained from approved sources and never harvested from waters contaminated with raw sewerage.
Biology References Fish & Fish, 1996, Hayward & Ryland, 1990, Hayward et al., 1996, Tebble, 1976, Gosling, 1992(c), Bayne, 1976b, Seed & Suchanek, 1992, Gosling, 1992(b), Bayne et al., 1976, Dare, 1976, Dare, 1982(b), Raffaelli et al., 1990, Craeymeersch et al., 1986, Marsh, 1986, Richardson & Seed, 1990, Bower, 1992, Bower & McGladdery, 1996, Shumway, 1992, Holt et al., 1998, Baird, 1966, Meire & Ervynck, 1986, Thiesen, 1972, Clay, 1967(d), Gosling, 1992(a), Gray et al., 1997, Carter & Seed, 1998, Seed, 1968, Thompson et al., 2000, Seed 1993, Seed, 1969a, Ambariyanto & Seed, 1991, Hayward & Ryland, 1990, Heidi Tillin, unpub data, Julie Bremner, unpub data
Distribution and Habitat
Distribution in Britain & IrelandVery common all around the coast of the British Isles, with large commercial beds in the Wash, Morecambe Bay, Conway Bay and the estuaries of south-west England, north Wales, and west Scotland.
Global distributionOccurs from the White Sea, south to southern France in the N.E. Atlantic. In the W. Atlantic it extends from the Canadian Maritimes south to North Carolina. It occurs on the coasts of Chile, Argentina, the Falkland Islands and the Kerguelen Isles.
Biogeographic rangeNot researchedDepth range
MigratoryNon-migratory / Resident
Distribution Additional InformationGlobal distribution
Previous records of Mytilus edulis on north African coasts, and in the Mediterranean were probably Mytilus galloprovincialis and Mytilus edulis is absent from the Pacific coast of North America (Gosling, 1992c; Seed, 1992). Previous records of Mytilus edulis from the Pacific coast of North America were probably Mytilus trossulus and/or Mytilus galloprovincialis (Seed, 1992; Seed pers comm.). Mytilus edulis has been reported from Iceland (Varvio et al., 1988). Mytilus edulis occurs on the east and west coasts of South America, and the Falkland Islands (Seed, 1992). Records of mussels form the Kerguelen Islands may be Mytilus edulis (MacDonald et al., 1992; Gosling, 1992c; Seed, 1992).

Factors affecting zonation
Although sometimes abundant in the subtidal Mytilus edulis is primarily an intertidal species. Mytilus edulis can withstand extreme wave exposure, maintaining byssal attachment in high energy environments (Seed & Suchanek, 1992). The upper limit of Mytilus edulis populations on rocky shores is determined by its tolerance of temperature and desiccation, which may be synergistic, i.e. sudden mass mortalities at the upper limit of intertidal mussel beds are often associated with prolonged periods of unusually high temperatures and desiccation stress (Seed & Suchanek, 1992). Recruitment or movement into cracks, crevices or pools provides some protection from extremes of temperature and desiccation as well as from storms. Mytilus edulis is relatively tolerant of extreme cold and freezing, surviving a drop in tissue temperature to minus 10 °C (Williams, 1970). However, Bourget (1983) noted that cyclic exposures to sublethal temperatures e.g. minus 8 °C every 12.4hrs resulted in death after 3-4 cycles. This suggests that Mytilus edulis can survive occasional, sharp frost events, but may succumb to consistent very low temperatures over a few days (see sensitivity to temperature change).

Mytilus edulis is generally unable to maintain attachment to steep or vertical rock surfaces, where they are typically replaced by barnacles and fucoids. Cycles of loss and recruitment may result in a patchy distribution of clumps of mussels on the shore.
The lower limit of distribution is strongly influenced by predation, primarily from starfish but also dog whelks and crabs. For example, on the east coast of England, the starfish Asterias rubens and the dog whelk Nucella lapillus eliminate mussels from the lower intertidal (Seed, 1969). In Ireland, however, the lower limit is probably controlled by the crabs Carcinus sp. and Liocarcinus sp., the dog whelk Nucella lapillus and the starfish Marthasterias glacialis (Kitching & Ebling, 1967).
Daly & Mathieson (1977) reported that the lower limit of Mytilus edulis populations at Bound Rock, USA, was determined by burial or abrasion by shifting sands. Burial or abrasion is probably an additional controlling factor on British coasts where mobile sediment, such as sand, cobbles or boulders, occur (Holt et al., 1998).
Subtidal populations often occur on sea mounts, dock pilings and offshore oil platforms, where they grow to a large size, probably due to the lack of predators (Seed & Suchanek, 1992).
Substratum preferencesBedrock, Large to very large boulders, Sandy mud, Muddy gravel, Rockpools, Biogenic reef, Artificial (e.g. metal/wood/concrete), Mixed, Muddy sand, Small boulders, Under boulders, Caves, Crevices / fissuresPhysiographic preferencesOpen coast, Strait / sound, Sealoch, Ria / Voe, Estuary, Enclosed coast / Embayment
Biological zoneUpper Eulittoral, Mid Eulittoral, Lower Eulittoral, Sublittoral Fringe, Upper InfralittoralWave exposureVery Exposed, Exposed, Moderately Exposed, Sheltered, Very Sheltered
Tidal stream strength/Water flowStrong (3-6 kn), Moderately Strong (1-3 kn), Weak (<1 kn)SalinityReduced (18-30 psu), Variable (18-40 psu), Full (30-40 psu)
Habitat Additional Information
AMBI Group (Borja et al., 2000)III 
Distribution References Fish & Fish, 1996, Hayward & Ryland, 1990, Hayward et al., 1996, Tebble, 1976, Gosling, 1992(c), Varvio et al., 1988, MacDonald et al., 1992, Bayne, 1976b, Seed & Suchanek, 1992, Williams, 1970, Bourget, 1983, Almada-Villela et al., 1982, Seed, 1969b, Kitching & Ebling, 1967, Holt et al., 1998, Daly & Mathieson, 1977, Suchanek, 1978, Almada-Villela, 1984, Clay, 1967(d), Gosling, 1992(a), Seed, 1992, Seed, 1995, Gray et al., 1997, Carter & Seed, 1998, Suchanek, 1985, Hayward & Ryland, 1990
Reproduction/Life History
Reproductive typeGonochoristic Developmental mechanismPlanktotrophic
Reproductive SeasonApril to SeptemberReproductive LocationWater column
Reproductive frequencyAnnual protracted Regeneration potentialNo
Life span21-50 yearsAge at reproductive maturity1-2 years
Generation time1-2 yearsFecundity
Egg/propagule size75 µm diameterFertilization typeExternal
Larvae/Juveniles
Larval/Juvenile dispersal potential>10kmLarval settlement periodSee additional information
Duration of larval stage1-6 months  
Additional InformationLife span
Longevity is dependant on locality and habitat. On the lower shore, few individuals probably survive more than 2-3 years due to intense predation, whereas high shore populations are composed of numerous year classes (Seed, 1969b). Specimens have been reported to reach 18-24 years of age (Thiesen, 1973). Mortality is size dependant and can be high, e.g. Dare (1976) reported annual mortalities of 74% in 25mm mussels and 98% in 50 mm mussels in Morecambe Bay, England.

Spawning
Spawning is protracted in many populations, with a peak of spawning in spring and summer. For example, in north east England, resting gonads begin to develop from October to November, gametogenesis occurring throughout winter so that gonads are ripe in early spring. A partial spawning in spring is followed by rapid gametogenesis, gonads ripening by early summer, resulting in a less intensive secondary spawning in summer to late August or September (Seed, 1969a). Mantle tissues store nutrient reserves between August and October, ready for gametogenesis in winter when food is scarce (Seed & Suchanek, 1992). Larvae spawned in spring can take advantage of the phytoplankton bloom. The secondary spawning, is opportunistic, depending on favourable environmental conditions and food availability. Gametogenesis and spawning varies with geographic location, e.g. southern populations often spawn before more northern populations (Seed & Suchanek, 1992). Reproductive strategies in Mytilus edulis probably vary depending on environmental conditions (Newell et al., 1982).

Fertilization
Fertilization is external. Fertilization can occur successfully between 5 -22°C and at salinities of 15 -40psu (Bayne, 1965; Lutz & Kennish, 1992). Fertilized eggs are 60-90µm in diameter (Lutz & Kennish, 1992).

Fecundity
Fecundity and reproductive effort increase with age and size, young mussels diverting energy to rapid growth rather than reproduction. Reproductive output is influenced by temperature, food availability and tidal exposure and can therefore vary from year to year. An individual female (ca 7mm) can produce 7-8 million eggs, while larger individuals may produce as many as 40 million eggs (Thompson, 1979).

Larval development
In optimal conditions larval development may be complete in less than 20 days but growth and metamorphosis in the plankton between spring and early summer, at ca. 10 °C, usually takes 1 month. However, it is not unusual for planktonic life to extend beyond 2 months in the absence of suitable substrata or optimal conditions (Bayne, 1965; Bayne, 1976a). Pediveligers can delay metamorphosis for up to 40 days at 10 °C (Lutz & Kennish, 1992) or for up to 6 months in some cases (Lane et al., 1985). The duration of the delay is mainly determined by temperature, with longer delays at low temperature (Strathmann, 1987). Larvae become less selective of substrata the longer metamorphosis is delayed.

Settlement
In many populations Mytilus edulis exhibits a two stage settlement, the pediveliger settling on filamentous substrates and then moving on to suitable adult substrata by bysso-pelagic drifting. However, McGrath et al. (1988) and King et al. (1990) found little evidence of bysso-pelagic drifting in populations in Norwegian fjords or the Baltic, and pediveligers settled directly into adult beds.
Pediveligers typically settle at ca. 260 µm (McGrath et al., 1988) but can delay metamorphosis until ca. 350 µm. Pediveligers can delay settlement for up to 7 weeks (Holt et al., 1998). Pediveligers test the substrata using their sensory foot. Settling pediveligers prefer discontinuities in the substrata (Chipperfield, 1953), and reportedly tend to avoid adults (Lane et al., 1985).
Primary settlement tends to occur on filamentous substrata, such as, bryozoans, hydroids, filamentous algae such as Polysiphonia sp., Corallina sp. and Mastocarpus sp., or the byssus threads of previously settled adults. Primary settlement may allow the pediveligers to avoid competition for food with adults or being inhaled by suspension feeding adults. Post-larvae may remain on their primary attachment until 1-2mm in size (sometimes larger), and many late post-larvae over-winter on algae, moving to adult substrata in spring, although many will leave the algae earlier due to winter storms or death of the algae (Seed & Suchanek, 1992). Newly settled mussels are termed 'spat'.

Dispersal
Dispersal is dependant on the duration of planktonic life. Maintenance of their position in the water column by active swimming ensures that larvae can be potentially dispersed over great distances by currents. In addition, post-larvae can become bysso-pelagic up to 2-2.5 mm in size, which may take ca. 2 months to achieve, during which time they may be transported significant distances by currents.

Recruitment
Recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Jørgensen (1981) estimated that larvae suffered a daily mortality of 13% in the Isefjord, Denmark. Lutz & Kennish (1992) suggested that larval mortality was approximately 99%. Larval mortality is probably due to adverse environmental conditions, especially temperature, inadequate food supply (fluctuations in phytoplankton populations), inhalation by suspension feeding adult mytillids, difficulty in finding suitable substrata and predation (Lutz & Kennish, 1992). First winter mortality in the Exe estuary averaged 68%, adults suffering 39% mortality after spawning and 24% due to bird predation (McGrorty, et al., 1990). Beukema (1992) reported recruitment failure in Mytilus edulis populations in the Wadden Sea after mild winters, which was thought to be due to a resultant increase in the number of small crabs or flatfish on the flats. Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment, possibly from the pool of young mussels on filamentous algae (Seed & Suchanek, 1992). Mytilus sp. is highly gregarious and final settlement often occurs around or in between individual mussels of established populations. Competition with surrounding adults may suppress growth of the young mussels settling within the mussel bed, due to competition for food and space, until larger mussels are lost (Seed & Suchanek, 1992).
Persistent mussels beds can be maintained by relatively low levels of recruitment. McGrorty et al., (1990) reported that adult populations were largely unaffected by large variations in spatfall between 1976-1983 in the Exe estuary.
Reproduction References Hayward et al., 1996, Bayne, 1965, Lutz & Kennish, 1992, Bayne, 1983, Hrs-Brenko & Calabrese, 1969, Bayne, 1976b, Jørgensen, 1981, Chipperfield, 1953, Lane et al., 1985, McGrath et al., 1988, Sprung, 1984, Thorson, 1950, Seed, 1976, King et al., 1990, Seed & Suchanek, 1992, Mackie, 1984, Newell et al.,1982, Thompson, 1979, Thiesen, 1973, Dare, 1976, Holt et al., 1998, Strathman, 1987, McGrorty et al., 1990, Beukema, 1992, Widdows, 1991, Beaumont & Budd, 1982, Beaumont & Budd, 1984, Clay, 1967(d), Gosling, 1992(a), Gray et al., 1997, Seed, 1969a, Bayne, 1976a, Eckert, 2003, Heidi Tillin, unpub data, Julie Bremner, unpub data