BIOTIC Species Information for Mytilus edulis
Researched byLizzie Tyler Data supplied byUniversity of Sheffield
Refereed byThis information is not refereed.
General Biology
Growth formBivalved
Feeding methodPassive suspension feeder
Active suspension feeder
Mobility/MovementTemporary attachment
Permanent attachment
Environmental positionEpilithic
Typical food typesBacteria, phytoplankton, detritus, and dissolved organic matter (DOM). HabitAttached
BioturbatorNot relevant FlexibilityNone (< 10 degrees)
FragilityIntermediate SizeMedium(11-20 cm)
HeightInsufficient information Growth RateSee additional information.
Adult dispersal potential<10m DependencyIndependent
General Biology 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,
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