• 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).

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.

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).

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).

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).