The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

Removal of the substratum, be it rock or sediment, will entail the removal of the entire population and its associated community. Therefore, an intolerance of high has been recorded. Recovery may occur rapidly through good annual recruitment. However, examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment result in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported. Recoverability will depend on recolonization by the movement of young or juvenile Mytilus edulis from high shore or filamentous algal populations or recruitment of larvae settling directly on the new substratum. A single good recruitment event may return the population to prior levels within 1 -5 years, and an intolerance of high has been recorded. However, recoverability may be protracted in some circumstances (see additional information below).

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

Although apparently sedentary, Mytilus edulis is able to move some distance to change its position on the shore or within a bed or to resurface when buried by sand (Holt et al., 1998). Burial of Mytilus edulis beds by large-scale movements of sand and resultant mortalities have been reported from Morecambe Bay, the Cumbrian Coast and Solway Firth (Holt et al., 1998). Daly & Mathieson (1977) suggested that the lower limit of Mytilus edulis populations at Bound Rock, USA, was determined by burial or abrasion by shifting sands. Dare (1976) noted that individual mussels swept or displaced from a mussel bed rarely survived, since they either became buried in sand or mud or were scattered and eaten by oystercatchers. Dare (1976) reported that mussel beds accumulated ca. 0.4-0.75 m of 'mussel mud' (a mixture of silt, faeces, and pseudo-faeces) between May and September 1968 and 1971 in Morecambe Bay. Young mussels moved upwards becoming lightly attached to each other, but many were suffocated (Dare, 1976). Therefore, it appears that mussels are able to move upwards through accumulated sediment, but that a proportion will succumb and so an intolerance of intermediate has been recorded.
Recovery may occur rapidly through good annual recruitment. However, examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported.

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Moore (1977) reported that Mytilus edulis was relatively tolerant of turbidity and siltation, thriving in areas that would be harmful to other suspension feeders. Mytilus edulis possesses efficient shell cleaning and pseudofaeces expulsion mechanisms to remove silt (Moore, 1977), although it should be noted that pseudofaeces production involves an energetic burden (Navarro & Widdows, 1997). De Vooys (1987) examined the removal of sand from the mantle cavity and reported rapid discharge within 15min, with an exponential decrease over the next 4 hrs and slow discharge over 48 hrs. Purchon (1937) reported that Mytilus edulis died after an average of 13 days exposure to ca. 1200 mg/l suspended sediment (mud) but survived the length of the experiment (un-stated but >25 days) at 440 mg/l. Widdows et al. (1998) also noted that feeding rate was not reduced by current velocities up to 70 cm/s per se but by the resultant suspended sediment (at >50 mg/l ). Therefore, Mytilus edulis is probably of low intolerance to a change in suspended sediment at the benchmark level ( ±100 mg/l for 1 month). Recoverability is recorded as immediate given the mussels ability to discharge sand form the mantle cavity reported by De Vooys (1987).

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

A decrease in suspended sediment, especially organic particulates, could potentially reduce the food available to Mytilus edulis and hence its growth rate. Therefore, an intolerance of low has been recorded. Similarly, a recovery of immediate has been recorded.

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

The upper limit of Mytilus populations is primarily controlled by the synergistic effects of temperature and desiccation (Suchanek, 1978; Seed & Suchanek, 1992; Holt et al., 1998). For example, on extremely hot days in the summers of 1974 -1976 on Strawberry Island, Washington State, Suchanek (1978) reported mass mortality of Mytilus trossulus (as edulis) at the upper edge of the mussel bed. Mortality decreased down the shore. The upper limit of mussels fluctuated, increasing up the shore in winter and decreasing again in summer (Suchanek, 1978).

British Mytilus edulis have a sustained upper thermal tolerance limit of 29 °C (Almada-Villela et al., 1982) and occur in the upper eulittoral. Holt et al. (1998) suggested that tolerance of high temperatures and desiccation explained the upper limit of Mytilus edulis on the high shore. Therefore, Mytilus edulis is likely to exhibit a relatively low intolerance to changes in desiccation at the level of the benchmark although individuals at the upper limit of the range are probably more vulnerable to desiccation.

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Mytilus edulis can only feed when immersed, therefore, changes in emergence regime will affect individuals ability to feed and their energy metabolism. Growth rates decrease with increasing shore height and tidal exposure, due to reduced time available for feeding and reduced food availability, although longevity increases (Seed & Suchanek, 1992; Holt et al., 1998). Therefore, there will be a position on the shore where the energetic cost of metabolism is not met by feeding. Baird (1966) estimated that the point of zero growth occurred at 55% emergence but this value will vary between shores depending on local conditions, e.g. wave splash (Baird, 1966; Holt et al., 1998).
Increased emergence will expose mussel populations to increased risk of desiccation and increased vulnerability to extreme temperatures, potentially reducing their upper limit on the shore, and reducing their extent in the intertidal. But Mytilus edulis inhabits a wide range of shore heights and is probably relatively tolerant of changes of emergence at the benchmark level (change in emergence of 1hr for 1 year). Therefore, an intolerance of low has been reported. Similarly, once the prior emergence regime returns, the population will probably recover prior condition with a few months.

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Mytilus edulis can only feed when immersed, therefore, changes in emergence regime will affect individuals ability to feed and their energy metabolism. Growth rates decrease with increasing shore height and tidal exposure, due to reduced time available for feeding and reduced food availability, although longevity increases (Seed & Suchanek, 1992; Holt et al., 1998). Therefore, there will be a position on the shore where the energetic cost of metabolism is not met by feeding. Baird (1966) estimated that the point of zero growth occurred at 55% emergence but this value will vary between shores depending on local conditions, e.g. wave splash (Baird, 1966; Holt et al., 1998).
Decreased emergence, may allow the population to colonize further up the shore but exposes the lower limit of the population to increased predation, so that the population may effectively, move up the shore. But Mytilus edulis inhabits a wide range of shore heights and is probably relatively tolerant of changes of emergence at the benchmark level. Therefore an intolerance of low has been reported. Once the prior emergence regime returns, the population will probably recover with a few months.

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Widdows et al. (1998) showed that Mytilus edulis beds in sheltered conditions (based on field annular flumes measurements) reduced sediment erosion. Widdows et al. (1998) also noted that feeding rate was not reduced by current velocities up to 70 cm/s per se but by the resultant suspended sediment (at >50 mg/l ). It should also be noted that mussels probably benefit from high current velocities to supply food (suspended particulates, benthic diatoms and phytoplankton).
As mussel beds increase in size and depth, individual mussels become increasingly attached to each other rather than the substratum. As a result, the bed may become destabilised and susceptible to removal by wave action or tidal scour, although mussels at the edge of the beds are often more strongly attached than mussels within the bed (Seed & Suchanek, 1992).

Young (1985) demonstrated that byssal thread production (and hence attachment) increased with increasing water agitation. Mussels were able to increase their byssal attachment by 25% within 8 hours of a storm commencing. Young (1985) also reported that mussels were able to withstand shock or surges of up to 16 m/s (ca 30 knots). Young (1985) concluded that mussels would be susceptible to sudden squalls and surges, which may sweep them off rocks. Therefore, storms may cause significant mortality in mussel beds (see wave exposure below).
Mytilus edulis populations are found in weak to strong tidal streams, suggesting low intolerance to change in water flow rate, although, their intolerance probably owes more to the nature of the substratum than the strength of their attachment. Individuals attached to solid substrata (rock) are likely to be less intolerant than individuals attached to boulders, cobbles or sediment. Overall, Mytilus edulis can attach and grow on a variety of substrata in a variety of water flow regimes, and an intolerance of low has been reported. It should be noted that on sedimentary shores, mussel beds are probably more intolerant of increased water flow due to removal of the sediment. Once the prior water flow regime returns, the population will probably recover within a few months.

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Mytilus edulis probably benefits from high current velocities to supply food (suspended particulates, benthic diatoms and phytoplankton). A decrease in water flow is likely to decrease food availability. However, Mytilus edulis populations are found in weak to strong tidal streams, suggesting low intolerance to change in water flow rate, although, their intolerance probably owes more to the nature of the substratum than the strength of their attachment. Overall, an intolerance of low has been reported due their distribution in a variety of water flow regimes. Once the prior water flow regime returns, the population will probably recover within a few months.

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

In the British Isles an upper, sustained thermal tolerance limit of about 29 °C was reported for Mytilus edulis (Read & Cumming, 1967; Almada-Villla et al., 1982). But Seed & Suchanek (1992) noted that European populations were unlikely to experience temperatures greater than about 25 °C. Bayne et al. (1976) demonstrated that between 10 -20 °C water temperature had little effect on scope for growth. Mytilus edulis is generally considered to be eurythermal and an intolerance of 'low' has been recorded. Similarly, once the prior temperature regime returns, the population will probably recover any loss of condition within a few months.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Mytilus edulis can withstand extreme cold and freezing, surviving when its tissue temperature drops to -10 °C (Williams, 1970; Seed & Suchanek, 1992) or exposed to -30°C for as long as six hours twice a day (Loomis, 1995). In the laboratory, median lethal temperatures (MLT) of -16 °C after 24 hrs were estimated for large individuals (>3mm) while juveniles (<1.5mm) had an MLT of -12.5 °C. As expected, reducing the exposure time increased the MLT (Bourget, 1983). Bourget (1983) also reported that cyclic exposure to otherwise sublethal temperatures, e.g. -8 °C every 12.4 hrs resulted in significant damage and 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. Mytilus edulis was relatively little affected by the severe winter of 1962/63, with 30% mortality reported from south-east coasts of England (Whitstable area) and ca. 2% from Rhosilli in south Wales (Crisp (ed.),1964). Crisp (ed.) (1964) noted that most mortality resulted from predation on individuals weakened or moribund due to the low temperatures rather than the temperature itself.
Loomis (1995) reported that freezing tolerance increased after acclimation, high salinity and aerial exposure or anoxia. Freezing tolerance increased in winter, presumably due to increased isolation from the environment (shell closure), commensurate anaerobic conditions and increased mantle cavity fluid salinity with respect to the environment. Increased freezing tolerance may result from the accumulation of amino acids (e.g. taurine, glycine and alanine) involved in maintenance of osmotic balance in high salinities, and the end products of anaerobic metabolism (e.g. strombine, and octopine), which have been shown to also act as cryoprotectants (Loomis, 1995) (see oxygenation). Although Mytilus edulis may be intolerant of prolonged freezing temperatures, it is generally considered to be eurythermal and an intolerance of 'low' at the level of the benchmark is recorded. Similarly, once the prior temperature regime returns, the population will probably recover any loss of condition within a few months.

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Increased turbidity may reduce phytoplankton primary productivity, therefore reducing the food available to Mytilus edulis but mussels use a variety of food sources and the effects are likely to be minimal. Therefore, this species is probably tolerant to changes in turbidity.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Decreased turbidity may increase phytoplankton primary productivity, therefore potentially increasing the food available to Mytilus edulis but mussels use a variety of food sources and the effects are likely to be minimal. Therefore, this species is probably tolerant to changes in turbidity.

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Mytilus edulis populations are found in sheltered to wave exposed shores, suggesting low intolerance to change in wave exposure. Their intolerance probably owes more to the nature of the substratum than the strength of their attachment. Individuals attached to solid substrata (rock) are likely to be less intolerant than individuals attached to boulders, cobbles or sediment. Lewis (1964) noted that Mytilus edulis are favoured by damp conditions. Therefore, as wave exposure increases on rocky shores, barnacles and fucoids are replaced by mussel-dominated communities.

Storms and tidal surges are known to destroy mussel beds, often over hundreds of hectares in the Wash, Morecambe Bay and the Wadden Sea. Mussel beds persist in sheltered areas whereas beds in exposed areas are more dynamic (Holt et al., 1998). With increasing wave exposure mussel beds become increasingly patchy and dynamic. Young (1985) demonstrated that byssus thread production (and hence attachment) was increased by water agitation. Mussels were able to increase their byssal attachment by 25% within 8 hours of a storm commencing. Young (1985) concluded that mussels would be susceptible to sudden squalls and surges, which may sweep them off rocks. Mytilus edulis beds may also be damaged by wave-driven logs or equivalent debris (Seed & Suchanek, 1992). Intense mussel settlement may lead to choking and death of underlying mussels causing the population to loosen its attachment to the substratum (Seed, 1969b). Competition for space, especially in areas of rapid growth, may lead to the formation of hummocks, in which individual mussels may not be attached directly to the substratum. As a result, the population may become unstable and vulnerable to removal by rough seas (Seed, 1969b). Although mussel populations are found from wave exposed to sheltered shores, their intolerance to wave exposure is partly dependent on their substratum, and the size and density of the mussel bed, therefore, an intolerance of intermediate has been recorded to represent the increased susceptibility of mussel beds to damage by wave action.

Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment result in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). A recoverability of 'high' has been recorded.

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Mytilus edulis populations are found in sheltered to wave exposed shores, suggesting low intolerance to change in wave exposure. Their intolerance probably owes more to the nature of the substratum than the strength of their attachment. Lewis (1964) noted that rocky shore Mytilus edulis populations are favoured by damp conditions. Therefore, as wave exposure increases on rocky shores, barnacles and fucoids are replaced by mussel dominated communities. However, on rocky shores, as wave exposure decreases mussels are replaced by barnacle and fucoid dominated shores, possibly due to increased desiccation and predation (presumably by the dogwhelk Nucella lapillus). On wave sheltered sedimentary shores decreased wave exposure (i.e. sheltered to very sheltered) is likely to have little affect on mussel beds. Therefore, sheltered shore mussels beds are probably of low intolerance to decreased wave exposure, and may less patchy and more stable (persistent). However, on rocky shores decreased wave exposure may lead to a reduction in population density and dominance of the shore by barnacles and fucoids, therefore an overall intolerance of intermediate has been recorded.
Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported.

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Mytilus edulis may detect slight vibrations in its immediate vicinity and probably detects predators by touch (on the shell) or by scent. Therefore, it is likely to be insensitive to noise disturbance at the levels of the benchmark. Birds are major predators and several species are highly intolerant of disturbance by noise. Noise at the level of the benchmark may disturb predatory birds, so that the mussel populations may benefit indirectly.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Mytilus edulis can probably detect changes in light commensurate with shading by predators but its visual acuity is probably very limited and it is unlikely to be sensitive to visual disturbance. However, birds are highly intolerant of visual presence and are likely to be scared away by increased human activity, reducing the predation pressure on the mussels. Therefore, visual disturbance may be of indirect benefit to mussel populations.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

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. Wave driven logs have been reported to influence Mytilus trossulus (as edulis) populations, causing the removal of patches from extensive beds that subsequently open the beds to further damage by wave action. It is likely that abrasion or impact at the level of the benchmark (a boat anchor being dragged through or landing on the population) would also damage or remove patches of the population.

No studies of the effects of trampling on British or Irish populations of Mytilus edulis were found. But the effects of trampling on Mytilus californianus beds in Australia were studied by Brosnan & Cumrine (1994). They exposed mussel beds at two sites to low levels of trampling, 250 steps for 1 day every month over a 1 year period, which compared to 228 steps/hr recorded at another site. They reported that a loose, mono-layer bed was highly susceptible, trampling resulting in patches of bare rock that then expanded, beyond the area trampled, due to wave action. A dense, two-layer bed was less susceptible, initially showing less disturbance, although the top layer was lost. However, mussels continued to be lost for a year after trampling had stopped, resulting in patches. Patch size increased two years after trampling stopped. They suggested that continuous trampling may result in the loss of the bed. In a heavily trampled site mussels were not common and were confined to crevices (Brosnan & Cumrine, 1994).

Overall, the intolerance of trampling appears to be dependent on the density and depth of the affected mussel bed. Brosnan & Cumrine (1994) observed little recruitment in bare patches in the mussel beds until trampling had ceased and in some cases no recruitment two years later. Storms and wave action (including wave-driven logs) often clear patches of mussels in beds but occur primarily in winter and are localised. Trampling is most likely in spring and summer (Brosnan & Cumrine, 1994). The combined effects of trampling and natural winter disturbances may result in the loss of mussel beds in the long term.

Mytilus californianus bears radiating ribs, and is a larger than Mytilus edulis with a divergent ecology (Seed, 1992). Nevertheless, the above evidence suggests that mussel beds are potentially intolerant of the effects of trampling, depending on trampling intensity and frequency. Therefore, physical disturbance due to sand abrasion, impact by an anchor or debris, or due to trampling when emersed, is highly likely to result in the loss of a proportion of the population and an intolerance of intermediate has been recorded.

Recovery may occur rapidly through good annual recruitment. However, examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Dare (1976) reported that individual mussels swept or displaced from mussel beds rarely survived, since they either became buried in sand or mud, or were scattered and eaten by oystercatchers. Mussels can attach to a wide range of substrata and should a mussel be displaced to a suitable substratum it is likely to be able to attach itself quickly using byssus threads. For example, Young (1985) reported that detached mussels produced 8 byssus threads within the first 24hrs, and between 8-11 byssus threads within 3 days at 13°C (an average of 3.5 threads/ individual/ day), depending on temperature and water agitation. Overall, displacement is likely to result in the loss of some individuals due to vulnerability to predation as well as the danger of smothering, hence an intolerance of intermediate has been recorded.
Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

The effects of contaminants on Mytilus sp. were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992). Mussels are suspension feeders and, therefore, process large volumes of water together with suspended particulates and phytoplankton. Mussels absorb contaminants directly from the water, through their diet and via suspended particulate matter (Widdows & Donkin, 1992), the exact pathway being dependant on the nature of the contaminant.

• Widdows and Donkin (1992) reported 50% mortality from a tissue burden of 20 µg/g TBT.
• Exposure of Mytilus edulis to detergent (BP1002) in seawater resulted in 100% mortality at 10 ppm detergent, although all survived at 5 ppm detergent (Smith, 1968).
• Liu & Lee (1975) reported a LC₅₀ of 250 µg/l of the herbicide trifluralin in Mytilus galloprovincialis
• Mytilus edulis has been reported to bioaccumulate the insecticide ivermecten, although no adverse effects were observed (Cole et al., 1999).
• Biphenyl (a dye carrier) reduced the feeding rate of Mytilus edulis by 50% at 0.3 mg/l (Donkin et al., 1989).
• PCBs accumulate in gonads, although tissue concentrations are significantly reduced after spawning, although this may affect the next generation (Hummel et al., 1989; Holt et al., 1995).
• Significant increases in the incidence of tumours (neoplasia) were reported in the US Mussel Watch programme in the presence of higher concentrations of combustion related poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium (Hillman, 1993; Holt et al., 1998).
• Mytilus edulis survived in a power station cooling water culvert, exposed to 0.1-0.2 mg/l hypochlorite, although their growth rates were reduced by about a third. Mussels were able to recover in hypochlorite free periods between chlorination dosing (Thompson et al., 1997). Mytilus edulis and Mytilus galloprovincialis were reported to suffer 100% mortality after 15-135 days continuous exposure to 0.2-1.0 mg/l hypochlorite (Khalanski & Borget, 1980; cited in Thompson et al., 1997).
• Holt et al. (1995) also report that mussels may be absent from areas of high boating activity, presumably due to TBT.

Widdows et al. (1995) compared 'scope for growth' (SFG) and chemical contaminants in tissues of mussels from 26 coastal and 9 offshore sites around the United Kingdom. They noted that polar organics (probably derived from phytoplankton) accounted for some reduction in SFG, while organo-chlorides showed a significant correlation with an unexplained component of the decline in SFG. However, TBT levels were only high enough to cause an effect (<10% reduction in SFG) at 8 study sites (Widdows et al., 1995). Mytilus edulis is probably relatively tolerant of contaminants.
Widdows & Donkin (1992) list tolerances of Mytilus edulis adults and larvae (Tables 8.2, 8.3, & 8.4) but note that lethal responses give a false impression of high tolerance, since the adults can close their valves and isolate themselves from the environment for days. They suggest that sublethal effects (shell growth and 'scope for growth') are more sensitive indicators of the effects of contaminants. Also, adults are ca. 4 times more sensitive than larvae to TBT (Widdows & Donkin, 1992, see larval sensitivity).

Overall, the above evidence of contaminant induced mortality suggests that a proportion of the population may be lost and an intolerance of intermediate has been recorded.

Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported.

Heavy metal contamination

Evidence

The effects of contaminants on Mytilus sp. were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992). Widdows & Donkin (1992) list tolerances of Mytilus edulis adults and larvae (Tables 8.2, 8.3, & 8.4) but note that lethal responses give a false impression of high tolerance, since the adults can close their valves and isolate themselves from the environment for days. They suggested that sublethal effects e.g. shell growth and 'scope for growth' (SFG), are more sensitive indicators of the effects of contaminants. Reported effects of heavy metals follow.

• Adult 15 day LC₅₀ to 50µg/l Cu (Widdows & Donkin, 1999).
• Crompton (1997) reported that adult bivalve mortalities occurred after 4-14 day exposure to 0.1-1 µg/l Hg, 1-10 µg/l Cu and Cd, 10-100 µg/l Zn but 1-10 mg/l for Pb and Ni.
• Widdows et al. (1995) reported 'no observed effect thresholds' on feeding or SFG in Mytilus edulis tissues of 150 µg Cd/g dry wt, 25 µg Cu/ g dry wt, (lethal at 60 µg Cu/g dry wt), 12 µg Hg/g dry wt, 10 mg Pb/g dry wt, and 300 µg Zn/g dry wt. However, the tissue concentration of heavy metals at the sites studied was not high enough to reduce SFG significantly.
• Mussels were reported to be missing from an wider area than other shore organisms on a Cumbrian shore in the vicinity of a phosphate rich effluent outfall contaminated by a number of heavy metals (Holt et al., 1998).
• Adults are ca >10 fold more intolerant than larvae to Cu, petroleum hydrocarbons and sewage sludge (Widdows & Donkin, 1992) (see larval sensitivity).

Overall, Mytilus edulis is probably relatively tolerant of heavy metal contamination. But the potential mortality indicated above suggest an intolerance of intermediate.
Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported.

Hydrocarbon contamination

Evidence

Widdows & Donkin (1992) list tolerances of Mytilus edulis adults and larvae (Tables 8.2, 8.3, & 8.4) but note that lethal responses give a false impression of high tolerance, since the adults can close their valves and isolate themselves from the environment for days. They suggested that sublethal effects e.g. shell growth and 'scope for growth' (SFG), are more sensitive indicators of the effects of contaminants.

• Widdows et al. (1995) demonstrated that toxic hydrocarbons, primarily poly-aromatic hydrocarbons, made a large contribution the decline in SFG observed along the North Sea coast. Hydrocarbons reduce clearance rate through 'non-specific narcosis'.
• Mussel populations in Sullom Voe experienced moderate hydrocarbon pollution and a reduced SFG but had sufficient capacity to grow, reproduce and maintain a viable population (Widdows et al., 1992).
• Widdows et al. (1987) examined the response of Mytilus edulis to high oil (water accommodated fraction of diesel oil) (125 ± 28 µg/l) and low oil (28 ±7 µg/l) over a 8 month period, and subsequent recovery. They observed a marked reduction in SFG (due to reduced feeding rate and food absorption efficiency), and a correlation between the reduction in SFG and the hydrocarbon tissue burden (Widdows et al., 1987; Widdows & Donkin, 1992; Widdows et al., 1995). Mussels exposed to high oil conditions showed a negative SFG and weight loss. During recovery, 22 days after removal to 'clean' sea water the high oil mussels depurated (removed) hydrocarbons more rapidly than low oil mussels, and showed an increased clearance rate and growth rate associated with 'catch-up' growth. Both high and low oil mussels recovered completely within 55 days.
• Widdows et al. (1987) also reported that high and low oil contamination of the experimental basins resulted in 100% mortality amongst mussels kept in the basins from autumn 1982 to summer 1983 and from spring 1983 to summer 1984 respectively.
• Widdows et al. (1992) reported the following tolerances of adult Mytilus edulis to hydrocarbons; a 4 day LC₅₀ of 1-10 mg/l of crude oil, and a 4 month LC₅₀ to 125 µg/l of diesel.
• A sunflower oil tanker spill off the Anglesey coast resulted in ingestion of oil droplets and subsequent mortalities after spawning (Mudge et al., 1993; Holt et al., 1998).
• Bokn et al., (1993) demonstrated that Mytilus edulis was lost from mesocosm experiments continuously dosed with 30.1 to 129.4 µg/l of the water accommodated fraction of diesel, and was the most intolerant of the intertidal species studied.
• Mytilus edulis dominated jetty piles immediately adjacent to an oil refinery effluent in Milford Haven, suggesting a high tolerance of hydrocarbon contamination (K. Hiscock, pers. comm.).

Overall, hydrocarbon tissue burden results in decreased SFG and in some circumstances may result in mortalities, reduced abundance or extent of Mytilus edulis. Therefore, an intolerance of intermediate has been recorded. Larval mytilids are less sensitive than adults (see larval sensitivity).
Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of high has been reported.

Radionuclide contamination

Evidence

The periostracum of Mytilus edulis was reported to concentrate uranium (Widdows & Donkin, 1992). Mussels have also been reported to bioaccumulate ¹⁰⁶Ru, ⁹⁵Zr, ⁹⁵Nb, ¹³⁷Cs and ⁹⁰Sr (Cole et al., 1999). While the above data demonstrates that Mytilus edulis can accumulate radionucleides, little information concerning the effects of radionucleides on marine organisms was found.

Changes in nutrient levels

Evidence

Butler et al. (1990) examined the effects of sewage sludge on adult and larval Mytilus edulis. Exposure of adults to 0.02 - 0.04% industrial/domestic sewage sludge resulted in a reduced respiration rate and a 50% decrease in net energy surplus after 4 weeks. There was no clear relationship between the tissue concentration of heavy metals and physiological stress and it was unclear whether the observed effect was due to increased levels of nutrients or contaminants within the sewage sludge. Mytilus edulis may benefit from moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material. The resultant increased food availability may increase growth rates, reproductive potential and decrease vulnerability to predators.
Long term and/or high levels of organic enrichment may result in deoxygenation (see oxygenation below) and algal blooms, which may have adverse effects indirectly. Algal blooms have reportedly had adverse effects on Mytilus edulis. Blooms of the toxic dinoflagellate Gyrodinium aureolum caused mortality of Mytilus edulis in Norway and sublethal effects on clearance rate and cellular damage in the UK (Holt et al., 1998). A bloom of Phaeocystis poucheri that produced copious amounts of glutinous material prevented Mytilus edulis from feeding, and hence reproductive failure in the Dutch Wadden Sea (Pieters et al., 1980; Holt et al., 1998). Landsberg (1996) also suggested that there was a correlation between the incidence of neoplasia or tumours in bivalves and out-breaks of paralytic shellfish poisoning in which bivalves accumulate toxins from algal blooms, although a direct causal effect required further research. Therefore, a proportion of the population may be lost as a result of nutrient enrichment, and an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Mytilus edulis is likely to encounter hyper-saline conditions in rock pools exposed on hot days, where evaporation can increase the salinity, e.g. Newell (1979) presented data on salinity fluctuations in rock pools, which reached salinities up to 42 psu. High shore rock pools show marked fluctuations in salinity. But Mytilus edulis is considered to be tolerant of a wide range of salinities (see Holt et al., 1998). Therefore, an intolerance of low, at the benchmark level, is recorded. On return to the prior salinity regime, Mytilus edulis will probably recover within a few days or weeks.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Mytilus edulis exhibits a defined behaviour to reducing salinity, initially only closing its siphons to maintain the salinity of the water in its mantle cavity, which allows some gaseous exchange and therefore maintains aerobic metabolism for longer. If the salinity continues to fall the valves close tightly (Davenport, 1979; Rankin & Davenport, 1981). In extreme low salinities, e.g. resulting from storm runoff, large numbers of mussels may be killed (Keith Hiscock pers comm.). In the long term (weeks) Mytilus edulis can acclimate to lower salinities (Almada-Villela, 1984; Seed & Suchanek, 1992; Holt et al., 1998). Almada-Villela (1984) reported that the growth rate of individuals exposed to only 13psu reduced to almost zero but had recovered to over 80% of control animals within one month. Mytilus edulis can also survive considerably reduced salinities, growing as dwarf individuals at 4-5psu in the Baltic. Differences in growth being due to physiological and/or genetic adaptation to salinity.

Mytilus edulis is an osmoconformer and maintains its tissue fluids iso-osmotic (equal ionic strength) with the surrounding medium by mobilization and adjustment of the tissue fluid concentration of free amino acids (e.g. taurine, glycine and alanine) (Bayne, 1976; Newell, 1989). But mobilizing amino acids may result in loss of protein, increased nitrogen excretion and reduced growth. Koehn (1983) and Koehn & Hilbish (1987) reported a genetic basis to adaptation to salinity at the aminopeptidase-1 locus, involved in the production of some free amino acids. In addition, Mytilus edulis thrives in brackish lagoons and estuaries, although, this is probably due to the abundance of food in these environments rather than the salinity (Seed & Suchanek, 1992).

Overall, Mytilus edulis can acclimate to a wide range of salinities and a change of salinity at the benchmark level is unlikely to adversely affect this species, and an intolerance of low has been recorded.

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

Mytilus edulis is regarded as euryoxic, and tolerant of a wide range of oxygen concentrations including zero (Zwaan de & Mathieu, 1992). Diaz & Rosenberg (1995) suggest it is resistant to severe hypoxia. Adult mytilids exhibited high tolerance of anoxia, e.g. Theede et al. (1969) reported LD₅₀ of 35 days for Mytilus edulis exposed to 0.21mg/l O₂ at 10°C, which was reduced to 25 days with the addition of sulphide (50 mg/l Na₂S .9H₂O). Mytilus edulis is capable of anaerobic metabolism. In aerial exposure (emersion) the mussel closes its valves, resulting in a low rate of oxygen exchange and consumption (Zwaan de & Mathieu, 1992; Widdows et al., 1979). Therefore, the mussel conserves energy and utilizes anaerobic metabolism. Anaerobic metabolism also increases at low temperatures and some of the end products of anaerobic metabolism may be cryoprotectant (see changes in temperature above).
Jorgensen (1980) observed, by diving, the effects of hypoxia (0.2 -1 mg/l) on benthic macrofauna in marine areas in Sweden over a 3-4 week period. Mussels were observed to close their shell valves in response to hypoxia and survived for 1-2 weeks before dying (Cole et al., 1999; Jorgensen, 1980). In hypoxic or anoxic conditions Mytilus edulis increases oxygen consumption until oxygen levels fall below 60% saturation, the proportion of anaerobic metabolism increases as the oxygen concentration falls below 90% saturation (Famme et al., 1981; Newell, 1989). In Mytilus galloprovincialis anaerobic metabolism increases once the oxygen concentration falls below 8kPa ( 3.5 mg/l) becoming maximal at and below 4kPa (1.76 mg/l) (Zwaan de & Mathieu, 1992). Anaerobic metabolism allows the mussel to maintain is metabolism close to aerobic levels (Newell, 1989), although it incurs an 'oxygen debt' in the process (Widdows et al., 1979). Although Mytilus edulis is highly tolerant of hypoxia at the benchmark level (2mg/l O₂ for 1 week), it incurs a metabolic cost and, hence, reduced growth, therefore, an intolerance of low has been recorded. Tolerance of anoxia increases during larval development (see larval sensitivity).

Once oxygen levels return to prior levels, Mytilus edulis will probably recover condition within a few weeks.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

Landsberg (1996) suggested that there was a correlation between the incidence of neoplasia or tumours in bivalves and out-breaks of paralytic shellfish poisoning in which bivalves accumulate toxins from algal blooms. However, demonstration of a direct causal effect requires further research.
Mytilus spp. hosts a wide variety of disease organisms. parasites and commensals from many animal and plant groups including bacteria, blue green algae, green algae, protozoa, boring sponges, boring polychaetes, boring lichen, the intermediary life stages of several trematodes, copepods and decapods (Bower, 1992; Bower & McGladdery, 1996; Gray et al., 1999). For example:

• The polychaete Polydora ciliata, bores into the shell, causing blisters, atrophy of adductor muscle, interferes with gamete production and reduces the strength of the shell, hence increasing their vulnerability to predation. Polydora ciliata has been, therefore, responsible for substantial mortalities among European mussel populations (Ambariyanto & Seed, 1991; Bower, 1992; Bower & McGladdery, 1996).
• The boring sponges Cliona celata and Cliona lobata, perforate the shell of mytilids leaving them vulnerable to predators. Infections of Cliona sp. may be confused with infections of the shell boring lichen Arthopyrenia sublitoralis reported in the Isefjord, Denmark.
• Mytilus edulis may also be infected by the copepod Mytilicola intestinalis (red worm disease). Mytilicola intestinalis may infect 100% of the mussel population at levels often greater than 30 copepods per mussel. Although its presence may reduce fecundity, it exhibits the features of a commensal rather than a harmful parasite.The mantle cavities of mussels may also be infected by pea crabs (Pinnotheridae), and Pinnotheres pisum occurs in Mytilus edulis and Mytilus galloprovincialis in European waters. Pea crabs feed on food collected by the host but may cause gill damage, shell thickening, emaciation, and reduction of reproductive capacity, and have been described as either commensals or parasites (Bower, 1992; Bower & McGladdery, 1996).
• The metacercarial stages of the parasitic trematode Gymnophallus spp., was reported to induce the formation of pearls in Mytilus edulis (Seed, 1969b; Fernandes & Seed, 1983).

Bower (1996) noted that mortality from parasitic infestation in Mytilus sp. was lower than in other shellfish in which the same parasites or diseases occurred. Seed (1969b) suggested that while parasites were a potential source of mortality in Mytilus sp. they were not thought to be of major significance on local (British) shores. Although Mytilus edulis populations, may suffer only limited mortality due to parasites and diseases when compared to mortality from other factors (see above and general biology), the potential for mortality suggests an intolerance of intermediate. Recoverability is likely to be high (see additional information below).

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

Mytilus edulis is an effective space occupier and few other species are able to out-compete it for space. However, the South American mytilid Aulocomya ater has been reported recently in the Moray Firth, Scotland in 1994 and again in 1997 (Holt et al., 1998; Eno et al., 2000; McKay, 1994). Aulocomya ater is thought to have a stronger byssal attachment than Mytilus edulis and may replace Mytilus edulis in more exposed areas if it reproduces successfully (Holt et al., 1998).

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Large mussel beds have been routinely fished for hundreds of years, and managed by local Sea Fishery Committees for the past hundred years (Holt et al., 1998). Mussel beds may be exploited by hand collection or dredging. Holt et al., (1998) suggest that when collected by hand at moderate levels using traditional skills the beds will probably retain most of their biodiversity (see importance). However, they also cite incidences of over-exploitation of easily accessible small beds by anglers for bait. Holt et al., (1998) suggest that in particular embayments over-exploitation may reduce subsequent recruitment leading to long term reduction in the population or stock. The relationship between stock and recruitment is poorly understood. Loss of stock may have significant effects on other species, e.g. in the Dutch Wadden Sea in 1990 the mussel stocks fell to unprecedented low levels resulting in death or migration of eiders, and oystercatchers seeking alternative prey such as the cockle Cerastoderma edule, the sand-gaper Mya arenaria and Baltic tellin Macoma balthica.
Therefore, extraction or fishing will inevitably result in loss of a proportion of the population so that an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information below).

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Holt et al. (1998) suggested that Mytilus sp. was probably less affected by incidental damage due to fisheries than other organisms, and reported that Mytilus edulis communities replaced Sabellaria spinulosa damaged by shrimp fishing. Therefore, an intolerance of low has been recorded.