Biodiversity & Conservation

CR.MCR.M.MytHAs

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum will include the removal of all the species within the biotope. Therefore, an intolerance of high has been recorded. Although a single good recruitment event may recolonize the substratum within a year, recovery may take up to 5 years, and is some circumstances significantly longer (see additional information below). Therefore, a recoverability of high has been recorded.
Smothering
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Intertidal Mytilus edulis beds have been reported to suffer mortalities as a result on smothering by large scale movements of sand or sand scour (Holt et al., 1998; Daly & Mathieson, 1977). Similarly, biodeposition within a mussel bed results in suffocation or starvation of individuals that cannot re-surface. Young mussels have been shown to move up through a bed, avoiding smothering, while many others were suffocated (Dare, 1976; Holt et al., 1998). This suggests that a proportion of the population may be able to avoid smothering in subtidal conditions, and, therefore, an intolerance of intermediate has been recorded. Although, Mytilus edulis is highly fecund, larval mortality is high. Larval development occurs within the plankton over ca 1 month (or more), therefore, whilst recruitment within the population is possible, it is likely that larval produced within the biotope are swept away from the biotope to settle elsewhere. Therefore, recovery is dependant on recruitment from outside the biotope and a recoverability of high has been reported (see additional information below).
Increase in suspended sediment
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Mytilus edulis has been reported to be relatively tolerant of suspended sediment and siltation and survived over 25 days at 440mg/l and on average 13 days at 1200mg/l (Purchon, 1937; Moore, 1977). Mytilus edulis also has efficient pseudofaeces discharge mechanisms (Moore, 1977; de Vooys, 1987). Similarly Asterias rubens flourishes in naturally turbid conditions and is capable of cleansing itself of adherent mud particles (Moore, 1977). However, both species probably suffer a metabolic cost resulting from the cleansing mechanisms, mucus production and interrupted or impaired feeding. Similarly, Urticina felina, Alcyonium digitatum and Balanus crenatus were considered to be of low intolerance to suspended sediment. In addition, the strong tidal streams characteristic of the biotope probably prevent suspended sediment settling out and hence reduces siltation. Therefore, a biotope intolerance of low, at the benchmark level, has been recorded. Hydroids, such as Sertularia spp. and Kirchenpaueria pinnata are likely to be more intolerant of siltation (Hiscock, 1983). However, greater increases in siltation may reduce the abundance of hydroids, bryozoans and anthozoans within the biotope especially on upward facing surfaces. The majority of the organisms within the biotope probably have mechanisms to deal with siltation and suspended sediment, so that recoverability of immediate has been recorded.
Decrease in suspended sediment
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A decrease in suspended sediment, especially organic particulates could potentially reduce the food available to Mytilus edulis and the other suspension feeders within the biotope. Therefore, an intolerance of low has been recorded.
Desiccation
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Circalittoral populations are unlikely to be exposed to desiccation.
Increase in emergence regime
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An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats.
Decrease in emergence regime
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An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action and tidal streams may be increased (see water flow rate below).
Increase in water flow rate
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The strong tidal streams characteristic of this biotope probably supply the community with adequate food in the form of particulates. This is a particular importance for passive suspension feeders such as hydroids and bryozoans. An increase in water flow may dislodge a proportion of the Mytilus edulis bed and increase competition for space from species adapted to very strong water flow rates such as Tubularia indivisa. Mytilus edulis populations are found from weak to strong tidal streams, suggesting a low intolerance to water flow rates. Similarly, Young (1985) reported that Mytilus edulis increased byssus thread production in response to increased agitation and water flow rates, and that Mytilus edulis was able to withstand surges of up to 16 m/s. However, Young (1985) also noted that mussels would be susceptible to sudden squalls and surges. Predation by Asterias rubens may also be decreased by increased water flow rates or wave exposure (Hiscock, 1983). Urticina felina and Alcyonium digitatum prefer areas of strong water flow, and Balanus crenatus is found in a wide range of water flow regimes. Species such as Molgula manhattensis and Flustra foliacea thrive in strong water flow but are found at low abundance in very wave exposed and very strong tidal streams (Hiscock, 1983). However, overall an intolerance of low has been recorded. Recoverability is likely to be rapid.
Decrease in water flow rate
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The strong tidal streams characteristic of this biotope probably supply the community with adequate food in the form of particulates. This is a particular importance for passive suspension feeders such as hydroids and bryozoans. Mytilus edulis tolerates a wide range of water flow rates. However, decreases in water flow rates are likely to increase siltation (see above) and increase predation pressure from crabs, lobsters and starfish such as Asterias rubens. The biotope is likely to suffer from competition from species adapted to more sheltered conditions. Therefore, an intolerance of intermediate has been recorded. Although, Mytilus edulis is highly fecund, larval mortality is high. Larval development occurs within the plankton over ca 1 month (or more), therefore, whilst recruitment within the population is possible, it is likely that larval produced within the biotope are swept away from the biotope to settle elsewhere. Therefore, recovery is dependant on recruitment from outside the biotope and a recoverability of high has been reported (see additional information below).
Increase in temperature
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Circalittoral populations are unlikely to experience rapid or extreme temperature changes due to natural events and may, therefore, be expected to be intolerant of acute temperature change. However, an upper, sustained temperature tolerance limit of about 29°C has been reported for Mytilus edulis in the United Kingdom (Read & Cumming, 1967; Almada-Villa et al., 1982). Seed & Suchanek (1992) noted that European populations were unlikely to experience temperatures greater than 25°C. Therefore, Mytilus edulis was consider to be of low intolerance to temperature change. Similarly, Urticina felina and Alcyonium digitatum were considered to be of low intolerance to temperature change. Balanus crenatus and Asterias rubens however, were assessed as highly intolerant of increased temperatures. Overall, the biotope has been assessed as of low intolerance to increased temperatures since the key species, Mytilus edulis is unlikely to be adversely affected. Recovery is likely to be rapid (see additional information below).
Decrease in temperature
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Circalittoral populations are unlikely to experience rapid or extreme temperature changes due to natural events and may, therefore, be expected to be intolerant of acute temperature change. However, Mytilus edulis tolerates decreases to in temperature and even freezing for short periods. Similarly, Balanus crenatus, Alcyonium digitatum, Asterias rubens and Urticina felina were unaffected by the severe winter of 1962/63 (Crisp, 1964). It appears, therefore, that most of the characterizing species within the biotope are tolerant of an acute short term temperature decrease and a biotope intolerance of low has been recorded. No information regarding the temperature tolerance of hydroids or bryozoans was found, and these groups may be more intolerant. Recovery is likely to be rapid (see additional information below).
Increase in turbidity
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Foliose algae have been reported in some records of this biotope (Hiscock, 1984). However, this biotope is primarily an animal dominated community, dependant on secondary production and not dependant on light. Therefore, the biotope is probably not sensitive to changes in turbidity and light attenuation.
Decrease in turbidity
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Foliose algae have been reported in some records of this biotope (Hiscock, 1984). However, this biotope is primarily an animal dominated community, dependant on secondary production and not dependant on light. Therefore, the biotope is probably not sensitive to changes in turbidity and light attenuation.
Increase in wave exposure
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Wave exposure causes oscillatory flow on the sea bed, the magnitude of which is attenuated with depth. Therefore, increases in wave exposure are likely to increase water flow rates in the circalittoral (see increases in water flow above). However, oscillatory water movement is potentially far more destructive than tidal streams due to the 'to and fro' motion is more likely to loosen mussels. Therefore, an intolerance of intermediate has been recorded.
Although, Mytilus edulis is highly fecund, larval mortality is high. Larval development occurs within the plankton over ca 1 month (or more), therefore, whilst recruitment within the population is possible, it is likely that larval produced within the biotope are swept away from the biotope to settle elsewhere. Therefore, recovery is dependant on recruitment from outside the biotope and a recoverability of high has been reported (see additional information below).
Decrease in wave exposure
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Wave exposure causes oscillatory flow on the sea bed, the magnitude of which is attenuated with depth. Therefore, decreases in wave exposure are likely to decrease water flow rates in the circalittoral, depending on the prevalent tidal streams. See increases in water flow rates above.
Noise
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Although, some fish species may be scared off or deterred from feeding by underwater noise, the majority of the species within the biotope are unlikely to be adversely affected by, or detect underwater noise.
Visual Presence
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None of the species within the biotope are likely to be adversely affected or detect changes in visual presence at the benchmark level.
Abrasion & physical disturbance
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Wave driven logs have been reported to influence Mytilus edulis populations, causing the removal of patches from extensive beds that subsequently open the beds to further damage by wave action (Holt et al., 1998). A similar effect could be caused by a vessel grounding.

Little information on physical disturbance in subtidal Mytilus spp. beds was found. Fishing activities, e.g. scallop dredging are know to physically disturb marine communities. However, benthic trawls tend to avoid rough ground, such as reefs and rocky areas. Modiolus modiolus beds have been reported to have declined off the Isle of Man due to scallop dredging, presumably because the scallop dredging activity had damaged the edges of denser beds over time (Jones, 1951; Holt et al., 1998). Benthic trawls, where they occur, may affect Mytilus edulis beds similarly. Scallop dredging and otter trawls has also been reported to damage Alcyonium digitatum (Hartnoll, 1998; Holt et al., 1998). Starfish, such as Asterias rubens have been reported to be damaged by benthic dredges, but have considerable regenerative capability, and, as scavengers, benefit from the presence of other damaged or killed animals (Emson & Wilkie, 1980; Gubbay & Knapman, 1999). Therefore, it is likely that abrasion or impact at the level of the benchmark (a passing scallop dredge) would damage or remove patches of the population and an intolerance of intermediate has been recorded. Recovery is dependant on recruitment of Mytilus edulis from outside the biotope and a recoverability of high has been reported (see additional information below).

Displacement
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Mytilus edulis is capable of re-attaching itself to suitable substrata once displaced. Dislodgement may result in increased risk of predation and some individuals may be lost if swept to unsuitable substrata. Overall, however, displacement will result in loss of mussels from this biotope. Displaced starfish are unlikely to be adversely affected and could probably return. Permanently attached species within the community such as hydroids, bryozoans and Alcyonium digitatum as likely to be lost as a result of displacement. Overall, a proportion of the mussel bed would probably survive displacement and an intolerance of intermediate has been recorded. However, other members of the community are probably more intolerant, resulting in a decline in species richness until they are able to recolonize. Recovery is dependant on recruitment of Mytilus edulisfrom outside the biotope and a recoverability of high has been reported (see additional information below).

Chemical Factors

Synthetic compound contamination
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The effects of contaminants on Mytilus edulis were extensively reviewed by Widdows & Donkin (1992) and Livingstone & Pipe (1992). Overall, Mytilus edulis is probably relatively tolerant of contaminants, although mortalities have been recorded (see species review for details). For example,
  • Widdows et al., (1995) noted that polar organics, and organo-chlorines reduced scope for growth in Mytilus edulis;
  • Mytilus edulis has been shown to accumulate PCBs and ivermecten (Hummel et al., 1989; Cole et al., 1999; Holt et al., 1995);
  • the presence of poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium gas been associated with an increase in tumours in Mytilus edulis (Hillman, 1993; Holt et al., 1998); and
  • mussels may be absent from areas of high boating activity, presumably due to TBT (Holt et al., 1998).
Mortality in Alcyonium digitatum was reported after exposure to the dispersant BP 1002 (Smith, 1968) whereas Smith (1968) found Urticina felina to be one of the more resistant species on the shore after the Torrey Canyon oil spill and Hoare & Hiscock (1974) reported it relatively close of a halogenated effluent discharge in Amlwch where other organisms were unable to survive. PCB exposure resulted in defective larvae in Asterias rubens (Besten et al., 1989). Barnacles, such as Balanus crenatus were considered to be highly intolerant of chemical contaminants (Holt et al., 1995). No information was found concerning the effect of contaminants hydroids, bryozoans or ascidians.
Therefore, chemical contamination may cause mortalities and sub-lethal effects in the Mytilus edulis bed but affect other members of the community to varying degrees, and an overall intolerance of intermediate has been recorded.
Recovery of the mussel beds will probably require recruitment from other areas, while most other members of the community will recolonize rapidly and a recoverability of high has been reported (see additional information below).
Heavy metal contamination
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Lethal threshold concentrations for several heavy metals have been determined in Mytilus edulis (see species review; Widdows & Donkin (1992) and Livingstone & Pipe (1992) for reviews). Mussels were also reported to be missing from a wider area of the Cumbrian coast than other organisms in the vicinity of a phosphate rich effluent contaminated by heavy metals (Holt et al., 1998). Widdows & Donkin (1992) noted that lethal responses give a false impression of high tolerance. However, Mytilus edulis is probably relatively tolerant of heavy metal contamination. Besten et al. (1989) suggested that cadmium (Cd) pollution posed a significant threat to populations of Asterias rubens since it affected reproduction. Little information concerning heavy metal toxicity was found for hydroids, bryozoans and ascidians.
Therefore, given the evidence of sub-lethal and lethal effects of heavy metals in Mytilus edulis a biotope intolerance of intermediate has been reported.
Hydrocarbon contamination
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The effects of contaminants on Mytilus edulis were extensively reviewed by Widdows & Donkin (1992) and Livingstone & Pipe (1992). Overall, Mytilus edulis is probably relatively tolerant of contaminants, although mortalities have been recorded (see species review for details). Circalittoral populations are protected from the immediate effects of oil spills by their depth. Therefore, hydrocarbon contamination in the circalittoral populations is limited to exposure to lighter oil fractions and PAHs in solution, as droplets as a result of wave exposure or adsorbed onto particulates.
  • Toxic hydrocarbons and PAHs contribute to a decline on the scope for growth in Mytilus edulis (Widows & Donkin, 1992; Widdows et al., 1995; ).
  • The presence of poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium gas been associated with an increase in tumours in Mytilus edulis (Hillman, 1993; Holt et al., 1998).
  • Mesocosm experiments have shown high mortalities of Mytilus edulis exposed to the water accommodated fraction of diesel (Widdows et al., 1987; Bokn et al., 1993).
  • Ingestion of droplets of sunflower oil, from a tanker spill off the Anglesey coast resulted in mortalities after spawning (Mudge et al., 1993; Holt et al., 1998).
  • Asterias rubens suffered mass mortalities after the Torrey Canyon oil spill and was reported to be lost from mesocosms treated with the water accommodated fraction of diesel (Smith, 1968; Bokn et al., 1993).
  • 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, Mytilus edulis is probably relatively tolerant of chronic hydrocarbon pollution. However, due to the incidence of mortality after exposure to diesel and oils Mytilus edulis was regarded as of intermediate intolerance to hydrocarbon contamination.
Little information was found concerning the effects of hydrocarbon pollution on hydroids, bryozoans, or ascidians. Although, Asterias rubens has been assessed as highly intolerant, the mussel bed may benefit from a reduction in starfish predation, and an overall biotope intolerance of intermediate has been recorded.
Recovery is probably dependant on Mytilus edulis recruitment and a recoverability of high has, therefore, been recorded (see additional information below).
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material, is likely to increase food availability for all the suspension feeders within the biotope. Therefore, 'tolerant*' has been recorded. However, long term or high levels of organic enrichment may result in deoxygenation and algal blooms. Mytilus edulis has been reported to suffer mortalities due to algal blooms of Gyrodinium aureolum and Phaeocystis poucheri (Holt et al., 1998). Circalittoral populations may be too deep to be affected by feeding on toxic algae. However, death of toxic and non-toxic algal blooms may result in large numbers of dead algal cells collecting on the sea bottom, resulting in local de-oxygenation as the algal decompose. Although, Mytilus edulis is probably tolerant of anoxic conditions other members of the community may be more intolerant (see oxygenation below).
Increase in salinity
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The biotope is sublittoral and present on the open coast in full salinity conditions. Increase in salinity is therefore considered not relevant.
Decrease in salinity
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Mytilus edulis thrives in brackish lagoons and estuaries, however, 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. However, many of the associated species in this biotope are restricted to open coast situations and are most likely intolerant of change in salinity. Therefore, an intolerance of intermediate is indicated. Once conditions return to normal, most of the associated species will recolonize from planktonic stages. A recoverability of high is given.
Changes in oxygenation
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Mytilus edulis was regarded to be tolerant of a wide range of oxygen concentrations including zero (Zwaan de & Mathieu, 1992; Diaz & Rosenberg, 1995; see species review). However, echinoderms such as Asterias rubens are highly intolerant of anoxic conditions. Similarly, Alcyonium digitatum and Balanus crenatus were considered to be highly intolerant of anoxia. Little information regarding the tolerance of ascidians and hydroids to hypoxia was found. Although Mytilus edulis is likely to tolerate hypoxic conditions, an intolerance of intermediate has been recorded due to the intolerance of the other members of the community. It should be noted that in the presence of strong to moderate tidal streams, anoxic conditions are unlikely to occur unless combined with reduced water flow rates. Recoverability of the associated species is likely to be rapid (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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The diseases and parasites of Mytilus edulis were reviewed by Bower (1992) and Bower & McGladdery (1996) (see the species review).
The boring sponge Cliona spp. has been reported from Modiolus modiolus beds and may affect subtidal Mytilus edulis beds. Similarly subtidal beds may be affected by the boring polychaete Polydora ciliata. Both of the above boring species weaken the shell of the victim and makes them more vulnerable to predation. Polydora ciliata also causes blisters, atrophy of muscle tissue and interferes with gamete production and has resulted in substantial mortalities in European mussel populations. Asterias rubens may be parasitised by the ciliate Orchitophyra stellarum (Vevers, 1951; Bouland & Clareboudt, 1994) resulting in castration of males, and subsequent reduction in population size (Vevers, 1951).
Therefore, an intolerance of intermediate has been recorded. Recovery of the mussel beds will be dependant on recruitment from other populations and a recoverability of high has been recorded (see additional information below).
Introduction of non-native species
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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., 1997; 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
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Large mussel beds in the intertidal and subtidal have been routinely fished for hundreds of years, and managed by local Sea Fishery Committees for the past hundred years (Holt et al., 1998). Some shallow subtidal populations are found in turbid areas and are essentially circalittoral, and represented by this biotope.
Subtidal mussel beds may be exploited dredging. 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 Cerastoderma edule, Mya arenaria, and Macoma balthica.
Extraction of Mytilus edulis is likely to remove much of the epifaunal and infaunal community, resulting in a decline in species richness. Overall, an intolerance of intermediate has been recorded at the benchmark level of extraction. However, recovery is likely to occur within 5 years and a recoverability of high has been recorded (see additional information below).

Additional information icon Additional information

Recoverability
Larval supply and settlement could potentially occur annually, however, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). Mytilus edulis is highly fecund but larval mortality is high. Larval development occurs within the plankton over ca 1 month (or more), therefore, whilst recruitment within the population is possible, it is likely that larval produced within the biotope are swept away from the biotope to settle elsewhere. Therefore, recovery is probably dependant on recruitment from outside the biotope.
While good annual recruitment is possible, recovery may take at least 5 years. However, it should be noted that in certain circumstances and under some environmental conditions recovery may take significantly longer. Overall, Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchanek, 1992).
The other characterizing species are likely to recolonize the substratum rapidly (see recruitment processes).

This review can be cited as follows:

Tyler-Walters, H. 2001. Mytilus edulis beds with hydroids and ascidians on tide-swept moderately exposed circalittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=208&code=1997>