Biodiversity & Conservation

LR.HLR.MusB.MytB

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
(View Benchmark)
Removal of the substratum will result in loss of the sessile barnacle and mussels and their associated community, and the loss of the biotope. Therefore, an intolerance of high has been recorded. Recovery is variable but likely to take up to 5-10 years depending on location and a recoverability of moderate has been recorded (see additional information below).
Smothering
(View Benchmark)
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. Significant sediment cover of the middle to lower intertidal in a South Californian shore, resulting from fresh water runoff, caused substantial decline in Corallina spp. die back of barnacles and Pelvetia spp. due to smothering, although Corallina spp. was able to expand up the shore in the following 6 months (Seapy & Littler, 1982). Red turf forming algae were relatively unaffected (Seapy & Littler, 1982).

Monterosso (1930) showed experimentally that chthamalid barnacle species can survive complete smothering by petroleum jelly for approximately two months, by respiring anaerobically. Complete smothering caused by the Torrey Canyon oil spill yielded similar results; a few Semibalanus balanoides died, yet Chthamalus stellatus / Chthamalus montagui seemed unaffected, while at Booby's bay more than 90% had managed to clear an opening in the oil film (Smith, 1968).

Limpets and small littorinids will probably be highly intolerant, being unable to move up through a 5 cm layer of sediment and hence suffocate. Patella vulgata is absent from silted shores. Mobile amphipods and small crustaceans will probably be able to move through the smothering sediment and avoid damage. Smothering by sediment and the resultant scour will also interfere with larval and algal recruitment and feeding in grazers.

Overall, the extent and abundance of the key species and some species of red algae are likely to be reduced. A severe reduction in the abundance of limpets and littorinid grazers is likely to result in a bloom of ephemeral algae and fucoids, resulting in smothering of surviving barnacles and mussels, and a major change in the community and loss of the biotope as described. Therefore, an intolerance of high has been recorded. Recoverability is likely to be moderate.
Increase in suspended sediment
(View Benchmark)
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), although increased suspended sediment may reduce feeding efficiency (Widdows et al., 1998). Increased suspended sediment may reduce growth rates in barnacles due to the energetic costs of cleaning sediment particles from feeding apparatus. Semibalanus balanoides, Patella vulgata and Nucella lapillus are also expected to have low intolerance to an increase in suspended sediment because they are found in turbid estuaries (e.g. the Severn estuary). Palmaria palmata occurs in areas of high suspended sediment such as the Bay of Fundy (see review). An increase in suspended sediment will increase turbidity (see below) and sediment scour. Siltation is unlikely to be significant in this biotope due to the high degree of water movement caused by extreme to moderate wave exposure. Mussel patches may accumulate sediment, which would probably decrease their species richness (Tsuchiya & Nishihira, 1985, 1986). Therefore a biotope intolerance of low has been recorded. Recovery of condition is likely to be very high.
Decrease in suspended sediment
(View Benchmark)
A decrease in suspended sediment, especially organic particulates could potentially reduce the food available to Mytilus edulis, barnacles and the other suspension feeders within the biotope. Turbidity is likely to be reduced (see below). However, few if any adverse affects may result and an intolerance of low has been recorded to represent the potential reduction in food availability. Recoverability is likely to be very high.
Desiccation
(View Benchmark)
Desiccation is a major factor governing the upper limit of rocky shore communities. The upper limit of Mytilus edulis population is primarily controlled by the synergistic effects of temperature and desiccation (Suchanek, 1978; Seed & Suchanek, 1992; Holt et al., 1998). Desiccation tolerance of Semibalanus balanoides varies considerably with the size of the barnacle and its position on the shore. Desiccation tolerance increases with shore height and increasing body size but Semibalanus balanoides is prevented from growing higher on the shore due to its desiccation tolerance. Semibalanus balanoides is replaced by more desiccation tolerant Chthamalus stellatus higher on the shore.
Overall, the upper limit of most of the species in the biotope is likely to be reduced. Red foliose algae may be particularly intolerant and restricted to the bottom of the shore, and mobile amphipods will probably migrate to damper refuges and the lower shore, while Corallina officinalis was considered to be highly intolerant (see MarLIN reviews for details). Thus the upper limit of the biotope is likely to be depressed down the shore to be replaced by a barnacle dominated community with an absence or very low abundance of limpets and whelks (e.g. £ELR.Bpat.Lic£) and cause a loss of species richness due to loss of some red algae. Therefore, an intolerance of intermediate has been recorded. A recoverability of high has been recorded (see additional information below).
Increase in emergence regime
(View Benchmark)
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). An increase in emersion would subject the species in the biotope to greater desiccation and nutrient stress, leading to reduced growth and a depression in the upper limit of the species distribution on the shore. Only those barnacles at the extremes of their physiological limits will die. Limpets are able to move down the shore although the loss of a home scar can increase the species vulnerability to predation. Thus, the biotope is likely to be lost only at the upper limit of its range and so a rank of intermediate has been recorded. A change in the level of emergence on the shore may also affect the lower distribution limit of all the key species as competition increases lower down the shore and probably allow the biotope to colonize further down the shore. Recoverability is likely to be high (see additional information below).
Decrease in emergence regime
(View Benchmark)
A decrease in emergence will reduce exposure to desiccation and extremes of temperature and allow the resident Mytilus edulis to feed for longer periods and hence grow faster. Semibalanus balanoides is likely to be able to compete with chthamalids further up the spore. The decrease in emergence may allow red algae to increase in abundance. But at the lower extent of the biotope, a decrease in emergence is likely to result in an increase in macroalgal growth and increased predation, favouring lower shore biotopes (e.g. £MLR.R£, £ELR.Him£ or £ELR.Coff£). the biotope will effectively move further up the shore in the long term, although the extent or species richness of the biotope may be reduced during the process. Therefore an intolerance of low has been recorded. Recoverability is likely to be very high.
Increase in water flow rate
(View Benchmark)
The biotope is characteristic of extreme to moderate wave exposed conditions where water movement from wave action will greatly exceed the strength of any possible tidal flow. The biotope is therefore considered to be not sensitive.
Decrease in water flow rate
(View Benchmark)
The biotope is characteristic of extreme to moderate wave exposed conditions where water movement from wave action will greatly exceed the strength of any possible tidal flow. The biotope is therefore considered to be not sensitive.
Increase in temperature
(View Benchmark)
Increased temperature is likely to favour chthamalid barnacles rather than Semibalanus balanoides (Southward et al. 1995). Chthamalus montagui and Chthamalus stellatus are warm water species, with a northern limit of distribution in Britain so are likely to be tolerant of long term increases in temperature, while Semibalanus balanoides is boreal and at its southern limit the British Isles. Thus, an increase in temperature may lead to a change in the dominant species of barnacle but result in a change in biotope.

Most other species within the biotope are eurythermal (e.g. Mytilus edulis, Patella vulgata and Nucella lapillus) and are hardy intertidal species that tolerate long periods of exposure to the air and consequently wide variations in temperature. In addition, most species are distributed to the north of south of the British Isles and unlikely to be adversely affected by long term temperature changes at the benchmark level (see MarLIN reviews). Corallina officinalis, however, experienced severe damage during the unusually hot summer of 1983 (Hawkins & Hartnoll, 1985). Nucella lapillus is probably relatively tolerant of temperature change within the normal range for the British Isles, but an acute temperature change (e.g. 5 °C) will probably interfere with feeding activity and in summer may result in direct mortality or indirect mortality due to heat coma and desiccation. However, a reduction in dog whelk abundance is likely to increase longevity in barnacles and mussels, favouring dominance by either species but not adversely affecting the biotope.

Therefore, an increase in temperature at the benchmark level is likely to be result in changes in the dominant barnacle species, and a decrease in abundance of Nucella lapillus and Corallina officinalis but otherwise only sub-lethal effects. Thus, the biotope is recorded as having low intolerance to an increase in temperature. Recoverability is probably very high.
Decrease in temperature
(View Benchmark)
A decrease in temperature will favour the boreal Semibalanus balanoides rather than the southern chthamalid barnacles which may lead to change in species dominance. However, a change in the species of barnacle will not change the biotope.

Patella vulgata is largely unaffected by short periods of extreme cold. Ekaratne & Crisp (1984) found adult limpets continuing to grow over winter when temperatures fell to -6°C, and stopped only by still more severe weather. Mytilus edulis is eurythermal and tolerant of freezing conditions can probably survive occasional, sharp frost events, but may succumb to consistent very low temperatures over a few days. However, Mytlius edulis was relatively unaffected by the severe winter of 1962/63 (Crisp, 1964). Nucella lapillus can probably survive temperatures as low as 3 °C and possibly 0 °C, although evidence for duration is lacking, the effects of low temperatures being sub-vital (see MarLIN reviews). Bousfield (1973), reported that amphipod tolerance to extremes of temperature is low. However, they probably derive protection within the macroalgal fronds or mussel matrix.

Overall, the dominant characterizing species will probably survive short term acute or long term chronic decreases in temperature at the benchmark level, while some mobile species may be lost by migration, reducing species richness. Therefore, an intolerance of low has been recorded to represent sublethal effects on growth and reproduction.
Increase in turbidity
(View Benchmark)
Increased turbidity may reduce phytoplankton primary productivity, therefore reducing the food available to Mytilus edulis and other suspension feeders. However, mussels and barnacles use a variety of food sources and the effects are likely to be minimal, and these species are probably not sensitive to changes in turbidity. Increased turbidity will decrease photosynthesis and primary productivity in seaweeds when immersed but they will probably be able to compensate when emersed. Therefore an intolerance of low has been recorded.
Decrease in turbidity
(View Benchmark)
Decreased turbidity may increase phytoplankton primary productivity, therefore potentially increasing the food available to Mytilus edulis and other suspension feeders but mussels and barnacles use a variety of food sources and the effects are likely to be minimal, and these species are probably not sensitive to changes in turbidity. Macroalgae may benefit from decreased turbidity resulting in rapid growth, especially of ephemeral green algae. Increase algal growth could smother the mussels and barnacles. But limpet grazers are abundant and will probably compensate for the increased growth. Therefore, an intolerance of low has been recorded.
Increase in wave exposure
(View Benchmark)
This biotope occurs from moderately to extremely wave exposed habitats. Increased wave exposure will probably result in an increase in the patchy nature of mussels and spaces within the barnacles cover due to older individuals being removed by wave action. In addition, dog whelks will be increasingly restricted to sheltered cracks and gullies, reducing predation on barnacles and mussels. The biotope may colonize a wider area as spray reduces the effects of desiccation further up the shore. More wave exposure tolerant species of algae, e.g. Porphyra species will dominate the red algal population.
The biotope is characteristic of wave exposed habitats and unlikely to be adversely affected by an increase in wave exposure and not sensitive has been recorded.
Decrease in wave exposure
(View Benchmark)
This biotope occurs from moderately to extremely wave exposed habitats. More wave sheltered rocky shore habitats tend to be dominated by macroalgae, especially fucoids (e.g. Fucus vesiculosus and Fucus serratus). Therefore, a decrease in wave exposure from e.g. moderately exposed to very sheltered is likely to result in marked changes in the community and loss of the biotope. The biotope may become replaced by typically sheltered biotopes, e.g. £SLR.Fves£, £SLR.Asc£, or £SLR.Fser£, depending on location. Therefore, an intolerance of high has been recorded. Recoverability is likely to be moderate (see additional information below).
Noise
(View Benchmark)
Mytilus edulis can probably detect slight vibrations in its immediate vicinity, however, it probably detects predators by touch (on the shell) or by scent. Limpets react to vibration but only within a very limited range. Similarly most of the invertebrates within the community are not sensitive to noise at the benchmark level. However, wildfowl are a major predator and several species are highly intolerance of noise. Therefore, noise at the level of the benchmark may disturb predatory wildfowl, so that the mussel and limpet populations may benefit indirectly. Overall, the biotope is unlikely to be adversely affected.
Visual Presence
(View Benchmark)
Most macro-invertebrates have poor or short range visual perception and are unlikely to be affected by visual disturbance such as by boats or humans on the shore. The biotope is therefore, considered to be not sensitive to the factor. However, wildfowl are highly intolerant of visual presence and are likely to be scared away by increased human activity, therefore reducing the predation pressure on the mussels and limpets especially. Therefore, visual disturbance may be of indirect benefit to mussel and limpet populations.
Abrasion & physical disturbance
(View Benchmark)
Physical disturbance due to wave driven debris (e.g. logs) or sediment abrasion will remove patches of mussels (see Daly & Mathieson, 1977, Seed & Suchanek, 1992) and probably areas of barnacles.

Rocky shore communities may also suffer physical disturbance due to trampling. Brosnan & Cumrine (1994) reported that beds of mussels were intolerance of trampling, depending on the thickness and density of the bed. In areas of intense trampling, mussels were uncommon and restricted to crevices. Trampling also reduced recruitment, and hence recovery rates (Brosnan & Cumrine, 1994). Small patches of mussels may be more susceptible to trampling damage. Brosnan & Cumrine (1994) also reported that barnacles were crushed by trampling and that Mastocarpus stellatus was intolerance of moderate trampling. Coralline algal turf was adversely affected by trampling, which resulted in a 50% reduction in turf height in one study (Brown & Taylor, 1999; Schiel & Taylor, 1999). Schiel & Taylor (1999) and Fletcher & Frid (1996) reported that trampling resulted in loss of species richness and formation of bare space in intertidal macroalgal assemblages. Amphipods, gastropods or small limpets may be crushed by trampling.

Therefore, a proportion of the population or mussels and barnacles may be lost, together with members of the associated community and macroalgae due to trampling (see species reviews for details) and an intolerance of intermediate has been recorded. Recovery is probably high (see additional information below). Severe abrasion (e.g. the barge stranding described by Bennell, 1981) will be similar to substratum removal in effect (see above).
Displacement
(View Benchmark)
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 oyster-catchers. Mussels can attach to a wide range of substrata and, if displaced, are likely to be able to attach themselves using byssus threads to a suitable substratum quickly. While a proportion of the mussels themselves would survive, displacement would result in loss of its associated community.

Barnacles are permanently fixed to the substratum and would die if removed. Similarly, macroalgal are permanently attached to their substratum, unable to reattach, and would be swept to deep water or the strandline and lost. Limpets may be dislodged by trampling and have difficulty reattaching if their foot is damaged (Prof. Steve J. Hawkins pers comm.). Displaced individuals with their foot exposed to the air will die due to desiccation if not predation. Therefore, a proportion of the limpet population may be lost. Mobile gastropods, isopods and amphipods will probably find other refuges or food sources on the shore and be little affected.

Displacement of key species would result in loss of their associated community, so that while the mussels may survive, their associated community, the barnacles and their associated community and hence the biotope would be lost. Therefore, an intolerance of high has been recorded. Recovery is probably moderate (see additional information below).

Chemical Factors

Synthetic compound contamination
(View Benchmark)
Chemical contaminants are likely to have a variety of effects depending the specific nature of the contaminant and the species group(s) affected.
  • Barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved (Holt et al., 1995).
  • Similarly, most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods, isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999). The pesticide ivermectin is very toxic to crustaceans, and has been found to be toxic towards some benthic infauna such as Arenicola marina (Cole et al., 1999).
  • Hoare & Hiscock (1974) reported that Patella vulgata was excluded from sites within 100-150m of the discharge of acidified, halogenated effluent in Amlwch Bay.
  • Limpets are also extremely intolerance of aromatic solvent based dispersants used in oil spill clean-up. During the clean-up response to the Torrey Canyon oil spill nearly all the limpets were killed in areas close to dispersant spraying. Viscous oil will not be readily drawn in under the edge of the shell by ciliary currents in the mantle cavity, whereas detergent, alone or diluted in sea water, would creep in much more readily and be liable to kill the limpet (Smith, 1968).
  • Loss of limpet grazers, as well as other gastropods and amphipod grazers, after the Torrey Canyon and other oil spills, results in a bloom of ephemeral green and brown algae, including fucoids (Suchanek, 1993; Raffaelli & Hawkins, 1999). After the Torrey Canyon spill, the resultant bloom smothered surviving barnacles, resulting in additional mortality.
  • Nucella lapillus is highly intolerance of TBT contamination, which may kill females at concentrations above 5 ng Sn/l, and cause imposex and hence reduced reproductive capacity at lower concentration and subsequently cause population decline due to natural mortality and poor recruitment (see MarLIN review for details).
  • The effects of contaminants on Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarised in the MarLIN review and Holt et al. (1998). A variety of chemical contaminants have been shown to produce sublethal effects and reduce scope for growth (e.g. PCBs, and organo-chlorides) (Widdows et al., 1995), while others (e.g. the detergent BP1002, the herbicide trifluralin and TBT) cause mortalities.
  • Red algae are probably intolerance of chemical contamination. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil contamination, although the filamentous forms were the most sensitive. Laboratory studies of the effects of oil and dispersants on several red algae species, including Palmaria palmata (Grandy, 1984 cited in Holt et al., 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. Cole et al. (1999) suggested that herbicides, such as simazina and atrazine were very toxic to macrophytes.
  • In addition, Hoare & Hiscock (1974) noted that almost all red algae were excluded from Amlwch Bay, Anglesey by acidified halogenated effluent discharge.
The Torrey Canyon oil spill probably represents a worst case scenario, which resulted in mass mortality of a wide variety of species including the space occupying barnacles and their grazers, and subsequent major fluctuations in the community over the next 2-15 years (Southward & Southward, 1978; Hawkins & Southward, 1992; Suchanek, 1993; Raffaelli & Hawkins, 1999). Overall, an intolerance of high has been recorded. Recoverability is likely to be moderate but may take longer in some locations (see additional information below).
Heavy metal contamination
(View Benchmark)
Heavy metal contamination affects different taxonomic groups and species to varying degrees.
  • The effects of contaminants on Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarised in the MarLIN review. Heavy metals were reported to cause sublethal effects and occasionally mortalities in mixed effluents.
  • Barnacles may tolerate fairly high level of heavy metals in nature, for example they possess metal detoxification mechanisms and are found in Dulas Bay, Anglesey, where copper reaches concentrations of 24.5 µg/l, due to acid mine waste (Foster et al., 1978; Rainbow, 1984).
  • Bryan (1984) suggested that adult gastropod molluscs (e.g. Littorina littorea, Patella vulgata and Nucella lapillus) were relatively tolerant of heavy metal pollution.
  • Crustaceans are generally regarded to be intolerant of cadmium (McLusky et al., 1986). In laboratory investigations Hong & Reish (1987) observed 96 hour LC50 (the concentration which produces 50% mortality) of between 0.19 and 1.83 mg/l in the water column for several species of amphipod.
  • Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. However, it is generally accepted that adult fucoids are relatively tolerant of heavy metal pollution (Holt et al., 1997).
  • Overall, heavy metals are likely to cause sublethal effects in the key species, their grazers and invertebrate predators but cause mortalities in some members of the community, and perhaps occasional mortality in mussels and barnacles in severe cases. Therefore, an intolerance of intermediate has been recorded, albeit at very low confidence. Recoverability is probably high (see additional information below).
Hydrocarbon contamination
(View Benchmark)
Hydrocarbon contamination, e.g. from spills of fresh crude oil or petroleum products, may cause significant loss of component species in the biotope, through impacts on individual species viability or mortality, and resultant effects on the structure of the community (Suchanek, 1993; Raffaelli & Hawkins, 1999).
  • The effects of contaminants on Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarised in the MarLIN review and Holt et al. (1998). Overall, hydrocarbon tissue burden results in decreased scope for growth and in some circumstances may result in mortalities, reduced abundance or extent of Mytilus edulis (see review).
  • Littoral barnacles (e.g. Semibalanus balanoides) have a high resistance to oil (Holt et al., 1995) but may suffer some mortality due to the smothering effects of thick oil (Smith, 1968).
  • Gastropods (e.g. Littorina littorea and Patella vulgata) and especially amphipods have been shown to be particularly intolerant of hydrocarbon and oil contamination (see Suchanek, 1993).
  • Similarly, laboratory studies of the effects of oil and dispersants on several red algae species (Grandy 1984 cited in Holt et al. 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination.
Loss of limpet grazers, as well as other gastropods and amphipod grazers, after the Torrey Canyon and other oil spills, results in a bloom of ephemeral green and brown algae, including fucoids (Smith, 1968; Suchanek, 1993; Raffaelli & Hawkins, 1999). After the Torrey Canyon spill, the resultant bloom smothered surviving barnacles, resulting in additional mortality.

Oil spills (e.g. the Torrey Canyon oil spill) result in mass mortality of a wide variety of species including the space occupying barnacles and their grazers, and subsequent major fluctuations in the community over the next 2-15 years (Southward & Southward, 1978; Hawkins & Southward, 1992; Suchanek, 1993; Raffaelli & Hawkins, 1999). Therefore, an intolerance of high has been recorded. Recoverability is likely to be moderate but may take longer in some locations (see additional information below).
Radionuclide contamination
(View Benchmark)
Insufficient information
Changes in nutrient levels
(View Benchmark)
A slight increase in nutrient levels could be beneficial for barnacles and mussels by promoting the growth of phytoplankton levels and therefore increasing zooplankton levels. Limpets and other grazers would also benefit from increased growth of benthic microalgae. However, Holt et al. (1995) predict that smothering of barnacles or mussels by ephemeral green algae is a possibility under eutrophic conditions.

Nutrient enrichment may also lead to eutrophication and associated increases in turbidity and suspended sediments (see above), deoxygenation (see below) and the risk of algal blooms.

Mussels are suspension feeders and accumulate toxins from toxic algae resulting in closure of shellfish beds (Shumway, 1992). The toxic algal blooms themselves have been shown to cause tumours, sublethal effects, reproductive failure and to be highly toxic to Mytilus edulis, and result in mass mortalities in the dog whelk Nucella lapillus (Pieters et al., 1980; Shumway, 1990; Landsberg, 1996; Holt et al., 1998; Gibbs et al., 1999).

Therefore, algal blooms or smothering by ephemeral algae may result in loss of a proportion of the biotope and its associated species and an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information).
Increase in salinity
(View Benchmark)
This biotope occurs in full salinity and is unlikely to experience an increase in salinity, save due to short term evaporation of interstitial water.
Decrease in salinity
(View Benchmark)
Mytilus edulis and Semibalanus balanoides are tolerant of a wide range of salinity (see MarLIN reviews for details). Barnacles can survive periodic emersion in freshwater, e.g. from rainfall or fresh water run off, by closing their opercular valves (Foster, 1971b). They can also withstand large changes in salinity over moderately long periods of time by falling into a "salt sleep". Similarly, most of the characterizing species (e.g. Littorina littorea and Patella vulgata) are found in a wide range of salinities and are probably tolerant of variable or reduced salinity. The intertidal interstitial invertebrates and epifauna probably experience short term fluctuating salinities, with reduced salinities due to rainfall and freshwater runoff when emersed.
Prolonged reduction in salinity, e.g. from full to reduced due to e.g. freshwater runoff, is likely to reduce the species richness of the biotope due to loss of less tolerant red algae and some intolerant invertebrates. However, the dominant species will probably survive and the integrity of the biotope is likely to be little affected. Areas of freshwater runoff in the intertidal promote the growth of ephemeral greens, probably due to their tolerance of low salinities and inhibition of grazing invertebrates. Therefore, an intolerance of low has been recorded, together with a decline in species richness. Recoverability is likely to be very high (see additional information below).
Changes in oxygenation
(View Benchmark)
In moderately wave exposed to exposed habitats the resultant water movement and turbulence probably provides adequate oxygenation so that deoxygenation at the benchmark is unlikely to occur except under extreme circumstances.

Biological Factors

Introduction of microbial pathogens/parasites
(View Benchmark)
Mytilus species host a wide variety of disease organisms. parasites and commensals from many animal and plant groups including bacteria, blue green algae, protozoa, boring sponges, boring polychaetes, boring lichen, the intermediary life stages of several trematodes, the copepod Mytilicola intestinalis (red worm disease) and decapods e.g. the pea crab Pinnotheres pisum (Bower, 1992; Bower & McGladdery, 1996). Bower (1992) noted that mortality from parasitic infestation in Mytilus sp. was lower than in other shellfish in which the same parasites or diseases occurred. Mortality may result from the shell boring species such as the polychaete Polydora ciliata or sponge Cliona celata, which weaken the shell increasing the mussels vulnerability to predation (see MarLIN review for details). Barnacles are parasitised by a variety of organisms and, in particular, the cryptoniscid isopod Hemioniscus balani , in which heavy infestation can cause castration of the barnacle. Intertidal gastropods often act a secondary hosts for trematode parasites of sea birds. For example, Nucella lapillus may be infected by cercaria larvae of the trematode Parorchis acanthus. Infestation causes castration and continued growth (Feare, 1970b; Kinne, 1980; Crothers, 1985).
Overall, the occurrence of diseases and parasites are probably highly variable but significant infestations may result in loss of the proportion of the mussel or barnacle population and important members of the community, either through mortality or reproductive failure. Therefore, an intolerance of intermediate has been recorded. Recovery is likely to be high (see additional information below).
Introduction of non-native species
(View Benchmark)
The Australasian barnacle Elminius modestus was introduced to British waters on ships during the second world war. The species does well in estuaries and bays, where it can displace Semibalanus balanoides and Chthamalus montagui. However, its overall effect on the dynamics of rocky shores has been small as Elminius modestus has simply replaced some individuals of a group of co-occurring barnacles (Raffaelli & Hawkins, 1999).
The South American mytilid Aulocomya ater was reported recently in the Moray Firth, Scotland in 1994 and again in 1997 (McKay, 1994; Holt et al., 1998; Eno et al., 1997). 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). However, there is no evidence of competition at present. Overall, there is little evidence of this biotope being adversely affected by non-native species.
Extraction
(View Benchmark)
The only regularly harvested species to occur in this biotope are Mytilus edulis and Patella vulgata. Holt et al., (1998) suggest that when collected by hand at moderate levels using traditional skills mussel beds will probably retain most of their biodiversity. 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. Prolonged un-regulated collection may result in loss of the bed e.g. a small bed close to a road on Anglesey was almost eliminated by anglers and bait diggers over a period of years (Holt et al., 1998).

Patella vulgata is occasionally harvested by hand, without regulation, for human consumption. The delicate balance between limpets and algae is easily disturbed by even a small, localised temporary absence of limpets (Southward & Crisp, 1956; Southward, 1964; Hawkins, 1981; Hawkins et al., 1983). Removal of limpets at the benchmark level of 50% is likely to result in significant changes in community composition.

The edible winkle Littorina littorea is also harvested by hand, without regulation, for human consumption. In some areas, notably Ireland, collectors have noted a reduction in the number of large snails available (see MarLIN review). Removal of Littorina littorea will reduce their grazing of macroalgae but limpets and other littorinids will probably compensate.

Overall, there is likely to be a reduction of the extent of the mussels in the biotope and its associated species, while removal of limpets may have significant effects on the community. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high.

Additional information icon Additional information

Recoverability
Mytilus edulis is highly fecund but larval mortality is high. Larval development occurs within the plankton over ca 1 month (or more), with high dispersal potential.

Patches of mussels on the high shore and population of juveniles on filamentous substrata (e.g. macroalgae) probably contribute to local recruitment (Holt et al., 1998). Larval supply and settlement could potentially occur annually, however, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). Once settled, Mytilus edulis can reproduce within its first year if growth conditions allow.

On rocky shores, gaps in mussel beds are often colonized by barnacles and fucoids, barnacles enhancing subsequent recruitment of mussels. Cycles of loss and recruitment leads to a patchy distribution of mussels on rocky shores. High intertidal and less exposed sites recovered slower than low shore, more exposed sites. Several long term studies showed that gaps took a long time to heal, but in some cases enlarged (presumably due to wave action and predation), with little recovery within 3-5 years, leading to estimated recovery times of 8-34 years (Pain & Levin, 1981) or several hundreds of years (Seed & Suchanek, 1992). Mytilus edulis populations were considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchanek, 1992). While good annual recruitment is possible, recovery may take at least 5 years but significantly longer in certain circumstances and some environmental conditions.

Bennell (1981) observed that barnacle populations removed when the surface rock was scraped off in a barge accident at Amlwch, North Wales returned to pre-accident levels within 3 years. However, barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors, such as wind direction and the presence of adults as an inducement for larvae to settle, therefore populations may take longer to recover.

Studies of community recovery after the Torrey Canyon oil spill have suggested that sites affected by oil alone recovered rapidly, within 3 years. However, areas that were treated with dispersants resulted in death of a large number of invertebrates, especially grazing gastropods and barnacles due to smothering by the subsequent bloom of macroalgae. Recovery was variable, with heavily impacted areas taking 2->10 years to regain their prior species richness, whilst many shores recovered in 5-8 years while it was estimated that the worst affected shores would take ca 15 years (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999).

Recruitment in mobile species may be rapid once the mussel matrix develops. Macroalgae such as ephemeral greens (e.g. Ulva spp.) with recruit rapidly and Fucus vesiculosus recruits readily to cleared areas of the shore and full recovery takes 1-3 years (Holt et al., 1997).

Overall, the rate of recovery is probably dependant on the degree and nature of the disturbance. Where the dominant species (barnacles and mussels) are reduced or removed but other members of the community remain or adults can recruit from the surrounding area, recovery will probably be rapid, within ca 5 years. Similarly, if other members of the community alone are reduced or removed (e.g. mobile epifauna or macroalgae) recovery is likely to be rapid. However, recovery from a significant impact, especially the mass mortality of grazing gastropods, is likely to result in marked affects on the community, that may take between 5-10 years to recover and in some cases about 15 years.

This review can be cited as follows:

Tyler-Walters, H. 2002. Mytilus edulis and barnacles on very exposed eulittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=203&code=2004>