Barnacles and Littorina littorea on unstable eulittoral mixed substrata

29-11-2002
Researched byJacqueline Hill Refereed byThis information is not refereed.
EUNIS CodeA2.431 EUNIS NameBarnacles and Littorina spp. on unstable eulittoral mixed substrata

Summary

UK and Ireland classification

EUNIS 2008A2.431Barnacles and Littorina spp. on unstable eulittoral mixed substrata
EUNIS 2006A2.431Barnacles and Littorina spp. on unstable eulittoral mixed substrata
JNCC 2004LR.FLR.Eph.BLitXBarnacles and Littorina spp. on unstable eulittoral mixed substrata
1997 BiotopeLR.SLR.FX.BlitBarnacles and Littorina littorea on unstable eulittoral mixed substrata

Description

Banks of cobbles and pebbles which are too unstable or too small to support fucoids are usually colonised by the barnacles Semibalanus balanoides (often with Elminius modestus) with Patella vulgata on larger rocks and often dense aggregations of Littorina littorea. Between the cobbles and pebbles Mytilus edulis often occurs, but always at low abundance (compare with SLR.MytX). Juvenile Carcinus maenas occur between pebbles, and where patches of sediment occur, infaunal species such as the lugworm Arenicola marina and cockle Cerastoderma edule may be present. Fucoids are rare in this biotope. This biotope covers a wide range of wave exposures from mobile exposed and unstable shores to more sheltered stony shores in which fucoids are unable to attach and Mytilus edulis is infrequent. [Further data required to split this biotope into two: open coast cobble sites and estuarine stony sediment sites]. (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Recorded distribution in Britain and Ireland

Present on all rocky coasts of Britain but most common on the west coast. Recorded from two sites on the west coast of Ireland.

Depth range

-

Additional information

None entered

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JNCC

Habitat review

Ecology

Ecological and functional relationships

  • The SLR.BLlit biotope is found in a range of wave exposures. In exposed locations disturbance is high creating small scale succession events so that only fast growing opportunistic algal species such as Ulva are able to grow. However, the abundance of green algae in the biotope is low because of the grazing activity of Littorina littorea which occur in high abundance. In sheltered locations the substrata is more stable and fucoid sporelings may settle but are removed by the grazing activity of limpets and Littorina littorea. Thus, because of the impact of disturbance and/or grazing, algal cover is very low in the whole range of exposure in which the biotope is found.
  • The pebble and cobble beaches of SLR.BLlit have a poor fauna in comparison to open shore locations on bedrock, presumably as a result of siltation and the instability of the substratum. There is a covering of barnacles on the cobbles and pebbles and on larger stable boulders and rock Patella vulgata is present in high abundance. Littorina littorea, which is tolerant of muddy and silty conditions, can be found in large aggregations and often cluster on the tops of small stones. Although Mytilus edulis is less common on cobbles and pebbles than on larger boulders or bedrock, the species may serve to enhance the stability of the substratum.
  • Algal cover in the biotope is low and limited mostly to opportunistic green species such as Ulva spp. and Ulva spp.
  • In extremely sheltered locations, even the smallest stones are relatively stable but remain unoccupied by algal sporelings so that barnacles settle (Lewis, 1964; Raffaelli & Hawkins, 1999).
  • Littorina littorea is often the dominant grazing gastropod on the lower shore eating soft macrophytes and microalgae. Experiments in Helgoland (Janke, 1990) suggest that Littorina grazing can exclude the green alga Ulva and reduce the settlement and growth of Fucus species. Cover by opportunistic species like Ulva may be kept in check by littorinid grazing.
  • A dense covering of barnacle species is effective in limiting the efficiency of limpet grazing which adversely affects limpet growth. Bulldozing by grazing limpets may cause high post-settlement mortality of barnacles (Jenkins et al., 2000).
  • The crab Carcinus maenas is a predator of young Littorina littorea.
  • The characterizing species of the sediment beneath the pebbles and cobbles are infaunal such as the obligate deposit feeding Arenicola marina.

Seasonal and longer term change

Rocky shore communities are often highly variable in time, due to the combined influences of physical disturbance, competition, grazing, predation and variation in recruitment. Barnacle dominated rocky shores demonstrate dynamic temporal changes, mediated by relatively random events such as recruitment intensity, and the abundance of grazers and predators (Hawkins et al., 1992; Raffaelli & Hawkins, 1999). Settlement of Semibalanus balanoides takes place in the spring and Chthamalus spp. in the summer and autumn. Seasonal fluctuations in the abundance of Ulva spp. may also be seen.

Habitat structure and complexity

Habitat complexity in this biotope is relatively limited in comparison to some rocky shore biotopes. However, the mixed nature of pebbles and cobbles, boulders, rocks and coarse sediment does create some complexity. Larger cobbles and boulders provide substratum and shelter for a variety of species such as small crabs and gammarid amphipods. Beneath boulders and the largest cobbles and pebbles (if free of sediment) underboulder communities may be present. Smaller pebbles and cobbles will be too small and too unstable (e.g. subject to overturn) for some encrusting species to persist.

Productivity

In the absence, or low abundance, of macroalgae, production in this biotope is mostly secondary production by suspension and deposit feeders. Primary production will be limited to microalgae growing on rock surfaces. Detrital input will be important for the suspension feeding barnacles and mussels. Rocky shores can make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains. In general rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Hill et al., 1998). However, in the SLR.BLlit biotope, faunal species may not attain the same biomass that may be found on stable rocky substrata on the open coast, so secondary productivity is likely to be lower.

Recruitment processes

Most species present in the biotope possess a planktonic stage (gamete, spore or larvae) which float in the plankton before settling and metamorphosing into the adult form. This strategy allows species to rapidly colonize new areas that become available such as in the gaps often created by storms. Thus, for organisms such as those present in this biotope, it has long been evident that recruitment from the pelagic phase is important in governing the density of populations on the shore (Little & Kitching, 1996). Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.
  • Littorina littorea can breed throughout the year but the length and timing of the breeding period are extremely dependent on climatic conditions. Also, estuaries provide a more nutritious environment than the open coast (Fish, 1972). Sexes are separate, and fertilisation is internal. Littorina littorea sheds egg capsules directly into the sea. Egg release is synchronized with spring tides and occurs on several separate occasions. In estuaries, the population matures earlier in the year and maximum spawning occurs in January (Fish, 1972). Fecundity value is up to 100,000 for a large female (27mm shell height) per year. Female fecundity increases with size. Larval settling time or pelagic phase can be up to six weeks. Males prefer to breed with larger, more fecund females (Erlandsson & Johannesson, 1992). Parasitism by trematodes may cause sterility in Littorina littorea.
  • Barnacle settlement and recruitment can be highly variable because it is dependent on a suite of environmental and biological factors, such as wind direction and success depends on settlement being followed by a period of favourable weather. Long term surveys have produced clear evidence of barnacle populations responding to climatic changes. During warm periods Chthamalus spp. predominate, whilst Semibalanus balanoides does better during colder spells (Hawkins et al., 1994). Release of Semibalanus balanoides larvae takes place between February and April with peak settlement between April and June. Release of larvae of Chthamalus montagui takes place later in the year, between May and August.
  • Recruitment of Patella vulgata fluctuates from year to year and from place to place. Fertilization is external and the larvae is pelagic for up to two weeks before settling on rock at a shell length of about 0.2mm. Winter breeding occurs only in southern England, in the north of Scotland it breeds in August and in north-east England in September.
  • Mytilus edulis recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment (Seed & Suchanek, 1992). Mytilus sp. is highly gregarious and final settlement often occurs around or in-between individual mussels of established populations.
  • The infaunal polychaete Arenicola marina has high fecundity and the eggs develop lecithotrophically within the sediment or at the sediment surface. There is no pelagic larval phase and the juveniles disperse by burrowing. Recruitment must occur from local populations or by longer distance dispersal of postlarvae in water currents or during periods of bedload transport.
  • Ulva is a rapidly growing opportunistic species which can colonize bare substrata soon after it is created.

Time for community to reach maturity

No specific information was found concerning time taken for the community to reach maturity. However, the characterizing species of the SLR.BLlit biotope are widespread, highly fecund and quick to grow and mature and so the community would be expected to reach maturity within 5 years. For example, Bennell (1981) observed that barnacles that were 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, so populations may take longer to recruit to suitable areas. Littorina littorea is widespread and often common or abundant. Littorina littorea is an iteroparous breeder with high fecundity that lives for several (at least 4) years. Breeding can occur throughout the year and larvae form the main mode of dispersal. The planktonic larval stage is long (up to 6 weeks) although larvae do tend to remain in waters close to the shore. Most of the other species in the biotope have planktonic larvae and so should colonize suitable areas. Therefore, it seems likely that the biotope would reach maturity within five years. However, in newly created substrata, initial absence of grazing prosobranchs may allow first green, then brown algae to grow and dominate the shore until removed by scour or old age. In such cases the establishment of SLR.Bllit may take longer than five years.

Additional information

None

Preferences & Distribution

Recorded distribution in Britain and IrelandPresent on all rocky coasts of Britain but most common on the west coast. Recorded from two sites on the west coast of Ireland.

Habitat preferences

Depth Range
Water clarity preferences
Limiting Nutrients Not relevant
Salinity
Physiographic
Biological Zone
Substratum
Tidal
Wave
Other preferences Unstable substrata

Additional Information

This biotope is found in a range of wave exposure regimes from exposed to extremely sheltered.

Species composition

Species found especially in this biotope

    Rare or scarce species associated with this biotope

    -

    Additional information

    None

    Sensitivity reviewHow is sensitivity assessed?

    Explanation

    Barnacles, represented by Semibalanus balanoides, and Littorina littorea are important characterizing species as, in their absence, the SLR.BLlit biotope would not be recognized. Littorina littorea and Patella vulgata are key functional species because their grazing activity stops the development of algal cover. Other species that may be presents such as the epifaunal Patella vulgata, Littorina saxatilis and Mytilus edulis and the infaunal Arenicola marina and Cerastoderma edulis are not considered to be species particularly indicative of the sensitivity of the SLR.BLlit biotope as, in their absence, the biotope would become impoverished but would still be recognized as barnacles and Littorina littorea on unstable eulittoral mixed substrata.

    Species indicative of sensitivity

    Community ImportanceSpecies nameCommon Name
    Key functionalLittorina littoreaCommon periwinkle
    Key functionalPatella vulgataCommon limpet
    Important characterizingSemibalanus balanoidesAn acorn barnacle

    Physical Pressures

     IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
    High High Moderate Major decline High
    All key and important species in the biotope are highly intolerant of substratum loss. The barnacles are permanently attached to the substratum so populations would also be removed along with the substratum. Epifaunal grazers such as Littorina littorea and Patella vulgata are also likely to be removed because although mobile they cannot move rapidly enough to avoid the factor. The infaunal organisms, such as Arenicola marina and Cerastoderma edule, in sediment patches will also be removed. Therefore, the intolerance of the biotope to substratum loss is high because almost all faunal groups will be removed. Recovery is likely to be high - see additional information below for rationale.
    High High Moderate Minor decline Moderate
    Smothering by 5 cm of sediment would bury adult barnacles and prevent feeding although it is likely that barnacles can withstand smothering for some period of time. Monterosso (1930) showed experimentally that the chthamalid barnacles 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 and the chthamalids seemed unaffected. In some areas more than 90 % of the barnacles had managed to clear an opening in the oil film (Smith, 1968). Although oil had very little effect on individuals, it is likely that smothering by sediment can clog breathing apparatus. Smothering of Littorina littorea and limpets for one month is likely to interfere with locomotion, grazing and respiration. If the sediment is fluid and mobile, the molluscs are unlikely to be able to move through the layer of sediment and will probably die. Sediment will have an especially adverse effect on the settlement of larvae and spat. Infaunal organisms will be unaffected but suspension feeding mussels may be killed by smothering. The biotope occurs on sheltered shores so natural removal of sediment by wave action and water flow may take a long time. Therefore, as some of the key species are likely to be lost the intolerance of the biotope is recorded as high. Most characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area so recovery should be high - see additional information below.
    Low Immediate Not sensitive No change High
    Increased suspended sediment may reduce growth rates in barnacles due to the energetic costs of cleaning sediment particles from feeding apparatus. However, at the benchmark increase of 100mg/l for a month effects are likely to be minimal. Elminius modestus is more tolerant of turbidity than Semibalanus balanoides and Chthamalus species and may become the dominant barnacle. However, this will not alter the nature of the biotope. Littorina littorea also has low intolerance to an increase in suspended sediment because the species is found in turbid estuaries where suspended sediment levels are high. Other species in the biotope such as Patella vulgata and the infaunal organisms will be not sensitive. Therefore, at the level of the benchmark, the biotope is considered to have low intolerance. On return to pre-impact conditions recovery will be rapid. Normal feeding and cleaning rates will resume as soon as normal conditions return.
    Low Very high Not relevant No change High
    A decrease in suspended sediment may reduce food supplies for suspension feeding barnacles which may affect growth rates. However, for a period of a month any effects are likely to be minimal. None of the other key species in the biotope require a supply of suspended sediment particles for feeding or for activities such as tube building so the biotope is likely to have low intolerance to a decrease in the levels of suspended sediment for a month. Normal feeding and growth rates will resume as soon as normal conditions return.
    Low High Low No change Moderate
    A change in desiccation equivalent to a change in position of one vertical biological zone on the shore would adversely affect the biotope only where it is already near its upper limit on the shore because the key species, although relatively tolerant of desiccation, can only tolerate an increase up to a critical level of water content. Semibalanus balanoides may be replaced by the more desiccation tolerant Chthamalus montagui at the upper limits of the biotope. However, this will not alter the nature of the biotope. The upper range of littorinids and limpets and may be reduced as they move down the shore to avoid increasing desiccation. The overall impact is that 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 littorinids and limpets. Thus, the biotope is likely to be lost only at the very upper limit of its range and so a rank of low has been recorded. Recovery is likely to be high - see additional information for rationale.
    Low High Low No change Moderate
    An increase of one hour in the period of 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. However, only those barnacles at the extremes of their physiological limits will die. Littorinids and limpets are able to move down the shore to avoid the factor, although the loss of a limpets home scar can increase the species vulnerability to predation. Thus, the biotope is likely to be lost only at the very upper limit of its range and so a rank of low 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. Recovery is likely to be high - see additional information for rationale.
    Low High Low No change Moderate
    A decrease in the period of emersion will immerse animals at the bottom of the biotope in seawater for longer which may increase growth rates as the supply of oxygenated water and nutrients increase. However, competition from other species may increase and the biotope could change to another more species rich biotope. The overall effect could simply be a moving of the biotope up the shore so intolerance is assessed as low. Recovery is expected to be high - see additional information below for rationale.
    Intermediate Very high Low Minor decline Moderate
    Semibalanus balanoides can tolerate a wide range of water flow rates and because they are attached to the substratum will not be washed off. However, water flow rate is very important in determining the growth rate because of the supply of particles for feeding (Crisp, 1960). Littorina littorea is found in areas with water flow rates from negligible to strong. Increases in water flow rates above 6 knots may cause snails in less protected locations (e.g. not in crevices etc) to be continually displaced into unsuitable habitat so that feeding may become sub-optimal. Thus, populations of Littorina littorea are likely to reduce and so an intolerance of intermediate is given. In very strong water flow smaller cobbles and pebbles may be removed changing the nature of the substratum. On return to normal current levels the growth rate of the species would be quickly resumed.
    Not sensitive* No change High
    Semibalanus balanoides can tolerate a wide range of water current rates. However, water flow rate is very important in determining the growth rate and a decrease in water flow (where tidal flow is the main source of water movement) would lower growth rates (Crisp, 1960). On return to normal current levels the growth rate of the species would be quickly resumed. Littorina littorea is found in areas with water flow rates from negligible to strong so that a decrease in flow rate are not likely to have any effect. The intolerance of the biotope is recorded to be low. On return to normal conditions recovery will be rapid.
    Low Very high Very Low No change High
    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. Thus, an increase in temperature may lead to a change in the dominant species of barnacle. However, such a change will not alter the nature of the biotope. Littorina littorea distribution extends south from the British Isles where temperatures are higher and also survives in upper shore rockpools where temperature may exceed 30 °C. However, at water temperatures above about 20 °C growth rate is reduced. Therefore, the species is expected to have low intolerance to increases in temperature at the level of the benchmark. Normal metabolic rate can be re-established rapidly on return to better conditions. The other species in the biotope such as Patella vulgata and Mytilus edulis also have low intolerance to increases in temperature. Infaunal species are likely to be protected from temperature increases. Therefore, the impact on the biotope of temperature increases at the benchmark level are likely to be sub-lethal effects on growth and fecundity and so an intolerance rank of low is reported. On return to normal temperatures original metabolic activity will rapidly resume so recoverability is set to very high.
    Low Very high Moderate No change High
    A decrease in temperature will favour Semibalanus balanoides over other barnacle species (Southward et al. 1995). Adult Littorina littorea can easily tolerate sub-zero temperatures and the freezing of over 50 % of their extracellular body fluids. In colder conditions an active migration may occur down the shore to a zone where exposure time to the air (and hence time in freezing temperatures) is less. Other species in the biotope also how low intolerance to decreases in temperate. Long term chronic temperature decreases may slow down growth. Therefore, a benchmark decrease in temperature is likely to result in sub-lethal effects only and so the biotope is assessed as having low intolerance. On return to normal temperatures original metabolic activity will rapidly resume so recoverability is set to very high.
    Tolerant Not relevant Not relevant No change High
    The biotope is predominantly an assemblage of faunal species so the light attenuation effects of an increase in turbidity would not significantly affect the community. The little algal growth that is present is not likely to be significantly affected by a change in light attenuation because much photosynthesis occurs during emersion when the factor is not relevant. The impacts on suspension feeding organisms are addressed under 'suspended sediment' above.
    Tolerant Not sensitive* No change High
    The biotope is predominantly an assemblage of faunal species so the light attenuation effects of an increase in turbidity would not significantly affect the community. The little algal growth that is present is not likely to be significantly affected by a change in light attenuation because much photosynthesis occurs during emersion when the factor is not relevant. The impacts on suspension feeding organisms are addressed under 'suspended sediment' above.
    Intermediate High Low Minor decline High
    The SLR.BLlit biotope is only found on sheltered shores and is assessed as having high intolerance to increased wave exposure because of the nature of the substratum and its impact on the biota. The cobbles and pebbles in the biotope are likely to move much more as a result of increased wave oscillation. Epilithic species would probably accrue damage and Littorina littorea will become dislodged or damaged. Littorina littorea regularly have to abandon optimal feeding sites in order to avoid wave-induced dislodgement. This will result in a decreased growth rate (Mouritsen et al., 1999). Increases in wave exposure will probably cause a decrease in population size of Littorina littorea. Thus, the intolerance of the biotope to increased wave exposure is established as intermediate. However, recovery is likely to be good because key species are iteroparous and have planktonic larvae and so should be able to recolonize shores within a few years - see additional information below.
    Not sensitive* No change High
    The biotope normally occurs in locations that are extremely sheltered from wave action so an assessment of a further decrease is considered to be not relevant.
    Tolerant Not relevant Not relevant No change High
    None of the selected key or important species in the biotope are recorded as sensitive to noise although limpets do respond to vibration. However, the biotope as a whole is likely to be not sensitive to changes in noise levels at the level of the benchmark.
    Tolerant Not relevant Not relevant No change High
    Although many of the species in the biotope have eyes, visual perception is quite limited. The biotope is likely to be not sensitive to visual disturbances such as boats and humans on the shore.
    Intermediate High Low Minor decline Moderate
    The biotope is susceptible to abrasion and physical impact from trampling. Light trampling pressure, of 250 steps in a 20x20 cm plot, one day a month for a period of a year, has been shown to damage and remove barnacles (Brosnan & Crumrine, 1994). Trampling pressure can thus result in an increase in the area of bare rock on the shore (Hill et al., 1998). However, this biotope is characterized by unstable substrata, and probably experiences periodic episodes of severe abrasion from boulders turned or mobilised by wave action. Therefore, the species present are in all probability of at least intermediate intolerance to abrasion, a proportion of their populations being removed by abrasion. However, recovery is likely to be high (see additional information below) so that the biotope as a whole is probably of low sensitivity to abrasion.
    High High Moderate Decline High
    Intolerance to displacement is high because barnacles are permanently attached to the substratum and cannot re-establish themselves if detached. The epifaunal species, such as Littorina spp. and Patella vulgata, can re-establish themselves if displaced although they may be at risk of increased desiccation and predation if they are not returned foot down. Loss of barnacles results in a significant change in community composition and nature of the biotope so intolerance is recorded as high. Recovery is expected to be high - see additional information below.

    Chemical Pressures

     IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
    Intermediate High Low Decline Moderate
    Barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved (Holt et al., 1995). For example, in areas treated with dispersants following the Torrey Canyon oil spill, most Semibalanus balanoides were killed (Smith, 1968). Gastropod molluscs are known to be intolerant of endocrine disruption from synthetic chemicals such as tri-butyl tin (Cole et al., 1999). However, Littorina littorea is tolerant of high TBT levels (Oehlmann et al., 1998) and has been found to be well suited for TBT effect monitoring because the species exists in sufficient numbers for sampling even in regions where a relatively high level of contamination exists. The intolerance of Littorina littorea to other pollutants is unknown. However, since barnacles are intolerant of some pollutants the intolerance of the biotope is recorded as intermediate. Recovery is likely to be high - see additional information below.
    Heavy metal contamination
    Low Very high Very Low No change Moderate
    Intolerance of the biotope is low because the key species are fairly robust in terms of heavy metal pollution. Barnacles are able to concentrate heavy metals in their tissues and most of the information available suggests that adult gastropod molluscs are rather tolerant of heavy-metal toxicity (Bryan, 1984). Winkles may absorb metals from the surrounding water by absorption across the gills or from the diet, and evidence from experimental studies on Littorina littorea suggest that the diet is the most important source (Bryan et al., 1983). Littorina littorea has been suggested as a suitable bioindicator species for some heavy metals in the marine environment (Bryan et al., 1983). Recovery from sub-lethal effects will be very high as metabolism and growth return to normal.
    Hydrocarbon contamination
    High Moderate Moderate Decline High
    Littoral barnacles have a high resistance to oil (Holt et al., 1995). However, after the Torrey Canyon oil spill, some mortality of barnacles was caused by the oil although most had been able to form a hole in the covering of oil and were 'in good order' (Smith, 1968). Significant reductions in densities of Semibalanus balanoides were observed after the Exxon Valdez oil spill in 1989, especially at high and mid shore (Highsmith et al., 1996). Although barnacles survived on most shores, up to 98 % reduction in barnacle cover resulted from treatment by hot-water washing. However, recovery on most rocky shores was reported to have progressed considerably by July 1992 (Houghton, et al. 1996). Experience of and observations from oil spills such as the Sea Empress and Amoco Cadiz suggest that gastropod molluscs, such as Littorina littorea, are highly intolerant of hydrocarbon pollution. Patella vulgata is also known to be very intolerant of hydrocarbons (see species review). With the loss of grazers in the biotope algae may flourish, particularly the opportunistic greens such as Ulva in the early stages, significantly changing the nature of the biotope. Intolerance is, therefore, set to high. If the mollusc grazers do not recover green algae may then become replaced with the slower growing fucoids and the nature of the biotope will remain altered for a significant time. Recovery and return to SLR.BLlit may take many years and so a rank of moderate is given.
    Radionuclide contamination
    No information No information No information Insufficient
    information
    Not relevant
    There is insufficient information to make an assessment.
    Changes in nutrient levels
    Low Very high Very Low No change Moderate
    Little data exists on the effects of increased nutrients on the biotope. A slight increase in nutrient levels could be beneficial for barnacles by promoting the growth of phytoplankton levels and therefore increasing zooplankton levels. However, Holt et al. (1995) predict that smothering of barnacles by ephemeral green algae is a possibility under eutrophic conditions. Littorina littorea occurs on all British and Irish coasts, including lower salinity areas such as estuaries where nutrient loading is likely to be higher than elsewhere. Higher nutrient levels may benefit the algal substrata and food used by the snail. Overall, intolerance of the biotope is expected to low at the level of the benchmark and recovery is likely to be rapid.
    Not relevant Not relevant Not relevant Not relevant Not relevant
    The biotope occurs in areas of full salinity although will be subject to some variability because of rainfall in the intertidal. However, there are no reports of the biotope occurring in hypersaline areas such as rockpools where evaporation in the summer causes salinity to increase. In the laboratory, Semibalanus balanoides was found to tolerate salinities between 12 and 50 psu (Foster, 1970). Young Littorina littorea inhabit rock pools where salinity may increase above 35psu. Thus, key species may be able to tolerate some increase in salinity. However, in the field it is unlikely that the biotope would be subject to an increase in salinity and so the factor is ranked as 'not relevant'.
    Low Very high Moderate No change High
    Semibalanus balanoides can tolerate salinities between 12 and 50 psu, below this cirral activity ceases (Foster, 1970). 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". In this state motor activity ceases and respiration falls, enabling animals to survive in freshwater for three weeks (Barnes, 1953). Elminius modestus is more tolerant of lowered salinities than Semibalanus balanoides and so may replace it as the dominant barnacle. However, this will not change the nature of the biotope. Littorina littorea is found in waters of full, variable and reduced salinities and so populations are not likely to be highly intolerant of decreases in salinity. Therefore, it appears that the biotope would have low intolerance to a decrease in salinity. On return to normal conditions recovery is likely to be very rapid.
    Low Immediate Not sensitive No change High
    Semibalanus balanoides can respire anaerobically, so it can tolerate some reduction in oxygen concentration (Newell, 1979). When placed in wet nitrogen, where oxygen stress is maximal and desiccation stress is low, Semibalanus balanoides has a mean survival time of 5 days (Barnes et al., 1963). Littorina littorea can tolerate long periods of oxygen deprivation and so populations are unlikely to be significantly affected by deoxygenation at the benchmark level. Therefore, since the key species can probably survive low levels of oxygen for a week the intolerance of the biotope is recorded as low.

    Biological Pressures

     IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
    Low High Low No change Moderate
    Barnacles are parasitised by a variety of organisms and, in particular, the cryptoniscid isopod Hemioniscus balani. Heavy infestation can cause castration of the barnacle. Levels of infestation within a population vary. Once infected, recovery of an individual barnacle is unlikely. Parasitism by trematodes may cause sterility in Littorina littorea. There were no reported occurrences found of the biotope being affected by these or any other infestations so intolerance is reported to be low. However, there is always the potential for species in the biotope to be affected by pathogens or parasites so intolerance may change.
    Low High Low No change Moderate
    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). Although no other non-native species have been observed to compete with or prey upon the other species in the biotope there is always the potential for this to occur.
    Intermediate High Low Decline Moderate
    The key functional species Littorina littorea is 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. Large scale removal of Littorina littorea may allow a proliferation of opportunistic green algae, such as Ulva, on which it preferentially feeds. The community structure within the biotope is likely to be altered but some individuals are likely to remain. On balance, an intolerance of intermediate has been suggested with a high recovery.
    High High Moderate Decline High

    Additional information

    Recoverability
    Recovery of the biotope should be high as most species are abundant and widely distributed, are iteroparous and have planktonic larvae. For example, 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. Littorina littorea is widespread and often common or abundant. Littorina littorea is an iteroparous breeder with high fecundity that lives for several (at least 4) years. Breeding can occur throughout the year. The larvae form the main mode of dispersal with a long planktonic stage (up to 6 weeks) although larvae do tend to remain in waters close to the shore. Most of the other species in the biotope have planktonic larvae and so can recolonize the affected area. Therefore, it seems likely that within five years the community should be able to return to a pre-impact state so recovery is set to high. However, in cases where grazing prosobranchs are lost there is likely to be growth of first green and then brown algae which may then come to dominate the shore until removed by scour or old age. In such cases the re-establishment of SLR.Bllit may take longer than five years and a rank of moderate has been given.

    Importance review

    Policy/Legislation

    UK Biodiversity Action Plan Priority

    Exploitation

    Littorina littorea has some commercial value and is gathered by hand at a number of sites, particularly in Scotland and Ireland. Much demand comes from the French market (Irish Shellfish Association AGM, 1997)

    Additional information

    -

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    Citation

    This review can be cited as:

    Hill, J.M. 2002. Barnacles and Littorina littorea on unstable eulittoral mixed substrata. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/340

    Last Updated: 29/11/2002