|Researched by||Dr Heidi Tillin & Charlotte Marshall||Refereed by||This information is not refereed.|
Shallow pools on mixed cobbles, pebbles, gravel and sand characterized by abundant hydroids. Species present may include Obelia geniculata, Obelia dichotoma, Obelia longissima, Sertularia cupressina, Tublaria indivisa and Thuiaria thuja. The difficulty in identifying hydroids suggests many more species may be also be present. Other species typically found in this biotope include ephemeral green algae (Ulva sp.), red algae (Chondrus crispus and Coralline algae) and the winkle Littorina littorea. Within the pools, patches of sand may be occupied by the lugworm Arenicola marina and sand mason worms Lanice conchilega. These pools are often associated with mussel beds (MytX), with Mytilus edulis frequently recorded within the pools. Barnacles (Semibalanus balanoides and Austromius (Elminius) modestus) and the keel worm Spirobranchus (syn. Pomatoceros) triqueter may be attached to shells and small stones. Mobile species typical of rock pool habitats, such as Crangon crangon and Pomatoschistus minutus will also be found within the pool (JNCC, 2015).
|Depth Range||Mid shore|
|Water clarity preferences|
|Limiting Nutrients||Data deficient|
|Salinity preferences||Full (30-40 psu)|
|Physiographic preferences||Enclosed coast / Embayment|
|Biological zone preferences||Eulittoral|
|Substratum/habitat preferences||Cobbles, Pebbles, Gravel / shingle, Sand|
|Tidal strength preferences|
|Wave exposure preferences||Moderately exposed, Sheltered|
|Other preferences||Enclosed coasts (inlets, harbours)|
This MarLIN sensitivity assessment has been superseded by the MarESA approach to sensitivity assessment. MarLIN assessments used an approach that has now been modified to reflect the most recent conservation imperatives and terminology and are due to be updated by 2016/17.
|Community Importance||Species name||Common Name|
|High||Very high||Low||Major decline||High|
|The majority of the species associated with this biotope are attached permanently to the substratum and the removal of this substratum would result in the loss of the biotope. Ulva intestinalis, if detached from the substratum, may be buoyed up by gas and float to the surface where it continues to grow. However, the survival of this species in isolation will not constitute the biotope and therefore intolerance has been assessed as high. Recoverability is likely to be very high (see additional information).|
|High||Very high||Low||Major decline||Moderate|
|Obelia longissima forms long flexible colonies so that smothering material is likely to bend the colony flat against the substratum. In addition, local hypoxic conditions are also likely to inhibit growth. Although hydranths are likely to regress and portions of the colony are likely to die or be reabsorbed, parts of the colony are likely to become dormant, or otherwise survive for a period of at least a month. Ulva intestinalis is highly intolerant to smothering due to its filamentous form. It is likely to be completely smothered at the benchmark level and photosynthesis would be prevented due to lack of light. Furthermore, the thin fronds of the algae may start to rot. Littorina littorea will normally die if it cannot reach the surface within 24 hours (Chandrasekara & Frid, 1998). Under 5 cm of the mixed substratum associated with LR.H, it is possible that they may not reach the surface in this time. However, the journey to the surface is facilitated in well oxygenated sediments that contain fluid. The mussel Mytilus edulis and acorn barnacle Semibalanus balanoides may only be of intermediate intolerance to smothering (see MarLIN reviews) but these species are not characteristic of the biotope. |
On balance, intolerance has been assessed as high since the characterizing species are likely to experience high levels of mortality. Recoverability is expected to be very high (see additional information).
|Low||Immediate||Not sensitive||No change||Low|
|An increase in suspended sediment may have a deleterious effect on the suspension feeding community in LR.H. It is likely to clog their feeding apparatus to some degree, resulting in a reduced ingestion over the benchmark period and, subsequently, a decrease in growth rate. For hydroids especially this could potentially lead to a reduction in overall biomass. Moreover, because the rockpool has a 'pulsed' influx of water, the suspended sediment is likely to settle between tides and, over the course of one month, increase the depth of sediment in the pool. Some smaller immobile species including barnacle and tubeworms may be temporarily smothered. Furthermore, hydranths toward the lower reaches of the hydroid colonies may be smothered and regress. On balance, intolerance has been assessed as low since, at most, there may be a reduction in the overall hydroid standing biomass and this will not affect the recognizable biotope. Hydroids exhibit remarkable powers of regeneration and Obelia longissima (as commissularis) rapidly heals cut ends of stolons or branches within 1-2 min, and new growth can rapidly occur from the cut end or both ends of an excised piece of stolon (Berrill, 1949). Assuming the majority of the colonies remained, recovery has been assessed as immediate.|
|Tolerant*||Very high||Not sensitive||No change||Low|
|A decrease in suspended sediment is likely to benefit the community associated with LR.H. The suspension feeders may be able to feed more efficiently due to a reduction in time and energy spent cleaning feeding apparatus. Over the course of the benchmark the hydroids may increase in abundance and the mussels may experience an enhanced scope for growth. Therefore, tolerant* has been suggested.|
|High||Very high||Low||Major decline||Moderate|
|LR.H is found in the eulittoral. Moving the biotope up one vertical biological zone on the shore, in combination with the shallow nature of the pool, could mean that this biotope has the potential to dry out, especially if this shift coincided with hot, dry or windy weather. Alternatively, the biotope would be more at risk from freezing if the shift coincided with cold temperatures and icy winds.
Ulva intestinalis can survive several weeks of living in completely dried out rock pools, while becoming completely bleached on the uppermost layers, but remaining moist underneath the bleached fronds. The Littorina littorea may be found at the high tide level on the shore and will probably be able to find some refuge underneath the damp fronds of the Ulva intestinalis. During long periods of exposure to desiccating influences, Littorina littorea forms a dried mucus seal around the shell aperture thereby reducing evaporation. In contrast, Obelia longissima is highly intolerant desiccation and, at the benchmark level, the hydroids in LR.H are likely to experience mass mortality. Due to the fact that hydroids characterize this biotope, intolerance has been assessed as high. Even where the colonies are totally destroyed and/or removed, remaining resting stages or colony fragments, together with rapid growth and potentially good recruitment should result in rapid recovery. Recoverability has been assessed as very high.
|An increase in emergence would mean that this shallow rockpool would be at greater risk of desiccation (see above) and extremes of temperature (see below), since the pool would be exposed to the influences of air temperature for longer. The Ulva intestinalis may dry out and become bleached in the upper reaches of the rock pool although the majority of the plants would survive. The common periwinkles could move down into the wetter reaches of the pool when the pool wasn't immersed by the tide. The hydroids would most likely experience a decline in abundance since those in the upper reaches of the pool would die if the rockpool started to dry out. Overall, the biotope would probably remain but over a smaller area. Accordingly, intolerance has been assessed as intermediate but with a very high recovery (see additional information).|
|Tolerant*||Not sensitive||Rise||Very low|
|A decrease in emergence would mean that this shallow rock pool would be at less risk of desiccation. In addition, depending on the nature of the surrounding bedrock, the rockpool may become slightly deeper. As a result, it is possible that species diversity could increase as, for example, other hydroids colonized the pool. This could result in increased competition between the suspension feeders but, on the whole, LR.H is likely to be tolerant* of a decrease in emergence at the benchmark level.|
|High||Very high||Low||Major decline||Very low|
|LR.H is found in shallow eulittoral rock pools and is not expected to experience any water flow, unless they covered by the tide, apart from wind driven water movement. An increase in water flow rate and the benchmark level is likely to flush much of the sediment from the pool. This would result in the vast majority of the hydroids, ephemeral green algae, lugworms and sand masons being lost. The Littorina littorea, Mytilus edulis and Semibalanus balanoides may well remain but this would not constitute a recognizable biotope in terms of LR.H. Intolerance has therefore been assessed as high. Recoverability is expected to be very high.|
|LR.H is found in shallow eulittoral rock pools and is not expected to experience any water flow, unless they covered by the tide, apart from wind driven water movement. Obelia longissima and Littorina littorea have both been recorded in very weak tidal streams (no information on Ulva intestinalis) and tolerant has been suggested.|
|Due to the fact that LR.H is found in shallow eulittoral rockpools, the associated community must be adapted, to a certain degree, to frequent and often rapid changes in temperature. Air temperatures can be greatly elevated on hot days and due to the shallow nature of the pool, the water is likely to heat up quickly. Ulva intestinalis, Littorina littorea and Mytilus edulis all occur to the south of the British Isles and, because they are often found in upper shore rockpools, are likely to be tolerant to both chronic and acute increases in temperature. Semibalanus balanoides is pre-eminently a boreal species, adapted to cool environments. Reproduction in Semibalanus balanoides is inhibited by temperatures greater than 10 °C (Barnes, 1989) and it has been assessed as being of intermediate intolerance to an increase in temperature at the benchmark level (See MarLIN review).
Cornelius (1995b) suggested that numerous records in the Indo-Pacific were probably attributable to Obelia longissima and it is unlikely to be adversely affected by chronic temperature change at the benchmark level within the British Isles. However, Berrill (1949) reported that hydranths did not start to develop unless the temperature was less than 20 °C. Furthermore, Berrill (1948) reported that Obelia species were absent from a buoy in July and August during excessively high summer temperatures in Booth Bay Harbour, Maine, USA and the abundance of Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20 °C during this period (see MarLIN review). Therefore it is likely that Obelia longissima is highly intolerant to an acute rise in temperature at the benchmark level since temperatures in excess of 20 °C can reasonably be expected over summer in a shallow eulittoral rockpool. As Berrill (1948) suggested, other hydroids may be equally intolerant.Overall, therefore, intolerance of LR.H to an acute increase in temperature has been assessed as high. Recoverability is expected to be very high (see additional information).
|Due to the fact that LR.H is found in shallow eulittoral rockpools, the associated community must be adapted, to a certain degree, to frequent and often rapid changes in temperature. These pools may even freeze over during the coldest winter months.|
Kosevich & Marfenin (1986) reported that Obelia longissima was active all year round in the White Sea. Similarly, its northern limit lies in the Arctic Circle (Cornelius, 1995b; Stepanjants, 1998) suggesting that it probably tolerant of the lowest temperatures it is likely to encounter in Britain and Ireland. However, growth rates are reduced at low temperatures. Berrill (1949) reported that stolons grew, under optimal nutritive conditions, at less than 1 mm in 24 hrs at 10-12 °C, 10 mm in 24 hrs at 16-17 °C, and as much as 15-20 mm in 24 hrs at 20 °C.
Mytilus edulis can also withstand extreme cold and freezing, surviving when its tissue temperature drops to -10 °C (Williams, 1970; Seed & Suchanek, 1992) or exposed to -30°C for as long as six hours twice a day (Loomis, 1995). Bourget (1983) reported that cyclic exposure to otherwise sublethal temperatures, e.g. -8 °C every 12.4 hrs resulted in significant damage and death after 3-4 cycles. This suggests that Mytilus edulis can survive occasional, sharp frost events, but may succumb to consistent very low temperatures over a few days. Although Mytilus edulis may be intolerant of prolonged freezing temperatures, it is generally considered to be eurythermal.
Semibalanus balanoides is pre-eminently a boreal species, adapted to cool environments. An exceptional tolerance to cold is acquired in December and January and is lost between February and April. The median lethal temperature in January was -17.6 °C in air for 18 hours, whereas animals in June could only withstand -6.0 °C (Crisp & Ritz, 1967). Semibalanus balanoides was not affected during the severe winter of 1962-63 in most areas, except the south east coast which suffered 20-100% mortality. (Crisp, 1964).
On balance, intolerance of LR.H to a decrease in temperature at the benchmark level has been assessed as intermediate to reflect the possibility that some pool higher up the shore may freeze over. Recovery is expected to be very high (see additional information).
|Low||Immediate||Not sensitive||No change||Moderate|
|An increase in turbidity may reduce primary production in the rockpool. For pools further up the shore that have less contact with the sea to replenish the water and hence suspended matter, this may lead to a reduction on ingestion for suspension feeders. Over the course of the benchmark this may lead to a reduced scope for growth and Ulva intestinalis may also experience a slight reduction in growth. Intolerance has been assessed as low with an immediate recovery.|
|Tolerant*||Not sensitive||No change||Low|
|A decrease in turbidity is likely to enhance primary productivity within the biotope. This will directly benefit Ulva intestinalis and indirectly benefit Littorina littorea, Obelia longissima, Mytilus edulis and Semibalanus balanoides through secondary productivity. Bourget et al. (in press), for example, noted that for any given water temperature on buoys in the Gulf of St Lawrence, water transparency and primary production influenced the biomass of fouling organisms, including Obelia longissima. Biomass was reported to increase with increasing transparency up to a transparency of 15 m after which it decreased again (see Figure 2, Bourget et al., in press). Increased transparency was presumably correlated with increased primary production and hence food availability. Tolerant* has been recorded.|
|High||Very high||Low||Major decline||Low|
|An increase in wave exposure at the benchmark level is likely to adversely affect LR.H biotopes in the lower eulittoral. Both Ulva intestinalis and Littorina littorea are out of their preferred habitat in very wave exposed locations. If Littorina littorea were dislodged they are likely to be damaged, and may therefore become more susceptible to predation. Small patches of Mytilus edulis may also be susceptible to dislodgement in very exposed conditions. Dare (1976) noted that individual mussels swept or displaced from a mussel beds rarely survived, since they either became buried in sand or mud, or were scattered and eaten by oystercatchers. Obelia longissima is found in habitats with all levels of wave exposure because the branches and stems are flexible and probably able to withstand oscillatory flow (see Hunter, 1989). Semibalanus balanoides is tolerant of all levels of wave exposure. However, in LR.H pools in the lower eulittoral, very wave exposed shores would probably mean that the sediment would be flushed from the shallow pools, effectively removing the substratum (see above). In this case, the relative tolerance of Obelia longissima to increased wave exposure is irrelevant. It is likely that the entire biotope could be lost in the lower eulittoral and accordingly intolerance has been assessed as high. Recovery is expected to be very high.|
|A decrease in wave exposure is likely to adversely affect LR.H rockpools and it likely that, at the benchmark level, a different biotope will develop. The existence of LR.H is, in some respects, dependent on the influence of wave exposure. LR.H is dominated by ephemeral hydroids and seaweeds which thrive due to the disturbed nature of the habitat which prevents their competitive exclusion by late successional species. A reduction in wave exposure would remove this disturbance and therefore allow succession to take place in which the hydroids and ephemeral seaweeds would probably be out-competed by longer lived species. LR.H would be lost and, accordingly, intolerance has been assessed as high. Recoverability is expected to be very high (see additional information).|
|Tolerant||Not relevant||Not sensitive||Not relevant||Moderate|
|None of the important characterizing species in LR.H are thought to have effective mechanisms for detecting noise and are likely to be tolerant of noise at the benchmark level.|
|Tolerant||Not relevant||Not sensitive||Not relevant||Moderate|
|None of the important characterizing species in LR.H are thought to have effective mechanisms for detecting visual presence and are likely to be tolerant of visual presence at the benchmark level.|
|The existence of LR.H is, in some respects, dependent on the influence of physical disturbance such as sand scour. LR.H is dominated by ephemeral hydroids and seaweeds which thrive due to the disturbed nature of the habitat which prevents their competitive exclusion by late successional species. However, abrasion by an anchor or fishing gear could potentially destroy parts of the biotope, depending on the size of the pool and on the size off the impact. The delicate filamentous fronds of Ulva intestinalis will easily be scraped off the surface of the rock. Parts of the delicate Obelia longissima colonies are also likely to be removed. However, the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies (see displacement). If the shells of Littorina littorea or Mytilus edulis are damaged, the risk of predation and desiccation will increase. In most cases, it is very likely that some part of each population will remain and therefore, intolerance has been assessed as intermediate. Recovery is likely to be very high (see additional information).|
|Intermediate||Very high||Low||Minor decline||Moderate|
|If Littorina littorea was picked up and moved somewhere else it is unlikely that it would experience any adverse effects. Despite the other two characterizing species, Obelia longissima and Ulva intestinalis, being permanently attached to the substratum, they can be remarkably tolerant of displacement if replaced in water. Fragmentation is thought to be a possible mode of asexual reproduction in hydroids (Gili & Hughes, 1995). Therefore, it is possible that a proportion of displaced colonies (or fragments thereof) may attach to new substrata and survive. Cornelius (1995b) noted that detached specimens of Obelia longissima sometimes continue to grow if entangled in the intertidal. Ulva intestinalis, if detached from the substratum, may be buoyed up by gas and float to the surface where it continues to grow. Mytilus edulis may survive displacement (see MarLIN review) but Dare (1976) noted that individual mussels swept or displaced from mussel beds rarely survived, since they either became buried in sand or mud, or were scattered and eaten by oystercatchers. Furthermore, Semibalanus balanoides will not survive displacement. However, these two species are not characteristic of the biotope and their loss would not affect the visible biotope. On balance, an intolerance of intermediate has been recorded since it is likely that some hydroid colonies may die and not all of the displaced Ulva intestinalis will survive. Recoverability will be very high (see additional information).|
|No information||No information||No information||Insufficient
|Apart from Ulva intestinalis, no information was found concerning the effects of radionuclides on the characterizing and other important species in LR.H and no assessment of sensitivity has been made. Ulva sp. are known to be able to acquire large concentrations of radioactive substances from surrounding water. In the vicinity of the Sellafield nuclear plant, England, Ulva (as Enteromorpha) sp. accumulated zirconium, niobium, cerium and plutonium-239, however the species appeared to be unaffected by the radionuclides (Clark, 1997).|
|In a shallow rockpool such as those associated with LR.H, an influx of nutrients could lead to an increase in the abundance of Ulva intestinalis since nitrogen enrichment has been shown to enhance its growth (Kamer & Fong, 2001). An increase in the standing biomass of Ulva intestinalis would benefit Littorina littorea which grazes on it. Mytilus edulis may also benefit from moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material. The resultant increased food availability may increase growth rates, reproductive potential and decrease vulnerability to predators. In contrast, Obelia longissima may be competitively displaced by the Ulva intestinalis, leading to a reduction in the abundance of hydroids. In respect of the possibility of a reduction in hydroid coverage, an intermediate intolerance has been suggested. Recoverability is likely to be very high since excess nutrients are likely to be utilized quickly.|
|Low||Very high||Very Low||No change||Low|
|High air temperatures cause surface evaporation of water from pools, so that salinity steadily increases, especially in pools not flooded by the tide for several days. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity, and values ranging from 5-30 psu have been recorded in rockpools over a period of 24 hrs (Ranade, 1957). As a consequence of such a regime, the entire LR.H community will be adapted, to a certain degree, to fluctuating salinities. For instance, during the summer, owing to excessive evaporation brine precipitation may occur in rockpools containing Ulva intestinalis and salinity has been reported to rise as high as 180 psu (Reed & Russell, 1979). At the benchmark level, an intolerance of low has been suggested to reflect the different experiences rockpools at the top and bottom of the eulittoral are likely to have.|
|Low||Very high||Moderate||No change||Low|
|High rainfall will reduce salinity in rock pools and may create a surface layer of brackish/nearly fresh water for a period. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity, and values ranging from 5-30 psu have been recorded in rockpools over a period of 24 hrs (Ranade, 1957). As a consequence of such a regime, the entire LR.H community will be adapted, to a certain degree, to fluctuating salinities. Ulva intestinalis, for instance, is considered to be a remarkably euryhaline species, tolerant of extreme salinities ranging from 0 psu to 136 psu although reduced salinity can affect the growth rate of Ulva intestinalis. Obelia longissima, Littorina littorea and Semibalanus balanoides are also found in areas of reduced salinity. Mytilus edulis exhibits a defined behaviour to reducing salinity, initially only closing its siphons to maintain the salinity of the water in its mantle cavity, which allows some gaseous exchange and therefore maintains aerobic metabolism for longer. In extreme low salinities, e.g. resulting from storm runoff, large numbers of mussels may be killed (Keith Hiscock, pers comm.). In the long term (weeks) Mytilus edulis can acclimate to lower salinities (Almada-Villela, 1984; Seed & Suchanek, 1992; Holt et al., 1998). Almada-Villela (1984) reported that the growth rate of individuals exposed to only 13 psu reduced to almost zero but had recovered to over 80 percent of control animals within one month. Mytilus edulis can also survive considerably reduced salinities, growing as dwarf individuals at 4-5psu in the Baltic. At the benchmark level, an intolerance of low has been suggested to reflect the different experiences rockpools at the top and bottom of the eulittoral are likely to have.|
|Low||Immediate||Not sensitive||No change||Low|
|Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (Gili & Hughes, 1995). Although no information was found on oxygen consumption in Obelia longissima, Sagasti et al. (2000) reported that epifaunal species (including several hydroids and Obelia bicuspidata) in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O2/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition.|
Littorina littorea can endure long periods of oxygen deprivation. The snails can tolerate anoxia by drastically reducing their metabolic rate down to 20% of normal (MacDonald & Storey, 1999).Mytilus edulis is regarded as euryoxic, tolerant of a wide range of oxygen concentrations including zero (Zwaan de & Mathieu, 1992). Diaz & Rosenberg (1995) suggest it is resistant to severe hypoxia. Adult mytilids exhibited high tolerance of anoxia and Mytilus edulis is capable of anaerobic metabolism. Jorgensen (1980) observed, by diving, the effects of hypoxia (0.2 -1 mg/l) on benthic macrofauna in marine areas in Sweden over a 3-4 week period. Mussels were observed to close their shell valves in response to hypoxia and survived for 1-2 weeks before dying (Cole et al., 1999; Jorgensen, 1980).
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).No information was found concerning the effects of reduced oxygen concentration on Ulva intestinalis. On balance, Mytilus edulis and Littorina littorea are tolerant of hypoxia at the benchmark level (2 mg/l O2 for 1 week) although such a reduction in oxygen concentration will incur a metabolic cost and, hence, reduced growth. Accordingly, an intolerance of low has been recorded. Once oxygen levels return to prior levels, both species are likely to recover condition within a few weeks.
|No information||No information||No information||Insufficient
|No information was found concerning the effects of microbial pathogens on two of the characterizing species (Ulva intestinalis and Littorina littorea) but Obelia species are infected by a number of parasites at various stages in their life cycles (see MarLIN review of Obelia longissima). However, no negative effects have been noted from such an infestation. Mytilus spp. and Semibalanus balanoides also host various microbial pathogens. Mytilus edulis host a wide variety of disease organisms, parasites and commensals from many animal and plant groups including bacteria, blue green algae, green algae, protozoa, boring sponges, boring polychaetes, boring lichen, the intermediary life stages of several trematodes, copepods and decapods (See MarLIN review of Mytilus edulis). However, Mytilus edulis and Semibalanus balanoides are not characterizing species and their viability will not affect the recognizable biotope. Overall, insufficient information was available to make an assessment of sensitivity.|
|Not relevant||Not relevant||Not relevant||Not relevant||Not relevant|
|There are no alien species recorded in LR.H and no assessment of intolerance has been made.|
|Intermediate||Very high||Low||No change||Low|
|Of the important characterizing species, only the common periwinkle Littorina littorea is known to be targeted for extraction. This species 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. Due to the shallow nature of the pools associated with LR.H and the fact they are likely to occur in accessible places, it would be easy for this species to be extracted from the biotope. However, only large individuals would be removed and smaller ones would probably be left behind. Intolerance has been assessed as intermediate with a very high recoverability as adult snails would most probably crawl from nearby rock pools (see additional information).
The 'other' important species, Mytilus edulis, has been fished for hundreds of years although the extraction of this species is unlikely to affect the recognizable biotope. Mussel beds may be exploited by hand collection or dredging. Holt et al., (1998) suggest that when collected by hand at moderate levels using traditional skills the beds will probably retain most of their biodiversity. However, they also cite incidences of over-exploitation of easily accessible small beds by anglers for bait. Holt et al., (1998) suggest that in particular embayments over-exploitation may reduce subsequent recruitment leading to long term reduction in the population or stock.
|Not relevant||Not relevant||Not relevant||Not relevant||Not relevant|
Ulva intestinalis is generally considered to be an opportunistic species, with an 'r-type' strategy for survival. The r-strategists have a high growth rate and high reproductive rate. The species is also capable of dispersal over a considerable distance. Ulva intestinalis is amongst the first multicellular algae to appear on substrata that have been cleared following a disturbance. Following the Torrey Canyon oil spill in March 1967, for instance, species of the genus Ulva (then Enteromorpha) rapidly recruited to areas where oil had killed the herbivores that usually grazed on them, so that a rapid greening of the rocks (owing to a thick coating of Ulva) was apparent by mid-May (Smith, 1968). The rapid recruitment of Ulva to areas cleared of herbivorous grazers was also demonstrated by Kitching & Thain (1983). Following the removal of the urchin Paracentrotus lividus from areas of Lough Hyne, Ireland, Ulva (as Enteromorpha) grew over the cleared area and reached a coverage of 100% within one year. Therefore, evidence suggests that Ulva intestinalis is likely to have a considerable ability for recovery within a year.In the common periwinkle, the larvae form the main mode of dispersal. Littorina littorea is an iteroparous breeder with high fecundity that lives for several years. Breeding can occur throughout the year and the planktonic larval stage is long (up to 6 weeks) although larvae do tend to remain in waters close to the shore. Therefore recruitment and subsequent recovery rates should be high. Although adult immigration is usually an unlikely means of recovery, given their slow crawling, it may be possible in LR.H due to the likelihood of similar rockpools and Littorina littorea populations in close proximity.
Seed & Suchanek (1992) reviewed studies of recovery of 'gaps' (naturally or artificially induced) in mussel beds in Mytilus species. On rocky shores, gaps 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. Overall, Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Seed & Suchanek, 1992; Holt et al., 1998). Larval supply and settlement could potentially occur annually but settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). Therefore, while good annual recruitment is possible, recovery may take at least 5 years, although in certain circumstances and under some environmental conditions recovery may take significantly longer.
Bennell (1981) recorded recovery of Semibalanus balanoides populations within 3 years on a site cleared of barnacles in North Wales. Barnacle recruitment is, however, dependent on a suite of environmental and biological factors and, therefore, populations may take longer to recover.
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Last Updated: 25/02/2016