Biodiversity & Conservation

CR.MCR.SfR.Pid

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The key structuring species, Pholas dactylus, is highly intolerant of substratum loss because once removed from its burrow it cannot excavate a new chamber and is likely to die. Recovery should be good because most characterizing species have planktonic larvae and so recolonization should be possible within five years.
Smothering
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The key structural species Pholas dactylus is relatively tolerant of smothering by silt for it has been recorded from gently sloping chalk bedrock largely overlain by mud or silt 1-5cm deep, anoxic below the surface (Knight, 1984). Polydora ciliata is also found in areas of high siltation. However, many of the other species, such as Urticina felina and the many sessile suspension feeders like the sponge Halichondria panicea, though tolerant of turbid waters, are likely to be killed by a 5cm deep layer of silt. Species diversity can be expected to decline leaving a preponderance of Pholas dactylus and Polydora ciliata.
Increase in suspended sediment
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The biotope occurs in silty turbid conditions so must tolerate or require some degree of siltation. Polydora ciliata, for example, requires suspended sediment in order to construct the tubes in which it lives and piddocks create sediment in the process of burrowing. Other species in the biotope, such as the sponge Halichondria panicea, the anemone Urticina felina and polychaetes Pomatoceros triqueter, Sabellaria spinulosa and Lanice conchilega are all tolerant of some siltation. A significant decrease in siltation levels may reduce food input to the biotope resulting in reduced growth and fecundity of suspension feeding animals. Conversely, increases in suspended sediment may benefit this species if availability of organic particles increases. However, very high levels of silt may clog respiratory and feeding organs of some suspension feeders such as sea squirts and may result in a minor decline in faunal species diversity.
Decrease in suspended sediment
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Desiccation
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Although the biotope itself is circalittoral, the key structural species Pholas dactylus is also found in the intertidal. Other key species in the biotope, Polydora ciliata, Halichondria panicea and Urticina felina, can also be found intertidally, typically on the lower shore. However, other species in the biotope, such as Alcyonium digitatum are entirely subtidal and would be highly intolerant of desiccation. Thus, exposure of the biotope to an hour of air and sunshine may cause the loss of some species although the biotope as a whole would probably remain physically and functionally intact. Recolonization of those species affected by desiccation would probably take place within five years so recovery is assessed as high. During this recovery time the biotope will probably continue to exist albeit with slightly fewer species.
Increase in emergence regime
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The key species in the biotope (Pholas dactylus, Polydora ciliata, Urticina felina and Halichondria panicea) as well as many of the other species in the biotope are found intertidally (e.g. Pomatoceros triqueter, Balanus crenatus and Molgula manhattensis) and can tolerate some level of emergence. However, other species in the biotope, such as Alcyonium digitatum and Tubularia indivisa are entirely subtidal and would be highly intolerant of emergence. Thus, exposure of the biotope to an hour of air and sunshine may cause the loss of some species although the biotope as a whole would probably remain physically and functionally intact. Recolonization of those species affected by emergence would probably be rapid as most have planktonic larvae, although the anemone Urticina felina has poor dispersal and takes a long time to recover. Recovery within five years should be possible. During this time the biotope will probably continue to exist albeit with slightly fewer species.
Decrease in emergence regime
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Increase in water flow rate
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The biotope occurs in areas of weak to moderately strong water flow rates and so should be fairly tolerant of changes. Changes in water flow rate affect siltation levels and feeding of suspension and deposit feeders. Pholas dactylus occurs where the surface of the rock was scoured clean, and where it was covered with a layer of silty sediment (Wood & Wood, 1986). In areas of very strong tidal flow water movements may interfere with suspension feeding resulting in reduced growth and fecundity and the possible loss of some species. is dependent on water movement for a supply of suspended particles which it uses to construct its tube. Reductions in water flow rate may reduce the amount of suspended sand grains available. This may limit growth of the worms or reduce the density of worms that can be supported in a particular area.
Decrease in water flow rate
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Increase in temperature
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The key structuring species in the biotope, Pholas dactylus, is a southern species and occurrence in Britain represents the northern limit of its range. Long term increases in temperature may allow the species to extend its geographical range further north. Pholas dactylus spawning appears to be temperature dependent and so a long term drop in temperature may cause Pholas dactylus to be replaced by piddocks tolerant of cooler water such as Barnea candida and Zirfaea crispata so the overall nature of the biotope is unlikely to change significantly. The other key species in the biotope extend into much cooler and warmer waters than found in Britain so are likely to be tolerant of long term changes in temperature.
Decrease in temperature
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Increase in turbidity
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The biotope is predominantly found in turbid waters and is therefore, likely to be tolerant of changes in turbidity. Few of the species are likely to be highly intolerant of changes in turbidity although decreases in turbidity may affect food supply to suspension feeding organisms impairing growth and fecundity. Resulting changes in light attenuation may affect the distribution of red algae often found in the biotope. On return to normal conditions recovery is likely to be good.
Decrease in turbidity
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Increase in wave exposure
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The biotope is found in areas of moderate wave exposure. The chalk or clay habitat is soft and friable and an increase in wave exposure is likely to erode some of the substratum enabling only short lived species to survive. Species diversity is therefore likely to decline.
Decrease in wave exposure
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Noise
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Although some species may respond to vibration the biotope as a whole is not likely to be affected by noise disturbance.
Visual Presence
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Most macroinvertebrates have poor or short range perception and although some are likely to respond to shading caused by predators the biotope as a whole is unlikely to be sensitive to visual disturbance. However, the common piddock Pholas dactylus does react to changes in light intensity by withdrawing its siphon which may be an adaptive response to avoid predation by shore birds and fish (Knight, 1984).
Abrasion & physical disturbance
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Piddocks in burrows near the surface of the rock are likely to be damaged or killed by abrasion but many will be protected within their burrow. Some individuals of Polydora ciliata are also likely to be killed but surviving animals can migrate to affected areas. Species in the biotope that are upright and protrude above the substratum will also be damaged or killed (e.g. the sponge Halichondria panicea, hydroids, Alcyonium digitatum etc.). Therefore, an intolerance of intermediate has been recorded and species diversity will decline. Recovery will be good because most component species have pelagic larvae or can migrate into the area.
Displacement
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Many of the species in the biotope are permanently attached to the substratum and will die if displaced from their attachment. The key structuring species, the piddock, cannot excavate a new chamber if removed from its new burrow and will also die. Therefore intolerance to displacement is high. Recovery should be good because most characterizing species have planktonic larvae and so recolonization should be possible within five years.

Chemical Factors

Synthetic compound contamination
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Insufficient information is available to make an assessment. Polydora ciliata, the key functional species in this biotope has been recorded as thriving in the area surrounding a bromide extraction plant (Hoare & Hiscock, 1974).
Heavy metal contamination
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Insufficient information is available to make an assessment. Experimental studies with various species suggests that polychaete worms, including Polydora ciliata, are quite tolerant to heavy metals (Bryan, 1984).
Hydrocarbon contamination
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Insufficient information is available to make an assessment.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Insufficient information is available to make an assessment. However, Polydora ciliata is tolerant of high nutrient conditions and is found in high abundance close to sewage outfalls for example.
Increase in salinity
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Many of the species in the biotope are found in the intertidal where some reduced salinity must be experienced from precipitation run-off. However, all species are fully marine species and a long term change in salinity is likely to be detrimental to most species. Urticina felina, a characterizing species in this biotope is likely to be highly intolerant of reductions in salinity.
Decrease in salinity
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Changes in oxygenation
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Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l. Although Pholas dactylus may be tolerant of low oxygenation in an intertidal habitat, sub-tidal levels of 2mg/l for one week may be detrimental. On return to normal conditions however, recovery is likely to be rapid.

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information.
Introduction of non-native species
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The American piddock, Petricola pholadiformis, was introduced into Britain at the end of the nineteenth century, and is especially common in reduced salinity waters around the mouth of the Thames (Eno et al., 1997). It does not therefore seem likely to affect the MCR.Pid biotope. There is no information on other non-native species affecting the biotope.
Extraction
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Pholas dactylus is known to be harvested in Britain although not to a great extent. In Italy, harvesting of piddocks has had a destructive impact on habitats and has now been banned (E. Pinn, pers. Comm. To MarLIN). In Britain, collection of piddocks is thought to have a similarly destructive effect. People have been known to go out onto the shore and, with the use of a hammer and chisel, excavate the piddocks from the soft rock (K. Hiscock, pers. Comm.). This would be catastrophic for the biotope. The stability of the soft rock would be reduced and potentially lead to the loss of the vast majority of piddocks that inhabit the top few centimetres of the substratum down to a depth of 10 cm. Farming methods are being investigated as an alternative and it is therefore possible that further targeted extraction could be a future possibility.

If there is a continued increase in the marine aquarium trade for cold water species then Urticina felina could be a potential target species for extraction. Urticina felina is a slow growing anemone with poor dispersive abilities. It may take several years for recovery to occur but removal and recovery of this species may not have an important role in the viability and functioning of the biotope.

Overall an intermediate intolerance has been suggested because extraction of piddocks is probably rare. Recoverability is likely to be high (see additional information).

Additional information icon Additional information

Recoverability
Although specific information regarding the intolerance of the biotope to a range of chemical factors is not available, the biotope is found predominantly in the South East of England, where waters are turbid due in part to the continuous dissolution of chalk from the rocks, but also from input from rivers, sewage and other discharges. Therefore, the species within the biotope may be tolerant of low level pollution including nutrients.

This review can be cited as follows:

Hill, J.M. 2008. Piddocks with a sparse associated fauna in upward-facing circalittoral very soft chalk or clay. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 20/12/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=152&code=2004>