Biodiversity & Conservation

SS.IMU.MarMu.PhiVir

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The important characterizing species associated with this biotope rank high to substratum loss as they are infaunal, burrowing species. Philine aperta has fast growth and reproductive rates and could recolonize from other areas as the species is common. Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain, however information from other species suggest that this species is likely to be slow growing with patchy and intermittent recruitment. Therefore, full recovery of this biotope and the important characterizing species will take many years.
Smothering
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Species within the biotope are able to deal with small and temporary increases in silt deposition as they have the ability to self-clean. Depositions of thick silt, however, are likely to smother individuals to an extent where they are unable to self-clean or dig out and may also leave a substratum unsuitable for recolonization. Philine aperta has fast growth and reproductive rates and could recolonize from other areas as the species is common. Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain, however information from other species suggest that this species is likely to be slow growing with patchy and intermittent recruitment. Therefore, full recovery of this biotope and the important characterizing species will take many years and so a rank of moderate is reported.
Increase in suspended sediment
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The dominant trophic groups associated with this biotope are suspension feeders and deposit feeders. Suspension of fine silt and clay fractions of the sediment resulting from activities such as dredging may clog feeding structures. Effects, however, are uncertain. Species within the biotope are able to deal with small and temporary increases in silt as they have the ability to self-clean.
Decrease in suspended sediment
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Desiccation
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This biotope is found in the infralittoral zone and so not likely to be affected by desiccation. The important characterizing species, if exposed to air, are likely to be highly intolerant.
Increase in emergence regime
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This biotope is found in the infralittoral zone and so not likely to be affected by emergence. The important characterizing species, if exposed to air, are likely to be highly intolerant.
Decrease in emergence regime
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The biotope is found in the infralittoral zone so a decrease in emergence is not relevant.
Increase in water flow rate
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The biotope is only found in areas of weak or very weak tidal streams and so is likely to be intolerant of increases in water flow. Some tidal flow is necessary for the horizontal supply of small and light nutritious particles by resuspension and advective transport, influencing the growth rate of suspension-feeding benthos (Dauwe, 1998). However, some suspension feeders in the biotope will be unable to feed if the water flow rate increases by two categories in the water flow scale (see benchmarks). The sea pen Virgularia mirabilis for example, will retract into the sediment at water currents speeds greater than 0.5m/s (i.e. 1 knot) (Hiscock, 1983). If water speeds remain at this level or above, sea-pens will be unable to extend above the sediment, will be unable to feed and will probably die. The sea slug Philine aperta, which lives on the surface of the sediment, and the deposit feeding Amphiura chiajei would probably be washed away by strong water movement. Therefore, a long term increase in water flow rates would probably result in the loss of many of the key species, and hence the biotope, so intolerance is reported to be high.
Decrease in water flow rate
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The biotope exists in habitats such as sea lochs, where tidal streams are already very weak so a decrease in flow rate would result in almost non-moving water. In these enclosed or semi-enclosed water bodies, negligible water flow may result in some deoxygenation of the overlying water and the loss of some intolerant species. The sea pen Virgularia mirabilis for example, has high intolerance to deoxygenation and may die. Tidal currents keep most of the organic particles in the sediment in suspension which can support suspension feeders even in low organic content sediments. Therefore, if water movement becomes negligible suspended organic particles available to filter feeders such as the sea pens will decline. Growth and fecundity will be affected and over a period of a year may result in the death of sea pens. The overall impact on the biotope is likely to be the loss of a few key species such as sea pens and so intolerance is assessed as high. Recovery would probably take longer than five years and so is assessed as moderate - see additional information below for rationale.
Increase in temperature
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Insufficient information
Decrease in temperature
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Increase in turbidity
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Philine aperta is a burrowing species and Virgularia mirabilis is insensitive to light therefore an increase or decrease in light levels caused by changing turbidity levels will have little or no effect on the biotope and associated species. Nephrops norvegicus is most active at night in shallow depths and during the day in deep water, suggesting that a particular level of light is of some importance.
Decrease in turbidity
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Increase in wave exposure
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This biotope is associated with low-energy conditions and so is unlikely to be effected by wave exposure. If wave exposure increased Philine aperta can burrow and Virgularia mirabilis, if uprooted, can reburrow provided suitable substratum remains for recolonization.
Decrease in wave exposure
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Noise
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The important characterizing species associated with this biotope respond to sound vibrations and can retract into the sediment. Feeding will resume once the disturbing factor has passed.
Visual Presence
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The important characterizing species associated with this biotope have limited visual perception and therefore unlikely to be sensitive to visual presence.
Abrasion & physical disturbance
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The important characterizing species associated with this biotope can retract into the sediment and displaced individuals that are not damaged will reburrow. Virgularia mirabilis is able to retract into the sediment and so some individuals may be able to avoid some forms of abrasion or physical disturbance. Sea pens retract slowly and are likely to be sensitive to abrasion by trawling for instance, which is likely to break the rachis of Virgularia mirabilis. Species obtained by dredges were invariably damaged (Hoare & Wilson, 1977). However, the densities of Virgularia mirabilis were similar in trawled and untrawled sites in Loch Fyne and no changes in sea pen density was observed after experimental trawling over a 18 month period in another loch (Howson & Davies, 1991; Tuck et al. , 1998; Hughes, 1998). Hughes (1998) concluded that Virgularia mirabilis and Pennatula phosphorea, which can withdraw into the sediment, were probably less susceptible to the effects of damage by fishing gear than Funiculina quadrangularis, which is unable to withdraw. In an investigation into the effect of shellfish traps on benthic habitats (Eno et al. , 1996), creels were dropped on sea pens and left for extended periods to simulate the effects of smothering which could occur during commercial operations. The sea pens consistently righted themselves following removal of the pots. However, predation may increase and the viability of a population may be reduced whilst regeneration occurs. Philine aperta is soft bodied and likely to be damaged or displaced by a passing scallop dredge.

Overall, a proportion of the species populations are likely to be lost and an intolerance of intermediate has been recorded. For recoverability, see additional information below.

Displacement
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The important characterizing species associated with this biotope have the ability to reburrow, provided they have not been damaged. However, predation risk is increased whilst individuals are reburrowing.

Chemical Factors

Synthetic compound contamination
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Fine sediments associated with this biotope are ideal for accumulation of pollutants and bioturbation by burrowing organisms may help to redistribute them. Suspension feeders have the ability to accumulate pollutants, however, effects are uncertain.
Heavy metal contamination
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Fine sediments associated with this biotope are ideal for accumulation of pollutants and bioturbation by burrowing organisms may help to redistribute them. Suspension feeders have the ability to accumulate pollutants, however, effects are uncertain.
Hydrocarbon contamination
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Fine sediments associated with this biotope are ideal for accumulation of pollutants and bioturbation by burrowing organisms may help to redistribute them. Suspension feeders have the ability to accumulate pollutants, however, effects are uncertain.
Radionuclide contamination
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Fine sediments associated with this biotope are ideal for accumulation of pollutants and bioturbation by burrowing organisms may help to redistribute them. Suspension feeders have the ability to accumulate pollutants, however, effects are uncertain.
Changes in nutrient levels
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Nutrient enrichment would have an impact most likely through resultant hypoxia (low oxygen concentrations).
Increase in salinity
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The biotope is found in fully marine conditions and does not extend into estuaries so is likely to be intolerant of decreases in salinity. The key species are highly intolerant of salinity changes although Jones et al. (2000) suggest that Virgularia mirabilis appears to be somewhat tolerant of occasional lowering of salinity.
Decrease in salinity
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Changes in oxygenation
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Well oxygenated water is required for burrowing species associated with this biotope. Sea pen faunal communities are absent from areas which are de-oxygenated and characterized by a distinctive bacterial community. Therefore they are likely to be highly intolerant of anoxia and hypoxia may result in reduced viability of species populations such as Amphiura filiformis. Philine aperta has fast growth and reproductive rates and could recolonize from other areas as the species is common. Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain, however information from other species suggest that this species is likely to be slow growing with patchy and intermittent recruitment. Therefore, full recovery of this biotope and the important characterizing species will take many years.

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information
Introduction of non-native species
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Insufficient information
Extraction
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It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. However, fishing for Nephrops norvegicus may lead to exploitation of the biotope. The important characterizing species associated with this biotope can retract into the sediment and displaced individuals that are not damaged will reburrow. Virgularia mirabilis is able to retract into the sediment and so some individuals may escape damage from the trawl. Sea pens retract slowly and are likely to be sensitive to abrasion by trawling for instance, which is likely to break the rachis of Virgularia mirabilis. Species obtained by dredges were invariably damaged (Hoare & Wilson, 1977). However, the densities of Virgularia mirabilis were similar in trawled and untrawled sites in Loch Fyne and no changes in sea pen density was observed after experimental trawling over a 18 month period in another loch (Howson & Davies, 1991; Tuck et al. , 1998; Hughes, 1998). Hughes (1998) concluded that Virgularia mirabilis and Pennatula phosphorea, which can withdraw into the sediment, were probably less susceptible to the effects of damage by fishing gear than Funiculina quadrangularis, which is unable to withdraw. In an investigation into the effect of shellfish traps on benthic habitats (Eno et al., 1996), creels were dropped on sea pens and left for extended periods to simulate the effects of smothering which could occur during commercial operations. The sea pens consistently righted themselves following removal of the pots. However, predation may increase and the viability of a population may be reduced whilst regeneration occurs. Philine aperta is soft bodied and likely to be damaged or displaced by a passing scallop dredge.

Overall, a proportion of the species populations are likely to be lost and an intolerance of intermediate has been recorded. For recoverability, see additional information below.

Additional information icon Additional information

Recoverability
No evidence on community development was found. Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain. However, information from other species suggest that this species is likely to be slow growing with patchy and intermittent recruitment and so recovery from loss of this species is likely to longer than five years. Philine aperta is thought to live for 3-4 years and spawns egg masses, which release pelagic larvae, for several months between the spring and summer so recovery is likely to fairly rapid. Individuals can also migrate in from outside areas. However, since one the key species, Virgularia mirabilis may not recover from loss within a five year period, the recovery rank for the biotope is set to moderate.

This review can be cited as follows:

Hill, J.M. & Wilson, E. 2005. Philine aperta and Virgularia mirabilis in soft stable infralittoral mud. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 01/08/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=202&code=1997>