Biodiversity & Conservation

SS.IMU.MarMu.AreSyn

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum would result in removal of the habitat and its associated community, therefore an intolerance of high has been recorded.
Recovery will depend on recruitment from similar habitats which, however, have a restricted distribution. Although, Arenicola marina is widely distributed the characteristic species of synaptid found in this biotope have a restricted distribution and their larvae may be retained within the isolated bays or lagoons that they inhabit. Therefore, a recoverability of moderate has been recorded (see additional information below).
Smothering
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Arenicola marina is a sub-surface deposit feeder that derives the sediment it ingests from the surface. It rapidly reworks and mixes sediment. It is unlikely to be perturbed by smothering by 5 cm of sediment. Leptosynapta bergensis has a similar lifestyle and is also unlikely to be intolerant of smothering at the level of the benchmark. Labidoplax media lives in the top few cm of sediment and will incur an energetic cost, due to its relatively small size, returning to the top of the sediment after smothering, therefore an intolerance of low has been recorded. However, it is likely to be more intolerant of smothering by sediment outside its habitat preferences e.g. coarser material or impermeable materials.
Recovery will depend on the animals return to their preferred feeding position, therefore a recovery of very high has been recorded (see additional information below).
Increase in suspended sediment
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This biotope forms in sheltered areas with low water flow where fine silts and muds are deposited. Deposit feeders are unlikely to be perturbed by increased concentrations of suspended sediment since they live in sediment and are probably adapted to re-suspension of sediment by wave action, during storms or runoff. Increased siltation by fine materials, however, may modify the nature of the substratum and render it less suitable for Arenicola marina in examples of this biotope where this species is abundant, resulting in a reduced abundance of the lugworm. However, the synaptid holothurians or the biotope as a whole are unlikely to be adversely affected. Surface deposit feeders may benefit from the influx of organic particulates and detritus, therefore, not sensitive* has been recorded at the benchmark level. Prolonged or extreme increases in siltation may increase the height of the bed, raising the biotope into the intertidal, and exposing the biotope to emergence and desiccation (see below).
Decrease in suspended sediment
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Decreases in siltation, may result in reduced food supply for deposit feeders, which are partly dependant on organic particles and detritus collected on the sediment surface for food. However, the sediment is rich in organic material and therefore an intolerance of low has been recorded (see additional information below).
Desiccation
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The biotope is infralittoral, extending between 0 and 5m in depth, and is unlikely to be exposed to the air. In addition, the fine sediments characteristic of this biotope, have a low permeability and, therefore, hold water near the surface, and would protect even surface dwelling organisms from the direct effects of desiccation. Arenicola marina and Leptosynapta bergensis are protected from desiccation since they inhabit deep, water filled burrows. Therefore, not relevant has been recorded.
Increase in emergence regime
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This biotope occurs in the shallow subtidal and may be affected by an increase in emergence. Increased emergence may expose the biotope to temperature extremes and an increased risk of desiccation. The soft bodies of holothurians may be particularly intolerant. Increased emergence is unlikely to adversely affect Arenicola marina, which reaches high abundances on intertidal mudflats. However, emergence may increase the exposure of Labidoplax media to drying and wildfowl predation. Leptosynapta sp. will not be able to irrigate their burrows during emersion, increasing the risk of hypoxia and anoxia as well wildfowl predation. Overall, therefore, although the lugworm may not be adversely affected, the synaptid holothurians are likely to be lost from the biotope. The biotope is likely to be replaced by an intertidal mudflat biotope dominated by lugworm. Therefore, an intolerance of high has been recorded (see additional information below).
Decrease in emergence regime
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A decrease in emergence may allow the biotope to extend its range further up the shore. Therefore, it may benefit and not sensitive* has been recorded.
Increase in water flow rate
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An increase in water flow will probably remove the surface layer of fine sediment and /or result in modification of the sediment type, reducing the proportion of fine sediment in favour of coarser sediment such a sands. Surface dwelling synaptids such as Labidoplax media may be physically removed or adversely affected by a change in sediment grain size. Arenicola marina is found in a high abundance in muddy sands and tolerates strong tidal flow, and is, therefore, unlikely to be adversely affected by changed sediment type. Leptosynapta bergensis was only recorded from fine sand and muds in the MNCR (JNCC, 1999). The change in the physical characteristics of the sediment resulting from increased water flow will probably severely reduce the abundance of synaptid holothurians, allow other species to colonize the habitat and result in a loss of the biotope. Therefore, an intolerance of high has been recorded.
A recoverability of moderate has been recorded (see additional information below).
Decrease in water flow rate
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This biotope occurs in areas of weak to very weak tidal streams and any further decrease in water flow is unlikely.
Increase in temperature
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Increased temperatures may affect infauna indirectly, by stimulating increased bacterial activity, increased oxygen consumption and, therefore, depletion of oxygen from the interstitial waters resulting in reduced oxygen levels (hypoxia) or absence of oxygen (anoxia) (see deoxygenation) in the sediment (Hayward, 1994). Sommer et al. (1997) examined sub-lethal effects of temperature in Arenicola marina and suggested a critical upper and lower temperature of 20 °C and 5 °C respectively in North Sea specimens. Above or below these critical temperatures specimens resort to anaerobic respiration. Sommer et al. (1997) noted that specimens could not acclimate to a 4 °C increase above the critical temperature. Therefore, Arenicola marina is probably intolerant of a short term acute change in temperature of 5 °C although it is unlikely to be directly affected due to its infaunal habit. However, temperature change may adversely affect reproduction, for example, spawning can be inhibited in gravid adults maintained above 15 °C and temperature change may affect maturation, spawning time, synchronization of spawning and reproduction in the long-term (Bentley & Pacey, 1992; Watson et al., 2000). Therefore, temperature change may affect lugworm recruitment in the long term.
Little information was found concerning temperature tolerance in synaptid holothurians. Both Labidoplax media and Leptosynapta bergensis are essentially boreal species, being restricted to the northern shores of the British Isles. Therefore, these species may be intolerant of increase of temperature, especially short term acute change. Hence, an overall intolerance of intermediate has been recorded.
A recoverability of high has been suggested (see additional information below).
Decrease in temperature
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Sommer et al. (1997) examined sub-lethal effects of temperature in Arenicola marina and suggested a critical upper and lower temperature of 20 °C and 5 °C respectively in North Sea specimens. Above or below these critical temperatures specimens resort to anaerobic respiration. Therefore, Arenicola marina is probably intolerant of a short term acute change in temperature of 5 °C although it is unlikely to be directly affected due to its infaunal habit.
Little information was found regarding temperature tolerance in synaptid holothurians. Both Labidoplax media and Leptosynapta bergensis are essentially boreal in distribution being essentially limited in distribution to northern habitats in the British Isles and may not be affect by a long term decrease in temperature. The shallow, enclosed situations they are found in are subject to significant decreases in temperature driven by air temperatures because of their enclosed site. They are therefore likely to be tolerant of sharp decreases in temperature. Therefore, an overall biotope intolerance of low has been recorded, albeit at low confidence.
A recoverability of high has been suggested (see additional information below).
Increase in turbidity
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This biotope probably exists in naturally turbid conditions. However, an increase in turbidity and hence decreased light penetration may result in a decrease in primary production by phytoplankton and microphytobenthos and ephemeral algae. However, the potential decrease in food available is probably insignificant given the high organic content of the sediment and resident microbial population. Therefore, not sensitive has been recorded.
Decrease in turbidity
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This biotope probably exists in naturally turbid conditions. A decrease in turbidity will result in increased phytoplankton and microphytobenthos primary production. Decreased turbidity may increase the growth of ephemeral algae such as Derbesia sp. and a resultant increase in primary productivity and detritus. Algal mats may increase hypoxic and anoxic conditions under the mat, at the sediment surface. However, synaptid holothurians and lugworms are probably tolerant of hypoxic conditions (see deoxygenation below). Therefore a recoverability of low has been recorded.
Increase in wave exposure
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This biotope is characteristic of very wave sheltered to ultra wave sheltered bays and lagoons. Increased wave action will disrupt feeding and burrowing activities, especially for species near to the sediment surface, such as Labidoplax media, which may be physically moved with the sediment. More importantly, increased wave action will change the sediment characteristics, changing porosity and sediment sorting, removing fine material and resulting in an increase in the coarseness of the sediment. Infauna are sensitive to change in sediment type as many are adapted to burrow through certain grades of sediment (Trueman & Ansell, 1969; Elliot et al., 1998). Increased wave exposure may also erode the sediment bed. An increase in wave exposure is likely to result in a significant change in the sediment characteristics and its associated fauna, and probably the loss of synaptid holothurians and the biotope. Therefore, an intolerance of high has been recorded.
Decrease in wave exposure
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This biotope is characteristic of very wave sheltered to ultra wave sheltered bays and lagoons. An additional decrease in wave exposure is unlikely.
Noise
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Arenicola marina and synaptids, as well as other species in the biotope, are unlikely to be sensitive to noise.
Visual Presence
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Arenicola marina lives in a burrow and therefore in permanent darkness. Labidoplax media lives in the surface of the sediment and is know to burrow quickly if exposed on the sediment surface, although this may be geotactic rather than phototactic response. Leptosynapta sp. are known to swim, but only in darkness suggesting that they can respond to light. However, their visual range is probably very limited.
Abrasion & physical disturbance
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Arenicola marina lives in sediment to a depth of 20-40 cm and is therefore protected from most sources of abrasion and physical disturbance caused by surface action. Leptosynapta sp. also lives in burrows in the sediment. Labidoplax media lives in the top few cm of the sediment are, therefore, more vulnerable to abrasion and are likely to be removed or damaged by a passing scallop dredge.. However, the characteristic species of this biotope are all soft bodied and a proportion of the population is likely be damaged by any activity (e.g. anchors, dredging) that penetrates the sediment. Therefore, an intolerance of intermediate has been recorded. A recoverability of high has been suggested (see additional information below).
Displacement
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Displacement from the sediment is likely to expose Arenicola marina and synaptids to an increased risk of predation. However, once on the substratum surface Arenicola marina, terebellids and synaptid holothurians are capable of burrowing back into the sediment and passive migration to suitable sediment. Trueman & Ansell (1969) regarded apodous as efficient burrowers and reported that Leptosynapta sp. required only 5-6min for complete burial (Clark, 1899 cited in Trueman & Ansell, 1969). Therefore, an intolerance of low has been recorded.

Chemical Factors

Synthetic compound contamination
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The xenobiotic ivermectin (used to control parasitic infestations in livestock including sea lice in fish farms), degrades slowly in marine sediments (half life >100 days). Ivermectin was found to produce a 10 day LC50 of 18µg ivermectin /kg of wet sediment in Arenicola marina. Sub-lethal effects were apparent between 5 - 105 µg/kg. Cole et al. (1999) suggested that this indicated a high intolerance.
Little information on the toxicity of synthetic chemicals to synaptid holothurians was found. Newton & McKenzie (1995) suggested that echinoderms tend to be very intolerant of various types of marine pollution but gave no detailed information. Cole et al. (1999) reported that echinoderm larvae displayed adverse effects when exposed to 0.15mg/l of the pesticide Dichlorobenzene (DCB). Smith (1968) demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus. Therefore, the larvae of synaptid holothurians may also be intolerant of synthetic chemicals.
Overall, therefore, members of this biotope may be sensitive synthetic chemicals to varying degrees and adverse effects on larvae may reduce recruitment in the long term resulting in the loss of as proportion of the population. Therefore, an intolerance of intermediate has been recorded, albeit at very low confidence.
A recoverability of high has been suggested (see additional information below).
Heavy metal contamination
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Sediment may act as a sink for heavy metals contamination, so that deposit feeding species may be particularly vulnerable to heavy metal contamination through ingestion of particulates.
At high concentrations of Cu, Cd or Zn the blow lug left the sediment (Bat & Raffaelli, 1998). The following toxicities have been reported in Arenicola marina:
  • no mortality after 10 days at 7 µg Cu /g sediment, 23µg Zn/g and 9g Cd /g;
  • median lethal concentrations (LC50) of 20 µg Cu/g, 50 µg Zn/g, and 25 µg Cd/g (Bat & Raffaelli, 1998).
However, Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se the least toxic.
Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were intolerant of heavy metals, e.g. the intolerance of larvae of Paracentrotus lividus to copper (Cu) had been used to develop a water quality assessment. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu). Sea-urchins, especially the eggs and larvae, are used for toxicity testing and environmental monitoring (reviewed by Dinnel et al. 1988). Crompton (1997) reported that mortalities occurred in echinoderms after 4-14day exposure to above 10-100 µg/l Cu, 1-10 mg/l Zn and 10-100 mg/l Cr but that mortalities occurred in echinoderm larvae above10-100 µg/ l Ni.
Therefore, although the polychaete members of the biotope may be relatively resistant to heavy metal contamination it appears that echinoderms and their larvae may be more intolerant. Therefore, an intolerance of intermediate has been recorded.
A recoverability of high has been suggested (see additional information below).
Hydrocarbon contamination
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Sheltered embayments and lagoons, where this biotope is found, are particularly vulnerable to oil pollution, which may settle onto the sediment and persist for years (Cole et al., 1999). Subsequent digestion or degradation of the oil by microbes may result in nutrient enrichment and eutrophication (see nutrients below). Although, protected from direct smothering by oil by its depth, the biotope is relatively shallow and would be exposed to the water soluble fraction of oil, water soluble PAHs, and oil adsorbed on to particulates.
Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. Crude oil and oil: dispersant mixtures were shown to cause mortalities in Arenicola marina (see review). Prouse & Gordon (1976) found that blow lug was driven out of the sediment by waterborne fuel oil concentrations of >1 mg/l or sediment concentration of >100 µg/g.
Crude oil and refined oils were show to have little effect on fertilization in sea urchin eggs but in the presence of dispersants fertilization was poor and embryonic development was impaired (Johnston, 1984). Sea urchin eggs showed developmental abnormalities when exposed to 10-30mg/l of hydrocarbons and crude oil : Corexit dispersant mixtures have been shown to cause functional loss of tube feet and spines in sea urchins (Suchanek, 1993).
Although, no direct information on synaptid holothurians was found , it seems likely that adults and especially larvae are intolerant of hydrocarbon contamination. Together with the likely intolerance of polychaetes and overall biotope intolerance of intermediate has been suggested.
A recoverability of high has been suggested (see additional information below).
Radionuclide contamination
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Insufficient information
Changes in nutrient levels
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Increases in nutrients and low oxygen conditions result in anaerobic conditions. However, this biotope is characterized by organically rich mud and low oxygen conditions. Moderate enrichment, is likely to increase food availability and hence, abundance of deposit feeding organisms.
However, eutrophication is likely to result in algal mats, which create anoxic conditions in the sediment underneath, changes in the microphytobenthos, and with increasing enrichment a reduction in species richness, the sediment becoming dominated by pollution tolerant polychaetes, e.g. Manayunkia aestuarina. In extreme cases the sediment may become anoxic and defaunated (Elliot et al., 1998). Algal blooms may also occur in eutrophic areas and have been implicated in mass mortalities of lugworms, e.g. in South Wales where up to 99% mortality was reported (Holt et al. 1995; Olive & Cadman, 1990; Boalch, 1979).
Therefore, although moderate enrichment may be beneficial, a marked increase in nutrients levels, in an already organic rich biotope may have adverse affect, resulting in a reduction in the abundance of the several members of the community and an intolerance of intermediate has been recorded at the level of the benchmark.
A recoverability of high has been suggested (see additional information below).
Increase in salinity
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Increased salinity is unlikely to occur in this biotope, unless exposed to a hypersaline effluent. Arenicola marina exposed to hyper-osmotic shock (47 psu) loose weight, but are able to regulate and gain weight within 7-10 days (Zebe & Schiedek, 1996). Echinoderms are also likely to loose weight due to osmosis in hypersaline conditions, however, little information was found.
Decrease in salinity
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Once the salinity of the overlying water drops below about 55%seawater (about 18psu) Arenicola marina stops irrigation, and compresses itself at the bottom of its burrow. Once normal salinities return they resume usual activity (Shumway & Davenport, 1977; Rankin & Davenport,1981; Zebe & Schiedek, 1996). This behaviour, together with their burrowing habitat, enabled the lugworm to maintain its coelomic fluid and tissue constituents at a constant level, whereas individuals exposed to fluctuating salinities outside their burrow did not (Shumway & Davenport, 1977). Environmental fluctuations in salinity are only likely to affect the surface of the sediment, and not deeper organisms, since the interstitial or burrow water is little affected. However, lugworms may be affected by low salinities after heavy rains. Arenicola marina was able to osmoregulate intracellular and extracellular volume within 72 - 114 hrs by increased urine production and increased amino acid concentration in response to hypo-osmotic shock (low salinity) (see Zebe & Schiedek, 1996). However, Arenicola marina is unable to tolerate salinities below 24 psu and is excluded from areas influenced by freshwater runoff or input (e.g. the head end of estuaries) where it is replaced by Hediste diversicolor (Hayward, 1994).
Echinoderms are generally regarded as stenohaline and most species are exclusively marine (Binyon, 1966; Pawson, 1966; Stickle & Diehl, 1987; Lawrence, 1996). However, some euryhaline species have been reported and local adaptation may occur in some species (see Binyon, 1966 and Stickle & Diehl, 1987 for reviews). Leptosynapta inhaerens was reported to be euryhaline but only known in full salinity waters in European waters (Binyon, 1966). Connor et al. (1997a) reported this biotope to be characteristic of full salinity. However, this biotope was reported to occur at 24-35psu and 18-35psu in Scottish lagoons (Thorpe et al. 1998; Thorpe, 1998).
Leptosynapta sp. may derive some protection from short term reduction in salinity due to their burrowing habitat, however, Labidoplax media is more vulnerable.
Overall, therefore, the synaptids are probably able to tolerate salinities as low as 18psu. This biotope may experience low salinities during heavy rains. The synaptids may be able to acclimate to reduced salinity but are likely to be more intolerant of short term acute change, although the water retained by the sediment will act as a buffer to extreme change. For example, Lawrence (1996) cites several examples of mass moralities in echinoderms due to sudden increases in river discharge or localized heavy rains. Overall, therefore, a proportion of the lugworm and synaptid populations may be lost due to short term acute change in salinity and intolerance of intermediate has been recorded.
Recovery will depend on recruitment and recolonization by survivors. The synaptids are hermaphroditic, mature at about 1 year old, have a pelagic larva (Nyholm, 1951) and are probably relatively fecund. Recolonization by swimming and burrowing is also possible. Similarly, Arenicola marina are thought to recolonize available habitat relatively quickly where adjacent populations are available. Therefore, a recoverability of high has been suggested (see additional information below).
Changes in oxygenation
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Intertidal populations of Arenicola marina are subject to reduced oxygen concentrations regularly at low tide. Arenicola marina is capable of anaerobic respiration. Transition from aerobic to anaerobic metabolism takes several hours and is complete within 6-8 hrs, although this is likely to be the longest period of exposure at low tide. Fully aerobic metabolism is restored within 60 min once oxygen is returns (Zeber & Schiedek, 1996). Arenicola marina was able to survive anoxia for 90 hrs in the presence of 10 mmol/l sulphide in laboratory tests (Zeber & Schiedek, 1996). At 16 °C Arenicola marina survived 72 hrs of anoxia but only 36 hrs at 20 °C. Juveniles have a lower tolerance of anoxia but are capable of anaerobic metabolism (Zebe & Schiedek, 1996). However, Arenicola marina has been found to be unaffected by short periods of anoxia and to survive for 9 days without oxygen (Borden, 1931 and Hecht, 1932 cited in Dales, 1958; Hayward, 1994). In addition, Diaz & Rosenberg (1995) considered Arenicola marina to be resistant of severe hypoxia.
Nilsson & Rosenberg (1994) exposed sediment cores, containing Labidoplax buskii and other benthic species to moderate (1mg O2/l) and severe (0.5mg O2/l) hypoxia for 20 days. Labidoplax buskii was seen to leave the sediment when the oxygen concentration was reduced to 1.6 mg O2/l (16% saturation) and were dead by the end of the experiment in both moderate and severe hypoxia experiments. However, little other information on tolerance of hypoxia in synaptid holothurians was found.
The fine muddy sediments found in this biotope are organic rich and the benthic macrofauna is probably adapted to a degree of hypoxia. Burrowing species such as Arenicola marina (see above) and Leptosynapta sp. burrow into anoxic sediment and may be tolerant of hypoxia. Using Labidoplax buskii and Arenicola marina as examples, the characterizing species would probably survive exposure to 2mg O2/l for one week (the benchmark) although they may incur a metabolic cost or reduced feeding during exposure. Therefore, an intolerance of low has been recorded.

Biological Factors

Introduction of microbial pathogens/parasites
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Ashworth (1904) recorded the presence of distomid cercariae and Coccidia in Arenicola marina from the Lancashire coast. However, no information concerning infestation or disease related mortalities in the lugworm or synaptid holothurians was found.
Introduction of non-native species
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No non-native species likely to compete with the characterizing species within this biotope were found.
Extraction
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Arenicola marina is the only species found in this biotope that may be subject to extraction for bait. Bait digging is highly unlikely since the biotope occurs in the shallow sublittoral, however, bait dredging may be possible. In addition, the abundances of lugworm associated with this biotope are probably not commercially viable.
However, should bait dredging occur then the other important characterizing species, the synaptids and terebellids would also be removed or damaged. Therefore, an intolerance of intermediate has been recorded (see Arenicola marina review).
Recovery will depend on recruitment and recolonization by survivors. The synaptids are hermaphroditic, mature at about 1 year old, have a pelagic larva (Nyholm, 1951) and are probably relatively fecund. Recolonization by swimming and burrowing is also possible. Similarly, Arenicola marina are thought to recolonize available habitat relatively quickly where adjacent populations are available. Therefore, a recoverability of high has been suggested (see additional information below).

Additional information icon Additional information

Recoverability

Rates of recover for Arenicola marina are derived from studies of bait digging (see species review for details). Recovery to pre-exploitative levels has been found to be within a few months (Fowler, 1999) whilst Beukema (1995) noted that it took 3 years for recovery to previous levels.

Overall recovery of lugworm populations is generally regarded as rapid. However, Fowler (1999) pointed out that recovery may take longer on a small pocket beach (or isolated area) with limited possibility of recolonization from surrounding areas. Therefore, if adjacent populations are available recovery will be rapid and a rank of 'very high' recoverability recorded. However, where the affected population is isolated or severely reduced (e.g. by long-term mechanical dredging), then recovery may be extended.

Little information concerning the recruitment of synaptid holothurians was found. Labidoplax buskii has pelagic larvae that spend about 10-12 days in the plankton. Using this species as an example, dispersal is potentially good where the local currents can transport larvae. Small species such as Labidoplax buskii and Labidoplax media may also colonize new areas of sediment if they were carried with sediment moved by water flow (bedload transport). Therefore, within a given area, synaptids may be self recruiting and may recolonize denuded areas by swimming or burrowing. For example, Labidoplax dubia and Leptosynapta inhaerens were reported to be able to swim at night (by sinusoidal movements of the entire body) (Pawson, 1966).

However, this biotope is characteristic of wave sheltered areas with weak or very weak tidal streams, and has a restricted distribution, therefore, recruitment to these isolated habitats from outside the area is likely to take some time and be dependant on sporadic events such as storms, suggesting a recoverability of moderate.


This review can be cited as follows:

Tyler-Walters, H. 2001. Arenicola marina and synaptid holothurians in extremely shallow soft mud.. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/07/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=108&code=1997>