The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Little direct evidence was found to assess the effects of increased temperature on piddocks and the assessment is based on distribution records and evidence for spawning in response to temperature changes. Pholas dactylus occurs in the Mediterranean and the East Atlantic, from Norway to Cape Verde Islands (Micu, 2007).    There is some evidence that temperature influences the timing of reproduction in Pholas dactylus, which usually spawns between July and August. Increased summer temperatures in 1982 induced spawning in July on the south coast of England (Knight, 1984). Similar observations have been made for other piddock species. Spawning of the piddock Petricolaria pholadiformis is initiated by increasing water temperature (>18 °C) (Duval, 1963a), so elevated temperatures outside of usual seasons may disrupt normal spawning periods. The spawning of Barnea candida was also reported to be disrupted by changes in temperature (Duval, 1963b). Disruption from established spawning periods, caused by temperature changes, may be detrimental to the survival of recruits as other factors influencing their survival may not be optimal, and some mortality may result. Established populations may otherwise remain unaffected by elevated temperatures.

Sensitivity assessment. The global distribution of Pholas dactylus suggests that this species can tolerate warmer waters than currently experienced in the UK and may therefore be tolerant of a chronic increase in temperature. Short-term acute increases may, (depending on timing) interfere with spawning cues which appear to be temperature driven.  The effects will depend on seasonality of occurrence and the species affected. Adult populations may be unaffected and, in such long-lived species, an unfavourable recruitment may be compensated for in a following year. Resistance to an acute change in temperature is therefore assessed as ‘High’ and recovery as ‘High’ (no impact to recover from) and the biotope is considered ‘Not Sensitive’.  It should be noted that the timing of acute changes may lead to greater impacts, temperature increases in the warmest months may exceed thermal tolerances whilst changes in colder periods may stress individuals acclimated to the lower temperatures.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Little direct evidence was found to assess the effects of increased temperature on piddocks and the assessment is based on distribution records and evidence for spawning in response to temperature changes. Pholas dactylus occurs in the Mediterranean and the East Atlantic, from Norway to Cape Verde Islands (Micu, 2007).    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.

Sensitivity assessment. The global distribution of Pholas dactylus suggests that this species can tolerate cooler waters than currently experienced in the UK and may therefore be tolerant of a chronic decrease in temperature. Short-term chronic increases may, (depending on timing) interfere with spawning cues which appear to be temperature driven.  The effects will depend on seasonality of occurrence and the species affected. Adult populations may be unaffected and, in such long-lived species, an unfavourable recruitment may be compensated for in a following year. Resistance to an acute change in temperature is therefore assessed as ‘High’ and recovery as ‘High’ (no impact to recover from) and the biotope is considered ‘Not Sensitive’.  It should be noted that the timing of acute changes may lead to greater impacts, temperature increases in the warmest months may exceed thermal tolerances whilst changes in colder periods may stress individuals acclimated to the lower temperatures.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Sensitivity assessment. No evidence for the range of physiological tolerances to salinity changes were found for piddocks and sensitivity to this pressure is not assessed based on 'No evidence'.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

No direct empirical evidence was found to assess this pressure for piddocks and the assessment is largely based on the reported distribution of the biotope which is recorded only from fully marine habitats (Connor et al., 2004). Biotopes found in the intertidal will naturally experience fluctuations in salinity where evaporation increases salinity and inputs of rainwater expose individuals to freshwater. Species found in the intertidal are therefore likely to have some form of behavioural or physiological adaptations to changes in salinity.

No information was found for the salinity tolerance of Pholas dactylus.  Barnea candida is reported to extend in to estuarine environments in salinities down to 20 psu (Fish & Fish, 1996).  Petricolaria pholadiformis is particularly common off the Essex and Thames estuary, e.g. the River Medway (Bamber, 1985) suggesting tolerance of brackish waters.  Zenetos et al. (2009) suggest that at all sites where Petricolaria pholadiformis has been found there is some freshwater inflow into the sea. According to the literature, the species in its native range inhabits environments with salinities between 29 and 35ppt, while in the Baltic Sea it is reported from salinities 10-30 psu (Gollasch & Mecke, 1996, cited from Zenetos et al. 2009). According to Castagna & Chanley (1973, cited from Zenetos et al. 2009) the lower salinity tolerance of Petricolaria pholadiformis is 7.5-10 psu. It thus appears that reduced salinity facilitates its establishment (Zenetos et al., 2009). No information was found for the salinity tolerance of Pholas dactylus.

Mytilus edulis is found in a wide range of salinities from variable salinity areas (18-35 ppt) such as estuaries and intertidal areas, to areas of more constant salinity (30-35ppt) in the sublittoral (Connor et al., 2004).  In addition, Mytilus edulis thrives in brackish lagoons and estuaries, although, this is probably due to the abundance of food in these environments rather than the salinity (Seed & Suchanek, 1992). Mytilus edulis was recorded to grow in a dwarf form in the Baltic sea where the average salinity was 6.5psu (Riisgård et al., 2013).  Mytilus edulis exhibits a defined behavioural response 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.  If the salinity continues to fall the valves close tightly (Davenport,1979; Rankin & Davenport, 1981).  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% of control animals within one month.  Observed differences in growth are due to physiological and/or genetic adaptation to salinity.Decreased salinity has physiological effects on Mytilus edulis; decreasing the heart rate (Bahmet et al., 2005), reducing filtration rates (Riisgård et al., 2013), reducing growth rate (Gruffydd et al., 1984) and reducing the immune function (Bussell et al., 2008).  Both Bahmet et al., (2005); Riisgård et al., (2013) noted that filtration and heart rates return to normal within a number of days acclimation or a return to the original salinity.  However, Riisgard et al., (2013) did observe that mussels from an average of 17 psu found it harder to acclimate between the salinity extremes than those from an average of 6.5 psu.  This observation may mean that mussels in a variable/ lower salinity environment are more able to tolerate change than those found at fully marine salinities. 

Sensitivity assessment.  A change in full to variable salinity (the pressure benchmark) may lead to changes in the composition of the piddock population with replacement of Pholas dactylus (if it is sensitive) by piddocks more tolerant of lower salinities particularly Petricolaria pholadiformis. Mytilus edulis is considered to be tolerant of a change in salinity at the pressure benchmark, based on evidence for salinity tolerances and recorded distribution.  Based on reported distributions of the biotope and piddocks it is considered that the benchmark decrease in salinity would result in decreased abundance of Pholas dactylus in biotopes that were previously fully marine. As the biotope is sublittoral other piddock species more adapted to the intertidal may not be able to colonise the habitat to the same abundance. Resistance is therefore assessed as 'Low' and resilience (following return to full salinity) as 'Medium', the biotope is therefore considered to have ‘Medium’ sensitivity to this pressure.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Established adult piddocks are, to a large extent, protected from direct effects of increased water flow, owing to their environmental position within the substratum. Increases or decreases in flow rates may affect suspension feeding by altering the delivery of suspended particles or the efficiency of filter feeding.  This biotope has been recorded from areas where tidal flows vary between 0.5 -1.5 m/s (Connor et al., 2004), suggesting that changes in  flow rates (increase or decrease) within this range will not negatively impact the biotope.  Adult piddocks may become exposed should physical erosion of the clay and chalk substratum occur at a greater rate than burrowing, and lost from the substratum. At higher densities bioerosion by piddocks may destabilise the substratum increasing vulnerability to erosion.  

 The most damaging effect of increased flow rate would be the erosion of the clay or soft chalk substratum as this could eventually lead to loss of the habitat. No evidence was found to assess the water velocities at which erosion of clay or chalk occurs. Some erosion will occur naturally and storm events and wave action may be more significant in loss and damage of substratum than surface water flow.

Sensitivity assessment. No direct evidence was found to assess this pressure at the benchmark. Based on the exposure of piddocks in this biotopes to water flows between 0.5 and 1.5 m/s (Connor et al., 2004), the biotope is considered to be unimpacted by changes within this range as long as these do not lead to increased erosion of the substratum. Resistance is therefore assessed as 'High' and resilience as 'High' (based on no impact to recover from), so that the biotope is considered to be ‘Not sensitive’.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Not relevant to sublittoral habitats.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

No direct evidence was found to assess sensitivity to this pressure.  The biotope typically occurs in moderately wave exposed locations (Connor et al., 2004). The piddocks are unlikely to be directly affected by changes in wave exposure, owing to their environmental position within the substratum, which protects them. On chalk and clay substrates, it is possible however, that wave action actively erodes the substratum at a faster rate than the piddocks can burrow leaving them exposed to predators or displaced. At higher densities bioerosion by piddocks may destabilise the substratum increasing vulnerability to erosion.  

Potentially the most damaging effect of increased wave heights would be the erosion of the clay substratum as this could eventually lead to loss of the habitat. Increased erosion would lead to the loss of habitat and removal of piddocks. No evidence was found to link significant wave height to erosion. Some erosion will occur naturally and storm events may be more significant in loss and damage of clays  than changes in wave height at the pressure benchmark.  Micu (2007) for example observed numerous Pholas dactylus that had been washed out of the clay substratum or exposed due to storm damage to the clay in the Romanian Black Sea.

Sensitivity assessment. No direct evidence was found to assess this pressure at the benchmark. Based on the occurrence of this biotope in moderately wave exposed habitats the piddocks and algal mat are considered to have 'High' resistance to changes at the pressure benchmark where these do not lead to increased erosion of the substratum.  Resilience is therefore assessed as 'High' and the biotope is considered to be 'Not sensitive', at the pressure benchmark.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Specific information concerning oxygen consumption and reduced oxygen tolerances were not found for important characterizing species within the biotope. Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg O₂/l and probable adverse effects below 2mg O₂/l. Duval (1963a) observed that conditions within the borings of Petricolaria pholadiformis were anaerobic and lined with a loose blue/black sludge. Similarly  Knight (1984) observed Pholas dactylus exposed to low oxygen concentrations  in burrows in areas of chalk bedrock, that were overlain with silt and were anoxic below the surface (Knight, 1984).  Piddocks may therefore have some tolerance of this pressure at the benchmark, however, insufficient information has been recorded to develop an assessment.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014).  No evidence was found to assess the sensitivity of piddocks to this pressure.  Nutrient enrichment that enhances productivity of phytoplankton may indirectly benefit the suspension feeding piddocks by increasing food supply.

Sensitivity assessment. The pressure benchmark is set at a level that is relatively protective and the biological assemblage is considered to be 'Not sensitive' at the pressure benchmark. Resistance and resilience are therefore assessed as 'High'.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

No evidence was found to assess this pressure. 

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

This biotope is characterized by the clay or soft chalk substratum which supports populations of burrowing piddocks. A change to a sedimentary, rock or artificial substratum would result in the loss of piddocks significantly altering the character of the biotope. The biotope is therefore considered to have 'No' resistance to this pressure, recovery of the biological assemblage (following habitat restoration) is considered to be 'Medium' (2-10 years). The biotope is dependent on the presence of clay or soft chalk, when lost restoration would not be feasible and recovery is therefore categorised as 'Very low'. Sensitivity is therefore assessed as 'High', based on the lack of recovery of the substratum. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

This biotope is characterized by the clay or soft chalk substratum which supports populations of burrowing piddocks. A change to a sedimentary substratum would result in the loss of piddocks significantly altering the character of the biotope. The biotope is therefore considered to have 'No' resistance to this pressure, recovery of the biological assemblage (following habitat restoration) is considered to be 'Medium' (2-10 years) but see caveats in the recovery notes. The biotope is dependent on the presence of soft chalk or clayt, when lost restoration would not be feasible and recovery is therefore categorised as 'Very low'. Sensitivity is therefore assessed as 'High', based on the lack of recovery on chalk or clay substratum. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

The removal of substratum to 30cm depth will remove the clay or chalk substratum, piddocks and the associated biological assemblage, in the impact footprint.  Resistance is therefore assessed as ‘None’, recovery of the biological assemblage (fwhere suitable substratum remains) is considered to be 'Medium' (2-10 years). The biotope is dependent on the presence of clay or chalk substratum, when lost restoration would not be feasible and recovery is therefore categorised as 'Very low'. Sensitivity is therefore assessed as 'High', based on the lack of recovery of clay or chalk habitats. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Within this biotope the sparse epifauna could be damaged and removed by surface abrasion.  Some species protruding from the surface may also be removed.  Although the piddocks are afforded some protection from surface abrasion by living in their burrows, the clay or chalk is soft which leaves many individuals, especially those near the surface of the clay, vulnerable to damage and death through exposure of burrows and habitat damage.   Micu (2007), for example, observed that after storms in the Romanian Black Sea, the round goby, Neogobius melanostomus, removed clay from damaged or exposed Pholas dactylus burrows to be able to remove and eat the piddocks.

The most significant impact may be on the substratum by removing  or damaging surface layers resulting in the clay or chalk being more vulnerable to erosion. Natural erosion processes are, however, likely to be on-going within this habitat type. Where abundant the boring activities of piddocks also contribute significantly to bioerosion, which can make the substratum habitat more unstable and can result in increased rates of coastal erosion (Evans 1968a, Trudgill 1983, Trudgill & Crabtree, 1987).  Pinn et al. (2005) estimated that over the lifespan of a piddock (12 years), up to 41% of the shore could be eroded to a depth of 8.5 mm. Surface erosion is therefore a natural part of the environmental processes the biotope experiences although rates could be enhanced by surface abrasion and disturbance.

Sensitivity assessment. Surface abrasion may remove epifauna and result in the loss of some piddocks and damage to habitat. Resistance is assessed as ‘Medium’ for piddocks and substratum.   As the substratum cannot recover, resilience is assessed as ‘Very Low’ and sensitivity of the overall biotope is considered to be ‘Medium’.  

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Penetration and disturbance below the surface of the substratum will damage and remove the dense algal mat and surface fauna and could damage and expose piddocks. Individuals in damaged burrows, or those that are removed from the substratum, are unlikely to be able to rebury and will be predated by fish and other mobile species (Micu, 2007). 

The most significant impacst arising from this pressure may be the damage and removal of the chalk and clay substratum. Where abundant the boring activities of piddocks can make the substratum habitat more unstable and can exacerbate erosion (Evans 1968a, Trudgill 1983, Brookes & Stevens, 1985, Trudgill & Crabtree, 1987).  Pinn et al. (2005) estimated that over the lifespan of a piddock (12 years), up to 41% of the shore could be eroded to a depth of 8.5 mm. Piddock burrowing can therefore  make the substratum more vulnerable to damage and removal.

Sensitivity assessment. Sub-surface penetration and disturbance will remove and damage the sparse epifauna and result in the loss of piddocks and damage to the habitat. Resistance is therefore assessed as ‘Low’ for the piddocks and substratum.  The piddocks are judged to have ‘Medium’ resilience (where suitable substratum remains) so that sensitivity of the piddocks is ‘Medium’. As the substratum cannot recover, resilience is assessed as ‘Very Low’ and sensitivity of the overall biotope is considered to be ‘High’.  

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

No direct evidence was found to assess this pressure. Increased suspended particles will decrease light penetration, may enhance food supply (where these are organic in origin), or decrease feeding efficiency (where the particles are inorganic and require greater filtration efforts). Very high levels of silt may clog respiratory and feeding organs of some suspension feeders.  Increased levels of particles may increase scour and deposition in the biotope depending on local hydrodynamic conditions, however, the piddocks are protected from scour within burrows and increased organic particles will provide a food subsidy.

Pholas dactylus occurs in habitats such as soft chalks where turbidity may be high (UK BAP, 2008) and is therefore unlikely to be affected by an increase in suspended sediments at the pressure benchmark. Piddocks, in common with other suspension feeding bivalves, have efficient mechanisms to remove inorganic particles via pseudofaeces. Experimental work on Pholas dactylus showed that large particles can either be rejected immediately in the pseudofaeces or passed very quickly through the gut (Knight, 1984).  Increased suspended sediments may impose sub-lethal energetic costs on piddocks by reducing feeding efficiency and requiring the production of pseudofaeces with impacts on growth and reproduction.

A significant decrease in suspended organic particles may reduce food input to the biotope resulting in reduced growth and fecundity of Pholas dactylus. However, local primary productivity may be enhanced where suspended sediments decrease, increasing food supply.  

Sensitivity assessment. No direct evidence was found to assess sensitivity to this pressure however, based on the occurrence of Pholas dactylus in turbid areas and evidence for the production of pseudofaeces by piddocks, resistance is assessed as ‘High’ and resilience as High (no impact to recover from). The biotope is therefore considered to be ‘Not sensitive’.  

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

The burrowing mechanisms of the piddocks Pholas dactylus and Barnea candida and other Pholads, mean that the burrows have a narrow entrance excavated by the juvenile. As the individual grows and excavates deeper the burrow widens resulting in a conical burrow from which the adult cannot emerge.  Piddocks cannot therefore emerge from layers of deposited silt as other more mobile bivalves can.

No examples of direct empirical evidence or experiments on mortality rates have been found. Sometimes the substratum in which piddocks reside is covered by a thin layer of loose sandy material, through which the piddocks maintain contact with the surface via their siphons.   Pholas dactylus is considered relatively tolerant of smothering by silt as it has been recorded from gently sloping chalk bedrock largely overlain by mud or silt 1-5cm deep, anoxic below the surface (Knight, 1984).  Pholas dactylus has also been found living under layers of sand in Aberystwyth, Wales, (Knight, 1984) and in Eastbourne, with their siphons protruding at the surface (Pinn et al., 2008). It is likely that the piddocks would be able to extend their siphons through loose material, particularly where tidal movements and the oscillatory water movements produced by waves shift sediments.  Barnea candida has also been found to survive being covered by shallow layers of sand in Merseyside (Wallace & Wallace, 1983). Wallace & Wallace (1983) were unsure as to how long the Barnea candida could survive smothering but noted that, on the coast of the Wirral, the piddocks have survived smothering after periods of rough weather. Where smothering is constant, survival can be more difficult. The redistribution of loose material following storms off Whitstable Street, in the Thames Estuary, is thought to be responsible for the suffocation of many Petricolaria pholadiformis. However, it was not known how deep the layer of 'loose material' was, nor how long it lasted for or what type of material it was made up of.

Indirect indications for the impacts of siltation are provided by studies of Witt (2004) on the impacts of harbour dredge disposal. Petricolaria (syn. Petricola) pholadiformis was absent from the disposal area, and Witt (2004) cites reports by Essink (1996, not seen) that smothering of Petricolaria (syn. Petricola) pholadiformis from siltation could lead to mortality within a few hours.  Hebda (2011) also identified that sedimentation may be one of the key threats to Barnea truncata populations.  At Agigea (Micu, 2007) reported that smothering of clay beds by sand and finer sediments had removed populations of Pholas dactylus. In this area sand banks up to 1m thick frequently shift position driven by storm events and currents (Micu, 2007). Similar smothering was described in the case of Barnea candida populations boring into clay beds (Gomoiu & Muller 1962, cited from Micu, 2007).

Sensitivity  assessment. As piddocks are essentially sedentary and as siphons are relatively short, siltation from fine sediments that add to existing silt layers could be lethal.  As the evidence suggests that Pholas  dactylus is present under deposits  up to the benchmark layer, resistance is assessed as ‘Medium’ where existing deposits are relatively thin. Effects may be mitigated where water currents and wave exposure rapidly removed the overburden and this will depend on local hydrodynamic conditions and the footprint of the deposit. Resilience is assessed as ‘Medium’ (2-10 years) for piddocks and sensitivity is therefore assessed as ‘Medium’. 

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

The burrowing mechanisms of the piddocks Pholas dactylus and Barnea candida and other Pholads, mean that the burrows have a narrow entrance excavated by the juvenile. As the individual grows and excavates deeper the burrow widens resulting in a conical burrow from which the adult cannot emerge.  Piddocks cannot therefore emerge from layers of deposited silt as other more mobile bivalves can.

No examples of direct empirical evidence or experiments on mortality rates in response to siltation have been found for piddocks. Indirect indications for the impacts of siltation are provided by studies of Witt (2004) on the impacts of harbour dredge disposal. Petricolaria pholadiformis (as Petricola pholadiformis was absent from the disposal area, and Witt (2004) cites reports by Essink (1996, not seen) that smothering of Petricola pholadiformis from siltation could lead to mortality within a few hours.  Hebda (2011) also identified that sedimentation may be one of the key threats to Barnea truncata populations.  At Agigea (Micu, 2007) reported that smothering of clay beds by sand and finer sediments had removed populations of Pholas dactylus. In this area sand banks up to 1m thick frequently shift position driven by storm events and currents (Micu, 2007). Similar smothering was described in the case of Barnea candida populations boring into clay beds (Gomoiu & Muller 1962, cited from Micu, 2007).

Sensitivity assessment. As piddocks are essentially sedentary and as siphons are relatively short, siltation from fine could be lethal.  Siltation at the pressure benchmark is considered to smother most or all of the piddocks and the surface fauna.  Resistance to siltation is therefore assessed as ‘None' although effects could be mitigated where water currents and wave exposure rapidly removed the overburden and this will depend on shore height and local hydrodynamic conditions. Resilience is assessed as ‘Medium’ (2-10 years) for piddocks and sensitivity is therefore assessed as ‘Medium’. 

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Not relevant.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Pholas dactylus can perceive and react to light (Hecht, 1928) however there is no evidence that this pressure would impact the biotope. 

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Not relevant.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

 ‘Not relevant’ to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’. 

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Pholas dactylus reacts quickly to changes in light intensity, after a couple of seconds, by withdrawing its siphon (Knight, 1984). This reaction is ultimately an adaptation to reduce the risk of predation by, for example, approaching birds (Knight, 1984). However, its visual acuity is probably very limited and it is unlikely to be sensitive to visual disturbance. Resistance and resilience are therefore assessed as ‘High’ by default.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

The species characterizing this biotope, Pholas dactylus and other piddocks, are not farmed or translocated and therefore this pressure is 'Not relevant' to this biotope.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

The friable nature of the substratum which is subject to on-going erosion means this biotope supports only a sparse epifauna and flora. This biotope is therefore unlikely to be invaded by sessile invasive non-indigenous species. As the biotope occurs subtidally and turbidity levels are often high this biotope likely to be unsuitable for invasive non-indigenous algae.  

The American piddock, Petricolaria pholadiformis is a non-native, boring piddock that was unintentionally introduced from America with the American oyster, Crassostrea virginica, not later than 1890 (Naylor, 1957). Rosenthal (1980) suggested that from the British Isles, the species has colonized several northern European countries by means of its pelagic larva and may also spread via driftwood, although it usually bores into clay, peat or soft rock shores. In Belgium and The Netherlands Petricolaria pholadiformis  almost completely displaced the native piddock, Barnea candida (ICES, 1972). However, this has not been observed elsewhere, and later studies have found that Barnea candida is now more common than Petricolaria pholadiformis in Belgium (Wouters, 1993) and there is no documentary evidence to suggest that Barnea candida has been displaced in the British Isles (J. Light & I. Kileen pers. comm. to Eno et al., 1997). This species is also unlikely to displace Pholas dactylus which is more likely to occur subtidally.

Although not currently established in UK waters, the whelk Rapana venosa, may spread to habitats. This species has been observed predating on Pholas dactylus in the Romanian Black Sea by Micu (2007).

Sensitivity assessment. Based on the lack of records of invasive non-indigenous species in this biotope, and the unsuitability of the habitat for algae and other attached epifauna this biotope is considered to have ‘High’ resistance to this pressure and, by default ‘High’ resilience, this biotope is therefore considered to be ‘Not sensitive’. This assessment may need revising in light of future invasions, e.g. the introduction of the whelk Rapana venosa.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

No evidence.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Piddocks may be removed as bait and across Europe they have traditionally been harvested for food, however high levels of habitat damage are associated with the removal of boring molluscs (Fanelli et al., 1994) and this practice has largely been banned. The most sensitive component of this biotope to targeted harvesting is the peat substratum which may be damaged and removed if piddocks were excavated from their burrows, this effect is considered through the physical damage pressures, abrasion and penetration and sub-surface damage.

Sensitivity assessment. Removal of piddocks will result in loss of targeted individuals and damage to the habitat. Resistance is assessed as ‘Low’ as piddocks are sedentary and burrow openings are readily detected.  Piddocks are predicted to recover within 2-10 years so that resilience is considered to ‘Medium’ and sensitivity is ‘Medium’. 

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

The sparse epifauna and the substratum may be removed or damaged by activities targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. It is unlikely that any targeted harvesting of other species occurs in this biotope or that it would unintentionally remove piddocks. Resistance of surface fauna and flora is assessed as 'Medium' and resilience as 'High' so that sensitivity is assessed as 'Low'.