The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

Loss of the substratum will remove the resident population of the burrowing razor shell Ensis ensis and so intolerance is high. However, razor clams are very mobile bivalves and will rapidly migrate and recolonize favourable areas. In addition, razor shells have a pelagic larva, with annual spatfalls (E. Fahy pers. comm.) so it seems likely that populations could recover within a year. However, given the sporadic nature of the abundance of spatfalls, recovery may be more protracted and the age distribution of the population may be skewed towards younger individuals than before. In the Setubal region of Portugal, signs of recovery of Ensis siliqua populations was observed about 18 months after the cessation of fishing. However, yields were still low and it was recommended that the closure of fishing should remain for full recovery (Gaspar & Dias, 1999). However, it is likely that recovery should be complete within five years although beds of larger adults would probably take up to about 10 years.

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

Ensis ensis lives buried in sand, can extend its siphons and rise in its burrow and so is likely to tolerate smothering by 5 cm of sediment.

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Holme (1954) reports that Ensis ensis is the more silt tolerant of the British Ensis species and is generally found at sheltered localities or from offshore and in sediments with a silt percentage of up to 16%. A decrease in siltation may affect growth and fecundity if the supply of organic particulate matter declines.

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

Ensis ensis is found in the intertidal at extreme low water so will be subject to desiccation only rarely. The shell gapes at both ends so that water loss cannot be prevented and the animal is therefore, likely to be highly intolerant of an increase in desiccation. If the animal is not surrounded by a burrow the shells open and the mantle splits. However, the species may burrow further into the sand during low tide to avoid desiccation although on a number of occasions Ensis ensis have been seen to come right out of the sand at low tide, and lie on the surface when a heavy mortality is likely to result (Holme, 1954). Ensis spp. may be held for some time out of water, provided the shells are kept closed (by being restrained by an elastic band for example) although periods are likely to be damaging. In years of good recruitment recolonization may occur within a year. However, recruitment is sporadic (see reproduction) and recovery may take longer but should be complete within five years.

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Ensis ensis is found in the intertidal at extreme low water being exposed briefly once or twice a month and is therefore likely to be highly intolerant of an increase in emergence. On a number of occasions Ensis ensis have been seen to come right out of the sand at low tide, and lie on the surface when a heavy mortality is likely to result. While some probably survive the exposure and burrow in again when the tide returns, many must be eaten by gulls (Holme, 1954). The species will probably tolerate a decrease in emergence which will probably allow the population to extend up the shore. In years of good recruitment recovery may occur within a year. However, recruitment is sporadic (see reproduction) and recovery may take longer but should be complete within five years.

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

In an investigation of an intertidal population of Ensis ensis in north Wales Henderson & Richardson (1994) found that the largest razor clams were found only at extreme low water, whilst smaller clams (<100mm) were collected further up the shore. The authors suggest that the area on the lower shore may be unsuitable for juveniles because they are exposed to the greatest tidal currents and may get washed away. Increased water flow is likely to make the sediment more mobile and individuals of Ensis ensis may be washed away. However, since the species is able to burrow deeper into the sediment during unsuitable conditions water flow rates would have to increase substantially to remove individuals and so intolerance is assessed as intermediate.

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Populations of the razor shell Ensis siliqua in the warmer waters of Portugal spawn several months earlier in the year than UK populations and are sexually mature at only one year old (Gaspar & Monteiro, 1998; Henderson & Richardson, 1994) compared to three in the UK. Therefore, it is likely that temperature is important for growth and fecundity of Ensis ensis. However, the species extends north and south of British populations and so is likely to be tolerant of a long term change in temperature of 2°C. The species is likely to be more intolerant of a rapid change in temperature of 5°C outside its normal temperature range. During the cold winter of 1962-63 when air temperatures fell below freezing for several weeks Crisp (1964) recorded very high levels of mortality of Ensis ensis and suggests that the razor shells lost the ability to burrow at lowered temperatures, and so were left exposed at the surface. At some sites live individuals were later found by digging so some protection is afforded by burrowing position. Ensis spp. are known to emerge from the sediment when shallow inshore waters become warm as happened in Torbay in 1999 (K. Hiscock pers. comm.). In years of good recruitment recolonization may occur within a year, however, recruitment is sporadic and may take several years when recruitment is poor.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Changes in light attenuation resulting from turbidity changes are not likely to affect the suspension feeding Ensis ensis. However, if increased turbidity is caused by silt particles additional feeding costs may be imposed and phytoplankton production may decline reducing food supplies. The species may benefit from increased nutritional value if turbidity is caused by organic particles.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

On exposed beaches where the sand is continually churned by waves, razor shells are absent. Wave scour caused by winter gales along the North Wales coast washed out some individuals of Ensis ensis although numbers were much lower than for some other fauna (Rees et al., 1976). Therefore, significant increases in wave exposure may cause the death of some individuals in a population and may limit individuals to below the low-water mark. On moderately wave exposed beaches Ensis ensis may be replaced by the larger Ensis siliqua (Holme, 1954). In years of good recruitment recolonization may occur within a year, however, recruitment is sporadic and may take several years when recruitment is poor.

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Ensis ensis can probably detect the vibration caused by predators and will withdraw its siphons. No information was found concerning the effect of noise or vibration on razor shell populations although the species is unlikely to be sensitive to noise or vibration.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Razor shells are unlikely to be sensitive to visual disturbance.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Ensis ensis has a thin brittle shell and so is highly intolerant of abrasion and physical disturbance. Eleftheriou & Robertson (1992) observed large numbers of Ensis ensis killed or damaged by dredging operations and Gaspar (1998) reports high levels of damage in Ensis siliqua from fishing. Therefore, an intolerance of high has been recorded. In years of good recruitment recolonization may occur within a year. However, given the sporadic nature of the abundance of spatfalls, recovery may be more protracted and the age distribution of the population may be skewed towards younger individuals than before. In the Setubal region of Portugal, signs of recovery of Ensis siliqua populations was observed about 18 months after the cessation of fishing. However, yields were still low and it was recommended that the closure of fishing should remain for full recovery (Gaspar & Dias, 1999). However, it is likely that recovery should be complete within five years although beds of larger adults would probably take up to about 10 years.

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Razor shells displaced from their burrow onto the surface of the sediment, as may be caused by a storm, can rapidly reburrow on return to a suitable substratum and so can survive. However, if the method of removal is stressful the species ability to reburrow on return to a suitable substratum may be impaired. For example, live specimens removed by dredging and then replaced on the substratum took a long time to reburrow and many were consumed by predatory crabs (Robinson & Richardson, 1998). Therefore, intolerance has been assessed as intermediate.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

High levels of mortality of Ensis spp. were found at places distant from shores treated with dispersants following the Torrey Canyon oil spill (Smith, 1968). Almost complete mortality of razor shells was found at stations more than a kilometre from the shore at a depth of about 20m. Experiments have shown that Ensis species are intolerant of only 0.5 ppm of detergent with high intolerance to the solvent rather than the surfactant element (Smith, 1968). On return to normal conditions recovery may occur within a year if recruitment is good. However, recruitment is sporadic (see reproduction) and recolonization may take longer but should be complete within five years.

Heavy metal contamination

Evidence

No specific information on the effect of heavy metals to razor shells could be found. However, in investigations of faunal distribution in the metal contaminated Restronguet Creek in the Fal estuary bivalve molluscs appear to be the most vulnerable (Bryan, 1984). The bivalve Scrobicularia plana, for example, is absent from large areas of the intertidal muds where, under normal conditions, it would account for a large amount of the biomass (Bryan & Gibbs, 1983). Bryan (1984) also reports that metal-contaminated sediments can exert a toxic effect on burrowing bivalves and so intolerance has been assessed as intermediate. Embryonic and larval stages of bivalve molluscs are the most vulnerable to heavy metals (Bryan, 1984). On return to normal conditions recovery may occur within a year if recruitment is good. However, recruitment is sporadic (see reproduction) and recovery may take longer but should be complete within five years.

Hydrocarbon contamination

Evidence

Ensis ensis is reported to bioconcentrate aromatics and is highly intolerant of hydrocarbons. Four days after the Sea Empress oil spill moribund razor shells (mostly Ensis siliqua) were the first organisms observed to have been affected (SEEEC, 1998). Hundreds of razor shells were protruding from the sand and most died in that position over the next few days. Glegg & Rowland (1996) observed dead razor shells washed up on the shore a few days after the final break-up of the Braer wreck and about a million razor shells were seen after the Amoco Cadiz oil spill in Brittany (Southward, 1978). On return to normal conditions recolonization should be high. Although recruitment of Ensis ensis is sporadic recovery should be complete within five years. However, the age distribution of the population may be skewed towards younger individuals than before.

Radionuclide contamination

Evidence

Insufficient
information.

Changes in nutrient levels

Evidence

Although no specific information regarding the response of Ensis ensis to changes in nutrient levels the species is not characteristic of habitats at the upper end of the organic gradient and so is assessed as having intermediate intolerance.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Ensis ensis does not occur in water of reduced salinity, although its absence from estuaries may sometimes be due to the lack of deposits of suitable grade (Holme, 1954). The species concentrates K and Ca (Kinne, 1971) and can probably tolerate a degree of salinity reduction because it will be subject to periodic precipitation in the intertidal. Intolerance has therefore, been assessed as intermediate. One means of collecting Ensis spp. is to sprinkle salt on their burrows causing them to rise to the surface.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

Ensis species typically occur in sands which are not black below the surface i.e. where conditions are oxygenated and not reducing. Where seaweed or other organic matter gets buried in and incorporated in the sand, resulting in a black layer containing ferrous sulphide, Ensis is absent. However, the species can tolerate sands which are slightly reducing, in which there is a grey layer below the surface, such as occurs on beaches of firm fine sand in which the organic content is not high, but there is little circulation of water (Holme, 1954), and so intolerance is assessed as intermediate.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

No information on diseases of Ensis spp. was found. However, mortalities of the Pacific razor clam Siliqua patula, explained by infection with Rickettsia-like organisms, have been reported in several locations in the US (Elston, 1986).

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

The American razor shell Ensis americanus (synonym: Ensis directus) has spread from its point of introduction in the German Bight in 1978 into southern North Sea countries. The species, which is native to the Atlantic coast of North America was found in Britain in 1989 on Holme beach, Norfolk. The long-lasting pelagic larval stage is assumed to be transported with water currents and has spread rapidly in southern North Sea countries and is now found at sites along the British east coast south from the Humber and along the English Channel west as far as East Sussex (Eno et al., 1997). The species lives in brackish as well as marine conditions so may be filling a niche in estuaries not already occupied (Urk van, 1987). Ensis americanus is also found in much finer and unstable sand than Ensis ensis and so the two species may not be in direct competition. Armonies & Reise (1999) report that there were no significant interactions between Ensis americanus and resident species.

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Traditionally Ensis ensis has been hand collected for food, bait and personal use. In Scotland, some subtidal razor clam beds are dense enough to be exploited commercially and recently the species has been harvested by suction dredger (Fowler, 1999; Robinson & Richardson, 1998). Ensis ensis has a pelagic larva so it seems likely that the population could recolonize within a year. However, given the sporadic nature of recruitment in Ensis ensis, recovery may be more protracted but should be complete within five years. However, the age distribution of the population may be skewed towards younger individuals than before. In the Orkneys for example, where Ensis arcuatus beds are subject to repeated dredging, populations have a significantly smaller average length than those at an un-fished site (Robinson & Richardson, 1998).

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Ensis ensis has no known obligate relationships with other species.