|Researched by||Jacqueline Hill||Refereed by||This information is not refereed|
|Other common names||Sword razor, Narrow jackknife clam, Common razor clam||Synonyms||-|
Razor shells have an elongate and fragile shell with valves gaping at both ends. The shell is smooth on the outside and whitish in colour with vertical and horizontal reddish-brown or purplish-brown markings separated by a diagonal line. The periostracum is olive-green. The inner surface is white with a purple tinge and the foot is pale red-brown. The presence of razor shells in sand is indicated by keyhole-shaped openings made by the short, united siphons which extend just above the sediment surface when the animal is suspension feeding.
There are three species of razor shell in Britain and Ireland: Ensis ensis, Ensis siliqua and Ensis arcuatus. Ensis ensis is slender, with a slightly curved elongate shell up to 130mm long. In Ensis siliqua both dorsal and ventral margins are straight and adults are up to 200mm long. Ensis arcuatus grows up to 150mm long and the dorsal margin is straight, the ventral margin is curved. It may be particularly difficult to distinguish between species in juvenile individuals.
The sensitivity and recoverability information has been compiled primarily using information regarding the common razor shell Ensis ensis.
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|Phylum||Mollusca||Snails, slugs, mussels, cockles, clams & squid|
|Class||Bivalvia||Clams, cockles, mussels, oysters, and scallops|
|Typical abundance||See additional information|
|Male size range||up to 13cm|
|Male size at maturity||>10cm|
|Female size range||>10cm|
|Female size at maturity|
|Body flexibility||None (less than 10 degrees)|
|Characteristic feeding method|
|Typically feeds on||Suspended organic detritus|
|Is the species harmful?||No|
Ensis ensis is an edible species and therefore non-toxic. However, Ensis species are thought to be especially at risk of Amnesic Shellfish Poisoning (ASP) (Edward Fahy pers. comm.).
Abundance of Ensis sp. varies from high to low density. In favourable conditions - such as the lee of rocks, rocks and islands for Ensis arcuatus on the western coast, individuals are found in high densities in 'beds' which interchange individuals with the surrounding areas where they occur in a more dispersed pattern (Fahy et al. in press).
Size range given for Ensis ensis. Ensis siliqua males and females up to 20 cm and Ensis arcuatus males and females up to 15 cm.
Growth in the first winter is 2-4 cm. The three species have similar growth patterns but with different asymptotic lengths. In Ensis siliqua males grow faster than females. Growth rates are higher in the summer, when the food supply is abundant, than in the winter when the temperature and food supply are both reduced. Ensis ensis also show a neap-spring lunar growth pattern with smaller growth bands during spring tides when animals are emersed for longer (Henderson & Richardson, 1994). The growth rate given is the maximum rate in the first year or two of life. Thereafter growth falls to 2-3 cm/year (Robinson & Richardson, 1998).
|Biological zone preferences|
|Substratum / habitat preferences|
|Tidal strength preferences|
|Wave exposure preferences|
|Depth range||to a depth of 60m|
|Other preferences||No text entered|
|Reproductive frequency||Annual episodic|
|Fecundity (number of eggs)|
|Generation time||Insufficient information|
|Age at maturity||3 years minimum|
|Season||Summer - Summer|
|Life span||11-20 years|
|Duration of larval stage||1-2 months|
|Larval dispersal potential||No information|
|Larval settlement period|
The MarLIN sensitivity assessment approach used below has been superseded by the MarESA (Marine Evidence-based Sensitivity Assessment) approach (see menu). The MarLIN approach was used for assessments from 1999-2010. The MarESA approach reflects the recent conservation imperatives and terminology and is used for sensitivity assessments from 2014 onwards.
|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.|
|Tolerant||Not relevant||Not sensitive||High|
|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.|
|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.|
|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.|
|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.|
|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.|
|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.|
|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.|
|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.|
|Tolerant||Not relevant||Not sensitive||Moderate|
|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.|
|Tolerant||Not relevant||Not sensitive||Moderate|
|Razor shells are unlikely to be sensitive to visual disturbance.|
|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.|
|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.|
|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.|
|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.|
|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.|
|No information||No information||No information||Not relevant|
|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.|
|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.|
|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.|
|No information||No information||No information||Not relevant|
|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).|
|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.|
|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).|
|Not relevant||Not relevant||Not relevant||Not relevant|
|Ensis ensis has no known obligate relationships with other species.|
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|National (GB) importance||-||Global red list (IUCN) category||-|
Centre for Environmental Data and Recording, 2018. IBIS Project Data. Occurrence dataset: https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.
Cofnod – North Wales Environmental Information Service, 2018. Miscellaneous records held on the Cofnod database. Occurrence dataset: https://doi.org/10.15468/hcgqsi accessed via GBIF.org on 2018-09-25.
Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) data for Great Britain and Ireland - restricted access. Occurrence dataset: https://doi.org/10.15468/4bsawx accessed via GBIF.org on 2018-09-25.
Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) records for Great Britain and Ireland. Occurrence dataset: https://doi.org/10.15468/aurwcz accessed via GBIF.org on 2018-09-25.
Environmental Records Information Centre North East, 2018. ERIC NE Combined dataset to 2017. Occurrence dataset: http://www.ericnortheast.org.uk/home.html accessed via NBNAtlas.org on 2018-09-38
Fenwick, 2018. Aphotomarine. Occurrence dataset http://www.aphotomarine.com/index.html Accessed via NBNAtlas.org on 2018-10-01
Fife Nature Records Centre, 2018. St Andrews BioBlitz 2015. Occurrence dataset: https://doi.org/10.15468/xtrbvy accessed via GBIF.org on 2018-09-27.
Kent Wildlife Trust, 2018. Kent Wildlife Trust Shoresearch Intertidal Survey 2004 onwards. Occurrence dataset: https://www.kentwildlifetrust.org.uk/ accessed via NBNAtlas.org on 2018-10-01.
Lancashire Environment Record Network, 2018. LERN Records. Occurrence dataset: https://doi.org/10.15468/esxc9a accessed via GBIF.org on 2018-10-01.
Merseyside BioBank., 2018. Merseyside BioBank (unverified). Occurrence dataset: https://doi.org/10.15468/iou2ld accessed via GBIF.org on 2018-10-01.
National Trust, 2017. National Trust Species Records. Occurrence dataset: https://doi.org/10.15468/opc6g1 accessed via GBIF.org on 2018-10-01.
NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.
Norfolk Biodiversity Information Service, 2017. NBIS Records to December 2016. Occurrence dataset: https://doi.org/10.15468/jca5lo accessed via GBIF.org on 2018-10-01.
OBIS (Ocean Biodiversity Information System), 2022. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2022-11-27
Outer Hebrides Biological Recording, 2018. Invertebrates (except insects), Outer Hebrides. Occurrence dataset: https://doi.org/10.15468/hpavud accessed via GBIF.org on 2018-10-01.
South East Wales Biodiversity Records Centre, 2018. SEWBReC Molluscs (South East Wales). Occurrence dataset: https://doi.org/10.15468/jos5ga accessed via GBIF.org on 2018-10-02.
This review can be cited as:
Last Updated: 02/11/2006