Limaria hians beds in tide-swept sublittoral muddy mixed sediment

12-08-2003
Researched byDr Harvey Tyler-Walters & France Perry Refereed byDr Jason Hall-Spencer
EUNIS CodeA5.434 EUNIS NameLimaria hians beds in tide-swept sublittoral muddy mixed sediment

Summary

UK and Ireland classification

EUNIS 2008A5.434Limaria hians beds in tide-swept sublittoral muddy mixed sediment
EUNIS 2006A5.434Limaria hians beds in tide-swept sublittoral muddy mixed sediment
JNCC 2004SS.SMx.IMx.LimLimaria hians beds in tide-swept sublittoral muddy mixed sediment
1997 BiotopeSS.IMX.FaMx.LimLimaria hians beds in tide-swept sublittoral muddy mixed sediment

Description

Mixed muddy gravel and sand often in tide-swept narrows in the entrances or sills of sea lochs with beds or 'nests' of Limaria hians. The Limaria hians forms woven 'nests' or galleries from byssus and fragments of seaweeds so that the animals themselves cannot be seen from above the seabed. Modiolus modiolus sometimes occur at the same sites lying amongst the Limaria hians bed. Other fauna associated with this biotope include hydroids such as Kirchenpaueria pinnata, Nemertesia spp. and Plumularia setacea, mobile crustaceans (e.g. Hyas araneus) and echinoderms (Crossaster papposus, Ophiothrix fragilis and Ophiocomina nigra). Sometimes seaweeds occur if the beds are in shallow enough water. (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Recorded distribution in Britain and Ireland

The biotope is recorded from a few locations on the west coast of Scotland, and in Mulroy Bay, Co. Donegal, Ireland. For a full species distribution see Limaria hians.

Depth range

0-5 m, 10-20 m, 20-30 m, 50-100 m

Additional information

Little information on the ecology of this biotope was found, and the review presented has been based on the survey data and few detailed studies available (JNCC, 1999; Minchin, 1995, Hall-Spencer & Moore, 2000b, Trigg, 2009; Trigg & Moore, 2009; and Trigg et al., 2011).

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Further information sources

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JNCC

Habitat review

Ecology

Ecological and functional relationships

The information below is based on survey data and the few detailed studies available: Gilmour (1967), Minchin (1995), Connor et al. (1997), JNCC (1999), and Hall-Spencer & Moore (2000b).
  • Limaria hians is an active suspension feeder on phytoplankton, bacteria, and detritus.
  • The carpet of byssus threads, coarse sediment, and shell produced by Limaria hians provides refugia, and substratum for attachment for a wide variety of sessile and sedentary species.
  • Other suspension feeders include sponges, hydroids (e.g. Kirchenpaueria pinnata, Nemertesia spp., and Tubularia spp.), bryozoans (e.g. Bugula spp.) , soft corals (e.g. Alcyonium digitatum), epifaunal and infaunal bivalves (e.g. Modiolus modiolus and Mya truncata respectively), tube worms (e.g. Pomatoceros triqueter), ascidians and small crustaceans. If present, the brittlestars, e.g. Ophiura spp. or Ophiopholis aculeata, may also suspension feed.
  • Kelp (e.g. Laminaria hyperborea) and foliose red algae (e.g. Delesseria sanguinea or Phycodrys rubens) probably provide primary production in the form of detritus and dissolved organic matter, and via grazing by amphipods, isopods, chitons, gastropods (e.g. Gibbula cineria, Tectura virginea or Calliostoma zizyphinum) or sea urchins (e.g. Psammechinus miliaris). Suspension feeders, including Limaria hians obtain primary productivity from phytoplankton and benthic and epiphytic microalgae.
  • The faunal turf of hydroids and bryozoans is probably grazed by echinoderms (e.g. Henricia oculata and Echinus esculentus) or gastropods (e.g. Calliostoma zizyphinum) or preyed on by polychaetes (e.g. the sea mouse Aphrodite aculeata), nudibranchs (e.g. Onchidoris sp.) and pycnogonids (e.g. Achelia echinata) (Gordon, 1972; Salvini-Plawen, 1972: Ryland, 1976).
  • Mobile predators include crabs such as Cancer pagurus and Necora puber, which probably eat a variety of epifauna including gastropods, small crustacea and bivalves. The nests of Limaria hians provide a defence against most predators (Merrill & Turner, 1963: Gilmour, 1967). In addition, Limaria hians can also discard its tentacles leaving a predator with a sticky, unpalatable meal while the rest of the animal makes its escape by swimming. The tentacles of Limaria hians (especially the longer tentacles) secrete an adhesive and irritant mucus, that has been shown to deter a variety of predators from crabs to fish; e.g. the flounder was reported to spit out Limaria hians (Gilchrist, 1896; Merrill & Turner, 1963; Gilmour, 1967). However, the starfish Asterias rubens and Marthasterias glacialis prey on a wide range of epifauna and molluscs including Limaria hians (Minchin, 1995).
  • Fish such as juvenile cod Gadus morhua, small-spotted catshark (dogfish) Scyliorhinus canicula, and dragonets Callionymus lyra and gobies probably prey on mobile and sessile epifauna on the reef and may take damaged specimens of Limaria hians (JNCC, 1999; Hall-Spencer & Moore, 2000b).
  • Starfish, brittlestars, hermit crabs (e.g. Pagurus bernhardus), crabs and the common whelk Buccinum undatum probably act as epifaunal scavengers within this biotope (JNCC, 1999; Hall-Spencer & Moore, 2000b).
  • The nest galleries support a number of detrivores, deposit feeders, scavengers and predators. For example, polychaetes (e.g. Polynoe spp. and Flabelligera affinis), flatworms and small crustaceans are probably scavengers within the nests, preyed on by polychaetes such as Glycera lapidum and Nephtys hombergi and ribbon worms) (Hall-Spencer & Moore, 2000b).

Seasonal and longer term change

Seasonal change
In the Plymouth area, Limaria hians breeds between early spring to late summer, the larvae present in the water column from August to the following April, with a peak of larval abundance in October at Plymouth or in late summer in Mulroy Bay, Co. Donegal (Lebour, 1937b, MBA, 1957; Allen, 1962; Minchin, 1995).

The macroalgae in the biotope may be expected to show seasonal changes in growth and development; for examples see Delesseria sanguinea and Laminaria hyperborea reviews. In temperate waters most bryozoan species tend to grow rapidly in spring and reproduce maximally in late summer, depending on temperature, day length and the availability of phytoplankton (Ryland, 1970). Several species of bryozoans and hydroids demonstrate seasonal cycles of growth in spring/summer and regression (die back) in late autumn/winter, over-wintering as dormant stages or juvenile stages (see Ryland, 1976; Gili & Hughes, 1995; Hayward & Ryland, 1998). For example, the fronds of Bugula species are ephemeral, surviving about 3-4 months but producing two frond generations in summer before dying back in winter, although the holdfasts are probably perennial (Eggleston, 1972a; Dyrynda & Ryland, 1982). The hydroid Tubularia indivisa is annual, dying back in winter (Fish & Fish, 1996), while the uprights of Nemertesia antennina die back after 4-5 months and exhibit three generations per year (spring, summer and winter) (see reviews; Hughes, 1977; Hayward & Ryland, 1998; Hartnoll, 1998). Many of the bryozoans and hydroid species are opportunists (e.g. Bugula flabellata) adapted to rapid growth and reproduction (r-selected), taking advantage of the spring/summer phytoplankton bloom and more favourable (less stormy) conditions (Dyrynda & Ryland, 1982; Gili & Hughes, 1995). Some species such as the ascidians Ciona intestinalis and Clavellina lepadiformis are effectively annual (Hartnoll, 1998). Therefore, the biotope is likely to demonstrate seasonal changes in the abundance or cover of the epifauna and macroalgae.

Temporal change
Hall-Spencer & Moore (2000b) studied a Limaria hians bed over a 5 year period, while Minchin (1995) reported the that Limaria hians was abundant in the Moross Channel, Mulroy Bay from 1978-1982 and had been recorded from Mulroy Bay a hundred years previously. This suggests that Limaria hians beds, once established, are probably relatively stable unless affected by human impacts or storms (see sensitivity).

Habitat structure and complexity

The gaping file shell Limaria hians can build extensive nests made of shell, stones debris and maerl (when present) interlaced by several hundred byssus threads, and lined by mucus, mud and their faeces (Gilchrist, 1896; Hall-Spencer & Moore, 2000b). Nests may be constructed by expansion of smaller burrows, in gravel, shell sand or laminarian holdfasts, or may be simply composed of byssus threads (see Merrill & Turner (1963) and Gilmour (1967) for details). Nests are about the maximum gape of shell in diameter by about twice the length of the animal, with holes for the entrance and exit of water. Nests vary in size and complexity with individual Limaria hians being recorded from nests of 2-5 cm diameter, while larger nests of up to 25 cm diameter and 10 cm in length consisted of numerous ventilated holes and galleries (Gilmour, 1967; Tebble, 1976; Hall-Spencer & Moore, 2000b). Hall-Spencer & Moore (2000b) reported that six of these large nests contained 24-52 small and 25-40 large individuals of Limaria hians, with adult individuals occupying single galleries with two ventilation holes, while juveniles occupied complex galleries with multiple ventilation holes. Limaria hians can also occur individually or in small numbers, for example in kelp holdfasts, or under stones intertidally (Jason Hall-Spencer, pers com.).

This biotope is characterized by dense populations of Limaria hians where the nests coalesce into a carpet or reef over the sedimentary substratum, and in which individual Limaria hians are not visible (Connor et al., 1997; JNCC, 1999). For example, in the Creag Gobhainn area of Loch Fyne, Limaria hians formed a reef 10-20cm high, composed of >700 individuals/m² and covering several hectares (Hall-Spencer & Moore, 2000b) and 216-584 /m² were reported in the Moross Channel, Mulroy Bay in 1980 (Hobson, 1980 cited in Minchin, 1995). The carpet of nests covers and hence stabilizes the substratum. In addition, the carpet of nests provides substratum for the attachment for a diverse array of sessile and sedentary invertebrates, niches and refugia for mobile epifauna, and the nests themselves support a burrowing infauna and scavengers. The exact composition of the associated community probably varies with location depending on the species present in the surrounding area.

  • In shallow examples of this biotope the Limaria hians carpet provides substratum for macroalgae, including the kelps Saccorhiza polyschides, Laminaria digitata and Saccharina latissima (studied as Laminaria saccharina and their associated flora and fauna (e.g. see EIR.LhypR) , which would otherwise be unable to attach to a sedimentary substratum (Minchin, 1995).
  • Sessile epifauna attached to the nests and any available hard substrata such as stones include, sponges (e.g. Esperiopsis fucorum), hydroids (e.g. Kirchenpaueria pinnata, Nemertesia spp., and Tubularia spp.), soft corals (e.g. Alcyonium digitatum), anemones (e.g. Urticina felina and Metridium senile), bryozoans (e.g. Bugula spp.), barnacles (e.g. Balanus crenatus), amphipods (e.g. Ampelisca spp. and Jassa spp.), ascidians (e.g. Ciona intestinalis and Corella parallelogramma), tube worms (e.g. Pomatoceros triqueter), and bivalves (e.g. the horse mussel Modiolus modiolus and scallops Pecten maximus and Chlamys varia) (Connor et al., 1997; JNCC, 1999; Hall-Spencer & Moore, 2000b).
  • Mobile epifauna include flatworms, ribbon worms (Nemertea), polychaetes (e.g. the sea mouse Aphrodite aculeata), pycnogonids, amphipods, shrimp, hermit crabs (e.g. Pagurus bernhardus), crabs (e.g. Cancer pagurus, Hyas araneus, and Necora puber), gastropods (e.g. Gibbula spp., Calliostoma zizyphinum, and Buccinum undatum), nudibranchs (e.g. Onchidoris spp.), sea urchins (e.g. Psammechinus miliaris), brittlestars (e.g., Ophiothrix fragilis and Ophiocomina nigra), and starfish (e.g. Asterias rubens, Crossaster papposus and Marthasterias glacialis) (Connor et al., 1997; JNCC, 1999; Hall-Spencer & Moore, 2000b).
  • The galleries of the nests also supported scavengers such as scale worms (e.g. Polynoe sp.) and predatory polychaetes (e.g. Lepidonotus squamatus and Glycera lapidum), while the polychaetes Flabelligera affinis and the bivalve Mysella bidentata were associated with the faeces-lined walls of nest galleries (Hall-Spencer & Moore, 2000b).
  • Hall-Spencer & Moore (2000b) reported that the sediment underneath the Limaria hians bed supported a diverse infaunal community including burrowing bivalves (e.g. Mya truncata, Dosinia exoleta and Tapes rhomboides), the heart urchin Echinocardium pennatifidum and the holothurian Thyonidium drummondi. The high infaunal biodiversity observed in their study area was attributed to the porosity of the Limaria hians beds and the locally strong currents, that allowed adequate exchange of oxygenated water and nutrient. Examples of this biotope that occur in areas of low water movement (i.e. weak currents and wave sheltered conditions) may not exhibit such as diverse community.

Productivity

Little information on productivity was found. However, phytoplankton, benthic microalgae, kelps and other macroalgae probably make an important contribution to primary productivity where abundant. Dame (1996) suggested that dense beds of bivalve suspension feeders increase turnover of nutrients and organic carbon in estuarine (and presumably coastal) environments by effectively transferring pelagic phytoplanktonic primary production to secondary production in the sediments (pelagic-benthic coupling). The Limaria hians beds probably also provide secondary productivity in the form of tissue, faeces and pseudofaeces.

Recruitment processes

Limaria hians is dioecious (Ansell, 1974) and can reproduce in its second summer (Minchin, 1995). Hrs-Benko (1973) reported that Limaria hians in the northern Adriatic were sexually active throughout the year, with a main spawning period between late spring and summer, while Minchin (1995) noted that settlement normally occurred in August to September in Mulroy Bay. Veligers of Limaria hians were collected between August and the following April in the Plymouth area, absent in early summer with a peak in abundance in October (Lebour, 1937b). Limaria hians veligers are distinctive and triangular in shape (80-320 µm in length). Larvae reach 320 µm in length within a few weeks in the laboratory, after which metamorphosis occurs, suggesting that the veligers could spend at least a few weeks in the plankton. Newly metamorphosed juveniles grow rapidly, reaching 2 mm in length within about 2.5 months (Lebour, 1937b). Minchin (1995) noted that Limaria hians laid down two growth rings per year after their first year, and reported a mean shell length of 2 cm in their third summer. Hrs-Benko (1973) noted that individuals >2.7-3 cm in size died in the Adriatic population, while Minchin (1995) recorded 5 year classes and 6 year old specimens in Mulroy Bay. However, Limaria hians populations are dependant on recruitment to maintain their abundance as recruitment failure in the Moross Channel, Mulroy Bay , associated with tri-butyl tin (TBT) contamination, resulted in loss of the resident population (Minchin, 1995).

The associated macroalgae, epifauna and interstitial fauna probably depend on locality and recruit from the surrounding area. Many hydroids and most bryozoans, ascidians and probably sponges have short lived plankton or demersal larvae with relatively poor dispersal capabilities. Exceptions include Nemertesia antennina and Alcyonium digitatum and hydroids that produce medusoid life stages, which probably exhibit relatively good dispersal potential. Hydroids and bryozoans are opportunistic, rapid growing species, with relatively widespread distributions, which colonize rapidly and are often the first groups of species to occur on settlement panels. Sponges and anemones may take longer to recruit to the habitat but are good competitors for space. Recruitment in epifauna communities is discussed in detail in the faunal turf biotopes MCR.Flu, CR.Bug and in Modiolus modiolus beds (MCR.ModT).

Mobile epifaunal species, such as echinoderms, crustaceans, and amphipods are fairly vagile and capable of colonizing the community by migration from the surrounding areas, probably attracted by the refugia and niches supplied by the Limaria hians carpet. In addition, most echinoderms and crustaceans have long-lived planktonic larvae with high dispersal potential, although, recruitment may be sporadic, especially in echinoderms.

Time for community to reach maturity

The time taken for the biotope to develop would depend on the time required for the Limaria hians population to increase in abundance and develop a carpet or bed of byssal nests. Colonization by macroalgae, epifauna, and mobile species would probably be rapid and may enhance development of the byssus reef (Jason Hall-Spencer pers comm.). The recovery of the Limaria hians beds in Moross Channel, Mulroy Bay was studied by Minchin (1995). The population was reduced to only <2% of its 1980 abundance by 1986 (Minchin et al., 1987). In the follow up study, Minchin (1995) reported that after successful spat falls in 1989 onwards, the population, carpet and associated community had returned to their 1980 state by 1994, presumably due to recruitment from a few surviving old specimens or populations in other areas of Mulroy Bay. Therefore, it is likely that a recognizable biotope could develop within 5 years once successful recruitment of Limaria hians occurred. However, the associated community may take longer to develop, especially in the case of long-lived species.

Additional information

None entered

Preferences & Distribution

Recorded distribution in Britain and Ireland

The biotope is recorded from a few locations on the west coast of Scotland, and in Mulroy Bay, Co. Donegal, Ireland. For a full species distribution see Limaria hians.

Habitat preferences

Depth Range 0-5 m, 10-20 m, 20-30 m, 50-100 m
Water clarity preferencesData deficient
Limiting Nutrients Data deficient
Salinity Full (30-40 psu), Variable (18-40 psu)
Physiographic
Biological Zone Lower infralittoral, Upper circalittoral, Upper infralittoral
Substratum Coarse clean sand, Mixed, Muddy gravel, Muddy gravelly sand, Muddy sand, Muddy sandy gravel
Tidal Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave Extremely sheltered, Sheltered, Very sheltered
Other preferences Data deficient

Additional Information

This biotope has been recorded from 4-98 m on mixed muddy gravel or sand, coarse sands, muddy maerl, and bedrock in areas with weak to strong tidal streams and wave sheltered to extremely wave sheltered habitats (Connor et al., 1997; JNCC, 1999; Hall-Spencer & Moore, 2000b). It is probable that the Limaria hians carpet does not occur in shallow depths in wave exposed locations. It occurs at high densities in the Creag Gobhainn area of Loch Fyne (Hall-Spencer & Moore, 2000b) and Moross Channel, Mulroy Bay, Ireland (Minchin, 1995), and is very common in Loch Sunart (Howson, 1996).

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope

-

Additional information

Beds of Limaria hians provide stable substrata in otherwise sedimentary habitats and support a diverse epifauna and infauna (Hall-Spencer & Moore, 2000b). The MNCR recorded 324 species within this biotope, although not all species were present in all records of the biotope (Connor et al., 1997a). Hall-Spencer & Moore (2000b) reported 19 species of macroflora and 265 species of invertebrate macrofauna in only six Limaria hians nests from one site in Loch Fyne, Scotland. Recently, Trigg et al. (2011) recorded 282 species (across 16 phyla) of epiflora, epifauna and infauna, from only two sites in Scotland. 

Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

The biotope is characterized by a dense bed or reef of the flame shell Limaria hians. Each bivalve secretes byssus threads that are interwoven to form a carpet of threads, coarse sediment, shell and organic matter. This porous layer lies on top of the mixed muddy gravel found within this biotope. Beds of Limaria hians provide stable substratum in otherwise sedimentary habitats and support a diverse epifauna and infauna (Hall-Spencer & Moore, 2000). The species found within the community are widespread and characteristic of the wave sheltered and tide swept situations where Limaria hians are found. The Marine Nature Conservation Review (MNCR) recorded 324 species within this biotope, although not all species were present in all records of the biotope (Connor et al., 1997a). Hall-Spencer & Moore (2000b) reported 19 species of macroflora and 265 species of invertebrate macrofauna in only six Limaria hians nests from one site in Loch Fyne, Scotland.

Loss or degradation of the Limaria hians beds in this biotope would result in reclassification of the sedimentary biotope and the loss or degradation of the associated assemblage, and destabilization of the sediment (Minchin, 1995). Therefore, Limaria hians is considered to be the key characterizing and key structural species and the sensitivity review focuses on the sensitivity of this species and the beds they create. The sensitivity of other associated species found within this biotope are not given in this assessment as none are thought to be pivotal to the recruitment or maintenance of Limaria hians bed.  Pressure effects on other components of the community will be described where relevant to the pressures.

Resilience and recovery rates of habitat

The Limaria hians biotope is recorded from the west coast of Scotland, Orkney and Mulroy Bay, Ireland. It is possible that the highly camouflaged nature of these nests has led to a lack of data regarding their location (Trigg, 2009). In a number of the localities where this biotope is found its extent is decreasing (Hall-Spencer, 1998 and unpublished data). Wide scale declines of this biotope have been recorded in the Clyde Sea and off the Isle of Man, and it has disappeared from prior strongholds such as the Skelmorlie Bank, Stravanan Bay and Tan Buoy, Great Cumbrae (Hall-Spencer and Moore, 2000b). Limaria hians is not known to be subject to direct exploitation for either commercial or recreational use. However, Aequipectin opercularis, Pecten maximus and Buccinum undatum are exploited commercially and are associated with SS.SMx.IMx.Lim biotope (Connor et al., 2004).

Natural disturbance events that reduce the cover of Limaria hians byssus mat within beds were recorded by Minchin (1995) and Trigg et al. (2011).  In the strong tidal currents associated with Limaria hians beds, kelp hold fasts can tear free from the substratum removing sections of the byssus mat (Minchin, 1995; Hall-spencer & Moore, 2000; Trigg et al., 2011).  These disturbance events immediately reduce the biodiversity of the area when compared with unaffected mat in the same reef (Trigg et al., 2011). After a disturbance event species succession will return the bed to a mature state. The state of flux occurring during succession can positively affect biodiversity (Townsend et al., 1997). If disturbance is to frequent then regime shifts can occur.

Impacted Limaria hians reefs may recover through peripheral movement of nest materials (Trigg & Moore, 2009) and settlement of juveniles on suitable substratum (Minchin, 1995; Trigg, 2009). It is also possible that translocation of dislodged, undamaged adults could contribute to recovery (Hall-Spencer & Moore, 2000b). Juvenile recruitment is the most important mechanism for recovery of significantly impacted reefs. This recruitment in turn is dependent on the production of larvae from mature specimens or populations within the vicinity of a damaged area.

Limaria hians is dioecious (Ansell, 1974) and can reproduce in its second summer (Minchin, 1995) with a life expectancy up to six years. Within the northern Adriatic, Limaria hians are sexually active throughout the year, with a peak in reproduction in between late spring and summer (Hrs-Benko, 1973). Minchin (1995) noted that settlement normally occurred in August to September in Mulroy Bay. Veligers of Limaria hians were collected between August and the following April in the Plymouth area (Lebour, 1937b). However, Trigg (2009) suggest that there could be variability in the reproductive cycles of Limaria hians even within similar geographical locations. Very little information is available on the mortality rates of Limaria hians.   A study of the size-frequency within the population between April 2006 and June 2007 recorded a decline in population density during May in both years (Trigg, 2009). The observed decline could indicate a natural fluctuation within the population when mortality rates are high after spawning (Trigg, 2009).

Information on recovery rates is limited and is largely based on observation of recovery from Mullroy Bay (Minchin, 1995) and modelled predictions (Trigg & Moore, 2009). Experiments tracking recovery of deliberately cleared patches also provide some information on recovery from small-scale disturbance. For example, Minchin (1995) reported that once recruitment began again in 1989, recovery was rapid. By 1994 the population and an extensive carpet of byssal nests indicated recovery to the earlier 1980 state. Young saithe were again present sheltering in a re-established kelp cover, suggesting that the community as a whole had also recovered. Recently, Trigg & Moore (2009) simulated a scallop dredge event on the extensive Limaria hians bed off Port Appin on the west coast of Scotland.  They cleared 0.25 m2 sections within an area of habitat with 100% Limaria hians cover.  They reported that re-growth generally occurred from peripheral nest material but that recovery within these test patches was not as thick as the undisturbed bed (Trigg & Moore, 2009).  Trigg & Moore (2009) estimated that regrowth occurred at a rate of 3.2 cm/annum, when the re-growth in the test areas was converted to a linear front.  They further estimated that it would take 117 years for a Limaria hians bed to recovery from a 7.5 m dredge running through the bed (Trigg & Moore, 2009). However, this prediction does not allow for the effect of any other pressures, or the return of the bed to maximum density.

Resilience assessment. Observations of Limaria hians recovery from both physical destruction and inhibition of recruitment found that the presence of a local healthy population contributed significantly to the recovery of a site (Trigg & Moore, 2009; Minchin, 1995) . Recovery rates are therefore site-specific and depend on the presence of source population to allow recolonization. This ability to recruit from a nearby population does not mean that recovery to previous habitat structure is quick.  Trigg & Moore (2009) predicted that it would take in excess of 117 years for a Limaria hians bed to fully recover from a single pass with a dredge. Minchin (1995) recorded a shorter recovery time of 12 years from a bed reduced to 2% of its original density. However, Hall-Spencer (pers comm) suggested that this short recovery time may be an exception to a rule. Therefore, where a population of Limaria hians experiences some mortality but the habitat is not changed (e.g. Medium resistance), then the habitat has the potential to recover within a several years depending on recruitment, a resilience of 'Medium' (2-10 years).  But where the population experiences significant mortality and/or the carpet (bed) is damaged (e.g. resistance is Low or None), then the recovery is likely to be prolonged, a resilience score of ‘Very Low’ (at least 25 years, prolonged or negligible), based on the recovery rates modelled by Trigg & Moore (2009) and the evidence that Limaria hians beds are in decline in UK waters.

Note - resilience and the ability of a habitat to recover from human induced pressures is subject to a number of factors. These include, but are not limited to; environmental conditions of the site, the frequency of disturbance events, the intensity of the disturbance and the number of pressures being exerted on a site at any one time. Recovery of impacted populations will always be mediated by stochastic events and processes acting over different scales. Local habitat conditions, further impacts and processes such as larval-supply and recruitment between populations are examples of such scales. Full recovery is defined as the return to the state of the habitat that existed prior to impact. This does not necessarily mean that every component species has returned to its prior condition, abundance or extent. It is more important that the relevant functional components are present and the habitat is structurally and functionally recognisable. It should be noted that the recovery rates are only indicative of the recovery potential. 

Hydrological Pressures

 ResistanceResilienceSensitivity
High High Not sensitive
Q: Medium
A: Low
C: NR
Q: High
A: High
C: High
Q: Medium
A: Low
C: Low

Limaria hians has been recorded from the Lofoten Isles in Norway, to as far south as the Canary Isles in the Azores. Therefore, it is unlikely to be affected by long term changes in temperature at the benchmark level in British waters. The subtidal situation of Limaria hians beds protects the organism from extreme changes in temperature. However it is likely that other species within the community may be affected, for example boreal species (e.g. Balanus crenatus and Modiolus modiolus) may increase in abundance with a decrease in temperature.

Sastry (1966) considered environmental conditions, such as temperature, influential to the reproductive cycle of bivalves. Studies on another member of the Limidae family, Ctenoides scaber, found that when water temperature and food abundance increased, gonad size within the organisms studied also increased (Dukeman et al., 2005). Any significant decrease in temperature may restrict the spawning period of Limaria hians. However, as the location of Limaria hians in the British Isles puts it in the middle of its range this is unlikely.

Sensitivity assessment. The community composition found on the Limaria hians beds may differ with temperature changes. However the presence of Limaria hians is unlikely to change with an increase in temperature at the benchmark level, due to Limaria hians being in the middle of its range in the British Isles. A resistance of ‘High’ and a resilience of ‘High’ give a sensitivity assessment of ‘Not sensitive’ at the benchmark level. 

High High Not sensitive
Q: Medium
A: Low
C: NR
Q: High
A: High
C: High
Q: Medium
A: Low
C: Low

Limaria hians has been recorded from the Lofoten Isles in Norway, to as far south as the Canary Isles in the Azores. Therefore, it is unlikely to be affected by long term changes in temperature at the benchmark level in British waters. The subtidal situation of Limaria hians beds protects the organism from extreme changes in temperature. However it is likely that other species within the community may be affected, for example boreal species (e.g. Balanus crenatus and Modiolus modiolus) may increase in abundance with a decrease in temperature.

Sastry (1966) considered environmental conditions, such as temperature, influential to the reproductive cycle of bivalves. Studies on another member of the Limidae family, Ctenoides scaber, found that when water temperature and food abundance increased, gonad size within the organisms studied also increased (Dukeman et al., 2005). Any significant decrease in temperature may restrict the spawning period of Limaria hians. However, as the location of Limaria hians in the British Isles puts it in the middle of its range this is unlikely.

Sensitivity assessment. The community composition found on the Limaria hians beds may differ with temperature changes. However the presence of Limaria hians is unlikely to change with an increase in temperature at the benchmark level, due to Limaria hians being in the middle of its range in the British Isles. A resistance of ‘High’ and a resilience of ‘High’ give a sensitivity assessment of ‘Not sensitive’ at the benchmark level. 

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Limaria hians can be found in both fully marine and variable salinity regimes (Conner et al., 2004). As this biotope occurs in full salinity, it is unlikely to encounter increases in salinity.

Sensitivity assessment. No evidence is available on the impact of an increase in salinity on Limaria hians. This biotope is also unlikely to experience an increase in salinity as it is only found in fully saline conditions. Therefore, this pressure is probably 'Not relevant'. 

Medium Medium Medium
Q: Low
A: NR
C: NR
Q: High
A: Medium
C: Medium
Q: Low
A: Low
C: Low

Limaria hians can be found in both fully marine and variable salinity regimes (Conner et al., 2004). Shallow sites may be vulnerable to reductions in salinity due to extreme freshwater runoff during heavy rainfall, e.g. in enclosed water masses such as lochs.

Several bivalves have been shown to be able to increase the concentration of free amino acids in their cytoplasm to compensate for decreases in salinity (Berger & Kharazova, 1997). As salinity falls most bivalves close their shells to isolate themselves from the surrounding environment. Limaria hians has a gaping shell that cannot be fully closed, and numerous, hydrostatic tentacles that cannot be withdrawn. Therefore, although it may exhibit an unknown degree of physiological tolerance, it is unlikely to be able to tolerate reduced or prolonged periods of variable salinity.

Sensitivity assessment. No evidence is available on the impact of a decrease in the salinity on Limaria hians. However,  the biotope is found in variable salinity regimes, so it is likely that Limaria hians have some tolerance to a decrease in salinity, but not for extended time periods. Resistance, resilience and sensitivity have all been scored as ‘Medium’. 

High High Not sensitive
Q: Medium
A: Low
C: NR
Q: High
A: High
C: High
Q: Medium
A: Low
C: Low

This biotope occurs in weak to moderately strong tidal streams (Connor et al., 2004). A change in water flow may affect settlement success of larvae. Studies of both epiphytic and infaunal organisms have found that larvae can settle at a range of flow rates (Abelson et al., 1994, Eckman et al., 1990, Mullineaux & Butman, 1991, Pawlik & Butman, 1993). It is likely that larvae use different flow rates as physical cues as to whether they have settled in the correct habitat (Abelson & Denny, 1997). To date there is no information on how Limaria hians larvae respond to changes in water flow rate. However significant changes in water flow rate could determine the larval settlement success rates. 

Limaria hians feeds on suspended organic matter such as microalgae, phytoplankton, bacteria and detritus (Trigg, 2009). It is an active suspension feeding, where by passing water is diverted into the mantle cavity of the organism, and then over its gill filaments (Ward et al., 2004). Changes in flow rate would change the amount of suspended particulate matter passing the organism in the water column, the water pressure over the gill filaments and consequently the stress levels of the organism (Cahalan et al., 1989). Investigations into the effects of water flow velocity on scallops have found that flow rates have an effect on the growth rates (Cahalan et al., 1989, Eckman, 1987, Wildish et al., 1987). No information is available on water flow rates and growth rates for Limaria hians.

A reduction in flow rate would lead to lighter sediment particles falling out of suspension. This would lead to greater siltation on the Limaria hians bed (see siltation). Reduced water flow would also reduce the availability of oxygen to the organism (see deoxygenation), and  a reduction in the speed at which waste produced by Limaria hians would be removed from the nest matrix.  However, a decrease in water flow form e.g. weak to negligible, may have an adverse effect where the biotope is subject to some wave action (e.g. in wave sheltered conditions). 

If water flow was to increase to strong or very strong is likely that physical damage to the bed would occur. Additional drag caused by emergent epifauna attached to the carpet (notably kelps) is likely to cause the carpet to be removed in lumps. Holes in the carpet, may then allow mobilization of the sediment, resulting in further damage (Minchin, 1995). Loss of the byssal mat will entail loss of the associated community; although individual flame shells will probably survive and be transported elsewhere. Increased flow over the bed could potentially remover lighter sediment fractions, leaving only coarser sediment, boulders and bedrock.

Sensitivity assessment.  An increase in water flow rate, may physically disturb the bed, where the change is outside the biotopes normal range of water flow. Examples of the habitat at the limits of the range of water flow are are likely to be most sensitive to change. However, a change in water flow of 0.1-0.2 m/s, is unlikely to adversely affect the biotope.  Therefore, resistance and reilience are 'High' and the biotope is 'Not sensitive' at the benchmark level.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: 
C: NR

Individual Limaria hians can be found from the low shore to the subtidal. However the SS.SMx.IMx.Lim biotope is only found in fully subtidal areas (Connor et al., 2004). The biotope has been recorded from the depth band 5-10 m to 20-30 m (Connor et al., 2004).

If for any reason the more shallow examples of the biotope became exposed at extreme low tides it is likely that they would be sensitive to any increase in exposure time. Very little information is available on the tolerance of Limaria hians to any of the consequences of increased exposure times, so it’s unclear what factors Limaria hians are most sensitive to. Possibilities include effects of desiccation, fluctuating temperatures or reduced feeding times. An increase in water depth is unlikely to cause a negative impact on the Limaria hians. It is possible that an increase in depth could lead to a decrease in light availability. A decrease in light availability could reduce the diversity of algae.

Sensitivity assessment. As the reefs are found in fully subtidal areas this pressure considered ‘Not Relevant’ to reefs of Limaria hians. Consequently, the resistance, resilience and sensitivity are all categorised as ‘Not Relevant’.

Low Very Low High
Q: Low
A: Medium
C: High
Q: High
A: Medium
C: Medium
Q: Low
A: Low
C: Medium

This biotope has been recorded from extremely wave sheltered to wave sheltered (Connor et al., 2004). However, it probably occurs at greater depth with increasing wave exposure, since the effect of wave action on water movement decreases with depth (Hiscock, 1983) The oscillatory nature of wave induced water movement is potentially damaging, especially where foliose macroalgae (e.g. kelps) attached to the carpet increase drag. An increase in wave exposure from moderately exposed to very exposed may result in disruption of the byssal carpet and mobilization of the substratum, especially where the biotope is found in shallower water.

Any decrease in wave exposure is unlikely, as the biotope has been recorded from extremely wave sheltered sites. The biotope would probably not be adversely affected as long as there was still water flow (see water flow).

Sensitivity assessment.  This is an wave sheltered biotope, so that even a small increase in significant wave height has the potential to damage shallow examples of the biotope. An increase in the wave exposure at the benchmark level may remove sections of the byssal carpet, its associated community and consequently the biotope. Therefore, resistance of ‘Low’ and a resilience of ‘Very Low’ have been given, resulting in a sensitivity of ‘High’.

Chemical Pressures

 ResistanceResilienceSensitivity
No evidence (NEv) No Evidence (NEv) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

In the Moross Channel, Mulroy Bay, an intensive settlement of Limaria hians spat occurred in 1982 followed by five years of failed settlement, which coincided with the use of TBT in fish farms in the area. Limaria hians samples in 1985 contained 0.2 µg/g tri-butyl tin oxide and similar levels were found in the Pacific oyster, scallops and mussels in the same area.Limaria hians larvae were detected again after the use of TBT was discontinued in 1985 (Minchin et al., 1987; Minchin, 1995). Minchin (1995) suggested that TBT contamination was the most likely cause of the disappearance of larvae from the plankton. Mytilus edulis continued to settle during the impacted period suggesting that Limaria hians was more intolerant.

Limaria hians populations are dependent on recruitment to maintain their abundance. A recruitment failure in Mulroy Bay associated with tri-butyl tin (TBT) contamination, resulted in loss 98% of the population between 1980 and 1986. Minchin (1995) studied the recovery of this population of Limaria hians and found that once recruitment began again in 1989, recovery was rapid, so that by 1994 the population and an extensive carpet of byssal nests indicated recovery to the earlier 1980 state. Young saithe were again present sheltering in a re-established kelp cover, suggesting that the community as a whole had also recovered. This suggests that where individuals survive and in the presence of good recruitment, a population may be able to regain its prior abundance within five years.

Minchin (1995) noted that good recruitment was necessary to maintain the byssal carpet. Poor recruitment resulted in weakening of the byssal carpet, which was pulled away in tufts due to drag by kelps in the strong currents, mobilization of the sediment and resultant smothering, and loss of the carpet, its attached kelps and associated community and the population was reduced to 1.6% of its 1980 abundance.

If TBT inhibits the settlement of Limaria hians larvae then it is accurate to give a resistance of ‘None’. The fast recovery recorded by Minchin (1995) suggests that that if TBT use is restricted then Limaria hians can recover. However, it is not clear if the speed of recovery seen in Mulroy Bay would occur everywhere.

Sensitivity assessment. Not sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Subtidal populations are protected from the direct effects of oil spills by their depth but are likely to be exposed to the water soluble fraction of oils and hydrocarbons, or hydrocarbons adsorbed onto particulates.

Suchanek (1993) noted that sub-lethal levels of oil or oil fractions reduce feeding rates, reduce respiration and hence growth, and may disrupt gametogenesis in bivalve molluscs. Widdows et al.(1995) noted that the accumulation of PAHs contributed to a reduced scope for growth in Mytilus edulis. However, no information on the responses of Limaria hians to hydrocarbons was found.

Laboratory studies of the effects of oil and dispersants on several red algae species, including Delesseria sanguinea (Grandy, 1984 cited in Holt et al., 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages.  O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination.

Sensitivity assessment. Not sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

No evidence (NEv) No Evidence (NEv) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. ‘Not Sensitive’ at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. There is ‘No Evidence’ available for the effect of this pressure on Limaria hians.

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. There is ‘No Evidence’ available for the effect of this pressure on Limaria hians.

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No information on the tolerance of hypoxia by Limaria hians was found. Most hydroids and bryozoans require adequate water flow and would probably be adversely affected by low oxygen concentration but may survive as dormant stages. Echinoderms appear to be intolerant (see reviews of Asterias rubens and Echinus esculentus) and would probably be killed or excluded by low oxygen concentrations. Similarly, some red algae may be intolerant (e.g. Delesseria sanguinea). However, while sessile and mobile epifauna may be killed, or lost by migration, the survival of the biotope is primarily dependent on the survival of Limaria hians.

Sensitivity assessment. There is ‘No Evidence’ available for the effect of this pressure on Limaria hians.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  Moderate increases in nutrient levels may benefit Limaria hians by increasing macroalgal and phytoplankton productivity. In turn these factors would increase the supply of materials used to bind into byssal nests, and increase the food supply. Similarly, increased availability of organic particulates may benefit the other suspension feeding members of the community, e.g. hydroids, bryozoans, sponges and ascidians.

Conversely nutrient enrichment could lead to increased turbidity (see ‘changes in suspended solids’) and decreased oxygen levels due to bacterial decomposition of organic material (see ‘deoxygenation’). However, Shumway (1990) reported the toxic effects of algal blooms on commercially important bivalves. This would suggest that prolonged or acute nutrient enrichment may have adverse effects on suspension feeding bivalves such as Limaria hians. A bloom of the toxic flagellate Chrysochromulina polypedis in the Skagerrak resulted in death or damage of numerous benthic animals, depending on depth.

Sensitivity assessment. The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014).  Therefore, the biotope is considered to be 'Not sensitive' at the pressure benchmark that assumes compliance with good status as defined by the WFD.

No evidence (NEv) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

The organic enrichment of a marine environment at this pressure benchmark leads to organisms no longer being limited by the availability of organic carbon. The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) and decreases in dissolved oxygen and uncharacteristic microalgae blooms (Bricker et al., 1999, 2008).

Johnston & Roberts (2009) undertook a review and meta analysis of the effect of contaminants on species richness and evenness in the marine environment. Of the 49 papers reviewed relating to sewage as a contaminant, over 70% found that it had a negative impact on species diversity, <5% found increased diversity, and the remaining papers finding no detectable effect. Although this finding was not specifically for this biotope it is still relevant as the meta analysis revealed that the effect of marine pollutants on species diversity were ‘remarkably consistent’ between habitats (Johnston & Roberts, 2009). It was found that any single pollutant reduced species richness by 30-50% within any of the marine habitats considered (Johnston & Roberts, 2009). Throughout their investigation there were only a few examples where species richness was increased due to the anthropogenic introduction of a contaminant. These examples were almost entirely from the introduction of nutrients, either from aquaculture or sewage outfalls. Of the 22 papers which were assessed for subtidal reefs, nearly 80% found that this form of marine pollution decreased the species diversity (Johnston & Roberts, 2009).

Sensitivity assessment. No empirical evidence was found to support the sensitivity assessment of Limaria hians to organic enrichment.  Therefore, 'No evidence' has been recorded. 

 

Physical Pressures

 ResistanceResilienceSensitivity
None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Sensitivity assessment. 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.  

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Senstivity assessment. Limaira hians beds are only recorded from sedimentary habitats. A permananet change from soft (sediment) to a hard (rock, or artificial) substratum will result in loss of the biotope. Therefore, the habitat is consider to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent change of substraum, so that 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.  

Medium Medium Medium
Q: High
A: Medium
C: Medium
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

The Limaria hians biotope is found on mixed muddy, sandy gravel (Connor et al., 2004). Trigg et al. (2011) commented that any one Limaria hians bed covers a range of sediment types. Sedimentary particles are bound up in the byssus nests created by these bivalves and contribute to their camouflage. The ability of Limaria hians to bind different sediment types into their byssus threads suggests that a change in the sediment composition may not decrease their ability to bind substrata. Although not proven with Limaria hians reefs, other complex bivalve reefs have been shown to slow down water flow (Crooks & Khim, 1999), leading to the deposition of finer sediments within the reef. This process would mean that well established Limaria hians reefs could change the sediment composition themselves and become muddier over time.

If the sediment composition changes it’s likely that the biodiversity of the area would decrease. Trigg et al. (2011) found that there was high infaunal polychaete diversity within the different sediment types. Consequently if you were to lose one of these sediment types you would lose a niche and its associated fauna.

Sensitivity assessment. The impact of the underlying sediment becoming either finer or coarser is not clear and there is also a possibility that the presence of the Limaria hians reef could over time change the substratum composition by decreasing the speed of water flow. A resistance of ‘Medium’ is given to represent the change in the associated infaunal community (e.g. from a coarse sediment community to a fine sediment community). Resilience is probably ‘Medium’ and the resultant sensitivity is ‘Medium’ at the benchmark level. 

None Very Low High
Q: Medium
A: 
C: High
Q: High
A: Medium
C: Medium
Q: Medium
A: Medium
C: High

Removal of the substratum would result in removal of the Limaria hians byssal carpet and the associated community (see abrasion / disturbance).

Sensitivity assessment. A resistance of ‘None’ has been given. If the substratum is removed then the overlying byssal nest would also be removed. Resilience would depend on the extent of substratum removal. Even if a small amount of substratum was removed, based on Trigg & Moores (2009) predictions for recovery within a healthy reef, it could take decades for the bed to recover fully. Therefore resilience has been assessed as ‘Very Low’, and overall sensitivity as ‘High’.

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium
Q: Medium
A: Medium
C: High

Evidence suggests that anthropogenic pressures have led to decreases in the abundance of Limaria. hians throughout the UK, most significantly in the second half of the 20th century (Trigg, 2009, Gilmour, 1967, Ansell, 1974, Tebble, 1974, Seaward, 1990, Hall-Spencer & Moore, 2000b). Hall-Spencer & Moore (2000b) concluded that Limaria hians beds were intolerant to physical disturbance by mooring chains, hydraulic dredges or towed demersal fishing gear. Hall-Spencer & Moore (2000b) reported that a single pass of a scallop dredge at Creag Gobhainn, Loch Fyne ripped apart and mostly removed the Limaria hians reef. Damaged file shells were consumed by scavengers (e.g. juvenile cod Gadus morhua, whelks Buccinum undatum, hermit crabs Pagurus bernhardus and other crabs) within 24 hrs. Hall-Spencer & Moore (2000b) noted that although Limaria hians was able to swim, the shell was thin and likely to be damaged by mechanical impact. Damage of the Limaria hians carpet would result in exposure of the underlying sediment and exacerbate the damage resulting in the marked loss of associated species (Hall-Spencer & Moore, 2000b).

Recently Trigg & Moore (2009) simulated a scallop dredge event on the extensive Limaria hians bed off Port Appin on the west coast of Scotland.  They cleared 0.25 m2 sections within an area of habitat with 100% Limaria hians cover.  They reported that re-growth generally occurred from peripheral nest material but that recovery within these test patches was not as thick as the undisturbed bed (Trigg & Moore, 2009).  Trigg & Moore (2009) estimated that regrowth occurred at a rate of 3.2 cm/annum, when the regrowth in the test areas was converted to a linear front.  They further estimated that it would take 117 years for a Limaria hians bed to recovery from a 7.5m dredge running through the bed (Trigg & Moore, 2009). However, this prediction does not allow for the effect of any other pressures, or the return of the bed to maximum density.

Species with fragile tests such as Echinus esculentus and the brittlestar Ophiocomina nigra and edible crab Cancer pagurus are reported to suffer badly from the impact of a passing scallop dredge (Bradshaw et al., 2000). Scavenging species would probably benefit in the short term, while epifauna would be removed or damaged with the byssal carpet.

Sensitivity assessment. A resistance of ‘None’ has been recorded. Resilience to such pressures are ‘Very Low’ and the overall sensitivity assessment is ‘High’

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium
Q: Medium
A: Medium
C: High

Penetration and or disturbance of the substratum would result in similar, if not identical results as abrasion or removal of the Limaria hians byssal carpet and the associated community (see abrasion / disturbance).

Sensitivity assessment. A resistance of ‘None’ has been given. If the substratum is removed then the overlying byssal nest would also be removed. Even if a small amount of substratum was removed, based on Trigg & Moores’ (2009) predictions for recovery within a healthy reef, it could take decades for the bed to recover fully. Therefore, resistance is ranked ‘None’, resilience has been assessed as ‘Very Low’ resulting in sensitivity being ‘High’.

Medium Medium Medium
Q: Low
A: Low
C: Low
Q: High
A: Medium
C: Medium
Q: Low
A: Medium
C: Medium

An increase in suspended sediment levels may adversely affect suspension feeding species by clogging feeding and respiratory structures, and may result in increased siltation depending on water movement. Minchin (1995) suggested that Limaria hians was common in areas free of silt and mud. But Limaria hians beds have been recorded on muddy sand and gravel in wave sheltered areas with weak tidal streams such as lochs, and presumably subject to suspended sediment and siltation. The byssal nest probably protects the residents from the direct effects of siltation.

Limaria hians beds are probably tolerant of a variety of suspended sediment and siltation regimes. However, an increase in suspended sediment loads is likely to reduce feeding efficiency of suspension feeders including Limaria hians and increase energetic costs in the form of sediment rejection currents, mucus and pseudofaeces in the Limaria hians.

Sensitivity assessment. Overall, a resistance of ‘Medium’ has been recorded with a resilience of ‘Medium’, giving a sensitivity score of ‘Medium’

Low Very Low High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: High

Minchin (1995) reported that degradation of the Limaria hians bed resulted in patches of exposed shell-sand. As the underlying substratum became exposed it became mobile and subsequently began to bury some of the surviving Limaria hians, which contributed to the decline of the bed. Smothering by 5 cm of sediment is likely to prevent water flow through the byssal nests of Limaria hians, preventing feeding and resulting in local hypoxia. Limaria hians  s capable of swimming, and some individuals may be able to evacuate their nests. However, a proportion of the Limaria hians may be lost.

Interstitial or infaunal species are unlikely to be adversely affected, although feeding may be interrupted and mobile species will avoid the effects. Small epifaunal species, who require water flow for gaseous exchange and feeding are likely to be killed by a large deposition of sediment. Some larger species may not be entirely smothered and continue growing.

Sensitivity assessment. Loss of a proportion of the gaping file shell population and resultant degradation of the byssal carpet and loss of some associated epifauna will result in the loss of species richness. ‘Low’ resistance and ‘Very Low’ resilience give a sensitivity score of ‘High’.

None Very Low High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: High

In light of the assessment given above, if a greater amount of sediment deposition occurred on a Limaria hians bed the consequences would only be more severe. If 30 cm of fine material were to be deposited onto a Limaria hians reef it is highly likely all the Limaria hians, interstitial and infaunal species would die of hypoxia. It could also be assumed that all but the largest epiphytes would also be affected by the inability to undergo gaseous exchange, photosynthesise or feed. This would remove most if not all of the species within the biotope, leading to a total loss of all ecosystem function.

Sensitivity assessment. Resistance given as ‘None’, resilience as ‘Very Low’ and sensitivity a ‘High’. 

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. There is ‘No Evidence’ available for the effect of this pressure on Limaria hians.

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. There is ‘No Evidence’ available for the effect of this pressure on Limaria hians.

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. ‘No Evidence’ as to whether Limaria hians can perceive sound, or whether this could cause negative stress to the animal. The vibrations caused by sound may cause an impact; however there is ‘No Evidence’ available to support an assessment. 

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. In this biotope, Limaria hians lives with a byssus nest and is unlikely to be exposed to changes in light levels.  An increase in light levels at the benchmark is unlikely to influence photosynthesis in macroalgae expect in the shallowest habitats, and even then a change of 0.1 lux is unlikely to be significant.  Changes in the duration of illumination may affect spawning in species that use moonlight or day length as cues but no evidence of this effect was found.  Therefore, the effects of this pressure are considered 'Not relevant'. 

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. This pressure is only applicable to mobile species such as fish and marine mammals, and not seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed. But seed dispersal is not considered under the pressure definition and benchmark. Consequently the effect of this pressure on Limaria hians has been considered ‘Not Relevant’. 

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. ‘Not Relevant’ for benthic habitats. Collision by grounding vessels is addressed under the ‘surface abrasion’ pressure. 

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. In this biotope, Limaria hians lives with a byssus nest and is unlikely to be exposed to visual disturbance. Therefore, the effects of this pressure are considered 'Not relevant'.

Biological Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group. Translocation has the potential to transport pathogens or non-native species to uninfected areas (see pressures ‘introduction of microbial pathogens’ and ‘introduction or spread of invasive non-native species’). The sensitivity of the ‘donor’ population to harvesting to supply stock for translocation is assessed for the pressure ‘removal of non-target species’.

Sensitivity assessment. This pressure is ‘Not relevant’ to Limaria hians. 

No evidence (NEv) No Evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Sensitivity assessment. There is ‘No evidence’ available for the effect of this pressure on Limaria hians.

Low Very Low High
Q: Medium
A: Medium
C: Low
Q: High
A: Medium
C: Medium
Q: Medium
A: Medium
C: Low

Limaria hians may be infested with 'oyster gill worms', trematodes of the genus Urastoma but they are considered to be harmless facultative commensals (Lauckner, 1983). Limaria hians may also act as secondary hosts for the metacercariae of digenean trematodes, which may cause sublethal effects or in extreme cases parasitic castration (Lauckner, 1983).

Sensitivity assessment. Although infected individuals may not recover from the sublethal affects of trematodes, the relatively short life span of Limaria hians may allow the population to recover rapidly. A resistance of ‘Low’ has been recorded with a resilience of ‘Medium’ and sensitivity of Medium.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

Exploitation of other species in the vicinity of Limaria hians beds is likely to be responsible for a significant decline in Limaria hians (Hall-Spencer, 1998, 1999, Hall-Spencer & Moore, 2000, A. Brand pers comm. taken from Hall-Spencer & Moore, 2000b, Wood, 1988). Lobsters and crabs are targeted by potting on the Loch Fyne Limaria hians beds, although no evidence of damage to the Limaria hians has been seen in this area (Hall-Spencer, pers comm.).

The scallop species Aequipectin opercularis and Pectin maximus  as well as the whelk Buccinum undatum are species which are associated with Limaria hians reefs as well as being commercially exploited species within the UK. The static or mobile gears that are used to target these species can cause direct physical impact. These impacts are assessed through the abrasion and penetration of the seabed pressures.

Aequipectin opercularis is recorded as occasional, Pectin maximus is recorded as rare and Buccinum undatum is recorded as occasional within SS.SMx.IMx.Lim (Connor et al., 2004).

However, the removal of the above species from the biotope may not have a significant negative impact on the community. While removal of this target species will reduce species richness there is no known obligate relationship between these species, and therefore the loss of the species are unlikely to adversely affect the resident Limaria hians population.

Sensitivity assessment. A ‘High’ resistance and resilience to this pressure is suggested and sensitivity is ranked ‘Not Sensitive’.

None Very Low High
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: Medium

SS.SMx.IMx.Lim may be directly removed or damaged by static or mobile gears that are targeting other species. Scallop dredging and other invasive forms of fishing have been reported to have decimated the previously extensive Limaria hians beds in the Clyde (Wood, 1988, Hall-Spencer, 1998, Hall-Spencer & Moore, 2000a, 2000b, Trigg & Moore, 2009). These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. The accidental removal of Limaria hians would be highly detrimental to the biotope as the biotope is characterized by this species.

Sensitivity assessment. The resistance of Limaria hians to accidental removal is ‘None’. The resilience of the species would depend on the extent of removal. A resilience score of ‘Very Low’ has been given in light of the recovery times predicted by Trigg & Moore (2009). This results in a sensitivity score of ‘High’.

Importance review

Policy/Legislation

Habitats of Principal ImportanceFile shell beds [Scotland]
Habitats of Conservation ImportanceFile shell beds
UK Biodiversity Action Plan PriorityFile shell beds
Priority Marine Features (Scotland)Flame shell beds

Exploitation

Limaria hians is not known to be subject to exploitation. However, Hall-Spencer & Moore (2000b) reported that exploitation of other species (scallops) found in the vicinity of Limaria hians beds was probably responsible for a significant decline in Limaria hians numbers (see sensitivity). Lobsters and crabs are also targeted by potting on the Loch Fyne Limaria hians beds but no evidence of damage has been seen (Hall-Spencer, pers comm.).

Additional information

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Citation

This review can be cited as:

Tyler-Walters, H. & Perry, F., 2015. Limaria hians beds in tide-swept sublittoral muddy mixed sediment. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/112

Last Updated: 10/08/2015

Tags: flame shell file shell carpet biogenic beds tide-swept tideswept tide swept epifauna