Philine aperta and Virgularia mirabilis in soft stable infralittoral mud

02-01-2005
Researched byDr Harvey Tyler-Walters & Jacqueline Hill Refereed byDr David Hughes
EUNIS CodeA5.343 EUNIS NamePhiline aperta and Virgularia mirabilis in soft stable infralittoral mud

Summary

UK and Ireland classification

EUNIS 2008A5.343Philine aperta and Virgularia mirabilis in soft stable infralittoral mud
EUNIS 2006A5.343Philine aperta and Virgularia mirabilis in soft stable infralittoral mud
JNCC 2004SS.SMu.IFiMu.PhiVirPhiline aperta and Virgularia mirabilis in soft stable infralittoral mud
1997 BiotopeSS.IMU.MarMu.PhiVirPhiline aperta and Virgularia mirabilis in soft stable infralittoral mud

Description

Stable muds, occasionally with small stones, with a high proportion of fine material (typically greater than 80%) may contain the opisthobranch Philine aperta and the sea pen Virgularia mirabilis. These muds typically occur in shallow water down to about 12-15 m where significant seasonal variation in temperature is presumed to occur. This habitat is restricted to the most sheltered basins in, for example, sea lochs. Although most records suggest full salinity conditions are prevalent, some sites may be subject to variable salinity. Philine aperta is the most characteristic species of this habitat, occurring in high densities at many sites, whilst Virgularia mirabilis, a species found more widely in muddy sediments, appears to reach its highest densities in this shallow mud but may not be present in all examples of this biotope. Other conspicuous species found in this shallow muddy habitat include Cerianthus lloydii, Pagurus bernhardus, Sagartiogeton spp. and Hydractinia echinata. Burrowing crustacean megafauna, characteristic of deeper mud, are rare or absent from this shallow sediment although Nephrops norvegicus may sometimes be recorded. This biotope has been primarily recorded on the basis of its epifauna and a few conspicuous infauna. Little data exists on the infaunal component of this biotope but it may include Nephtys spp., spionid polychaetes, Ampelisca spp. and the bivalves Nucula spp., Thyasira flexuosa, Mysella bidentata and Abra spp. In the south of Great Britain, the polychaete Sternaspis scutata is also characteristic of this biotope. This polychaete is rare in Great Britain (Sanderson 1996). This southern variant of the biotope is very restricted in the UK to Portland Harbour but is known to occur further south in the Gulf of Gascony and the Mediterranean. (Information from Connor et al., 2004).

Recorded distribution in Britain and Ireland

In the UK this biotope is almost confined to the most sheltered basins of certain sea lochs, with one further example known from Portland Harbour in southern England.

Depth range

5-10 m, 10-20 m

Additional information

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JNCC

Habitat review

Ecology

Ecological and functional relationships

  • The characterizing and other species in this biotope occupy space in the habitat but their presence is most likely primarily determined by the occurrence of a suitable substratum rather by interspecific interactions. Virgularia mirabilis and Philine aperta are functionally dissimilar and are not necessarily associated with each other but occur in the same muddy sediment habitats. There is no information regarding possible interactions between any of the other species in the biotope but there seems to little interdependence. Burrowing species which are present create tunnels in the sediment which themselves provide a habitat for other burrowing or inquilinistic species.
  • Virgularia mirabilis might be adversely affected by high levels of megafaunal bioturbation, perhaps by preventing the survival of newly settled colonies.
  • Many of the species living in deep mud biotopes are generally cryptic in nature and not usually subject to predation. Evidence of predation on Virgularia mirabilis by fish seems limited to a report by Marshall & Marshall (1882 in Hoare & Wilson, 1977) where the species was found in the stomach of haddock. Many specimens of Virgularia mirabilis lack the uppermost part of the colony which has been attributed to nibbling by fish. Observations by Hoare & Wilson (1977) suggest however, that predation pressure on this species is low. The sea slug Armina loveni is a specialist predator of Virgularia mirabilis.
  • Nephrops norvegicus is known to be eaten by a variety of bottom-feeding fish, including cod, haddock, skate and dogfish. Symbion pandora, a tiny sessile animal less than 1 mm long, lives commensally on the mouthparts of Nephrops norvegicus.
  • Brittlestars are common, with Amphiura chiajei predominating on finer muds. Most of these animals are deposit-feeders, ingesting tiny organic particles and feeding on the bacterial layer coating the sediment grains. If present in high abundance the burrowing and feeding activities of Amphiura chiajei can modify the fabric and increase the mean particle size of the upper layers of the substrata by aggregation of fine particles into faecal pellets. Such actions create a more open fabric with a higher water content which affects the rigidity of the seabed (Rowden et al., 1998(b)). Such destabilisation of the seabed can affect rates of particle resuspension.
  • The hydrodynamic regime, which in turn controls sediment type, is the primary physical environmental factor structuring benthic communities such as IMU.PhiVir. The hydrography also affects the water characteristics in terms of salinity, temperature and dissolved oxygen. It is also widely accepted that food availability (see Rosenberg, 1995) and disturbance, such as that created by storms, (see Hall, 1994) are also important factors determining the distribution of species in benthic habitats.

Seasonal and longer term change

  • Species such as the sea pen Virgularia mirabilis and Amphiura chiajei appear to be long-lived and are unlikely to show any significant seasonal changes in abundance or biomass. Seapen faunal communities appear to persist over long periods at the same location. Movement of the sea pen Virgularia mirabilis in and out of the sediment may be influenced by tidal conditions (Hoare & Wilson, 1977).
  • The numbers of some of the other species in the biotope may show peak abundances at certain times of the year due to seasonality of breeding and larval recruitment.

Habitat structure and complexity

The biotope has very little structural complexity with most species living in or on the sediment. Burrowing megafauna are generally rare or absent, therefore there will be few burrows available for colonization. Several species, such as the sea pen Virgularia mirabilis and the anemone Cerianthus lloydii, extend above the sediment surface. However, apart from a couple of species of nudibranch living on the sea pens and the tubiculous amphipod Photis longicaudata associated with Cerianthus lloydii (Moore & Cameron, 1999) these species do not provide significant habitat for other fauna. Excavation of sediment by infaunal organisms, such as errant polychaetes and Philine aperta, ensures that sediment is oxygenated to a greater depth allowing the development of a much richer and/or higher biomass community of species within the sediment.

Productivity

Productivity in subtidal sediments is often quite low. Macroalgae are absent from IMU.PhiVir and so productivity is mostly secondary, derived from detritus and organic material. Allochthonous organic material is derived from anthropogenic activity (e.g. sewerage) and natural sources (e.g. plankton, detritus). Autochthonous organic material is formed by benthic microalgae (microphytobenthos e.g. diatoms and euglenoids) and heterotrophic micro-organism production. Organic material is degraded by micro-organisms and the nutrients are recycled.

Recruitment processes

  • Philine aperta, Virgularia mirabilis and other major component species in sea pen biotopes appear to have a plankton stage within their life cycle.
  • The reproductive biology of British sea pens has not been studied, but in other species, for instance Ptilosarcus guerneyi from Washington State in the USA, the eggs and sperm are released from the polyps and fertilization takes place externally. The free-swimming larvae do not feed, and settle within seven days if a suitable substratum is available (Chia & Crawford, 1973). Dispersal of Virgularia mirabilis planulae is therefore likely to be limited to relatively short distances suggesting that populations may not be replenished from distant sources (David Hughes, pers. comm.). The limited data available from other species would suggest a similar pattern of patchy recruitment, slow growth and long life-span for Virgularia mirabilis.

Time for community to reach maturity

Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain, however information from other species suggest that this species is likely to be long-lived and slow growing with patchy and intermittent recruitment. Other burrowing species representative of this biotope vary in longevity and reproductive strategies.

Additional information

No text entered

Preferences & Distribution

Recorded distribution in Britain and IrelandIn the UK this biotope is almost confined to the most sheltered basins of certain sea lochs, with one further example known from Portland Harbour in southern England.

Habitat preferences

Depth Range 5-10 m, 10-20 m
Water clarity preferences
Limiting Nutrients Nitrogen (nitrates), Phosphorus (phosphates)
Salinity Full (30-40 psu)
Physiographic Enclosed coast / Embayment
Biological Zone Infralittoral
Substratum Mud
Tidal Very Weak (negligible)
Wave Extremely sheltered, Very sheltered
Other preferences

Additional Information

This biotope can be found in water less than 10 m deep in the sheltered inner basins of some sea lochs (Howson et al., 1994)

Species composition

Species found especially in this biotope

    Rare or scarce species associated with this biotope

    • Sternapsis scutata

    Additional information

    May contain the nationally rare polychaete Sternapsis scutata in southern Great Britain.

    Sensitivity reviewHow is sensitivity assessed?

    Sensitivity characteristics of the habitat and relevant characteristic species

    Philine aperta and Virgularia mirabilis are the main important characterizing species, giving the name to the biotope. Cerianthus lloydii is another characteristic member of the epifauna. Amphiura filiformis may be abundant but reaches higher abundance in SMU.IFiMu.BriAchi. Other members of the infauna are probably found in a range of other biotope in similar sediments, while the other species are mobile (e.g. crabs and hermit crabs) or restricted to stones or shells (e.g. Hydractinia). Connor et al. (2004) note that this biotope might represent a temporal variant of similar SMU biotopes as the abundance of Philine aperta may vary from year to year.

    Therefore, the assessment of sensitivity is based on the important characterizing species Philine aperta and Virgularia mirabilis and the mud habitat. The sensitivity of other species is discussed where relevant.

    Resilience and recovery rates of habitat

    Philine aperta is a simultaneous hermaphrodite, capable of producing both eggs and sperm (Lancaster, 1983). In Britain, spawning is thought to occur between April and September (Thompson, 1976; Lancaster, 1983).  It lays eggs in flask-shaped eggs in masses of up to 50,000. Eggs hatch within 3.5 to 8 days depending on temperature. The veliger larvae are ready for metamorphosis and settlement within 30-40 days (in the laboratory) (Thompson, 1976; Lancaster, 1983; Thompson, 1988; Hansen, 1991; Hansen & Ockelmann, 1991). It has a life span of 3-4 years (Thompson, 1976). Philine aperta is widely distributed around the coasts of Britain. 

    Little evidence was found to support this resilience assessment for Cerianthus lloydii. MES (2010) suggested that the genus Cerianthus would be likely to have a low recovery rate following physical disturbance based on long-life span and slow growth rate. The MES (2010) review also highlighted that there were gaps in information for this species and that age at sexual maturity and fecundity is unknown although the larvae are pelagic (MES 2010). No empirical evidence was found for recovery rates following perturbations for Cerianthus lloydii. This species has limited horizontal mobility and re-colonization via adults is unlikely (Tillin & Tyler-Walters, 2014).

    Little information on the reproduction and life history of Virgularia mirabilis was found.  Edwards & Moore (2009) noted that many sea pens exhibited similar characteristics.  Recent studies of oogenesis in Funiculina. quadrangularis and Pennatula phosphorea in Loch Linnhe, Scotland, demonstrated that they were dioecious, with 1:1 sex ratios, highly fecund, with continuous prolonged oocyte development and annual spawning (Edwards & Moore 2008; Edwards & Moore 2009).  In Pennatula phosphorea, oogenesis exceeded 12 months in duration, with many small oocytes of typically 50 per polyp giving an overall fecundity of ca 40,000 in medium to large specimens, depending on size.  However, <30% matured (synchronously) and were spawned in summer (July-August).  Mature oocytes were large (>500µm) which suggested a lecithotrophic larval development (Edwards & Moore, 2008).  In Funiculina. quadrangularis fecundity was again high, expressed as 500-2000 per 1 cm midsection, but not correlated with size, and again, only a small proportion of the oocytes (<10%) matured.  Unlike Pennatula phosphorea, annual spawning occurred in autumn or winter (between October and January).  Also the mature oocytes were very large (>800µm), which suggested a lecithotrophic larval development (Edwards &Moore, 2009).  In a study of the intertidal Virgularia juncea fecundity varied with length (46,000 at 50 cm and 87,000 at 70 cm), reached a maximum size of 200-300 µm in May and were presumed to be spawned between August and September (Soong, 2005). Birkland (1974) found the life span of Ptilosarcus gurneyi to be 15 years, reaching sexual maturity between the ages of 5 and 6 years; while Wilson et al. (2002) noted that larger specimens of a tall sea pen (Halipteris willemoesi) in the Bering Sea were 44 years old, with a growth rate of 3.6 - 6.1cm/year.

    Hughes (1998a) suggested that patchy recruitment, slow growth and long life-span were typical of sea pens.  Larval settlement is likely to be patchy in space and highly episodic in time with no recruitment to the population taking place for some years.  Greathead et al. (2007) noted that patchy distribution is typical for sea pen populations.  In Holyhead harbour, for example, animals show a patchy distribution, probably related to larval settlement (Hoare and Wilson, 1977). 

    Virgularia mirabilis was found to withdraw into its burrow rapidly (ca 30 seconds) and could not be uprooted by dragged creels (Hoare and Wilson 1977; Eno et al.., 2001; Ambroso et al. 2013).  In summary, British sea pen species have been found to recover rapidly from the effects of dragging, uprooting and smothering (Eno et al. 2001). Recovery from effects that remove a proportion of the sea pen population (e.g. bottom gears, hydrographic changes) will depend on recruitment processes and little is known about the life history and population dynamics of sea pens (Hughes 1998a).

    Resilience assessment. No information on the population dynamics of Philine aperta was found. However, it is highly fecund, with high potential larval dispersal range so that recruitment is probably good, and it is a mobile species, capable of recolonizing the affected area adjacent populations, especially as it is common in British waters.  Therefore, Philine aperta population s would probably recover within a couple of years (resilience is ‘High’).  However, there is little information regarding the resilience of Cerianthus lloydii. A resilience of ‘Medium’ (2 – 10 years) is suggested for all resistance levels (‘None’, ‘Low’, ‘Medium’ or ‘High’) based on expert judgement. Where Virgularia mirabilis survive impact undamaged, that is resistance is ‘High’, recovery is likely to be rapid; a resilience of ‘High’ (<2 years).  Where a proportion of the population is removed or killed, then the species has a high dispersal potential and long-lived benthic larvae, but larval recruitment is probably sporadic and patchy and growth is slow, suggesting that recovery may take many years: a resilience of ‘Low’ (>10 years).  Therefore, the resilience of the biotope is likely to be Low (>10 years) as Virgularia mirabilis and Cerianthus lloydii are likely to take many years to recover.  The assessment is based on the reproduction and life history characteristics of the important characteristic species, or similar species, rather than direct evidence. Therefore, while confidence in the quality of the evidence and its concordance is Medium, confidence its application in Low.

    Hydrological Pressures

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

    In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature between 5°C and 15°C (10°C) (Hughes, 1998a). Although, unusually warm summers or cold winters may change the temperatures outside this range, benthic burrowing species will be buffered from extremes by their presence in the sediment.

    Spawning, hatching, and time to metamorphosis are all temperature dependent in Philine aperta. Spawning occurs during the warmest months of the year (April to August) (Lancaster, 1983). Laboratory results showed hatching occurred after 3.5 days at 23°C and 8 days at 13°C (Thompson, 1976) and time to metamorphosis occurred after 35-40 days at 12-13°C and 30 days at 15°C (Hansen & Ockelmann, 1991). Philine aperta is widely distributed around the coasts of Britain, south to the Mediterranean, South Africa, Ceylon and the Philippines (Thompson, 1976).

    Sea pens can withdraw into their burrows for protection. No information was found on the upper limit of sea pens tolerance to temperature. Virgularia mirabilis is recorded from western Europe, the Mediterranean, from Norway and Iceland to Africa in the North Atlantic, and to the Gulf of Mexico in North America (Hughes, 1998a; OBIS 2015). Jones et al. (2000) suggested that Virgularia mirabilis was probably more tolerant of temperature change than other British sea pen species due to its abundance in shallow waters.

    Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to Biscay. Larvae, but not adults, have been recorded from the Mediterranean.  However, no further information on the temperature tolerance of Cerianthus lloydii was found.

    The distribution of Virgularia mirabilis, Cerianthus lloydii, and Philine aperta suggest that they are probably resistant of 2°C change in temperature for a year.  Exposure to short-term acute change of 5°C may interfere with reproduction in Philine aperta and may cause Virgularia mirabilis, Cerianthus lloydii to withdraw into their burrows temporarily. However, there is no evidence to suggest that mortality would result.  Therefore, a resistance of High is suggested but with Low confidence. Therefore, resilience is High, so that the biotope is probably Not sensitive at the benchmark level.

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

    In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature between 5°C and 15°C (10°C) (Hughes, 1998a). Although, unusually warm summers or cold winters may change the temperatures outside this range, benthic burrowing species will be buffered from extremes by their presence in the sediment.

    Spawning, hatching, and time to metamorphosis are all temperature dependent in Philine aperta. Spawning occurs during the warmest months of the year (April to August) (Lancaster, 1983). Laboratory results showed hatching occurred after 3.5 days at 23°C and 8 days at 13°C (Thompson, 1976) and time to metamorphosis occurred after 35-40 days at 12-13°C and 30 days at 15°C (Hansen & Ockelmann, 1991). Philine aperta is widely distributed around the coasts of Britain, south to the Mediterranean, South Africa, Ceylon and the Philippines (Thompson, 1976).

    Sea pens can withdraw into their burrows for protection. No information was found on the upper limit of sea pens tolerance to temperature. Virgularia mirabilis is recorded from western Europe, the Mediterranean, from Norway and Iceland to Africa in the North Atlantic, and to the Gulf of Mexico in North America (Hughes, 1998a; OBIS 2015). Jones et al. (2000) suggested that Virgularia mirabilis was probably more tolerant of temperature change than other British sea pen species due to its abundance in shallow waters.

    Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to Biscay. Larvae, but not adults, have been recorded from the Mediterranean.  Crisp (1964) reported that Cerianthus lloydii in North Wales were apparently unaffected by the severe winter of 1962/63. However, no further information on the temperature tolerance of Cerianthus lloydii was found.

    The distribution of Virgularia mirabilis, Cerianthus lloydii, and Philine aperta suggest that they are probably resistant of 2°C change in temperature for a year.  Exposure to short-term acute change of 5°C may interfere with reproduction in Philine aperta and may cause Virgularia mirabilis, Cerianthus lloydii to withdraw into their burrows temporarily. However, there is no evidence to suggest that mortality would result.  Therefore, a resistance of High is suggested but with Low confidence. Therefore, resilience is High, so that the biotope is probably Not sensitive at the benchmark level.

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

    No information on the salinity tolerance of the important characterizing species was found.  Cerianthus lloydii may be recorded from the intertidal at LWST, but is probably protected from changes in salinity due to its infaunal habitat, buffered by the salinity of the interstitial water of the sediment. Greathead et al. (2007) demonstrated that Virgularia mirabilis was the most ubiquitous of all three of the sea pens in Scotland, found in habitats nearer coastal areas and inner sea lochs. Jones et al. (2000) suggested that Virgularia mirabilis was more tolerant of reduced salinity than other British sea pens due to its distribution in shallower waters. No information on the salinity preferences of Philine aperta was found.

    An increase in salinity at the benchmark level, would result in a salinity of >40 psu, and as hypersaline water is likely to sink to the seabed, the biotope may be affected by hypersaline effluents. Ruso et al. (2007) reported that changes in the community structure of soft sediment communities due to desalinisation plant effluent in Alicante, Spain. In particular, in close vicinity to the effluent, where the salinity reached 39 psu, the community of polychaetes, crustaceans and molluscs was lost and replaced by one dominated by nematodes. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly but was more of a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. In several studies, echinoderms and ascidians were amongst the most sensitive groups examined (Roberts et al., 2010b).

    Sensitivity assessment. This biotope (IFiMu.PhiVir) is recorded from full and variable salinity regimes. However, although the biotope might occur in sea lochs subject to variable salinity, the benthos may not experience variable salinity at depth, and infauna are protected from short-term changes in salinity due to the salinity of the interstitial waters. An increase in salinity at the benchmark level would result in a salinity of >40 psu.  However, hypersaline effluent is likely to sink to the seabed and may affect the community. Based on the evidence from Ruso et al. (2007) and Roberts et al. (2010b) it is likely that the community will be degraded and, especially, Philine aperta  will leave the affected area or be killed.  The effect on sea pens and anemones is unknown. Therefore, a resistance of Low is suggested with Low confidence. Resilience is probably Medium so that the sensitivity is assessed as Medium.

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

    No information on the salinity tolerance of the important characterizing species was found.  Cerianthus lloydii may be recorded form the intertidal at LWST, but is probably protected from changes in salinity due to its infaunal habitat, buffered by the salinity of the interstitial water of the sediment. Greathead et al. (2007) demonstrated that Virgularia mirabilis was the most ubiquitous of all three of the sea pens in Scotland, found in habitats nearer coastal areas and inner sea lochs. Jones et al. (2000) suggested that Virgularia mirabilis was more tolerant of reduced salinity than other British sea pens due to its distribution in shallower waters. No information on the salinity preferences of Philine aperta was found.

    Sensitivity assessment. This biotope (IFiMu.PhiVir) is recorded from full and variable salinity regimes. However, although the biotope might occur in sea lochs subject to variable salinity, the benthos may not experience variable salinity at depth, and infauna are protected from short-term changes in salinity due to the salinity of the interstitial waters. A decrease in salinity at the benchmark level, would result in a reduced salinity regime. The majority of the characterizing species are only found in full salinity conditions.  Therefore, such a reduction in salinity probably results in mobile species leaving the biotope, the death of species that could not relocate, and a marked reduction in species richness.  Therefore, a resistance of Low is recorded based on expert judgement. Resilience is probably also Low so that sensitivity is assessed as High.

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

    The biotope (IFiMu.PhiVir) occurs in low energy environments with weak (<0.5 m/sec.) to very weak tidal streams (Connor et al. 2004), which are a prerequisite for the fine mud sediments characteristic of the biotope.  Virgularia mirabilis is also recorded from coarser sandier muds with small stones and shell fragments e.g. SS.SMu.CSaMu.VirOphPmax (Hughes, 1998a; Greathead et al. 2007), and is probably more tolerant of current or wave induced flow than other British sea pens. Hiscock (1983) examined the effects of water flow on Virgularia mirabilis.  As water flow rates increase, Virgularia mirabilis first responds by swinging polyps around the axial rod to face away from the current (at 0.12 m/s), then polyps face downstream.  With further increase in flow, the stalk bends over and the pinnae are pushed together to an increasing amount with increasing velocity of flow (at 0.33 m/s).  Finally, tentacles retract and at water speeds greater than 0.5 m/s (i.e. 1 knot) the stalk retracts into the mud (Hiscock, 1983).  If water speeds remain at this level or above the sea-pen will be unable to extend above the sediment, unable to feed and could die (Hill & Wilson, 2000).

    Cerianthus lloydii is recorded from biotopes with a wide range of water flow regimes, from very weak to strong flow and in muddy to mixed or coarse sediments (Connor et al., 1997a).Therefore, it is likely to tolerate changes in water flow regimes. However, Philine aperta is recorded from mud, muddy sand and sand (Thompson, 1976; Connor et al., 1997a).

    Sensitivity assessment. This biotope is only recorded in muds and in weak or very weak flow (Connor et al., 2004), so that a further decrease in flow is not relevant. Increased flow has the potential to modify the sediment, especially at the surface. A significant increase in water flow may winnow away the mud surface or even remove the mud habitat and hence the biotope if prolonged. An increase of 0.2 m/s may begin to erode the mud surface where the site is already subject to flow (e.g. weak flow at the seabed), based on sediment erosion deposition curves (Wright, 2001).  However, given the depth of mud that characterizes the biotope only the surface of the mud may be removed within a year. Cerianthus lloydii is unlikely to be impacted by a change in the sediment, and is a passive predator. Philine aperta is also found in coarser sediments but reaches a high abundance in this biotope, presumably due to the abundance of prey and or habitat stability.  Virgularia mirabilis may be directly affected by an increase in flow, especially if it exceeds 0.5 m/s.  Therefore, modification of the sediment, coupled with a reduction in the Virgularia mirabilis abundance may result in a loss this biotope as described by the classification.  Therefore, a resistance of Low is recorded. Resilience is probably also Low so that sensitivity is assessed as High.

    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

    The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

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

    The biotope (IFiMu.PhiVir) occurs in low energy environments sheltered or extremely sheltered from wave action (Connor et al. 2004), which are a prerequisite for the fine mud sediments characteristic of the biotope. Virgularia mirabilis occurs in coastal areas and inner sea lochs but these areas are still sheltered from wave action, and in sandier muds (e.g. the biotope SS.SMu.CSaMu.VirOphPmax) (Hughes, 1998a; Greathead et al. 2007), wave exposure was not recorded to be more than ‘sheltered’. Cerianthus lloydii is recorded from biotopes from wave exposed to extremely sheltered muddy to mixed or coarse sediments (Connor et al., 1997b). Therefore, it is likely to tolerate changes in wave action. However, Philine aperta is recorded from mud, muddy sand, and sand and very to extremely wave sheltered biotopes (Thompson, 1976; Connor et al., 1997b).

    Sensitivity assessment.  A decrease in wave exposure is unlikely in the sheltered habitats they inhabit.  An increase in wave exposure is likely to affect Virgularia mirabilis and Philine aperta species adversely, limiting or removing the shallower proportion of the population, and potentially modifying sediment and therefore habitat preferences in the longer-term.  However, a 3-5% increase in significant wave height (the benchmark) is unlikely to be significant. The benchmark level of change may be no more than expected during winter storms even in the sheltered waters favoured by this biotope. Therefore, resistance is recorded as High at the benchmark level. Hence, resilience is High and the biotope is assessed as Not sensitive at the benchmark level.

    Chemical Pressures

     ResistanceResilienceSensitivity
    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

    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

    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

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

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

    No evidence was found

    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

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

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

    Virgularia mirabilis is often found in sea lochs so may be able to tolerate some reduction in oxygenation. However, Jones et al. (2000) reported that sea pen communities were absent from areas which are deoxygenated and characterized by a distinctive bacterial community and Hoare & Wilson (1977) reported that Virgularia mirabilis was absent from sewage related anoxic areas of Holyhead harbour.

    Nilsson & Rosenberg (1994) examined the effects of hypoxia on muddy sediment cores in mesocosm experiments. Both moderate (ca 1 mg O2/l) and severe (ca 0.5 mg O2/l) hypoxia resulted in a significant reduction in species abundance after 6-7 days of hypoxia. Amphiura filiformis left the sediment as hypoxia increases, followed by Mysella bidentata (0.5-2 days later), Echinocardium cordatum left the sediment before moderate hypoxia was reached, and all Labidoplax buskii left the sediment at 1.6 mg O2/l, while Nephtys hombergii was the last species to leave the sediment. Almost all the Philine aperta left the sediment at both levels of hypoxia, and even escaped the experimental sediment cores, or died at the sediment surface. In moderate hypoxia most individuals survived but at severe hypoxia treatment only two individuals survived.

    Diaz & Rosenberg (1995) noted that anemones include species that were reported to be particularly tolerant of hypoxia (e.g. Cerianthus sp and Epizoanthus erinaceus). A major hypoxic event due a pyncocline in the Gulf of Trieste resulted in a mass mortality of benthos between 12 and 26th September 1983 (Stachowitsch, 1992), during which the oxygen levels fell below 4.2 mg/l, became anoxic, and hydrogen sulphide and ammonia were released (Faganeli et al., 1985). Amongst the epifauna, the even hypoxia resistant polychaetes and bivalves died after 4-5 days and the only organism to survive after one week were the anemones Cerianthus sp and Epizoanthus erinaceus, the gastropods Aporrhais pespelecani and Trunculariopsis trunculus and the sphinuculid Sipunculus nudis (Stachowitsch, 1992).

    Sensitivity assessment. The evidence suggests that severe hypoxic or anoxic conditions are likely to be detrimental to sea pen and Philine aperta, while Cerianthus lloydii may survive even anoxic conditions for a week.  Sea pens might be resistant of short term hypoxia due to their presence at depth in sheltered sea lochs but severe hypoxia may be detrimental. However, a reduction in oxygen levels to below 2 mg/l for a week will probably force Philine aperta to leave the affected area, and result in a significant reduction in its abundance and the abundance of other infauna.  

    Therefore, a resistance of Low is suggested to represent to loss of a small proportion of the sea pen population but a significant proportion of the Philine aperta population.  Resilience is probably Low due to time required for the sea pen population to recover, although the Philine aperta population would probably recover rapidly (< 2 years). Therefore, sensitivity is assessed as High.

    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

    Hoare & Wilson (1977) noted that Virgularia mirabilis was absent from part of the Holyhead Harbour heavily affected by sewage pollution.  However, the species was abundant near the head of Loch Harport, Skye, close to a distillery outfall discharging water enriched in malt and yeast residues and other soluble organic compounds (Nickell & Anderson, 1977; cited in Hughes, 1998a), where the organic content of the sediment was up to 5%.  Virgularia mirabilis was also present in Loch Sween in Scotland in sites where organic content was as high as 4.5% (Atkinson, 1989).

    No information was available on the effect of nutrient enrichment on Cerianthus lloydii or Philine aperta.  Borja et al. (2000) and Gittenberger and van Loon (2011) both assigned Cerianthus lloydii to their Ecological Group I, ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’ in of the  AZTI Marine Biotic Index (AMBI) index to assess disturbance (including organic enrichment).  The basis for their assessment and relation to the pressure benchmark is not clear.

    Sensitivity assessment. Sublittoral muds may be expected to be high in organic nutrients, and the presence of Virgularia mirabilis in areas of up to 4.5% organic carbon (Atkinson, 1989) suggest a resistance to organic enrichment at the benchmark level.  The high organic content suggests that nutrients are not limiting. But no evidence on the direct effects of nutrients in the form of nitrates, phosphates and silicates was found. Algal mats are associated with nutrient enrichment, but only in shallow waters but the biotope could be affected by the algal blooms that sink to the bottom when they die, although the main effects are organic enrichment and hypoxia.  However, the biotope is assessed as Not sensitive at the pressure benchmark of compliance with good status as defined by the WFD. 

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

    Hoare & Wilson (1977) noted that Virgularia mirabilis was absent from part of the Holyhead Harbour heavily affected by sewage pollution.  However, the species was abundant near the head of Loch Harport, Skye, close to a distillery outfall discharging water enriched in malt and yeast residues and other soluble organic compounds (Nickell & Anderson, 1977; cited in Hughes, 1998a), where the organic content of the sediment was up to 5%.  Virgularia mirabilis was also present in Loch Sween in Scotland in sites where organic content was as high as 4.5% (Atkinson, 1989). 

    No information was available on the effect of organic enrichment on Philine aperta.  Cerianthus lloydii was found near the centre of sewage sludge dumping groups at ca 10% organic carbon but was more abundant at intermediate nutrient enrichment (Hughes, 1998a). But Borja et al. (2000) and Gittenberger & van Loon (2011) both assigned Cerianthus lloydii to their Ecological Group I, ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’ in of the  AZTI Marine Biotic Index (AMBI) index to assess disturbance (including organic enrichment).  The basis for their assessment and relation to the pressure benchmark is not clear.

    Sensitivity assessment. Sublittoral muds may be expected to be high in organic nutrients, and the presence of Virgularia mirabilis in areas of up to 4.5% organic carbon (Atkinson, 1989) suggest a resistance to organic enrichment at the benchmark level.  Therefore, a precautionary resistance of Medium is suggested but with Low confidence, and as resilience is probably Low, a sensitivity of Medium is 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

    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

    If sedimentary substrata were replaced with rock substrata the biotope would be lost, as it would no longer be a sedimentary habitat and would no longer support sea pens and burrowing megafauna.

    Sensitivity assessment. Resistance to the pressure is considered ’None‘, and resilience ’Very low‘ or ‘None’ (as the pressure represents a permanent change) and the sensitivity of this biotope is assessed as ’High’.

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

    Virgularia mirabilis occurs in a number of biotopes, on substrata ranging from mud, sandy mud, and gravelly mud, with or with shell fragments or stones (Connor et al., 2004).  Greathead et al. (2007) suggested that the muscular peduncle of Virgularia mirabilis allowed it to occupy coarser muds than the other sea pens, and explained its presence in the Moray Firth and Firth of Forth, and its wider distribution in Scotland.  Greathead et al. (2007) noted that Pennatula phosphorea was absent in the North Minch while Funiculina quadrangularis and Virgularia mirabilis were present, but that Pennatula phosphorea was abundant in soft, adhesive mud with high silt-clay content in Loch Broom. This may suggest a preference for fine muds.  The MNCR only recorded Pennatula phosphorea from biotopes in ‘mud’. Greathead et al. (2007) also noted that Funiculina quadrangularis had the most restricted distribution, probably due to a preference of depth and soft deep muds of sheltered loch basins, where it was abundant.  Again, the MNCR only recorded Funiculina quadrangularis from biotopes in ‘mud’. However, it was also recorded from areas of muddy sand in the South and North Minches and in the Fladen Grounds but in deep water.  In addition, a 'mud' subtratum was the most important factor in a habitat suitability index model for sea pens developed by Greathead et al. (2015). In their model, habitat suitability for Funiculina quadrangularis increased with mud content up to a maximum at 90-100% mud. Pennatula phosphorea and Virgularia mirabilis also had their maximum habitat suitability at 100% mud.  All three species had zero habitat suitability at 0% mud. However, gravel content was also important. Virgularia mirabilis was the most tolerant of gravel content and was still recorded at 50% gravel while the were no records of Pennatula phosporea and Funiculina quadrangularis above 40% and 30% gravel respectively (Greathead et al., 2015).

    Cerianthus lloydii is recorded from biotopes in muddy to mixed or coarse sediments (Connor et al., 1997b). Therefore, it is likely to tolerate changes in sediment type. Philine aperta is recorded from mud, muddy sand and sand (Thompson, 1976; Connor et al., 1997b).

    Sensitivity assessment. While the important characteristic species are recorded from a range of sediment types, this biotope (IFiMU.PhiVir) is defined by its occurrence in mud.  Therefore, a change in sediment type by one Folk class (see Long, 2006), e.g. from mud to sandy mud and sand would result in loss of the biotope.  Therefore, a resistance of None is recorded.  As the change is permanent, resilience is Very low and sensitivity is assessed as High.

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

    Benthic trawls (e.g. rock hopper ground gear, otter trawls) will remove and capture sea pens (Tuck et al., 1998; Kenchington et al., 2011), albeit with limited efficiency. Nevertheless, dredging and suction dredging penetrates to greater depth and are likely to remove sea pens. Although Virgularia mirabilis will not be able to avoid activities that penetrate into the sediment. Assuming their burrows are only deep enough to hold the entire animal (see Greathead et al., 2007), then Virgularia mirabilis burrows are up to 40 cm deep.

    Cerianthus lloydii can also withdraw into the sediment, and its burrow is up to 40 cm deep. However, Philine aperta feeds at the surface and burrows to find prey (Thompson, 1976).

    Sensitivity assessment. Extraction of sediment to 30 cm (the benchmark) could remove most of the resident sea pens present, the burrowing sea anemones, mobile epifauna, and Philine aperta from the affected area.  Hence, the resistance is probably None. Resilience is probably Low, resulting in a sensitivity of High.

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

    Stable sedimentary habitats, such as mud were amongst the most vulnerable to fishing activities, e.g. otterboard trawling (Ball et al., 2000b; Collie et al., 2000). Tracks left by otterboards were visible 18 months after experimental trawls in Gareloch (Ball et al., 2000b). Ball et al., (2000b) concluded that trawling modified the benthic community due to an increase in opportunistic polychaetes. However, Kaiser et al. (2006) concluded that otterboards had a significant initial effect on muddy sands and muds, but that the effects were short-lived in  mud habitats.

    In experimental studies (Kinnear et al. 1996; Eno et al. 2001) sea pens were found to be largely resilient to smothering, dragging or uprooting by creels or pots.  Virgularia mirabilis withdrew very quickly into the sediment when exposed to pots or creels so that it was difficult to determine their response.  However, all sea pens recovered from being dragged over by pots or creels within 24-72 h, with exception of one individual Funiculina quadrangularis

    In Virgularia mirabilis withdrawal from physical stimulus is rapid (ca 30 seconds) (Hoare & Wilson, 1977; Ambroso et al., 2013).  Birkland (1974) maintained that the only way to capture all of the sea pens in an area (quadrat) was to remove them slowly by hand until no more emerged.  But several studies note that their ability to withdraw into the sediment in response to bottom towed or dropped gear (e.g. creels, pots, camera/video mounted towed sleds, experimental grab, trawl, or dredge) means that sea pen  abundance can be difficult to estimate (Birkeland, 1974; Eno et al., 2001; Greathead et al., 2007; Greathead et al., 2011).  The ability to withdraw also suggests that sea pens can avoid approaching demersal trawls and fishing gear.  This was suggested as the explanation for the similarity in the densities of Virgularia mirabilis in trawled and untrawled sites in Loch Fyne, and the lack of change in sea pen density observed after experimental trawling (using modified rock hopper ground gear) over a 18 month period in Loch Gareloch (Howson & Davies 1991; Hughes 1998a; Tuck et al. 1998).  Kenchington et al. (2011) estimated the gear efficiency of otter trawls for sea pens (Anthoptilum and Pennatula) to be in the range of 3.7 – 8.2%, based on estimates of sea pen biomass from (non-destructive) towed camera surveys.  However, species obtained by dredges were invariably damaged (Hoare & Wilson, 1977).  Hoare & Wilson (1977) noted that Virgularia was absent for areas of Holyhead Harbour disturbed by dragging or boat mooring, although no causal evidence was given (Hughes, 1998a).  Sea pens are potentially vulnerable to long lining.  Munoz et al. (2011) noted that small numbers of Pennatulids (inc. Pennatula sp.) were retrieved from experimental long-lining around the Hatton Bank in the north east Atlantic, presumably either attached to hooks or wrapped in line as it passed across the sediment.  Hixon & Tissot (2007) noted that sea pens (Stylatula sp.) were four times more abundant in untrawled areas relative to trawled areas in the Coquille Bank, Oregon, although no causal relationship was shown. 

    No information on the effects of abrasion or penetrative gear on Cerianthus lloydii or Philine aperta was found. Greathead et al. (2011) was not able to conclude if the variation in Cerianthus abundance in the Fladden Grounds was due to miscounting, its patchy distribution or fishing activity. 

    Sensitivity assessment. The reviews by Ball et al. (2000), Collie et al. (2000) and Kasier et al. (2006) suggest that stable sediments, e.g. muds are likely to be vulnerable to fishing activities. The evidence for Virgularia mirabilis suggests that its ability to withdraw into the sediment quickly would avoid surface abrasion from creels and pots but that dragging and mooring lines may be damaging, and individuals may be caught and removed by fishing lines (e.g. long-lines). Philine aperta feeds at the surface and burrows to find prey (Thompson, 1976) so that it might be susceptible to damage from passing gear or moorings. Therefore, a resistance of Medium is recorded due to the potential disturbance to the biotope as a whole.  As the impact may be limited (see Kenchington et al., 2011), a resilience of Medium is suggested and sensitivity is assessed as Medium.

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

    Stable sedimentary habitats, such as mud were amongst the most vulnerable to fishing activities, e.g. otter trawling (Ball et al., 2000; Collie et al., 2000). Tracks left by otter were visible 18 months after experimental trawls in Gareloch (Ball et al., 2000). Ball et al., (2000) concluded that trawling modified the benthic community due to an increase in opportunistic polychaetes. However, Kaiser et al. (2006) concluded that otter trawls had a significant initial effect on muddy sands and muds, but that the effects were short-lived in mud habitats.

    In Virgularia mirabilis withdrawal from the physical stimulus is rapid (ca 30 seconds) (Hoare & Wilson, 1977; Ambroso et al., 2013).  Birkland (1974) maintained that the only way to capture all of the sea pens in an area (quadrat) was to remove them slowly by hand until no more emerged.  But several studies note that their ability to withdraw into the sediment in response to bottom towed or dropped gear (e.g. creels, pots, camera/video mounted towed sleds, experimental grab, trawl, or dredge) means that sea pen  abundance can be difficult to estimate (Birkeland, 1974; Eno et al., 2001; Greathead et al., 2007; Greathead et al., 2011).  The ability to withdraw also suggests that sea pens can avoid approaching demersal trawls and fishing gear.  This was suggested as the explanation for the similarity in the densities of Virgularia mirabilis in trawled and untrawled sites in Loch Fyne, and the lack of change in sea pen density observed after experimental trawling (using modified rock hopper ground gear) over a 18 month period in Loch Gareloch (Howson & Davies 1991; Hughes 1998a; Tuck et al. 1998).  Kenchington et al. (2011) estimated the gear efficiency of otter trawls for sea pens (Anthoptilum and Pennatula) to be in the range of 3.7 – 8.2%, based on estimates of sea pen biomass from (non-destructive) towed camera surveys.  However, species obtained by dredges were invariably damaged (Hoare & Wilson, 1977).  Hoare & Wilson (1977) noted that Virgularia was absent for areas of Holyhead Harbour disturbed by dragging or boat mooring, although no causal evidence was given (Hughes, 1998a).  Sea pens are potentially vulnerable to long lining.  Munoz et al. (2011) noted that small numbers of Pennatulids (inc. Pennatula sp.) were retrieved from experimental long-lining around the Hatton Bank in the north east Atlantic, presumably either attached to hooks or wrapped in line as it passed across the sediment.  Hixon & Tissot (2007) noted that sea pens (Stylatula sp.) were four times more abundant in untrawled areas relative to trawled areas in the Coquille Bank, Oregon, although no causal relationship was shown. 

    No information on the effects of abrasion or penetrative gear on Cerianthus lloydii or Philine aperta was found. Greathead et al. (2011) were not able to conclude if the variation in Cerianthus abundance in the Fladden Grounds was due to miscounting, its patchy distribution or fishing activity. 

    Sensitivity assessment. The reviews by Ball et al. (2000), Collie et al. (2000) and Kasier et al. (2006) suggest that stable sediments, e.g. muds are likely to be vulnerable to fishing activities. The evidence for Virgularia mirabilis suggests that its ability to withdraw into the sediment quickly would avoid surface abrasion from creels and pots but that dragging and mooring lines may be damaging, individuals may be caught and removed by fishing lines (e.g. long-lines), and penetrative gear is likely to remove a proportion of the population. Philine aperta feeds at the surface and burrows to find prey (Thompson, 1976) so that it might be susceptible to damage from passing gear or moorings. Therefore, a resistance of Medium is recorded due to the potential disturbance to the biotope as a whole.  The resilience is probably Low so that sensitivity is assessed as Medium.

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

    The sea pen species assessed live in sheltered areas, in fine sediments, subject to high suspended sediment loads.  The effect of increased deposition of fine silt is uncertain but it is possible that feeding structures may become clogged.  When tested, Virgularia mirabilis quickly seized and rejected inert particles (Hoare & Wilson, 1977).  Hiscock (1983) observed Virgularia mirabilis secretes copious amounts of mucus which could keep the polyps clear of silt.  Kinnear et al. (1996) noted that another species of sea pen, Funiculina quadrangularis, was quick to remove any adhering mud particles by the production of copious quantities of mucus.  Virgularia mirabilis is also likely to be able to self-clean (Hiscock, 1983).  No indication of the suspended sediment load was given in any evidence found.  An increase in suspended sediment is unlikely to interfere with feeding in either Cerianthus lloydii or Philine aperta. Cerianthus lloydii is a passive predator while Philine aperta is an active predator that ploughs through the surface of the substratum looking for prey. Other members of the infaunal community are deposit feeders, predators or omnivores and unlikely to be affected.  However, an increase in turbidity and increased ,light attenuation may reduce the prevalence of microphytobenthos diatoms.

    If sea pen feeding is reduced by increases in suspended sediment the viability of the population will be reduced.  Once siltation levels return to normal, feeding will be resumed therefore recovery will be rapid. Overall, resistance is probably High, hence, resilience is also ‘High, and the biotope is probably Not sensitive at the benchmark level. 

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

    Natural accretion rates are potentially high in the sheltered muddy habitats. Hiscock (1983) observed Virgularia mirabilis secretes copious amounts of mucus, which could keep the polyps clear of silt and is also likely to be able to self-clean. Kinnear et al. (1996) noted that Funiculina quadrangularis was quick to remove any adhering mud particles by the production of copious quantities of mucus, once the source of smothering (in this case potting) was removed.  Virgularia mirabilis can burrow and move into and out of their own burrows.  It is probable therefore that deposition of 5 cm of fine sediment will have little effect other than to temporarily suspend feeding and the energetic cost of burrowing.

    In normal accretion, Cerianthus lloydii keeps pace with the accretion and, as a result, develops burrows much larger than the animal itself (Schäfer, 1972; Bromley, 2012). Schäfer (1972) reported that an increase in depositional rate led to an avoidance behaviour in Cerianthus lloydii.  The organism ceases tube building activity and instead the animal bunches its tentacles and intrudes its way up to the new surface, where it establishes a new burrow. However, no information on the depth of material through which is can burrow was given.

    Philine aperta plough through the surface of the substratum and creates furrows in its wake. Thompson (1976) suggested that it only burrowed into the substratum in pursuit of prey.

    Sensitivity assessment. The deposition of 5 cm of fine sediment is unlikely to affect the community adversely. Both Virgularia and Cerianthus can withdraw into their tube and can probably re-emerge through 5 cm of fines. Philine aperta is a large opisthobranch (up to 7 cm in length) that could probably move through a deposit of only 5 cm. The remaining infauna of polychaetes and bivalves are adapted to accreting environments and may be unaffected. However, no direct evidence was found. Therefore, a resistance of High is suggested, resulting in a resilience of High, so that the biotope is probably ‘Not sensitive’ at the benchmark level.  

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

    Natural accretion rates are potentially high in the sheltered muddy habitats. Hiscock (1983) observed Virgularia mirabilis secretes copious amounts of mucus, which could keep the polyps clear of silt and is also likely to be able to self-clean. Kinnear et al. (1996) noted that Funiculina quadrangularis was quick to remove any adhering mud particles by the production of copious quantities of mucus, once the source of smothering (in this case potting) was removed.  Virgularia mirabilis can burrow and move into and out of their own burrows.  It is probable therefore that deposition of 5 cm of fine sediment will have little effect other than to temporarily suspend feeding and the energetic cost of burrowing.

    In normal accretion, Cerianthus lloydii keeps pace with the accretion and, as a result, develops burrows much larger than the animal itself (Schäfer, 1972; Bromley, 2012). Schäfer (1972) reported that an increase in depositional rate led to an avoidance behaviour in Cerianthus lloydii.  The organism ceases tube building activity and instead the animal bunches its tentacles and intrudes its way up to the new surface, where it establishes a new burrow. However, no information on the depth of material through which is can burrow was given.

    Philine aperta plough through the surface of the substratum and creates furrows in its wake. Thompson (1976) suggested that it only burrowed into the substratum in pursuit of prey.

    Sensitivity assessment. The deposition of 30 cm of fine sediment is may affect the community adversely. Virgularia mirabilis and Cerianthus lloydii can burrow and move into and out of their own burrows, which can be up to 40 cm deep. It is probable, therefore, that deposition of 30 cm of fine sediment will have little effect other than to suspend feeding temporarily and the energetic cost of burrowing. However, Philine aperta lives primarily at the surface of the sediment, so that a sudden deposit of 30 cm of fine sediment may result in mortality of the opisthobranch, However, no direct evidence was found. Therefore, a resistance of Low is suggested due to the potential mortality of Philine aperta but with Low confidence.  The resilience is probably High based on the recovery of Philine aperta population so that the biotope is probably Low at the benchmark level.  

    Not Assessed (NA) Not Assessed (NA) Not assessed (NA)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    No evidence was found.

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

    No evidence was found

    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

    Some of the characterizing species associated with this biotope, in particular, the sea pens, may respond to sound vibrations and can withdraw into the sediment. Feeding will resume once the disturbing factor has passed. However, most of the species are infaunal and unlikely respond to noise disturbance at the benchmark level. Therefore, this pressure is probably Not relevant in this biotope.

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

    Shallow examples of this biotope develop a cover of microphytobenthic diatoms. so as increase in incident light may encourage their growth, while shading will inhibit their growth. Nevertheless, this biotope is dominated by deposit feeders and predators, so that the majority of the productivity is secondary. Therefore, the biotope is probably Not sensitive (resistance and resilience are High).

    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

    Not relevant–this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed.  But seed dispersal is not considered under the pressure definition and benchmark.

    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

    Not relevant to seabed habitats. 

    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

    Most species within the biotope are burrowing and have no or poor visual perception and are unlikely to be affected by visual disturbance such as shading. Epifauna such as crabs have well developed visual acuity and are likely to respond to movement in order to avoid predators. However, it is unlikely that the species will be affected by visual disturbance at the benchmark level.

    Biological Pressures

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

    No evidence of genetic modification, breeding, or translocation was found.

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

    In southern Britain, Sternapsis scutata is characterstic of this biotope (Connor et al., 2004).  Sternapsis scutata is a non-native polychaete that has extended its range in inshore muddy sediments in the south west of the UK (Shelley et al., 2008). However, in mesocosm experiments, little effect on biological functioning was detected after the introduction of the polychaete and a doubling of its biomass (Shelley et al., 2008). No direct evidence on the effect of non-native species on mud communities was found. However, this assessment should be revisited in the light of new evidence.

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

    No evidence was available on the effect of microbial pathogens on Cerianthus lloydii or Philine aperta or sea pens.

    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

    None of the characterizing species within this biotope are currently directly targeted in the UK and hence this pressure is considered to be ‘Not relevant’.

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

    The physical effects of fisheries or dredging activities are addressed under abrasion, penetration and extraction pressures above. No clear biological relationships between the important characteristic species were found. Therefore, removal of any one species may not affect other members of the community adversely.  However, is the important characterising species were removed as by-catch, the character of the biotope would change. A significant decline in the abundance of either Philine aperta or Virgularia mirabilis would result in loss of the biotope as recognised by the habitat classification.  Therefore, a resistance of Medium is recorded, albeit at Low confidence. As resilience is probably Low, sensitivity is assessed as Medium.

    Importance review

    Policy/Legislation

    Habitats Directive Annex 1Large shallow inlets and bays
    UK Biodiversity Action Plan Priority

    Exploitation

    • The fine sediments on which this biotope typically exists is not targeted for seabed extraction. Divers generally are not attracted to sedimentary habitats and so there is no likelihood of environmental damage by this means.
    • However, fishing for Nephrops norvegicus may lead to exploitation of the biotope. If trawls are used to extract Nephrops the biotope is likely to be significantly damaged as sea pens may be killed and Philine aperta and other species damaged or killed. However, creeling for Nephrops is unlikely to cause significant damage (Eno et al., 1996).

    Additional information

    -

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    Citation

    This review can be cited as:

    Tyler-Walters, H. & Hill, J.M. 2016. Philine aperta and Virgularia mirabilis in soft stable infralittoral mud. 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/202

    Last Updated: 19/05/2016

    Tags: Slender sea pen seapen lobe shell