Summary

UK and Ireland classification

Description

Stable muds, occasionally with small stones, with a high proportion of fine material (typically greater than 80%) may contain the opisthobranch Philine quadripartita 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 quadripartita 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, Kurtiella 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; JNCC, 2015).

Depth range

5-10 m, 10-20 m

Additional information

Records of Philine quadripartita in the British Isles were misidentified as Philine aperta (Price et al., 2011). Outwardly, most species of Philine are very similar in morphology and a detailed examination of their internal anatomy, especially the shape of the internal shell, gizzard and penial papilla, is required to differentiate the species (Price et al., 2011). Philine aperta is recorded from South Africa and Mozambique while Philine quadripartita is recorded from the North East Atlantic and the Mediterranean.

Listed By

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 quadripartita 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 quadripartita, 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 quadripartita, 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 lifespan 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

Habitat preferences

Depth Range 5-10 m, 10-20 m
Water clarity preferences
Limiting Nutrients Nitrogen (nitrates), Phosphorus (phosphates)
Salinity preferences Full (30-40 psu)
Physiographic preferences Enclosed coast or Embayment
Biological zone preferences Infralittoral
Substratum/habitat preferences Mud
Tidal strength preferences Very weak (negligible)
Wave exposure preferences 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 review

    Sensitivity characteristics of the habitat and relevant characteristic species

    Philine quadripartita 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 quadripartita may vary from year to year.

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

    Resilience and recovery rates of habitat

    Philine quadripartita (studies as 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 lifespan of 3-4 years (Thompson, 1976). Philine quadripartita 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 lifespan 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 lifespan 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 lifespan 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 quadripartita 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 quadripartita 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. An exception is made for permanent or ongoing (long-term) pressures where recovery is not possible as the pressure is irreversible, in which case resilience is assessed as ‘Very low’ by default. 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.