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information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Sparse sponges, Nemertesia spp. and Alcyonidium diaphanum on circalittoral mixed substrata

08-11-2016

Summary

UK and Ireland classification

UK and Ireland classification

Description

This biotope is found on moderately wave-exposed sand-scoured, circalittoral boulders, cobbles and pebbles that are subject to moderately strong tidal streams (referred to as lag-cobbles locally). It is characterized by sparse sponges and a diverse bryozoan and hydroid turf. The sparse sponge community is primarily composed of Dysidea fragilis andScypha ciliata. The mixed faunal turf is composed of Nemertesia antenninaNemertesia ramosaHalecium halecinum,Sertularia argenteaAlcyonium digitatumBugulina flabellataBugulina turbinataCrisularia plumosaFlustra foliacea,Cellepora pumicosaAlcyonidium diaphanumCellaria fistulosa and crisiid bryozoans. The anemones Epizoanthus couchiiSagartia elegans and Cerianthus lloydii may also be recorded. Echinoderms such as the starfish Asterias rubensCrossaster papposusHenricia oculata and the crinoid Antedon bifida. Other species present include the colonial ascidian Clavelina lepadiformis, the barnacle Balanus crenatus, the top shell Gibbula cineraria, the polychaeteSpirobranchus triqueter, the ascidian Morchellium argusProsthecareus vittatus and the crab Cancer pagurus. It is distributed off Pen Llyn and over considerable areas of the Irish Sea.

Depth range

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Additional information

Please note, molecular taxonomy of the genus Bugula (Fehlauer-Ale et al., 2015) identified several clear genera (clades ), Bugula sensu stricto (30 species), Bugulina (24 species), Crisularia (23 species) and the monotypic Virididentulagen. The following review was derived from information concerning species of Bugula where possible. The review assumes that, while their taxonomy has changed, the biology of Bugula sensu stricto and Bugulina remains similar. Hence, references to Bugula spp. in the text refer to Bugula sensu strictoBugulina, and Crisularia species.

Listed By

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Further information sources

Further information sources

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Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

CR.HCR.XFa.SpNemAdia is dominated by a dense hydroid and bryozoan turf with sparse sponges and is subject to sand scour.  The sand scour is probably responsible for the diversity of opportunistic hydroids and bryozoans.  SS.SSa.IFiSa.ScupHyd and SS.SMx.CMx.FluHyd are also dominated by a bryozoan and hydroid turf, on hard substrata (boulders, stone, etc.) subject to scour. Therefore the sensitivities are probably similar.

CR.HCR.XFa.SpNemAdia occurs on hard substrata and, in addition to the mixed faunal turf, includes a sponge component characterized by Dysidea fragilis and Scypha ciliata. This biotope is found on moderately wave-exposed sand-scoured, circalittoral boulders, cobbles and pebbles that are subject to moderately strong tidal streams. The mixed faunal turf is composed of Nemertesia antenninaNemertesia ramosaHalecium halecinum,Sertularia argenteaAlcyonium digitatumBugula flabellataBugula turbinataBugula plumosaFlustra foliacea,Cellepora pumicosaAlcyonidium diaphanumCellaria fistulosa and crisiid bryozoans (Connor et al., 2004).

SS.SSa.IFiSa.ScupHydm occurs on shallow sands with cobbles and pebbles which are exposed to strong tidal streams. It is characterized by colonies of hydroids, particularly Hydrallmania falcata along with Sertularia cupressina and Sertularia argentea. These hydroids are tolerant to periodic burial and scour by sand.(Connor et al., 2004).

SS.SMx.CMx.FluHyd is characterized by Flustra foliacea and the hydroid Hydrallmania falcate with other hydroids such as Sertularia argenteaNemertesia antennina and Nemertesia ramosa occurring on suitable, stable hard substrata.  It is similar, but experiences less sand scouring than SS.SSa.IFiSa.ScupHyd (Connor et al., 2004).

The sensitivity assessment is based on the sensitivity of the bryozoan and hydroid turf, and the sensitivity of the other species species is addressed where relevant.

Resilience and recovery rates of habitat

These biotopes are considered to have a high recovery potential. Sebens (1985, 1986) noted that bryozoans and hydroids covered scraped areas within four months in spring, summer and autumn. Hydroids exhibit rapid rates of recovery from disturbance through repair, asexual reproduction and larval colonization. Sparks (1972) reviewed the regeneration abilities and rapid repair of injuries. Fragmentation of the hydroid provides a route for short distance dispersal, for example, each fragmented part of Sertularia cupressina can regenerate itself following damage (Berghahn & Offermann, 1999). New colonies of the same genotype may, therefore, arise through damage to existing colonies (Gili & Hughes, 1995). Many hydroid species also produce dormant, resting stages that are very resistant of environmental perturbation (Gili & Hughes 1995).  Although colonies may be removed or destroyed, the resting stages may survive attached to the substratum and provide a mechanism for rapid recovery (Cornelius, 1995a; Kosevich & Marfenin, 1986).   The life cycle of hydroids typically alternates between an attached solitary or colonial polyp generation and a free-swimming medusa generation.  Planulae larvae produced by hydroids typically metamorphose within 24 hours and crawl only a short distance away from the parent plant (Sommer, 1992).  Gametes liberated from the medusae (or vestigial sessile medusae) produce gametes that fuse to form zygotes that develop into free-swimming planula larvae (Hayward & Ryland, 1994).  Planulae are present in the water column between 2 and 20 days (Sommer, 1992).

Hydroids are, therefore, classed as potential fouling organisms, rapidly colonising a range of substrata placed in marine environments and are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). 

For example, hydroids were reported to colonize an experimental artificial reef within less than 6 months, becoming abundant in the following year (Jensen et al., 1994).  In similar studies, Obelia species recruited to the bases of reef slabs within three months and the slab surfaces within six months of the slabs being placed in the marine environment (Hatcher, 1998). Cornelius (1992) stated that Obelia spp. could form large colonies within a matter of weeks. In a study of the long-term effects of scallop dredging in the Irish Sea, Bradshaw et al., (2002) noted that hydroids increased in abundance, presumably because of their regeneration potential, good local recruitment and ability to colonize newly exposed substratum quickly. Cantero et al. (2002) describe fertility of Obelia dichotoma, Kirchenpaureria pinnata, Nemertesia ramosa in the Mediterranean as being year-round, whilst it should be noted that higher temperatures may play a factor in this year round fecundity, Bradshaw et al. (2002) observed that reproduction in Nemertesia antennina occurred regularly, with three generations per year. It was also observed that presence of adults stimulate larval settlement, therefore if any adults remain, reproduction is likely to result in local recruitment.  It has also been suggested that rafting on floating debris as dormant stages or reproductive adults (or on ships hulls or in ship ballast water), together with their potentially long lifespan, may have allowed hydroids to disperse over a wide area in the long-term and explain the near cosmopolitan distributions of many hydroid species (Cornelius, 1992; Boero & Bouillon 1993).  For example, Halecium halecinum is an erect hydroid growing up to 25 cm and is found on stones and shells in coastal areas.  It is widely distributed in the Atlantic and is present from Svalbard to the Mediterranean (Hayward & Ryland, 1994; Palerud et al., 2004; Medel et al., 1998).   Nemertesia ramosa grows up to 15 cm and is found inshore to deeper water and is common throughout the British Isles and is distributed from Iceland to north-west Africa (Hayward & Ryland, 1994).  Hydrallmania falcata grows to 50 cm, grows on rock and shell, particularly in sandy areas and is found from the Arctic to the Mediterranean (Hayward & Ryland, 1994). 

Bryozoans are sessile fauna forming colonies through asexual budding following settlement of sexually produced larvae (Hayward & Ryland, 1995b).  Larvae have a short pelagic life time of up to about 12 hours (Ryland, 1976). Recruitment is dependent on the supply of suitable, stable, hard substrata (Eggleston, 1972b; Ryland, 1976; Dyrynda, 1994). Even in the presence of available substratum, Ryland (1976) noted that significant recruitment in bryozoans only occurred in the proximity of breeding colonies, although Hiscock (1981) described  Flustra foliacea colonizing the wreck of the MV Roberts, several hundreds of metres from any significant hard substrata, and hence a considerable distance from potentially parent colonies. 

Flustra foliacea is a coarse, foliaceous bryozoan which tends to be found on stones and shells, reaches 10 cm in height. It is common to all coasts in North-West Europe (Hayward & Ryland, 1995b) and is found across all coasts in the British Isles (NBN, 2015).    Stebbing (1974) noted that Flustra foliacea on the Gower peninsular, South Wales had an annual growth season between March and November, with a dormant winter period, when no growth occurred, leading to a line forming across the fronds which can be used to age specimens.  The species can regularly reach six years of age, although twelve year old specimens were reported off the Gower Peninsula (Stebbing, 1971a; Ryland, 1976).  Fortunato et al. (2013) compared numerous sets of growth data with their own observations and reported that colonies grow faster during the first couple of years (about 1.05 cm/year), slowing down afterwards, which could be due to the lateral growth of the fronds.  Colonies appeared to be able to regenerate areas of the frond which had been removed by grazing.  Silén (1981) found that Flustra foliacea could repair physical damage to its fronds with 5-10 days, concluding that, as long as the holdfast remains intact, Flustra foliacea would survive and grow back. Once settled, new colonies of Flustra foliacea take at least 1 year to develop erect growth and 1-2 years to reach maturity, depending on environmental conditions (Tillin & Tyler Walters, 2014). Four years after sinking off Lundy, the M.V. Roberts was found to be colonized by erect bryozoans and hydroids, including occasional Flustra foliacea (Hiscock, 1981).   Flustra foliacea colonies are perennial, and potentially highly fecund with increasing colony size as each zooid produces a single embryo (Tillin & Tyler Walters, 2014; Eggleston (1972a) with ca 10,000 larvae released from a specimen of Flustra foliacea within 3 hrs (Dalyell, cited in Hincks, 1880).  Whilst bryozoan larvae are typically very short-lived, limiting recruitment to the immediate area surrounding breeding colonies, specimens experiencing strong water movement would improve dispersal potential, and may explain reports of Flustra foliacea colonizing a wreck several hundreds of metres from any significant hard substrata, and hence a considerable distance from potentially parent colonies (Hiscock, 1981).  Flustra foliacea requires stable hard substrata (Eggleston, 1972; Ryland, 1976; Dyrynda, 1994) and the abundance of bryozoans is positively correlated with supply of stable hard substrata and hence with current strength (Eggleston, 1972b; Ryland, 1976). 

Alcyonidium diaphanum forms an erect colony that can grow up to 50 cm long but more usually 15 cm. It has a small encrusting base, which attaches to hard substratum. The size, colour and colony form varies widely around the British Isles (Ager, 2007). 

Bugula spp. are perennials that tend to form short-lived, large colonies in summer with significant die-back in late autumn and a dormant winter phase (Eggleston 1972a; Dyrynda & Ryland, 1982).   Reproduction occurs in summer/early autumn with some species such as Bugula flabellate reportedly having two generations of fronds capable of reproduction each year (Dyrynda & Ryland, 1982). Eggleston (1972a) reported that newly settled specimens from the first generation in the Isle of Man grew rapidly and contributed to the second generation. 

Little information on sponge longevity and resilience exists.  Reproduction can be asexual (e.g. budding) or sexual (Naylor, 2011) and individual sponges are usually hermaphroditic (Hayward & Ryland, 1994).  Short-lived ciliated larvae are released via the aquiferous system and metamorphosis follows settlement.  Growth and reproduction are generally seasonal (Hayward & Ryland, 1994). Rejuvenation from fragments can also be considered an important form of reproduction (Fish & Fish, 1996). Some sponges are known to be highly resilience to physical damage with an ability to survive severe damage, regenerate and reorganize to function fully again, however, this recoverability varies between species (Wulff, 2006). Marine sponges often harbour dense and diverse microbial communities, which can include bacteria, archaea and single-celled eukaryotes (fungi and microalgae), and comprise up to 40% of sponge volume.  The microbial community may have a profound impact on host biology (Webster & Taylor, 2012).   Many sponges recruit annually, growth can be quite rapid, with a lifespan of one to several years (Ackers, 1983). However, sponge longevity and growth is highly variable depending on the species and conditions (Lancaster et al., 2014). It is likely that erect sponges are generally longer lived and slower growing given their more complex nature than smaller encrusting or cushion sponges.  Fowler & Laffoley (1993) monitored marine nature reserves in Lundy and the Isles Scilly and found that a number of more common sponges showed great variation in size and cover during the study period.  Colonies appeared and vanished at some locations. Some large encrusting sponges went through periods of both growth and shrinkage, with considerable changes taking place from year to year. For example, Cliona celata colonies generally grew extremely rapidly, doubling their size or more each year. In some years, an apparent shrinkage in size also took place. In contrast, there were no obvious changes in the cover of certain unidentified thin encrusting sponges.  Dysidea fragilis is an encrusting sponge without spicules (5 cm across and up to 0.75 cm in height to lobose at up to 30 cm across).  Dysidea fragilis incorporates foreign material including hard material and sand grains (Hayward & Ryland, 1994). The skeleton is composed of spongin fibres making the consistency of Dysidea fragilis soft and elastic (Rowley, 2007).  It is found from the Arctic to the Mediterranean (Ackers et al., 1992; Mustapha et al., 2004) and is a ubiquitous species, present in superficial and deep, clean and polluted, marine and brackish waters (Aiello et al., 1996).  Galera et al. (2000) note that Dysidea avara is relatively fast growing and opportunistic.

Resilience assessment: The hydroids that characterize this biotope are likely to recover from damage very quickly. Based on the available evidence, resilience for the hydroid species is ‘High’ (recovery within two years) for any level of perturbation.  Depending on the season of the impact and level of recovery, recovery could occur within six months.

Bryozoans tend to be fast growing fauna that are capable of self-regeneration.  Dispersal of the larvae is limited and whilst it is likely that the bryozoan turfs would regenerate rapidly, within 2 years (resilience of ‘High’) from most levels of damage.  However, in the case of removal of 75% or more of the bryozoans/habitat (resistance of ‘None’), recovery could take longer and a resilience of ‘Medium’ (2-10 years) should be recorded in such cases.

Sebens (1985, 1986) found that the sponge Halichondria panicea reached pre-clearance levels of cover after 2 years. A few individuals of Alcyonium digitatum and Metridium senile colonized within 4 years (Sebens, 1986) and slower growing sponges would probably take longer to reach pre-clearance levels.  If the sponge community suffers significant mortality from a pressure (resistance of ‘None’, ‘Low’) resilience is assessed as ‘Medium’ (recovery within 2-10 years).  If resistance is assessed as ‘Medium’ or ‘High’ then resilience is assessed as ‘High’ (recovery within 2 years).

Hydrological Pressures

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

Gili & Hughes, (1995) reported that temperature is a critical factor stimulating or preventing reproduction and that most species have an optimal temperature for reproduction. However, limited evidence for thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope.  Berrill (1949) reported that growth in Obelia commissularis (syn. dichotoma) was temperature dependent but ceased at 27°C.  Hydranths did not start to develop unless the temperature was less than 20°C and any hydranths under development would complete their development and rapidly regress at ca 25°C.  Berrill (1948) reported that Obelia species were absent from a buoy in July and August during excessively high summer temperatures in Booth Bay Harbour, Maine, USA.  Berrill (1948) reported that the abundance of Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20°C during this period.  The upwelling of cold water (8-10°C colder than surface water) allowed colonies of Obelia sp. to form in large numbers.  Cantero et al. (2002) describe the presence and year-round fertility of Obelia dichotoma, Kirchenpaureria pinnata, Nemertesia ramosa and Halecium spp.in the Mediterranean. 

Bugula spp. grows and reproduces in the summer months, however, day length and/or the phytoplankton bloom characteristic of temperate waters are probably more important than temperature (Ryland, 1967; 1970; Tyler-Walters, 2005c).  Bugula turbinata is a predominantly southern species in British waters (Lewis, 1964; Hayward & Ryland, 1998) but has been recorded as far north as Shetland (NBN, 2015). A long term increase in temperature may increase its abundance in northern British waters and allow the species to extend its range. It occurs as far south as the Mediterranean (Rosso, 2003) and likely to tolerate increases of temperature, at the benchmark level.  Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over 9 years.  High temperature events were recorded, the first causing mass mortality among a number of species.  Alcyonidium diaphanum is commonly found across the British Isles and is probably widely distributed across North-West Europe (Fish  & Fish, 1996). 

Dysidea fragilis is found from the Arctic to the Mediterranean (Ackers et al., 1992; Mustapha et al., 2004).  Berman et al. (2013) monitored sponge communities off Skomer Island, UK over three years with all characterizing sponges for this biotope assessed.  seawater temperature, turbidity, photosynthetically active radiation and wind speed were all recorded during the study.

It was concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia, Stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied.  Morphological changes most strongly correlated with a mixture of water visibility and temperature.

Sensitivity assessment. None of the characterizing species are at their southern distribution limit and no evidence for mortality linked to increase in temperature in the British Isles could be found.  The biotope is therefore assessed as having resistance of ‘High’, Resilience of ‘High’ and is assessed as ‘Not sensitive’ at the benchmark level.

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

errill (1949) reported that for Obelia, stolons grew, under optimal nutritive conditions, at less than 1 mm in 24 hrs at 10-12 °C, 10 mm in 24 hrs at 16-17 °C, and as much as 15-20 mm in 24 hrs at 20 °C.  All important characterizing bryozoans (Alcyonidium diaphanum, Flustra foliaceaBugula plumosa and Bugula flabellata) have been recorded across the British Isles, from the Channel Isles to the northern coast of Scotland (NBN, 2015). Alcyonium digitatum is recorded from Iceland in the north, to Portugal in the south and it is unlikely that this species will be adversely affected by a long term temperature change in British waters (Budd, 2008).  Alcyonium digitatum was also reported to be apparently unaffected by the severe winter of 1962-1963 (Crisp, 1964b). 

The hydroids Obelia dichotoma, Halecium Halecinum and Nemertesia sp. were recorded in Svalbard ( Orjas et al., 2012).

Dysidea fragilis is found from the Arctic to the Mediterranean (Ackers et al., 1992; Mustapha et al., 2004).  Crisp (1964b) studied the effects of an unusually cold winter (1962-63) on the marine life in Britain, including Porifera in North Wales.   Whilst Dysidea fragilis was not mentioned in the study, Crisp found that Pachymastia johnstonia and Halichondria panicea were wholly or partly killed by frost and several species appeared to be missing including Amphilectus fucorum. Others, including Hymeniacidon perleve were unusually rare and a few species, including Polymastia boletiformis, were not seriously affected.  It should be noted that Crisp’s (1964b) general observations on all marine life stated that damage decreased the deeper the habitat and that the extremely cold temperatures (sea temperatures between 4-6°C colder than the 5 year mean over a period of 2 months) is more extreme than the benchmark level for assessment. 

Sensitivity assessment. The majority of characterizing species occur in boreal environments, with none at their northerly distribution limit.  Therefore, resistance is likely to be ‘High’ with resilience of ‘High’ and the biotope is probably ‘Not sensitive’ at the benchmark level.

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

Studies on hydroids in general have found that prey capture rates may be affected by salinity and temperature (Gili & Hughes, 1995) although no evidence was found for species that characterize this biotope.  Marin et al. (1998) described the presence of Dysidea fragilis in a hypersaline coastal lagoon (42-47 g/l) in La Mar Menor, Spain.  No other evidence could be found for characterizing sponges.  Soule & Soule (1979) cite Hastings (1927) who described the presence of 5 bryozoans in hypersaline conditions in the Suez Canal. ‘No evidence’ for mortality or tolerance of the characterizing bryozoans or hydroids in hypersaline conditions could be found.

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

This biotope is recorded in full salinity habitats (Connor et al., 2004).   Little evidence for the characterizing hydroids could be found. Stebbing, 1981 found that, for the hydroid Campanularia flexuosa, growth was inhibited in 70% seawater (ca 25‰) and that exposure to below 30% seawater (ca 10‰) was lethal after 3 days.

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata were fairly stenohaline and restricted to full salinity (30-35 ppt), noting that reduced salinities result in an impoverished bryozoan fauna. Flustra foliacea appears to be restricted to areas with high salinity (Tyler-Walters & Ballerstedt 2007; Budd 2008).  Dyrynda (1994) noted that Flustra foliacea and Alcyonidium diaphanum were probably restricted to the vicinity of the Poole Harbour entrance by their intolerance to reduced salinity. Although, protected from extreme changes in salinity due to their subtidal habitat, severe hyposaline conditions could adversely affect Flustra foliacea colonies.

Dysidea fragilis is a ubiquitous species, found in superficial and deep, clean and polluted, marine and brackish waters (Aiello et al., 1996).

Sensitivity assessment. The characterizing bryozoans are likely to be affected by a reduction in salinity, and species diversity is likely to decrease.  Resistance is assessed as ‘Low’, resilience is assessed as ‘High’ and sensitivity is ‘Low’.

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

Hayward & Ryland (1995b) noted that abundant communities of hydroids occur in narrow straits and headlands, which may experience high levels of water flow. Hydroids can bend passively with water flow to reduce drag forces to prevent detachment and enhance feeding (Gili & Hughes, 1995). The hydroid growth form also varies to adapt to prevailing conditions, allowing species to occur in a variety of habitats (Gili & Hughes, 1995). Hiscock (1979) assessed feeding behaviour of the hydroid Tubularia indivisa in response to different flow rates.  At flow rates <0.05 m/s, polyps actively moved tentacles.  Increasing the flow rate to 0.2 m/s increased capture rates but at higher flow rates from 0.5-0.9 m/s the tentacles were extended downcurrent and pushed together and feeding efficiency was reduced. In general, flow rates are an important factor for feeding in hydroids and prey capture appears to be higher in more turbulent conditions that prevent self-shading by the colony (Gili & Hughes, 1995). The capture rate of zooplankton by hydroids is correlated with prey abundance (Gili & Hughes, 1995), thus prey availability can compensate for sub-optimal flow rates. Water movements are also important to hydroids to prevent siltation, which can cause death (Round et al., 1961). Tillin & Tyler-Walters (2014) suggested that the range of flow speeds experienced by biotopes in which hydroids are found indicate that a change (increase or decrease) in the maximum water flow experienced by mid-range populations for the short periods of peak spring tide flow would not have negative effects on this ecological group. 

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to high mass transport of water such as the Menai Strait and tidal rapids generally support large numbers of bryozoan species (Moore, 1977a). Although, active suspension feeders, their feeding currents are probably fairly localized and they are dependent on water flow to bring adequate food supplies within reach (McKinney, 1986). A substantial decrease in water flow will probably result in impaired growth due to a reduction in food availability, and an increased risk of siltation (Tyler-Walters, 2005c).

Okamura (1984) reported that an increase in water flow from slow flow (1-2 cm/s) to fast flow (10-12 cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera

Flustra foliacea colonies are flexible, robust and reach high abundances in areas subject to strong currents and tidal streams Stebbing, 1971a; Eggleston, 1972b; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1983, 1985; Holme & Wilson, 1985). Dyrynda (1994) suggested that mature fronded colonies do not occur on unstable substratum due to the drag caused by their fronds, resulting in rafting of colonies on shells or the rolling of pebbles and cobbles, resulting in destruction of the colony. Dyrynda (1994) reported that the distribution of Flustra foliacea in the current swept entrance to Poole Harbour was restricted to circalittoral boulders, on which it dominated as nearly mono-specific stands. While, the pumping activity of the lophophores provide the greatest proportion of the colonies food requirements (Hayward & Ryland, 1998), the current generated is probably very localized and the colonies are likely to be dependant on water currents for food supply.  A significant decrease in water flow is likely to result in a decrease in the abundance of bryozoans. 

Sensitivity assessment. The biotope experiences moderate tidal streams and substantial increase or decrease would probably result in decline of the biotope.  However, a 0.1 – 02. m/s change (the benchmark) is unlikely to significantly impact the characterizing species.  Resistance is, therefore, assessed as ‘High’, resilience is assessed as ‘High’ and the biotope is assessed as ‘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

Changes in emergence are not relevant to this biotope as it is restricted to fully subtidal/circalittoral conditions-The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

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

Jackson (2004) reported that Nemertesia ramosa was intolerant of high wave exposure and only found in sheltered areas.   Faucci et al. (2000) recorded hydroid communities at two sites of different wave exposure and recorded the presence of Obelia dochotoma and Halecium spp. in both the exposed and sheltered sites, but only found Kirchenpaueria sp. in the sheltered site.  

Bugula spp. produce flexible erect tufts, which are likely to move with the oscillatory flow created by wave action. Bugula turbinata has been recorded from very wave exposed to very wave sheltered habitats (Tyler-Walters, 2005c).

Flustra foliacea occurs from very wave exposed to sheltered waters, although probably limited to deeper waters in very wave exposed conditions (Tyler-Walters & Ballerstedt , 2007) The oscillatory water flow generated by wave action may be more damaging than constant strong currents, e.g. strong wave action may generate an oscillatory flow of 2 m/sec at 20 m (Hiscock, 1983, 1985). Flustra foliacea is a common member of the flotsam, having been removed from its substratum by storms. Whilst the biotope is circalittoral, a severe increase in wave exposure (e.g. storms) could affect bryozoans colonies, especially on mobile substrata such as cobbles and pebbles).  Cocito et al. (1998) described a severe winter storm of 1993 had devastating effects on the same Flustra foliacea population, sweeping away most of the colonies down to 11 m.   

Roberts et al. (2006) studied deep sponge reef communities (18-20 m) in sheltered and exposed locations in Australia. They reported greater diversity and cover (>40% cover) of sponges in wave-sheltered areas compared with a sparser and more temporal cover in exposed sites (25% cover).  Erect sponges dominated the sheltered sites, while encrusting sponges dominated in exposed locations. 

Sensitivity assessment. Significant increase in wave exposure could affect the characterizing species due to increased scour and movement of mobile substrata.  A significant decrease may also affect the biotope.  However, a change at the benchmark level would be unlikely to affect the characterizing species.  Resistance is, therefore, assessed as ‘High’, resilience is assessed as ‘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

Although no information on the effects of heavy metals on the assessed hydroids was found, evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination.  Various heavy metals have been shown to have sublethal effects on growth in the few hydroids studied experimentally (Bryan, 1984). Stebbing (1981a) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1976) reported that 1 µg/l Hg2+ was stimulatory, although the effect was transitory, exposure resulting in reduced growth towards the end of his 11 day experiments. Cadmium (Cd) was reported to cause irreversible retraction of 50% of hydranths in Laomedea loveni after 7 days exposure at concentrations between 3 µg/l (at 17.5 °C and 10 ppt salinity) and 80 µg/l (at 7.5 °C and 25 ppt salinity) (Theede et al., 1979). Laomedea loveni was more tolerant of Cd exposure at low temperatures and low salinities. Karbe (1972, summary only) examined the effects of heavy metals on the hydroid Eirene viridula (Campanulidae). He noted that Cd and Hg caused cumulative effects, and morphological changes. Mercury (Hg) caused irreversible damage at concentrations as low as 0.02 ppm. He reported threshold levels of heavy metals for acute effects in Eirene viridula of 1.5-3 ppm Zn, 1-3 ppm Pb, 0.1-0.3 ppm Cd, 0.03-0.06 ppm Cu and 0.001-0.003 ppm Hg. Karbe (1972, summary only) suggested that Eirene viridula was a sensitive test organism when compared to other organisms.  Although no information on the effects of heavy metals on assessed hydroid species was found, the above evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination.

Bryozoans are common members of fouling communities and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1979; Holt et al., 1995).

Nevertheless, this biotope is considered to be '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

Filter feeders are highly sensitive to oil pollution, particularly those inhabiting the tidal zones which experience high exposure and show correspondingly high mortality, as are bottom dwelling organisms in areas where oil components are deposited by sedimentation (Zahn et al., 1981).

Oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sub-littoral habitats (Castège et al., 2014).

Banks & Brown (2002) found that exposure to crude oil significantly impacted recruitment in the bryozoan Membranipora savartii.

Tethya lyncurium concentrated BaP (benzo[a ]pyrene )to 40 times the external concentration and no significant repair of DNA was observed in the sponges, which, in higher animal, would likely lead to cancers. As sponge cells are not organized into organs the long-term effects are uncertain (Zahn et al., 1981).

Little information on the effects of hydrocarbons on bryozoans was found. Ryland & de Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. or other sessile fauna in Milford Haven or St. Catherine's Island, south Pembrokeshire. Houghton et al. (1996) reported a reduction in the abundance of intertidal encrusting bryozoa (no species given) at oiled sites after the Exxon Valdez oil spill. Soule & Soule (1979) reported that the encrusting bryozoan Membranipora villosa was not found in the impacted area for 7 months after the December 1976 Bunker C oil spill in Los Angeles Harbour. Additionally, Soule & Soule (1979) reported that Bugula neritina was lost from breakwater rocks in the vicinity (in December 1979) of the Bunker C oil spill and had not recovered within a year. However, Bugula neritina had returned to a nearby area within 5 months (May 1977) even though the area was still affected by sheens of oil. Furthermore, only three of eight recorded species two weeks after the incident were present in April within the affected breakwater area. By June all the species had been replaced by dense growths of the erect bryozoan Scrupocellaria diegensis.

Mohammad (1974) reported that Bugula spp. and Membranipora spp. were excluded from settlement panels near an oil terminal in Kuwait subject to minor but frequent oil spills. Encrusting bryozoans are also probably intolerant of the smothering effects of acute hydrocarbon contamination and pollution, resulting in suffocation of colonies and communities may be lost or damaged. Circalittoral communities are likely to be protected from the direct effects of oil spills by their depth. However, the biotope may be exposed to emulsified oil treated with dispersants, especially in areas of turbulence, or may be exposed to water soluble fractions of oils, PAHs or oil adsorbed onto particulates (Tyler-Walters, 2002).

Little information of the effects of hydrocarbons on hydroids was found although hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995).

Nevertheless, this biotope is considered to be '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

Species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Stebbing (1981a) cited reports of growth stimulation in Obelia geniculata caused by methyl cholanthrene and dibenzanthrene. Hoare & Hiscock (1974) suggested that the Bryozoa (as Polyzoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey, e.g. Electra pilosa occurred at lower abundance on laminarian holdfasts within the bay, compared to sites outside the affected area.

Nevertheless, this biotope is considered to be '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

'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

No benchmark proposed therefore sensitivity assessment has been assessed as 'Not sensitive' at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

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

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates this concentration is about 2 ml/l (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l.

Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (Gili & Hughes, 1995). Although no information was found on oxygen consumption for the characterizing hydroids, Sagasti et al. (2000) reported that epifaunal species, including several hydroids and bryozoans in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O2/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation. 

Sensitivity assessment. Whilst hydroids and bryozoans have been shown to tolerate short anoxic events (Sagasti et al., 2000) and the sand scoured nature of the biotope would likely result in occasional burial, an event at the benchmark level would likely result in quite significant mortality.  Resistance is therefore assessed as ‘Medium’, resilience as ‘High’ and sensitivity as ‘Low’.

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

Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids.

Hartikainen et al. (2009) reported that increased nutrient concentrations resulted in freshwater bryozoans achieving higher biomass.  O’Dea & Okamura (2000) found that annual growth of Flustra foliacea in western Europe has substantially increased since 1970.  They suggest that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).

Gochfeld et al. (2012) studied the effect of nutrient enrichment (≤0.05 to 0.07 μM for nitrate and ≤0.5 μM for phosphate)  as a potential stressor in Aplysina caulifornis, and its bacterial symbionts, and found that nutrient enrichment had no effects on sponge or symbiont physiology when compared to control conditions (

This study does contradict findings in Gochfeld et al. (2007) in which Aplysina spp. were virtually absent from a site of anthropogenic stress in Bocas del Toro, Panama, which experienced high rainfall and terrestrial runoff.  The author suggested that whilst this site did include elevated nutrient concentrations, other pressures and stresses could be contributing.

Rose & Risk (1985) described anincrease in abundance of Cliona delitrix in organically polluted section of Grand Cayman fringing reef  affected  by  the  discharge of untreated  faecal  sewage.  Ward-Paige et al. (2005) described greatest size and biomass of Clionids corresponding with highest nitrogen, ammonia and δ15N levels. 

However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with good status as defined by the WFD.

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

Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids.

O’Dea & Okamura (2000) found that annual growth of Flustra foliacea in western Europe has substantially increased since 1970.  They suggest that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).

Rose & Risk (1985) described increase in abundance of the sponge Cliona delitrix in an organically polluted section of Grand Cayman fringing reef affected by the discharge of untreated faecal sewage. 

De Goeij et al. (2008) used 13C to trace the fate of dissolved organic matter in the coral reef sponge Halisarca caerulea.  Biomarkers revealed that the sponge incorporated dissolved organic matter through both bacteria mediated and direct pathways, suggesting that it feeds, directly and indirectly, on dissolved organic matter.

Mayer-Pinto & Junqueira (2003) studies the effects of organic pollution on fouling communities in Brazil and found that some tolerance of polluted/unpolluted artificial reefs varied among bryozoan species.  It should be note that Bugula spp. preferred the polluted sites.

Sensitivity assessment. Whilst an increase in organic matter would likely be removed relatively rapidly by water movement in this biotope, such an increase would likely be beneficial to the characterizing species.  Resistance is therefore assessed as ‘High’, resilience as ‘High’ and the biotope is probably ‘Not sensitive’ at the benchmark level.

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

This biotope is characterized by the hard substratum provided by the pebbles and cobbles to which the key characterizing species can firmly attach to (Connor et al., 2004). A change to a mobile gravel or soft sedimentary substratum would significantly alter the character of the biotope. The biotope is considered to have 'No' resistance to this pressure based on a change to a soft sediment substratum, recovery of the biological assemblage (following habitat restoration) is considered to be 'Medium'. However, the pressure benchmark is considered to refer to a permanent change and recovery is therefore ‘Very low’.  Sensitivity is therefore 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

CR.HCR.XFa.SpNemAdi is characterized by the hard substratum provided by the pebbles and cobbles. SS.SMx.CMx.FluHydXF and SS.SSa.IFiSa.ScupHyd are dominated by hard substrata on sediment (Connor et al., 2004). A change to a mobile gravel or soft sedimentary substratum would significantly alter the character of the biotope. The biotope is considered to have resistance of 'None' to this pressure based on a change to a soft sediment substratum, recovery of the biological assemblage (following habitat restoration) is considered to be 'High'. However, the pressure benchmark is considered to refer to a permanent change and recovery is therefore ‘Very low’.  Sensitivity is therefore assessed as 'High'.

None Medium Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

The species characterizing these biotopes are epifauna occurring on the cobbles and pebbles that characterize this biotope (Connor et al., 2004).  Removal of the substratum would remove both the habitat (boulders, cobbles and pebbles) and the characterizing, attached species.

Sensitivity assessment. Biotope resistance is assessed as ‘None’ (in the extraction footprint), resilience (following habitat restoration, or where the underlying substratum remains the same) is assessed as ‘Medium’. Sensitivity is, therefore, assessed as ‘Medium’. Recovery will be prolonged (and sensitivity greater) where the entire habitat is removed and restoration (artificial or natural) to the previous state does not occur.

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

The species characterizing this biotope occur on the rock surface and therefore have no protection from surface abrasion. High levels of abrasion from scouring by mobile sands and gravels is an important structuring factor in this biotope (Connor et al., 2004) and may prevent succession. Where individuals are attached to mobile pebbles, cobbles and boulders rather than bedrock, surfaces can be displaced and turned over preventing feeding and leading to smothering.

The available evidence indicates that hydroids can be entangled and removed by abrasion.  Drop down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011).  The study showed that damage is incremental with damage increasing with frequency of trawls rather than a blanket effect occurring on the pass of the first trawls. The results indicated that epifaunal species, including the sponge Pachymatisma johnstoni, were highly damaged by the experimental trawl. Please note Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the Newhaven dredges. A fully loaded Newhaven dredge may cause higher damage to community as indicated in their study.

Re-sampling of grounds that were historically studied (from the 1930s) indicates that some species have increased in areas subject to scallop fishing (Bradshaw et al., 2002).  This study also found (unquantified) increase in abundance of tough stemmed hydroids including Nemertesia spp., its morphology may have prevented excessive damage.  Bradshaw et al. (2002) suggested that as well as having high resistance to abrasion   pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult. Hydroids may also recover rapidly as the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies.

Hydroid colonies were still present in the heavily fished area, albeit at lower densities than in the closed area. This may largely be because the Isle of Man scallop fishery is closed from 1st June to 31st October (Andrews et al., 2011), so at the time the samples were taken for the study in question, the seabed had been undredged for at least 3.5 months (Bradshaw et al., 2003). The summer period is also the peak growing/breeding season for many marine species.

Freese et al. (1999) studied the effects of trawling on seafloor habitats and associated invertebrates in the Gulf of Alaska.  They found that a transect following a single trawling event showed significantly reduced the abundance of ‘vase’ sponges (67% expressed damage) and ‘morel’ sponges (total damage could not be quantified as their brittle nature meant that these sponges were completely torn apart and scattered).   The ‘finger’ sponges, the smallest and least damaged of the sponges assessed, were damaged (14%) by being knocked over.

Tilnant (1979) found that, following a shrimp trawl in Florida, US, over 50% of sponges, including Neopetrosia, Spheciospongia, Spongia and Hippiospongia, were torn loose from the bottom.  Highest damage incidence occurred to the finger sponge Neopetrosia longleyi. Size did not appear to be important in determining whether a sponge was affected by the trawl.  Recovery was ongoing, but not complete 11 months after the trawl, although no specific data was provided.

Freese (2001) studied deep cold-water sponges in Alaska a year after a trawl event;  46.8% of sponges exhibited damage with 32.1% having been torn loose.  None of the damaged sponges displayed signs of regrowth or recovery.  This was in stark contrast to early work by Freese et al. (1999) on warm shallow sponge communities.  Impacts of trawling activity in Alaska study being much more persistent due to the slower growth/regeneration rates of deep, cold-water sponges. Given the slow growth rates and long lifespans of the rich, diverse fauna, it was considered likely to take many years for deep sponge communities to recover if adversely affected by physical damage (Freese, 2001).

Physical disturbance by fishing gear has been shown to adversely affect emergent epifaunal communities with hydroid and bryozoan matrices reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998). Heavy mobile gears could also result in movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998).   

Boulcott & Howell (2011) conducted experimental Newhaven scallop dredging over a circalittoral rock habitat in the sound of Jura, Scotland and recorded the damage to the resident community.

Sensitivity assessment

Given the sessile, erect nature of the sponges, hydroids and bryozoans, damage and mortality following a physical disturbance effect are likely to be significant, however some studies have brought into question the extent of damage to the faunal turf.  A proportion of the biotope occurs on cobbles, pebbles and mobile substrata, which could result in increased damage in abrasion events. 

The physiology of the bryozoans affords some protection in the event of abrasion events and recovery is likely to be rapid if stolons remain undamaged.  Based on the potential damage to sponges, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity is assessed as ‘Medium’.

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

This biotope is characterized by mobile pebbles and cobbles, pressures that lead to penetration and disturbance could damage associated species through abrasion and by overturning surfaces could result in smothering of fauna or reductions in respiration, feeding efficiency or fertilization of gametes in the water column. The biotope is, however, likely to be exposed to at least seasonal movement of substrata and this movement and scour maintains this biotope by preventing species that require more stable habitats from colonizing and developing stable populations (Connor et al., 2004). Evidence presented above for surface abrasion is considered equally relevant to this pressure as abrasion in this biotope is likely to lead to movement and displacement of mobile substrata.

Sensitivity assessment. The impact of pressures that disturb and penetrate the mobile substrata will depend on the footprint, duration and magnitude of the pressure. High levels of abrasion from scouring by mobile cobbles and pebbles is an important structuring factor in this biotope (Connor et al., 2004) but the persistence of the assemblage may depend on rapid recovery rather than high resistance (Gorzula, 1977).  Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity is assessed as ‘Medium’.

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

An increase in suspended sediment may have a deleterious effect on the suspension feeding community. It is likely to clog their feeding apparatus to some degree, resulting in a reduced ingestion over the benchmark period and, subsequently, a decrease in growth rate (Jackson, 2004). As the hydroids capture small prey in suspension (Gili & Hughes, 1995), a reduction in feeding efficiency could potentially lead to a reduction in overall biomass.

Nemertesia ramosa is a passive suspension feeder, extracting seston from the water column. Increased siltation may clog up the feeding apparatus, requiring energetic expenditure to clear. Recovery is likely to take only a few days. (Jackson, 2004)  

Bryozoans are suspension feeders that may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus.  However, Tyler-Walters & Ballerstedt (2007) reported Flustra foliacea as tolerant to increased suspended sediment based on its occurrence in areas of high suspended sediment e.g. abundant in turbid, fast flowing waters of the Menai Straits (Moore 1977). Also, communities dominated by Flustra foliacea were described on tide swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel (Holme & Wilson, 1985). Flustra foliacea is also characteristic of sediment-scoured, silty rock communities CR.HCR.XFa.FluCoAs and CR.MCR.EcCr.UrtScr (Connor et al., 2004). 

Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig 1979), many encrusting sponges appear to be able survive in highly sedimented conditions, and in fact many species prefer such habitats (Schönberg, 2015; Bell & Barnes 2000; Bell & Smith 2004).  Castric-Fey & Chassé (1991) conducted a factorial analysis of the subtidal rocky ecology near Brest, France and rated the distribution of species in varying turbidity (corroborated by the depth at which laminarians disappeared).  Cliona celata and Stelligera rigida were classed as indifferent to turbidity, Tethya aurantium, Pachymatisma johnstonia and Polymastia boletiformis (as Polymastia robusta) had a slight preference for clearer water, while Dysidea fragilis, Polymastia mamillaris, and Raspailia ramosa had a strong preference for turbid water.   Some sponges occur in harbours and estuaries, including Halichondria spp. and Hymeniacidon perleve (Ackers et al., 1992).  Storr (1976) observed the sponge Sphecispongia vesparium back washing to eject sediment and noted that other sponges (such as Condrilla nucula) use secretions to remove settled material.Tjensvoll (2013) found that the non-characterizing sponge Geodia barretti physiologically shuts down when exposed to sediment concentrations of 100 mg /l.  Rapid recovery to initial respiration levels directly after the exposure indicated that Geodia barretti can cope with a single short exposure to elevated sediment concentrations.

However, it should be noted that a laboratory study on the impact of elevated sedimentation rates on deep water sponges found that sediment load of 30 mg sed/l resulted in significantly higher sponge mortality compared with sponges exposed to 5 and 10 mg sed/l. although no additional information was provided (Hoffman & Tore Rapp, pers com cited in Lancaster et al., 2014).

Sensitivity assessment. Whilst increase in suspended sediment may result in extra energetic expenditure in cleaning, it is unlikely to increase mortality.  Although one report cited unpublished work that demonstrated increased sponge mortality at low suspended sediment concentrations(Lancaster et al., 2014), the majority of the literature reviewed suggested that a change in suspended solids is unlikely to cause significant mortality of the species considered in this study.  Therefore, resistance has been assessed as ‘High’, resilience as ‘High’ and the biotope is ‘Not Sensitive’ at the benchmark level. 

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

Holme & Wilson (1985) examined the fauna in a tide-swept region of the central English Channel. Flustra foliacea dominated communities were subject to sediment transport (mainly sand) and periodic, temporary, burial (ca <5 cm).

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al., 1961 reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement.

Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered, but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals, while others may just have the lower section of the main stem covered (Hayward & Ryland, 1994).  Obelia dichotoma stems grow to 5 cm, while polysiphonic structures can reach up to 35 cm in height.  Halecium halecinum can grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Some of the community is therefore likely to survive smothering by 5 cm.

Smothering by 5 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005).

Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig 1979), many encrusting sponges appear to be able survive in highly sedimented conditions, and in fact many species prefer such habitats (Schönberg, 2015; Bell & Barnes, 2000; Bell & Smith, 2004).  However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment;  16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina respectively.

The complete disappearance of the sea squirt Ascidiella aspera biocoenosis and ‘associated sponges’ in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014).   Ackers, (1992) noted that Dysidea fragilis incorporates sand grains.

Sensitivity assessment. Whilst 5 cm of deposition may bury some of the characterizing species, the biotope experiences moderate water flow and sediment is likely to be removed rapidly.  The biotope is sand scoured and occasional disposition events are likely to occur.  Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

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

Holme & Wilson (1985) examined the fauna in a tide-swept region of the central English Channel. Flustra foliacea dominated communities were subject to sediment transport (mainly sand) and periodic, temporary, burial (ca <5 cm).

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al., 1961 reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement.

Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered, but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals, while others may just have the lower section of the main stem covered (Hayward & Ryland, 1994).  Obelia dichotoma stems grow to 5 cm, while polysiphonic structures can reach up to 35 cm in height.  Halecium halecinum can grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994). 

Smothering by 30 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c).

Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig 1979), many encrusting sponges appear to be able survive in highly sedimented conditions, and in fact many species prefer such habitats (Schönberg, 2015; Bell & Barnes, 2000; Bell & Smith, 2004).  However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment;  16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina respectively.

The complete disappearance of the sea squirt Ascidiella aspera biocoenosis and ‘associated sponges’ in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014).   Ackers, (1992) noted that Dysidea fragilis incorporates sand grains.

Sensitivity assessment. The biotope occurs in sand scoured areas exposed to moderate water movement and deposited sediment would eventually be removed.  However, 30 cm of sediment would bury almost all characterizing species except for those on large boulders and would result in some mortality. Resistance is, therefore, assessed as ‘Medium’, resilience as ‘High’ and sensitivity as ‘Low’.

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) 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 evidence’ was found.

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

Stanley et al. (2014) studied the effects of vessel noise on fouling communities and found that the bryozoans Bugula neritina, Watersipora arcuate and Watersipora subtorquata responded positively.  More than twice as many bryozoans settled and established on surfaces with vessel noise (128 dB in the 30–10,000 Hz range) compared to those in silent conditions.  Growth was also significantly higher in bryozoans exposed to noise, with 20% higher growth rate in encrusting and 35% higher growth rate in branching species. No evidence could be found for the effects of noise or vibrations on the characterizing hydroids or sponges could be found.  The characterizing species are unlikely to be negatively affected by noise and resistance is therefore assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not Sensitive’.

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

Gili & Hughes (1995) reviewed the effect of light on a number of hydroids and found that there is a general tendency for most hydroids to be less abundant in well-lit situations, potentially due to increased competition with macroalgae.  Whilst hydroid larvae can be positively or negatively photoactic, the planulae of Nemertesia antennina show no response to light (Hughes, 1977).

Jones et al. (2012) compiled a report on the monitoring of sponges around Skomer Island and found that many sponges, particularly encrusting species, preferred vertical or shaded bedrock to open, light surfaces.   Flustra foliacea larvae are positively phototactic on release, swimming for only short periods (Hayward & Ryland, 1998), however, at the depths Flustra foliacea can occur, light may not be important.

Sensitivity assessment: Whilst sponges seem to favour shaded areas in which to settle, it is unlikely that changes at the benchmark pressure would result in mortality.  Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is, therefore, assessed as 'Not sensitive'. 

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

Barriers and changes in tidal excursion are 'Not relevant' to biotopes restricted to open waters.

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.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

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'.

Biological Pressures

 ResistanceResilienceSensitivity
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

Bugulina spp. are classed as fouling bryozoans, and may be found in the intake pipes of ships or power stations, and on ships hulls. The geographic distribution of Bugula spp. has been extended by transportation by shipping (Ryland, 1967). However, no information on transportation of Bugula turbinata was found.  Therefore, there is ‘No evidence’ on which to assess this pressure.

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

The high levels of scour in this biotope will limit establishment of all but the most scour resistant invasive non-indigenous species (INIS) from this biotope and no direct evidence was found for effects of INIS on this biotope.  A number of invasive bryozoans are of concern including  Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013).  However, there is currently ‘No evidence’ on which to assess this pressure.

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

Gochfeld et al. (2012) found that diseased sponges hosted significantly different bacterial assemblages compared to healthy sponges, with diseased sponges also exhibiting significant decline in sponge mass and protein content.  Sponge disease epidemics can have serious long-term effects on sponge populations, especially in long-lived, slow-growing species (Webster, 2007).  Numerous sponge populations have been brought to the brink of extinction including cases in the Caribbean with 70-95% disappearance of sponge specimens (Galstoff, 1942), the Mediterranean (Vacelet,1994; Gaino et al.,1992).  Decaying patches and white bacterial film were reported in Haliclona oculata and Halichondria panicea in North Wales, 1988-89, (Webster, 2007).  Specimens of Cliona spp. have exhibited blackened damage since 2013 in Skomer. Preliminary results have shown that clean, fouled and blackened Cliona all have very different bacterial communities. The blackened Cliona are effectively dead and have a bacterial community similar to marine sediments. The fouled Cliona have a very distinct bacterial community which may suggest a specific pathogen caused the effect (Burton, pers comm; Preston & Burton, 2015).  Hydroids exhibit astonishing regeneration and rapid recovery from injury (Sparks, 1972) and the only inflammatory response is active phagocytosis (Tokin & Yaricheva, 1959;1961, as cited in Sparks, 1972).  No record of diseases in the characterizing hydroids could be found. No evidence for disease in the characterizing bryozoans could be found.

Sensitivity assessment. Sponge diseases have caused limited mortality in some species in the British Isles, although mass mortality and even extinction have been reported further afield.  Whilst research is on-going into sponge disease in the UK, there is ‘No evidence’ of mass mortality in the important characterizing species.

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

Despite historic harvesting of the hydroid Sertularia cupressina in the Wadden Sea (Wagler et al., 2009), no evidence for harvesting of the characterizing hydroids could be found in the UK and targeted extraction is highly unlikely.  No evidence for commercial exploitation of bryozoans could be found.  Should removal of target species occur, the sessile, epifaunal nature of the characterizing species would result in little resistance to this pressure. Spongia officinalis (a Mediterranean species) has been targeted as a commercial species for use as bath sponges, although this species does not occur in the British Isles and no record of commercial exploitation of sponges in the British Isles could be found.  Hiscock (2003) stated that the greatest loss of Axinella dissimilis at Lundy might have been due to collecting during scientific studies in the 1970s. No indication of recovery was evident.  Axinella damicornis was harvested in Lough Hyne during the 1980s (for molecular investigations) and the populations were reduced to very low densities, which subsequently recovered very slowly, although they are now considered to be back to their original densities (Bell, 2007).  No evidence of targeted removal of the characterizing species could be found.

Sensitivity assessment

The characterizing species are sessile epifauna and would have no resistance to targeted extraction.  Based on the above observations, resistance is assessed as ‘None’ and resilience as ‘Medium’ with a resultant sensitivity of ‘Medium

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

This biotope may be removed or damaged by static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope.

Resistance is recorded as ‘Low’, resilience is recorded as ‘Medium’ and Sensitivity is ‘Medium’.

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Citation

This review can be cited as:

Readman, J.A.J., 2016. Sparse sponges, [Nemertesia] spp. and [Alcyonidium diaphanum] on circalittoral mixed substrata. 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/119

Last Updated: 31/03/2016