The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

No direct experimentation evidence on the effects of changing temperature on Acanella arbuscula was found but, evidence is available from the known distribution of the species and from some limited in situ growth measurements. Beazley & Kenchington (2012) state that Acanella arbuscula is found from Newfoundland to southeast Brazil in the Western Atlantic and from Iceland to the Mid Atlantic Ridge and Morocco in the North East Atlantic. In the North Atlantic, Acanella arbuscula has been recorded in Canadian, south Icelandic, Faroese and British waters, but not on the Norwegian shelf. This suggested that it is potentially limited in distribution by the cold (<0.5°C) water north of the Greenland-Scotland Ridge and north of the Davis Strait (Buhl-Mortensen et al., 2015). The authors use this information, combined with modelled near bottom temperatures, to infer a temperature preference of approximately 2-6°C, although specimens of Acanella arbuscula from British waters have been found at temperatures of 8-10°C (Buhl-Mortensen et al., 2015). Furthermore, investigations into the growth of bamboo corals, found increased internode radial growth with temperatures between 2-5°C, with rates plateauing at temperatures beyond ~5°C, with some regional differences (Thresher, 2009). Acanella arbuscula has also been recorded living at temperatures of 3.5°C in the north-west Atlantic and the Mid-Atlantic Ridge (Baker et al., 2012; Mortensen et al., 2008).

Sensitivity assessment.  Based on the wide-ranging distribution of the species and the presence of Acanella arbuscula at 8-10°C in British waters, up to 4°C higher than its inferred temperature preference of 2-6°C, an increase in temperature at the benchmark level of 5°C for one month or 2°C for one year, may not have a significant effect on the species in British and Irish waters. However, as a bathyal species, it is likely to have less acclimation to larger temperature increases. Hence, resistance is conservatively assessed as ‘Medium’.  Therefore, resilience is assessed as ‘Low’ and overall sensitivity as ‘Medium’. 

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

While no direct experimentation evidence on the effects of changing temperature on Acanella arbuscula has been undertaken, evidence is available from the known distribution of the species and from some limited growth measurements in situ. Beazley & Kenchington (2012) indicate that Acanella arbuscula is found from Newfoundland to southeast Brazil in the Western Atlantic and from Iceland to the Mid Atlantic Ridge and Morocco in the North East Atlantic. In the North Atlantic, Acanella arbuscula has been recorded in Canadian, S. Icelandic, Faroese and British waters, but not the Norwegian shelf, potentially limited in distribution by the cold water north of the Greenland-Scotland Ridge and north of the Davis Strait (Buhl-Mortensen et al., 2015). The authors use this information, combined with modelled near-bottom temperatures, to infer a temperature preference of approximately 2-6°C with occasional records at 1-3°C. Specimens of Acanella arbuscula from British waters have also been found at temperatures of 8-10°C (Buhl-Mortensen et al., 2015). Furthermore, investigations into the growth of bamboo corals, found increased internode radial growth with temperatures between 2-5°C, with rates plateauing at temperatures beyond ~5°C, with some regional differences (Thresher, 2009).

Sensitivity assessment.  Based on the wide-ranging distribution of the species, a decrease in temperature at the benchmark level of 5°C for one month or 2°C for one year, may not have a significant effect on the species. However, as the species is known to not occur in the colder (<0.5°C) waters of the Norwegian shelf it may have a higher sensitivity to colder temperatures. Furthermore, as a bathyal species, it is unlikely to have less acclimation to larger temperature decreases. Hence, resistance is conservatively assessed as ‘Medium’, resilience is assessed as ‘Low’ and overall sensitivity is assessed as ‘Medium’.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Yesson et al. (2012) modelled habitat suitability for Octocorallia species, including the suborder Calcaxonia, in which Isididae is a Family. They found salinity had the most predictive power for Calcaxonia out of nine chemistry variables. Frequency distribution plots from the study indicated a higher frequency of Calcaxonia species at around 34-36 psu (Yesson et al., 2012). The depths at which Acanella arbuscula occurs is typified by full salinity (30-35 psu) seawater and, they are not exposed to other salinities. Acanella arbuscula was also observed at several sites on the Mid-Atlantic Ridge by ROV, which measured the salinity between 35 – 35.2 (Mortensen et al., 2008).

Sensitivity assessment. Acanella arbuscula is found at depths where salinity rarely fluctuates and is therefore unlikely to be able to tolerate changes in salinity. If exposed to increased salinity, resistance is likely to be ‘Low’. Therefore,  resilience is assessed as ‘Low’, so the overall sensitivity is assessed as ‘High’.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

When undertaking habitat suitability modelling for Octocorallia species, including the suborder Calcaxonia, for which Isididae is a Family, Yesson et al. (2012) found salinity had the most predictive power for Calcaxonia out of nine chemistry variables. Frequency distribution plots from the study indicated a higher frequency of Calcaxonia species at around 34-36 psu (Yesson et al., 2012). The depths at which Acanella arbuscula occurs is typified by full strength (30-35 psu) seawater and as such, they are not exposed to other salinities. Acanella arbuscula was also observed at several sites on the Mid-Atlantic Ridge by ROV which measured the salinity between 35 – 35.2 (Mortensen et al., 2008).

Sensitivity assessment. Acanella arbuscula is found at depths where salinity rarely fluctuates and is therefore unlikely to be able to tolerate changes in salinity. If exposed to decreased salinity, resistance is likely to be ‘Low’. Therefore, resilience is assessed as ‘Low’, so the overall sensitivity is assessed as ‘High’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Morris et al. (2013) identified reduced flow regimes in the Whittard Canyon to potentially prevent erosion of sediments, and enable Acanella sp. to form in large patches in these areas. This suggestion could indicate that Acanella sp. prefers lower energy environments. Bottom current speeds of 5-10 cm/s (0.05-0.1 m/s) were observed in an area of bamboo corals Southwest of the Grand Banks of Newfoundland, Canada (Zedel & Fowler, 2009) and the presence of the bamboo coral reduced the bottom layer velocities of the benthic boundary layer (Zedel & Fowler, 2009).

Evidence on Isididae corals from ROV dives at the Manihiki Plateau in the SW Pacific found that current-facing ridge flanks had a higher abundance of Isidid coral coverage, due to the flow of organic material optimising feeding than the leeward side of the current flow where only some individuals were observed (Bashah et al., 2020). However, no corals were seen on areas of mobile sediments, which was thought to be partly due to stronger currents inhibiting the settling of coral larvae (Bashah et al., 2020).

Sensitivity assessment. As Acanella arbuscula assemblages are found in low flow regimes, they may be sensitive to increases in water flow at the benchmark level. However, the scale of this effect is unknown. As such, resistance is conservatively assessed as ‘Medium’, resilience is ‘Low’ and overall sensitivity is assessed as ‘Medium’. 

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Acanella arbuscula assemblage biotopes are found at mid and lower bathyal depths, recorded from 900 - 2500 m, and will not be affected by changes in the emergence regime. Therefore, the assessment at the pressure benchmark is ‘Not relevant’.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Acanella arbuscula assemblage biotopes are found at mid and lower bathyal depths, recorded from 900 - 2500m. They occur at depths at which even the wave action generated by storm conditions is unlikely to penetrate. Therefore, the assessment for the biotopes at the pressure benchmark is ‘Not relevant’.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is ‘Not assessed’ but the evidence is presented where available.

Although there is no evidence of the impacts of heavy metals in Acanella arbuscula there is evidence of the bioaccumulation of Cu, Zn, and Cd in Subergorgia suberosa, also in the Alyconacea order. Subergorgia exposed to heavy metal polluted water (13.67 µg/L Cu, 2.26 µg/L Zn, and 12.72 µg/L Cd) were subject to increased stress and, as a result, displayed necrosis, increased mucus secretion, tissue expansion, and increased mortality (Chan et al., 2012). In addition, a reduction in polyp extensions was seen which was likely due to the high concentrations of Cu and Zn (Chan et al., 2012). Reichelt-Brushett & Harrison (2005) found that exposure to dissolved metals impaired the success of fertilization in Scleractinian corals, so similar responses may be seen in other corals. 

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is ‘Not assessed’ but the evidence is presented where available.

There is a wealth of evidence surrounding hydrocarbon contamination on cold-water corals from the Gulf of Mexico after the Deepwater Horizon oil spill, although not specifically on Acanella arbuscula (Boehm & Carragher, 2012; Cordes et al., 2016; Fisher et al., 2014; Goodbody-Gringley et al., 2013; Hsing et al., 2013; Ruiz-Ramos et al., 2015; White et al., 2012). Potential effects of hydrocarbon contamination were observed in the Gulf of Mexico on a range of coral taxa, primarily scleractinians (Porites, Madrepora and Lophelia) as well as the octocorals Paramuricea and Callogorgia. Effects included tissue loss, sclerite enlargement, excess mucus production and a covering of hydrocarbon-based flocculate, as well as potential impacts on the larval stages of cold-water corals, which has significant implications for the resilience of the biotope to hydrocarbon contamination. In addition, there was concern over the longevity of pollution effects combined with the longevity and slow growth rate of cold-water coral communities. 

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is ‘Not assessed’ but the evidence is presented where available.

Following the Deepwater Horizon oil spill, millions of litres of chemical oil dispersants were used to aid in clean up operations, with a large quantity applied at depth. Laboratory studies on the impact of the dispersant Corexit 9500 on Swiftia exserta (from the same order as Acanella arbuscula) determined the concentration required to kill 50% of the sample within 96 hours (96 hr LC₅₀) was 70.27 mg/l for the dispersant alone but decreased to 41.04 mg/l when crude oil was also present (Frometa et al., 2017). The dispersant and oil-dispersant mix were observed to be more toxic to specimens than the oil alone, in Paramuricea type B3 and Callagorgia delta (DeLeo et al., 2016).

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence could be found regarding the effect of radionuclide contamination on the Acanella arbuscula assemblage biotopes. This pressure benchmark is assessed as ‘No Evidence’.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

No evidence could be found for the effects of the introduction of other substances on the Acanella arbuscula assemblage biotopes. This pressure benchmark is assessed as ‘No Evidence’.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Deoxygenation is known to be deleterious to benthic fauna, especially in light of globally changing oceanic conditions (Bijma et al., 2013). The bamboo coral Isidella tentaculum has been recorded living in conditions of low oxygen (0.63 ml/l (ca 0.88 mg/l) and 0.4 (ca 0.56 mg/l)  ± 0.2 ml/l (ca 0.28 mg/l)), with very low abundances observed within the severe hypoxic oxygen minimum zone core (<0.5 ml/l (ca 0.7 mg/l)) of north-east Pacific seamounts (Ross et al., 2020).

However, no specific evidence on the effects of deoxygenation on Acanella arbuscula was available. This pressure is therefore assessed as ‘No evidence’.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Bamboo corals are formed of high-Mg calcite (7-10 mol% MgCO₃ (Noé & Dullo, 2006)₎ internodes with proteinaceous gorgonin nodes around 4-8 mm thick (Roark et al., 2005). Radiocarbon analysis of calcite from bamboo coral samples in the Gulf of Alaska indicated that the inorganic carbon is precipitated from the surrounding seawater (Roark et al., 2005).

No specific evidence on the effects of nutrient enrichment on Acanella arbuscula assemblages could be found. Nevertheless, by definition, the biotopes are considered 'Not sensitive' at the pressure benchmark, which assumes compliance with good status as defined by the WFD.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Analysis of radiocarbon data of tissue and gorgonin samples from specimens of Isididae species at the Warwick Seamount in the Gulf of Alaska indicated that they rely primarily on particulate organic matter (POM) from the sea surface as a source of organic carbon (Roark et al., 2005). The study also identified that larger bamboo coral individuals likely grew faster than smaller ones due to the increased surface area available to the currents delivering the organic matter (Roark et al., 2005). Furthermore, Sherwood et al. (2008) found that ¹³C and ¹⁵N data indicated that sinking POM and subsequent resuspension was the major source of organic matter for Acanella arbuscula.

Sensitivity assessment.  Acanella arbuscula requires suspended organic matter to feed and support growth. The species is, therefore, likely to benefit from organic enrichment, meaning that resistance is assessed as ‘High’ so that resilience is ‘High’, and the biotopes are considered ‘Not sensitive’ at the benchmark level. However, due to very limited evidence, confidence in this assessment is Low.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have no resistance to this pressure and to be unable to recover from a permanent loss of available habitat. Hence, resistance is assessed as ‘None’, resilience as ‘Very low’ and sensitivity is assessed as ‘High’.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Acanella arbuscula is known to live on soft sediments (Mortensen et al., 2008) rather than on hard substrata like many cold-water corals, although Baker et al. (2012) recorded Acanella arbuscula in deep waters off Newfoundland, Canada on a range of bottom types. Furthermore, evidence from the Whittard Canyon found that whilst Acanella arbuscula dominated sedimentary habitats, it was also found in areas of sediment on slopes and mixed sediment or rock, albeit at reduced abundance, as the availability of suitable substrate to anchor into was reduced (Morris et al., 2013).   

Sensitivity assessment. If the sediment that characterizes the biotope was replaced with rock substratum, this would represent a fundamental change to the physical character of the biotope. Although evidence shows that Acanella arbuscula can occur on mixed substrata and rock, these were seen at reduced abundances, and this change in substratum would cause a reclassification of the biotope (i.e., loss of the biotope). Therefore, resistance is assessed as ‘None’, resilience as ‘Very low’, and sensitivity is assessed as ‘High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Acanella arbuscula occurs on soft substrata, using holdfast structures to anchor to the sediment (Baker et al., 2012; Wareham & Edinger, 2007). In Atlantic Canada, Mortensen et al. (2006) identified Acanella arbuscula only on mud habitats and on the Mid Atlantic Ridge, the species was also only found on soft sediments (Mortensen et al., 2008). However, Baker et al. (2012) recorded Acanella arbuscula in deep waters off Newfoundland, Canada on a range of bottom types and evidence from the Whittard Canyon found that whilst Acanella arbuscula dominated sedimentary habitats.  It was also found in areas of sediment on slopes and mixed sediment or rock, albeit at reduced abundance as the availability of suitable substratum was reduced (Morris et al., 2013).

Sensitivity assessment. A change in Folk class from ‘mud and sandy mud’ to ‘mixed’ or ‘sand and muddy sand’ would likely affect the characterizing species (Acanella arbuscula), which appears to prefer muddy substrata. In addition, this change in substratum may represent a fundamental change in the character of the biotope and would result in the reclassification of the biotope. Resistance is, therefore, assessed as ‘Low’, resilience as ‘Very low’ and the biotopes are considered to have ‘High’ sensitivity to a change in seabed type (by one Folk class).

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

As Acanella arbuscula assemblages are characterized by species with no mobility, removal of the substratum at the benchmark pressure would destroy the biotope within the affected area.  Since the removal of substratum pressure is likely to destroy the biotope, with slow recovery, the resistance of Acanella arbuscula assemblages is assessed as ‘None’, resilience as ‘Low’ and overall sensitivity to the pressure is assessed as ‘High’.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Yoklavich et al. (2018) found that, following groundfish trawling in the north-west Pacific, around 4% of corals were damaged or disturbed, of which 87% were bamboo corals occurring on rock substratum, identified as Isididae, Keratoisis spp. and Isidella tentaculum  Twenty per cent of the disturbed bamboo corals were sheared off close to the seabed. Other damage included uprooted, broken and dead bamboo corals (Yoklavich et al., 2018). On the southwest Grand Banks, in the northwest Atlantic, Edinger & Sherwood (2009) found dislodged colonies of Acanella arbuscula in a heavily trawled area.

Substantial evidence on the effects of abrasion from trawling on the Isididae coral Isidella elongata, which may be synonymous to Acanella arbuscula, is available from studies from the Mediterranean. For example, Maynou & Cartes, (2012) found trawling Isidella elongata caused direct impacts to the biological assemblages present, with the removal of corals, decreased invertebrate species diversity and reduction in fisheries production. Pierdomenico et al. (2018) found Isidella elongata had significantly higher abundances in areas of low trawling intensity and smaller colonies were more common than larger colonies in areas of higher trawling intensity. However, areas of low trawling intensity also contained a significant abundance of dead colonies (>2 colonies/m²) covered with high densities of epibionts, suggesting that slight fishing pressure may not be enough to completely remove colonies and the damage from fishing gear may favour colonisation by epibionts.

The effects of trawling were also highlighted by Mastrototaro et al. (2013) on a study in the Mediterranean where high densities of Isidella elongata colonies were found on an area between two seamounts with a bottom trawling ban at 480 – 615 m depth. In comparison, an area of high trawling impact nearby had a much lower density of Isidella elongata colonies, which were mainly young or damaged with a low number of branches (Mastrototaro et al., 2013). 

Based on anecdotal evidence, the meadows of Isidella elongata described in 1930 (Bo et al., 2008) on Mediterranean bathyal mud habitats have been almost completely removed from trawlable areas. However experimental trawling in non-fished areas showed high quantities (Sardà et al., 2004). Isidella elongata was also shown to display no recovery after trawling activities ceased, with little or no difference in abundance of Isidella elongata between actively trawled seamounts and on seamounts after 15 years of protection from fishing activities (Goode et al., 2020) 

Sensitivity assessment. Based on substantial evidence suggesting significant impacts of abrasion from trawling on the similar and possibly synonymous genus Isidella, the resistance is assessed as ‘None’. Therefore, resilience is assessed as ‘Low’ and overall sensitivity to the pressure is assessed as ‘High’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Damage to the sub-surface seabed is likely to be mechanically similar to surface abrasion (see above) whilst having additional deleterious effects, including damage to the holdfast structures of Acanella arbuscula. The sessile, embedded nature of the characterizing species is also likely to increase its sensitivity to sub-surface seabed damage.

Sensitivity assessment. Based on substantial evidence suggesting significant impacts of abrasion from trawling on similar and possibly synonymous genus Isidella, the resistance is assessed as ‘None’. Therefore, resilience is assessed as ‘Low’ and overall sensitivity to the pressure is assessed as ‘High’

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Acanella arbuscula communities found on mud substrata could be subject to high sediment loads in high current flow events. However, no evidence on the effect of changes in suspended solids on Acanella arbuscula or any other deep-sea Isididae was found. 

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Increased sedimentation can damage deep-sea corals, potentially blocking feeding apparatus and congesting polyps (Hecker et al., 1980 cited in Wareham & Edinger, 2007). While no direct evidence is available on the effects of burial at the benchmark level on Isididae, it is likely that given their anchorage into the sediment, lack of mobility, and the relatively small size (<30 cm) of colonies (Baker et al., 2012; Morris, 2011), deposition of 5 cm of material will be deleterious to the characteristic species.

Roberts et al. (2000) noted that suspension feeders including Acanella arbuscula may suffer the effects of smothering from resuspended sediments from trawling gear over a wide area. However, they did not provide further evidence regarding the quantity of material or the impacts on the organism. Furthermore, while investigating the impacts of trawling on Isidella elongata, Pierdomenico et al. (2018) suggested that resuspended sediment from fishing gear could cause smothering and burial of coral polyps which may lead to tissue necrosis and reduce larval recruitment, survival, and settlement.

Sensitivity Assessment. The evidence suggests that as a filter feeder Acanella arbuscula will be negatively affected by an increase in sedimentation.  However, there were no clear records of specific impacts. Sediment may block polyps and smother new recruits reducing survival of adult and larval stages, with no evidence of mechanisms to remove deposited sediments. Therefore, resistance is assessed as ‘Low’, albeit with low confidence. Resilience is probably ‘Low’, and overall sensitivity is assessed as ‘High’. 

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Increased sedimentation is known to be a threat to deep-sea corals, potentially blocking feeding apparatus and congesting polyps (Hecker et al., 1980 cited in Wareham & Edinger, 2007). While no direct evidence is available on the effects of burial at benchmark pressure on deep-sea Isididae, it is likely that given their anchorage into the sediment, lack of mobility, and the relatively small size (<30 cm) of colonies (Baker et al., 2012; Morris, 2011) deposition of 30 cm of material will result in complete burial and loss of the characterizing species.

Roberts et al. (2000) noted that suspension feeders including Acanella arbuscula may suffer the effects of smothering from resuspended sediments from trawling gear over a wide area, however, they did not provide further evidence regarding the quantity of material or the impacts to the organism. Furthermore, while investigating the impacts of trawling on Isidella elongata, Pierdomenico et al. (2018) indicated that resuspended sediment from fishing gear could cause smothering and burial of coral polyps which may lead to tissue necrosis and reduce larval recruitment, survival, and settlement.

Sensitivity Assessment. The evidence suggests that as a filter feeder Acanella arbuscula will be negatively affected by an increase in sedimentation however there were no clear records of specific impacts. Sediment may block polyps and smother new recruits reducing survival of adult and larval stages, with no evidence of mechanisms to remove deposited sediments. Therefore, resistance is assessed as ‘Low’, resilience as ‘Low’, and overall sensitivity is assessed as ‘High’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

A number of studies (e.g. Chapron et al., 2018; Courtene-Jones et al., 2019, 2017; La Beur et al., 2019; Taylor et al., 2016) have shown that microplastics are ingested by deep-sea invertebrates. However, research into the effects of litter and plastics in the deep sea is at an elementary stage (Cau et al., 2017; Chapron et al., 2018). Hence, no evidence was available on the effects of litter and/or microplastics on Acanella arbuscula. This pressure is, therefore ‘Not assessed’.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence was found regarding the effect of electromagnetic changes on the characterizing species. 

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

The Acanella arbuscula assemblage biotopes are characterized by invertebrates with no means to detect noise and as such will not be affected by changes in underwater noise. This pressure benchmark is assessed as ‘Not relevant'.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

The Acanella arbuscula assemblage biotopes are characterized by invertebrates with no means to detect light and as such will not be affected by changes in incident light. This pressure benchmark is assessed as ‘Not relevant’.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Acanella arbuscula is a sessile species, permanently attached to the substratum so that dispersal is dependent on larval mobility. However, it is unclear if this species uses internal fertilization and locally released, low-motility, planulae (Lawson, 1991) or uses broadcast spawning of gametes (Beazley & Kenchington (2012). There is some evidence its distribution is potentially restricted by the cold water north of the Greenland–Scotland Ridge and north of the Davis Strait, which suggests that its larvae are not transported widely in upper water masses (Buhl-Mortensen et al., 2015). However, this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal but larval dispersal is not considered under the pressure definition and benchmark. Therefore, this pressure benchmark is assessed as ‘Not relevant’.  

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

The Acanella arbuscula assemblage biotopes are characterized by sessile invertebrates and as such will not be affected by an increased risk in collision with artificial structures, shipping etc.  This pressure benchmark is assessed as ‘Not relevant’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

The Acanella arbuscula assemblage biotopes are characterized by invertebrates that are not reliant on vision and as such will not be affected by visual disturbance. This pressure benchmark is assessed as ‘Not relevant’. 

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

No evidence could be found on the effect of the translocation or introduction of genetically distinct organisms on the Acanella arbuscula assemblage biotopes.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

No evidence could be found regarding the effect of the 'Introduction or spread of invasive non-indigenous species' on the characterizing species. 

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

No evidence could be found for the effects of the introduction of microbial pathogens or disease vectors on the characterizing species.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

The characterizing species, Acanella arbuscula, is not a commercially or recreationally targeted species. This pressure is assessed as ‘Not Relevant’. 

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

It is likely that Acanella arbuscula is highly sensitive to removal through bycatch. Evidence from research trawling has shown that Acanella sp. can be captured in high densities (Jørgensen et al., 2013; Mortensen et al., 2008; Murillo et al., 2016, 2008).

In addition, a study from the Eastern Pacific found that Isididae corals were especially prominent in trawl bycatch, and video data revealed additional damage to corals left on the seabed including breakage, uprooting or shearing of the coral at the base (Yoklavich et al., 2018). Yoklavich et al. (2018) found that following groundfish trawling in the north-west Pacific around 4% of corals were damaged or disturbed, of which 87% were bamboo corals occurring on rock substratum, identified as Isididae, Keratoisis spp. and Isidella tentaculum  Other damage included uprooted, broken and dead bamboo corals (Yoklavich et al., 2018). This additional information from video surveys revealed that the effects of bycatch were not confined to specimens recovered with fishing gear. Whilst the disturbed corals from this study were living on rock, rather than the sediment characterizing this biotope, it still provides a useful insight into the effects of fishing gear on deep-sea Isididae corals.

Acanella arbuscula was also observed in by-catch in the north-west Atlantic from a variety of fishing gear, including pots, gillnets, longline, and trawls; with trawls contributing the greatest quantities of Acanella arbuscula (Wareham & Edinger, 2007). In addition, research into the effects of other fishing methods, such as longlining (Mytilineou et al., 2014; Orejas et al., 2009; Sampaio et al., 2012) and gillnetting (Clark et al., 2016; Fosså et al., 2002) have also found negative impacts.

Gilkinson & Edinger (2009) observed dislodged colonies of Acanella arbuscula in an area of heavy trawling. Shallower areas in the Desbarres Canyon on the southwest Grand Banks off Canada, where extensive trawl scours and fishing disturbance was evident, showed overturned rocks and uprooted and overturned Acanella arbuscula corals, lying on the sea bed. In addition, as Acanella is rooted into the sediment, is sessile and lives in stable deep waters, it is unlikely to survive being discarded from catches.

Sensitivity assessment. Acanella arbuscula lives on the surface of the seabed so is likely to be affected by the removal of non-target species. Based on evidence suggesting removal of Acanella sp. in research trawl gear, and prominence of Isididae corals in trawl bycatch, the resistance is assessed as ‘None’.  Therefore, resilience is assessed as ‘Low’ and overall sensitivity to the pressure is assessed as ‘High’.