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 evidence on the temperature tolerance of the Stauromedusae, and Haliclystus auricula, in particular, was found. It is recorded on both sides of the Atlantic in the northern hemisphere and ranges from the Arctic Ocean, south to the Azores (Zagal, 2008; OBIS 2017). It also occurs on seaweeds on the lower intertidal, although often protected by the seaweed from direct sunlight and hence desiccation (Corbin, 1979).  Therefore, it is likely to resist a 2°C change in temperature for a year in UK waters. It is a summer annual (Corbin, 1979) so that long-term changes in temperature may affect its seasonal growth and reproduction but the environmental factors that result in its seasonal abundance are not known (Corbin, 1979; Miranda et al., 2012). No information was found on the effect, if any, of short-term acute temperature change (e.g. by 5°C).

Sensitivity assessment. Therefore, it is probably resistant to a 2°C change in temperature for a year in UK waters and a resistance of 'High' is recorded, albeit with 'Low' confidence. Hence, resilience is assessed as 'High' and the species is recorded as 'Not sensitive' at the benchmark level.

Temperature decrease (local)

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

Evidence

No evidence on the temperature tolerance of the Stauromedusae, and Haliclystus auricula, in particular, was found. It is recorded on both sides of the Atlantic in the northern hemisphere and ranges from the Arctic Ocean, south to the Azores (Zagal, 2008; OBIS 2017). It also occurs on seaweeds on the lower intertidal, although often protected by the seaweed from direct sunlight and hence desiccation (Corbin, 1979).  Therefore, it is likely to resist a 2°C change in temperature for a year in UK waters. It is a summer annual (Corbin, 1979) so that long-term changes in temperature may affect its seasonal growth and reproduction but the environmental factors that result in its seasonal abundance are not known (Corbin, 1979; Miranda et al., 2012). No information was found on the effect, if any, of short-term acute temperature change (e.g. by 5°C).

Sensitivity assessment. Therefore, it is probably resistant to a 2°C change in temperature for a year in UK waters and a resistance of 'High' is recorded, albeit with 'Low' confidence. Hence, resilience is assessed as 'High' and the species is recorded as 'Not sensitive' at the benchmark level.

Salinity increase (local)

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

Evidence

Haliclystus auricula is recorded from full salinity conditions. It also occurs on seaweeds on the lower intertidal, although often protected by the seaweed from direct sunlight and hence desiccation that might result in localised increases in salinity (Corbin, 1979).  It is also found in lower shore rockpools, which are unlikely to experience the range of salinities typical of upper shore rockpools. It is unlikely to be exposed to hypersaline (>40) conditions but hypersaline effluents are probably detrimental. However, no evidence was found on which to base an assessment.

Salinity decrease (local)

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

Evidence

Haliclystus auricula is recorded from full salinity conditions. It also occurs on seaweeds on the lower intertidal, although often protected by the seaweed from direct sunlight (Corbin, 1979), although it may be exposed to rainfall and reduced salinity for short periods at low tide. It is also found in lower shore rockpools, which are unlikely to experience the range of salinities typical of upper shore rockpools. No evidence of salinity tolerance of the adult or other life stages was found. However, its position on the shore suggests that it could tolerate occasional rainfall at low tide but that a change from 'full' salinity to 'reduced' would be detrimental. Therefore, a resistance of 'Low' is suggested, with 'Low' confidence. Resilience is probably 'High' so sensitivity is assessed as 'Low'. 

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

Haliclystus auricula is recorded from the lower intertidal dominated by abundant fucoids and red seaweeds (e.g. Chondrus or Gigartina) and from seagrass beds (Corbin, 1979). It is, therefore, probably adapted to wave exposed to sheltered conditions in weak flow, that is, low to moderate energy habitats (see Connor et al., 2004). In addition, Clark (1878) noted that it needed some water flow in the laboratory.  Fenwick (2017) also remarked that the Stauromedusae in Mounts Bay, Cornwall were most likely to occur close to a constriction in the runnels and interlinked pools of the bay where the water flow was increased locally. Therefore, a significant change in water flow may be detrimental, partly as the species may be removed but mainly as the seaweed or plant substrata it requires would be removed or lost. However, a 1-2% change in water flow (the benchmark) is not likely to be significant. Therefore, a resistance of 'High' is recorded so that resilience is 'High' and the species is probably 'Not sensitive' at the benchmark level.

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

Haliclystus auricula is recorded from the lower intertidal dominated by abundant fucoids and red seaweeds and from seagrass beds (Corbin, 1979). Lower shore populations are likely to be affected by changes in emergence. A decrease in emergence will probably provide additional habitat. However, an increase in emergence (for a year) will increase the potential for desiccation and reduce the cover of its required macroalgal substratum. Therefore, a resistance of 'Low' is recorded. Resilience is probably 'High' so that sensitivity is assessed as 'Low'.

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

Haliclystus auricula is recorded from the lower intertidal dominated by abundant fucoids and red seaweeds (e.g. Chondrus or Gigartina) and from seagrass beds (Corbin, 1979). It is, therefore, probably adapted to wave exposed to sheltered conditions in weak flow, that is, low to moderate energy habitats (see Connor et al., 2004). Therefore, a significant change in wave action (e.g. to very exposed) may be detrimental, partly as the species may be removed but mainly as the seaweed or plant substrata it requires would be reduced in abundance or lost, presumably together with any resistant stages or early life stages, depending on the season. However, a 3-5% change in significant wave height (the benchmark) is not likely to be significant. Therefore, a resistance of 'High' is recorded so that resilience is 'High' and the species is probably 'Not sensitive' at the benchmark level.

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.

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.

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.

Radionuclide contamination

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

Evidence

No evidence was found.

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

This pressure is Not assessed.

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

Haliclystus and Calvadosia spp. have a preference for areas with flowing water, which suggests that low oxygen levels would be detrimental. However, no evidence was found.
 

Nutrient enrichment

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

Evidence

No information on the effects of nutrient enrichment on this species was found. However, its sensitivity is probably determined by the macroalgae and seagrass it requires for substratum.

Green algae species have been identified worldwide as species that occur in areas subject to increased nutrient input within the vicinity of sewage outfalls and at intermediately polluted sites (Littler & Murray, 1975; Bellgrove et al., 1997; Bellgrove et al., 2010).

Atalah & Crowe (2010) added nutrients to rockpools occupied by a range of algae including encrusting corallines, turfs of Mastocarpus stellatus, Chondrus crispus and Corallina officinalis and green and red filamentous algae. The invertebrates present were mostly Patella ulyssiponensis, the winkle Littorina littorea and the flat top shell Gibbula umbilicalis. Nitrogen and phosphorous enhancement were via the addition of fertilisers, as either 40 g/litre or 20 g/litre. The treatments were applied for seven month and experimental conditions were maintained every two weeks. The experimental treatments do not directly relate to the pressure benchmark but indicate some general trends in sensitivity.  The cover of green filamentous algae was significantly increased both by reduced grazing and increased nutrients, although the effect size was synergistically magnified by the combined effect of grazer removal and nutrients. Nutrient enrichment caused an absolute increase in the average cover of green filamentous algae of 19% (±3.9 S.E.) respect to the control treatments while the cover of red turfing algae was not affected by nutrient addition (Atalah & Crowe, 2010).

However, nutrient enrichment is cited as a threat to seagrass beds globally. A typical response to nutrient enrichment is a decline in seagrass populations in favour of macroalgae or phytoplankton (Baden et al., 2003).  The mechanisms responsible for seagrass decline under eutrophication are complex and involve direct and indirect effects relating to changes in water quality, smothering by macroalgal blooms (Den Hartog & Phillips, 2000), and competition for light and nutrients with epiphytic microalgae and with phytoplankton (Nienhuis, 1996). In the Mondego estuary (Portugal), eutrophication triggered serious biological changes, which led to an overall increase in primary production and to a progressive replacement of seagrass Zostera noltei beds by coarser sediments and opportunistic macroalgae (Cardoso et al., 2004a). Nutrients stimulate phytoplankton blooms that compete for nutrients but more importantly increase the turbidity and absorb light, reducing seagrass productivity (discussed in ‘changes in suspended solids’). In general terms, algae can out-compete seagrasses for water column nutrients since they have a higher affinity for nitrogen (Touchette & Burkholder, 2000). Short & Burdick (1996) found that excessive nitrogen loading stimulated the proliferation of algal competitors that caused shading and thereby stressed Zostera plants. Many seagrasses have a positive response to nitrogen and/or phosphorous enrichment (Peralta et al., 2003), but excessive loads can inhibit seagrass growth and survival, not only indirectly through light reduction resulting from increased algal growth but also directly in terms of the physiology of the seagrass. Direct physiological responses include ammonium toxicity and water column nitrate inhibition through internal carbon limitation (Touchette & Burkholder, 2000). In addition, Greening & Janicki (2006) found that in Florida, the USA, recovery of seagrass beds was incomplete 20 years after nutrient enrichment caused an eutrophication event.

Sensitivity assessment. Haliclystus and Calvadosia spp. can occur on a variety of macroalgae. In the intertidal hard rock habitats and rock pools, the macroalgal substratum for the species is unlikely to be removed by nutrient enrichment. Although the species may change in abundance adequate substratum is likely to be present, together with more grazers, on which the Stauormedusae may also feed (e.g. amphipods). Therefore, a resistance of ‘High’, and a resilience of ‘High’ are recorded and the species is probably ‘Not sensitive’ at the benchmark level in intertidal hard rock habitats.

The loss of seagrass beds worldwide has been attributed to nutrient enrichment, due in part to the likeliness of smothering by epiphytes, and the effects of reduced light penetration caused by eutrophication. Seagrass beds are regarded as highly sensitive to nutrient enrichment. Therefore, the Stauromedusae within seagrass beds may also be lost.  However, the benchmark of this pressure (compliance with WFD ‘good’ status) allows for a 30% loss of intertidal seagrass beds under the WFD criteria for good status. Therefore, at the level of the benchmark resistance of seagrass beds and hence the Stauromedusae that depend on the seagrass bed for substratum, to this pressure is assessed ‘Medium’. Resilience is assessed as ‘Medium’ so that sensitivity is assessed as ‘Medium’ in seagrass beds.

Organic enrichment

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

Evidence

No information on the effects of organic enrichment on this species was found. However, its sensitivity is probably determined by the macroalgae and seagrass it requires for substratum.

Organic enrichment and nutrient enrichment commonly co-occur, for example, sewage deposits or outputs from fish farms may enhance nitrogen and phosphorous and organic matter. Nutrient enrichment alters the selective environment by favouring fast-growing, ephemeral species such as Ulva lactuca and Ulva intestinalis (Berger et al., 2004, Kraufvelin, 2007). Rohde et al. (2008) found that both free growing filamentous algae and epiphytic microalgae can increase in abundance with nutrient enrichment. This stimulation of annual ephemerals may accentuate the competition for light and space and hinder perennial species development or harm their recruitment (Berger et al., 2003; Kraufvelin et al., 2007). Nutrient enrichment can also enhance fouling of Fucus fronds by biofilms (Olsenz, 2011). Nutrient enriched environments can not only increase algae abundance but the abundance of grazing species (Kraufvelin, 2007). High nutrient levels may directly inhibit spore settlement and hinder the initial development of Fucus vesiculosus (Bergström et al., 2003). Bellgrove et al. (2010) found that coralline turfs out-competed fucoids at a site associated with organic enrichment caused by an ocean sewage outfall. Therefore, in hard rock habitats, adequate substratum for the Stauromedusae is likely to remain so that resistance is recorded as 'High', resilience as 'High' and the species is probably 'Not sensitive' at the benchmark level in hard rock habitats.

Evidence on the effects of organic enrichment on Zostera species is limited but abundant for other seagrass species. Neverauskas (1987) investigated the effects of discharged digested sludge from a sewage treatment on Posidonia spp. and Amphibolis spp. in South Australia. Within 5 years the outfall had affected an area of approximately 1900 ha, 365 ha of which were completely denuded of seagrasses.  The author suggests that the excessive growth of epiphytes on the leaves of seagrasses was a likely cause for reduced abundance.  A subsequent study by Bryars & Neverauskas (2004) determined that 8 years after the cessation of sewage output, total seagrass cover was approximately 28% of its former extent. While these results suggest that seagrasses can return to a severely polluted site if the pollution source is removed, they also suggest that it will take many decades for the seagrass community to recover to its former state.

The effects of organic enrichment from fish farms were investigated on Posidonia oceanica seagrass beds in the Balearic Islands (Delgado et al., 1999).  The fish culture had ceased in 1991; however, seagrass populations were still in decline at the time of sampling.  The site closest to the former fish cages showed a marked reduction in shoot density, shoot size, underground biomass, sucrose concentration and photosynthetic capacities.  The shoot also had high P-concentration in tissues and higher epiphyte biomass compared to the other sites.  Since water conditions had recovered completely by the time of sampling, the authors suggest that the continuous seagrass decline was due to the excess organic matter remaining in the sediment (Delgado et al., 1999). It should be noted that coastal marine sediments where seagrasses grow are often anoxic and highly reduced due to the high levels of organic matter and slow diffusion of oxygen from the water column to the sediment. Seagrasses are adapted to these conditions but if the water column is organically enriched, plants are unable to maintain oxygen supply to the meristem and die fairly quickly. 

Evidence shows that seagrass beds found in proximity to a source of organic discharge were severely impacted with important losses of biomass.  Although no study was found on the British species, the evidence suggests that Zostera marina will be negatively affected by organic enrichment.  No evidence was found addressing the benchmark of this study, and the deposition of 100 gC/m²/year is considerably lower than the amount of organic matter discharged by sewage outlets and fish farms.  Therefore, resistance is assessed as ‘Medium’ to represent the partial loss of suitable substratum for the Stauromedusae.  Recovery is probably ‘Medium’ so that sensitivity is assessed as ‘Medium’ in seagrass beds.

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 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.

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

The species lives attached to macroalgae and plants (seagrass) in the lower littoral and shallow subtidal. In hard rock habitats, a change in seabed type from hard rock to sediment would result in the loss the most macroalgae and, hence, the species and any early life stages. Similarly, in sedimentary habitats, a change from sediment to hard rock substratum would result in loss of seagrass beds.  Based on the loss of species habitat (substratum), resistance is assessed as ‘None’.  The change is defined as permanent so that resilience is assessed as ‘Very low’ and sensitivity is assessed as ‘High’. Although no specific evidence is described, confidence in this assessment is ‘High’ due to the incontrovertible nature of this pressure.

Physical change (to another sediment type)

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

Evidence

This pressure is not 'Not relevant' where Haliclystus auricula occurs on macroalgae on hard rock habitats. However, change in the sediment type in seagrass beds would result in loss of the seagrass bed and, hence, suitable substrata from Haliclystus auricula and, presumably, loss of early life-stages. Therefore, a resistance of ‘None’ is recorded.  The change is defined as permanent so that resilience is assessed as ‘Very low’ and sensitivity is assessed as ‘High’ in seagrass beds.

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

This pressure is not 'Not relevant' where Haliclystus auricula occurs on macroalgae on hard rock habitats. However, in seagrass beds, the extraction of the sediment to 30 cm would result in the removal of Haliclystus auricula adults and early life-stages, its preferred substratum (seagrass and macroalgae), and the physical substratum (i.e the sediment). Therefore, a resistance of 'None' is suggested. Recovery will depend on the time take for the sediment to return and or suitable substratum (e.g. seagrass) to return and then be recolonized by Haliclystus auricula. In the case of seagrass beds, recovery is likely to be slow, if at all, and resilience is probably 'Very low' (see SS.SMp.SSgr.Zmar).  Therefore, sensitivity is assessed as ‘High’, where the species occurs in seagrass beds.

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

No evidence on the effect of abrasion (e.g. from trampling or vehicular access) of this species was found. Stauromedusae are small and soft-bodied and probably not physically robust. Similarly, their early life stages (e.g. stauropolyp, ‘microhydula’, encysted frustules, and overwintering planulae) are tiny and potentially susceptible to physical disturbance.  However, most importantly, they are likely to be lost if their macroalgal or seagrass substratum is damaged or removed by abrasion.

Most macroalgae are very flexible but not physically robust. The trampling of shores by humans will result in increased breakage of algal thalli, decreased thallus height and a net reduction in biomass (Tyler-Walters & Arnold, 2008). The effects of trampling are dependent on intensity, expressed as frequency and force per unit area of the impacting 'footprint' (see Liddle, 1997, Tyler-Walters & Arnold, 2008). Mechanical abrasion due to vehicles, jack-up-barges, or grounding vessels will exceed the abrasive 'intensity' of trampling by humans or livestock. Overall, the abundance of fucoids and red seaweeds are likely to be reduced (see reviews of LR.MLR.MusF.MytFR, LR.MLR.BF.FvesB for detail).

Similarly, seagrass beds are thought to be sensitive to abrasion due to trampling, vehicular access, potting, boating (wakes, anchor and mooring chains) and potting (see SS.SMp.SSgr.Zmar and LS.LMp.LSgr.Znol). A reduction in the abundance of seagrass will also result in loss of substratum in this species. Seagrass is also grazed by wildfowl, who can remove a significant proportion of the shoots.

Sensitivity assessment.  The sensitivity of Haliclystus auricula to abrasion is linked to the sensitivity of its preferred substratum; macroalgae in the intertidal and macroalgae and seagrass in the shallow subtidal. Therefore, if abrasion causes a significant reduction in the abundance of available substratum in the affected area, resistance is recorded as ‘Low’. It is difficult to know if removal of algal or plant substratum would also remove a significant proportion of the resistant or overwintering stages, although that seems likely. However, recovery is also dependent on the recovery of a suitable substratum. Therefore, a resilience of ‘Medium’ (2-10 years) is suggested to account for the return of suitable substratum (which is likely to vary between 1-2 years or 2-10 years depending on species) and subsequent recovery of the resident population from the surrounding area.  Hence, sensitivity is assessed as ‘Medium’. 

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

Hard rock is resistant of sub-surface penetration. Therefore, where this species' preferred substratum (i.e. macroalgae) occur on hard rock the pressure is recorded as 'Not relevant'.  The assessment for 'abrasion at the surface' only is, therefore, considered to equally represent sensitivity to this pressure. Please refer to ‘abrasion’ above for hard rock habitats.

Seagrass beds occur on sediments and are sensitive to penetrative activities that could remove the standing crop of leaves, and the rhizomes within the footprint of the activity, and hence, the substratum for this species (see LS.LMp.LSgr.Znol and SS.SMp.SSgr.Zmar). Recovery will, therefore, depend on the recovery of the seagrass bed, which is thought to be very slow and subsequent recolonization of the area by the Stauromedusae. Therefore, a resistance of 'None' is recorded to represent the loss of the substratum (seagrass), while resilience is recorded as 'Low' to represent to probably recovery of seagrass beds. Hence, sensitivity is assessed as 'High' in seagrass beds.

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

Haliclystus auricula and Calvadosia campanulata grow on macroalgae and seagrass.  In the intertidal, they grow on macroalgae on rock, and also in pools and runnels where some sediment may accumulate (Corbin, 1979; Fenwick, 2017). But no evidence on the effects of suspended sediments on the Stauromedusae was found. However, suspended sediments may adversely affect their preferred substratum.

Increases in the cover of sediment trapping, turf-forming red algae at the expense of canopy-forming species have been observed worldwide in temperate systems and have been linked to increased suspended solids linked to human activities worldwide (Airoldi, 2003).  For example, canopy-forming fucoids may be replaced by more sediment tolerant species such as Chondrus crispus.  As Haliclystus auricula and Calvadosia campanulata can grow on a variety of macroalgae an increase in suspended sediment may not be detrimental, especially as the macroalgae would probably recover within two years (i.e. 'High' resilience; for example  LR.MLR.MusF.MytFR).  Therefore, resistance is probably 'Medium', with a resilience of 'High' and a sensitivity of 'Low' on hard rock shores. 

However, seagrass beds are sensitive to changes in suspended sediments and, hence, turbidity. The decline of seagrass beds globally has been linked to increased turbidity (see SS.SMp.SSgr.Zmar). Therefore, where populations of this species occur in seagrass beds, resistance is probably 'Low' due to the loss of suitable seagrass substratum, and resilience is probably also 'Low' due to the time taken for seagrass beds to recover. Hence, sensitivity is probably 'High' if the population of Stauromedusae occurs in seagrass beds. 

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

Haliclystus auricula and Calvadosia campanulata grow on macroalgae and seagrass. In the intertidal, they grow on macroalgae on hard rock, and also in pools and runnels where some sediment may accumulate (Fenwick, 2017). No evidence on the effects of sedimentation or smothering on the Stauromedusae was found. However, sedimentation or smothering may adversely affect their preferred substratum.

Rocky shores. Increased abundance of algal turfs worldwide has been linked to sediment perturbations although not all the pathways and mechanisms of these effects are clear (Airoldi, 2003). However, even the most tolerant of organisms would eventually suffer from inhibition and mortality following smothering although the thresholds for these effects have has not been identified (Airoldi, 2003).In a review of the effects of sedimentation on rocky coast assemblages, Airoldi (2003) outlined the evidence for the sensitivity of coralline algae to sedimentation. The reported results are contradictory with some authors suggesting that coralline algae are negatively affected by sediments while others report that encrusting corallines are often abundant or even dominant in a variety of sediment impacted habitats (Airoldi, 2003 and references therein). Crustose corallines have been reported to survive under a turf of filamentous algae and sediment for 58 days (the duration of the experiment) in the Galapagos (species not identified, Kendrick, 1991). The crustose coralline Hydrolithon reinboldii has also been reported to survive deposition of silty sediments on subtidal reefs off Hawaii (Littler, 1973).

Atalah & Crowe (2010) added sediment to rockpools in controlled experiments. The rockpools were occupied by a range of algae including encrusting corallines, turfs of Mastocarpus stellatus, Chondrus crispus and Corallina officinalis and green and red filamentous algae. The invertebrates present were mostly Patella ulyssiponensis, the winkle Littorina littorea and the flat top shell Gibbula umbilicalis. Sediment treatment involved the addition of a mixture of coarse and fine sand of either 300 mg/cm²/month or 600 mg/cm² every 15 days (the depth of sediment was not reported). The experimental treatments do not directly relate to the pressure benchmark but indicate some general trends in sensitivity. In the pools, the chronic addition of both levels of sediment led to a significant decrease in grazers and crustose coralline algae also decreased. Sedimentation had no significant effect on the cover of green filamentous algae (Ulva sp.) but led to an increase in the mean cover of red turfing algae (Mastocarpus stellatus and Chondrus crispus and Corallina officinalis) from 11.7% (±1.0 S.E.) in controls to 26.1% (±4.7 S.E.) in sedimented assemblages, but there were no differences between the two levels of sedimentation. The cover of red filamentous algae (Ceramium spp. Gelidium spp.) was also significantly increased in the sedimentation experiments. The experimental results support the general trend of greater sensitivity of grazers and encrusting corallines to sedimentation than turf-forming algae.

Seagrass beds. Several studies have documented the deterioration of seagrass meadows by smothering due to excessive sedimentation. Consequences of enhanced sedimentation for seagrass beds depend on several factors such as the life history stage as well as the depth and timing of burial.

Early life stages of seagrass, smaller in size than adult plants, are most vulnerable to this pressure as even a small load of added sediment will lead to the complete burial. Vermaat et al. (1997) found that adult Zostera marina in the Dutch Wadden Sea was able to cope with sedimentation rates between 2 and 13 cm per year as the plant has the capacity to elongate vertical stems enabling it to raise the leaf canopy above the sediment load. A study in the USA, however, observed mortality of over 50% of plants of Zostera marina in field burial treatments of 4 cm (corresponding to 25% of plant height) for 24 days (Mills & Fonseca, 2003). Plants buried 75% or more of their height (16 cm) experienced 100% mortality indicating a low resistance of Zostera marina to burial. The differences observed between these two studies were probably caused by different phenotypes adapted to local conditions.

Sensitivity assessment.  On intertidal rocky shores exposed to wave action, 5 cm of deposited sediment is unlikely to persist for more than a few tidal cycles. However, sediment may be retained on wave sheltered shores and in sheltered rockpools. The evidence summarized above suggests that suitable substrata for Haliclystus auricula, Calvadosia campanulata and Calvadosia cruxmelitensis would remain in pools and on sheltered shores.  However, there is no information on the resistance of the stauromedusae themselves or their early life stages, which might be damaged or suffocated by a layer of sediment. Therefore, a resistance of ‘Medium’ is suggested, with a resilience of ‘High’ and sensitivity is assessed as ‘Low’ on hard rock shores.  However, seagrass beds may be adversely affected even under a sediment layer of only 5 cm, which will be retained in the sheltered conditions. Therefore, a resistance of ‘Low’ is suggested due to the potential loss of seagrass substratum, and a resilience of ‘Medium’ to represent to time take for the seagrass bed to recover.  Hence, sensitivity is assessed as ‘Medium’ where the population occurs in seagrass beds.

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

As above the duration of smothering by 30 cm of deposited material is dependent on the wave exposure of the shore. In wave exposed conditions the deposit is likely to be removed within a few tidal cycles while it may remain on sheltered shores and in sheltered rock pools.  The evidence summarized above suggests that suitable substrata for Haliclystus auricula, Calvadosia campanulata and Calvadosia cruxmelitensis would remain in pools and on sheltered shores (see evidence under ‘siltation (light)’ above).  However, there is no information on the resistance of the stauromedusae themselves or their early life stages, which might be damaged or suffocated by a layer of sediment. Therefore, a resistance of ‘Medium’ is suggested, with a resilience of ‘High’ and sensitivity is assessed as ‘Low’ on hard rock shores.  However, seagrass beds may be adversely affected by a sediment layer of 30 cm, which will be retained in the sheltered conditions (see evidence under ‘siltation (light)’ above). For example, Zostera marina experienced 100% mortality under 16 cm of sediment. Therefore, a resistance of ‘None’ is suggested due to the potential loss of seagrass substratum, and a resilience of ‘Very low’ to represent to time take for the Seagrass bed to recover.  Hence, sensitivity is assessed as ‘High’ where the population occurs in seagrass beds.

Litter

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

Evidence

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

Underwater noise changes

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

Evidence

No evidence was found

Introduction of light or shading

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

Evidence

Otto (1978) noted that spawning in Haliclystus stejnegeri was induced by exposure to light after an 8-hour dark period, while spawning was more intense in Haliclystus salpinx rather than induced under the same light regime (Otto, 1978). However, this observation does not provide any evidence about the response of Stauromedusae to increased light or shading in the natural environment. Therefore, no assessment was made.

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

Not relevant - this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Adult Haliclystus auricula are sedentary and unlikely to move far, and their early life stages are also benthic and non-motile (Clark, 1878; Miranda et al., 2012).  The dispersal of larval stages or propagules is not considered under the pressure definition and benchmark.

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

Not relevant’ to seabed habitats.  NB. Collision by interaction with bottom towed fishing gears and moorings are addressed under ‘surface abrasion’.

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 Stauromedusae are sedentary and do not exhibit an escape response. Visual disturbance by passing vessels or humans at the sea surface is probably '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 of translocation, breeding or species hybridization was found.

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 was found to suggest a positive or negative interaction between non-indigenous invasive species and Haliclystus auricula.

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 of microbial or other pathogens was found.

Removal of target species

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

Evidence

Not relevant. This species is not subject to a targetted commercial or recreational fishery.

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

Removal of macroalgae or seagrass would directly affect the population of Stauromedusae by removal of its substratum.  Recovery would depend on the recovery of a suitable substratum, which in the case of macroalgae could take between 1-2 or 2-10 years depending on species (for example see LR.MLR.MusF.MytFR). Hence a resistance of 'Low' is suggested, with a resilience of 'Medium' and a sensitivity of 'Medium' on hard rock shores. However, incidental removal of seagrass would take longer to recover, i.e. over 10 years (see SS.SMp.SSgr.Zmar).  Therefore, a resistance of 'Low' is suggested' with a resilience of 'Low' and a sensitivity of 'High' in seagrass beds.