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 information on temperature tolerance in Atrina fragilis was found, although it has been suggested that changes in seawater temperature are likely to affect larval recruitment patterns (Anon., 1999c). Atrina spp. are annual episodic or protracted spawners with one or two periods of spawning often associated with temperature and food supply (Chávez-Villalba et al., 2022). Stirling et al. (2018) concluded that Atrina fragilis in west Scotland exhibited peaks of spawning in summer and winter with low levels of spawning throughout the year. Wang et al. (2017) reported that Atrina pectinata in the Bohai Sea, China had a single spawning event; gametogenesis began in October, completed between June and July, and spawning occurred in August when temperatures and food availability were at their highest. 

The tropical pen shell Atrina maura was found to reach maturity more quickly at higher temperatures, taking only one month (normal maturation at lower temperatures of 20°C takes two months). However, with higher temperatures, oocytes are of poor quality than at cooler temperatures (Rodriguez-Jaramillo, 2001). Similarly, Leyva-Valencia et al. (2001) reported that activity, ingestion rates, scope for growth, and growth rates were highest at 29°C in juvenile Atrina maura and suggested an optimal temperature of 29°C or higher. No mortality occurred between 19 and 30°C, 10% mortality at 32°C and the authors reported a 96-hour LD₅₀ of 33.2°C. In flume experiments, Arrieche et al. (2010) noted that specimens of the tropical Atrina maura survived daily fluctuations of 8°C between 13°C and 33°C.  OBIS (July 2022) included records of Atrina fragilis from sites where sea surface temperature ranged from 5 to 20°C although most records were recorded from 10-15°C. 

Subtidal species such as Atrina fragilis are likely to exhibit lower temperature tolerance than intertidal species and are not likely to be resistant to rapid temperature change indicated in this benchmark. However, they occur from the Mediterranean to the Shetland Isles and are probably resistant to the range of temperatures that occur within that range.

Therefore, Atrina fragilis is probably resistant to a long-term change in temperature of 2°C for a year (see benchmark).  But shallow subtidal and sublittoral fringe populations may be adversely affected by short-term changes in temperature of 5°C for a month (see benchmark).  Therefore, a resistance of 'Medium' is recorded to represent the loss of the upper shore or shallow populations, albeit with 'Low' confidence. Resilience is probably 'Low' and sensitivity is assessed as 'Medium', albeit with 'Low' confidence. 

Temperature decrease (local)

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

Evidence

No information on temperature tolerance in Atrina fragilis was found, although it has been suggested that changes in seawater temperature are likely to affect larval recruitment patterns (Anon., 1999c). No information on temperature tolerance in Atrina fragilis was found, although it has been suggested that changes in seawater temperature are likely to affect larval recruitment patterns (Anon., 1999c). Atrina spp. are annual episodic or protracted spawners with one or two periods of spawning often associated with temperature and food supply (Chávez-Villalba et al., 2022). Stirling et al. (2018) concluded that Atrina fragilis in west Scotland exhibited peaks of spawning in summer and winter with low levels of spawning throughout the year.  Wang et al. (2017) reported that Atrina pectinata in the Bohai Sea, China had a single spawning event; gametogenesis began in October, completed between June and July, and spawning occurred in August when temperatures and food availability were at their highest. 

The tropical pen shell Atrina maura was found to reach maturity more quickly at higher temperatures, taking only one month (normal maturation at lower temperatures of 20°C takes two months). However, with higher temperatures, oocytes are of poor quality than at cooler temperatures (Rodriguez-Jaramillo, 2001). Similarly, Leyva-Valencia et al. (2001) reported that activity, ingestion rates, scope for growth, and growth rates were highest at 29°C in juvenile Atrina maura and suggested an optimal temperature of 29°C or higher. No mortality occurred between 19 and 30°C, 10% mortality at 32°C and the authors reported a 96-hour LD₅₀ of 33.2°C.  In flume experiments, Arrieche et al. (2010) noted that specimens of the tropical Atrina maura survived daily fluctuations of 8°C between 13°C and 33°C.  OBIS (July 2022) included records of Atrina fragilis from sites where sea surface temperature ranged from 5 to 20°C although most records were recorded from 10-15°C. 

Subtidal species such as Atrina fragilis are likely to exhibit lower temperature tolerance than intertidal species and are not likely to be resistant to rapid temperature change indicated in this benchmark. However, they occur from the Mediterranean to the Shetland Isles and are probably resistant to the range of temperatures that occur within that range.

Therefore, Atrina fragilis is probably resistant to a long-term change in temperature of 2°C for a year (see benchmark).  But shallow subtidal and sublittoral fringe populations may be adversely affected by short-term changes in temperature of 5°C for a month (see benchmark).  Therefore, a resistance of 'Medium' is recorded to represent the loss of the upper shore or shallow populations, albeit with 'Low' confidence. Resilience is probably 'Low' and sensitivity is assessed as 'Medium', albeit with 'Low' confidence. 

Salinity increase (local)

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

Evidence

Atrina fragilis occurs subtidally at full salinity but shallow subtidal populations may experience variable salinity. Dan Minchin (pers. comm.) suggested that Atrina fragilis may be exposed to reduced or variable salinities for brief periods.  A tropical pen shell Atrina maura, was found to have a wide range of halotolerance, from 16-50 (Leyva-Valencia et al., 2001). OBIS (July 2022) included records of Atrina fragilis from sites where sea surface salinity ranged from 30-35. Atrina fragilis is probably not resistant to hypersaline (>40) conditions for a year.  However, no information on the effects of hypersaline conditions or effluent on this or related species was found, and no assessment was made.

Salinity decrease (local)

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

Evidence

Atrina fragilis occurs subtidally at full salinity but shallow subtidal populations may experience variable salinity. Dan Minchin (pers. comm.) suggested that Atrina fragilis may be exposed to reduced or variable salinities for brief periods. A tropical pen shell Atrina maura was found to have a wide range of halotolerance, from 16-50 (Leyva-Valencia et al., 2001). OBIS (July 2022) included records of Atrina fragilis from sites where sea surface salinity ranged from 30 to 35.  

Kurihara et al. (2018) exposed juvenile Atrina pectinata (3.2 to 3.6 cm in length) to both rapid and gradual changes in a range of salinities at 12°C and 24°C in the laboratory. They reported that all juveniles survived 83 hours of gradual exposure to >=21.1 ppt at 24°C but all died after exposure <=18.6 ppt for <20 hours (with one exception).  At 12°C, all juveniles survived 83 hours of gradual exposure to >=17.2 ppt but all died after <33.6 hours at <=15.1 ppt. Rapid exposure to reduced salinity, by direct transfer of specimens into the required salinities, decreased the survival time.  At 24°C, all juveniles (except one) survived rapid exposure to >=17.7 ppt for 140 hours but all died rapid exposure to <=15.9 ppt for 13 to 20 hours. At 12°C, all specimens survived 140 hours at >=23.1 ppt and all died at <=21.3 ppt.  Kurihara et al. (2018) concluded that salinity tolerance was lower at 24°C than 12°C but that the difference was reduced during rapid changes in salinity. Kurihara et al. (2018) also suggested that juveniles were more sensitive to rapid changes in salinity than adults and report a period study that showed that adult Atrina pectinata survived rapid exposure to >15 ppt but died after exposure to <15 ppt for 96 hours at 28°C.

Sensitivity assessment. Atrina fragilis may be able to tolerate short periods of exposure to decreases in salinity but is probably not resistant to changes in salinity (e.g. from 'full' to 'reduced') for a year (see benchmark) based on the evidence from Atrina pectinata.  Therefore, resistance is assessed as 'Low', resilience as 'Low' and sensitivity 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

Atrina fragilis is known from weak (<0.5 m/s) to moderately strong currents (0.5-1.5 m/s).  For example in Knightstown, Valentia Island, the population is exposed to >2 knots (ca >1 m/s) on spring tides (Dan Minchin pers. comm.).  Stirling et al. (2016) reported that Scottish records occurred in areas with peak currents, during a mean spring tide, of 0.65 m/s on average and a maximum of 1.24 m/s. Stirling et al. (2016) suggested that extremely high currents (i.e. >2-3 m/s) would probably prevent the settlement of spat and the resultant resuspension of sediments would impact adults. Changes in current patterns may affect larval recruitment (Anon., 1999c). Arrieche et al. (2010) found that Atrina maura grew significantly larger above a flow rate of 7.3 cm/s in flume experiments. Arrieche et al. (2011) reported that ingestion rate, absorption rate, and scope for growth were highest at 1.6 cm/s (0.16 m/s) in Atrina maura under laboratory conditions. Scope for growth, ingestion and absorption rates were correlated with food supply at 0.8 and 1.6 cm/s and respiratory rate was positively correlated at 2.5 cm/s (0.25 m/s). 

However, an increase of 0.1-0.2 m/s for one year (the benchmark) may not be significant. A decrease in flow to 'very weak' or 'negligible' may be detrimental as water flow is important to provide a food supply for suspension feeders, as well as oxygenate the water column, especially in isolated waters. A reduction in food supply may well decrease growth and reproduction in this species, although as the species is long-lived, a change for one year (see benchmark) may not result in mortality.  Therefore, a resistance of 'High' is recorded, albeit with 'Low' confidence. Hence, resilience is 'High' and the species is assessed as '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

The majority of Atrina fragilis populations occur in the subtidal and in deep water and changes in emergence are Not relevant. However, Atrina fragilis has been recorded from the sublittoral fringe and shallow subtidal. It is possible that a sublittoral fringe population could be exposed to increased emergence, in which case the affected population would probably die.

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

Atrina fragilis occurs in sheltered or very sheltered waters (Anon 1999c; Butler et al. 1993) and can burrow into the substratum if partly uncovered by wave action or storms (Yonge, 1953).  However, a prolonged increase in wave action could remove some individuals from the substratum, which would not then be able to survive to re-establish themselves. Juveniles may be removed from sediment more easily than adults. 

García-March et al. (2007) reported that a bed of Pinna nobilis at 13 m Moraira Bay, in the Spanish Mediterranean suffered 13.6% mortality after a major storm in 2003 (greater than any recorded in the prior nine years), in which adults were broken or dislodged and died due to injury or exhaustion. The survivors were notably smaller (on average) than the dead specimens. García-March et al. (2007) examined the drag force on Pinna nobilis shells from this site and a shallow (6 m) site. They reported that the threshold for dislodgment in the deep population was ca 45 Newtons (N), which could be experienced at a water species of 1.25 m/s in large individuals with a relatively large surface area. The shallow area was populated by significantly smaller individuals, orientated with their dorsal-ventral surface in line with water flow, which reduced their mean drag force. García-March et al. (2007) suggested that the shallow site regularly experienced wave-mediated flow higher than the deep site, which selected the population for a smaller size (on average) while depth allowed individuals to grow larger. In the extreme storm, the deep site experienced more damage because it exposed the deeper water to higher than normal water speeds while the largest wave experienced during the storm broke before reaching the shallow site. 

Nevertheless, a change in significant wave height of 3-5% (the benchmark) is unlikely to be significant, especially as the depth of the population increases. Therefore, resistance is assessed as 'High', resilience as 'High' and this 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

Anon (1999c) suggested that Atrina fragilis may be affected by pollutants such as TBT (tri-butyl tin). Reid & Brand (1989) describe kidney gigantism and nephroliths (calcium or iron granules) in Pinna bicolor. Their role in removing excess calcium or heavy metals and potential detoxification is unclear. Ward & Young (1983) examined changes in epifauna of Pinna bicolor due to heavy metal contamination in Spence Gulf, South Australia. They stated that Pinna bicolor was tolerant of high concentrations of heavy metals in sediments near a lead smelter and contained high body loads of heavy metals. The occurrence of populations of this species in heavy metal contaminated sediment suggests that it is 'Not sensitive'. However, the body burden of Pinna bicolor was not given and no citation was provided for the information. The studied population may represent a localised adaptation.

Gongora-Gomez et al. (2018) reported that Atrina maura from an aquaculture farm in Gulf of California accumulated heavy metals in their body tissues but no information on any adverse effects was reported. They reported mean body burdens of ca 0.064 µg/g dwt Hg, ca 485.66 µg/g dwt Zn, 18.15 µg/g dwt Cd and 2.31 µg/g wwt Pb in soft tissues, and ca 0.058 µg/g dwt Hg, 64.83 µg/g dwt Zn, and1.82 µg/g dwt Cd in muscle. 

Anon (1999c) suggested that Atrina fragilis may be affected by pollutants such as TBT (Tri-butyl Tin). Inoue et al. (2006) identified a range of TBT contamination of 0.009 to 0.095 µg/g wwt in Atrina pectinata japonica collected from northern Kyushu, Japan. Inoue et al. (2007) reported that exposure of Atrina pectinata japonica to 1 µg/l TBT for 72 hours in the laboratory reduced energy metabolism but did not cause mortality during the experiment. 

Overall, information on the effects of heavy metal contamination and TBT exposure is limited to sublethal effects and no information on the native Atrina fragilis was found. Therefore, the evidence is probably not sufficient for an assessment. 

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

No evidence of the effects of hydrocarbons and PAHs on Atrina spp was found. 

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

No evidence of the effects of 'synthetic compounds' on Atrina spp was found. 

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

No evidence of the effects of 'other substances' on Atrina spp was found. 

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

Bivalves are generally resistant to hypoxia and can respire anaerobically.  Butler et al. (1993) state that Pinna bicolor and Pinna nobilis remain active at low oxygen concentrations (no value was given) and open their valves widely. Neither species stop pumping or respire anaerobically. Jaklin & Zahtila (1990, cited by Šimunović et al., 2010) reported a mass mortality in the northern Adriatic due to an anoxic event in November 1989. Subsequent diver surveys in January 1990 found many empty shells of Atrina fragilis in an area previously populated by 1-2 fan mussels per m² but suggested that 10% of the population survived. Nagasoe et al. (2020) examined the effects of hypoxia in juveniles, 1-year-old (mean of 9.4 cm in length) and 2 years old (mean of 14.6 cm in length) Atrina japonica for 96 hours (4 days) under laboratory conditions. As the oxygen concentration decreased, most individuals had open exhalent and inhalent siphons at or below 2 mg/l dissolved oxygen (DO) after 12 hours and under continued low oxygen the specimens emerged from the sediment exposing more than half their shell length. Nagasoe et al. (2020) reported 96-hour LD₅₀s of 0.84 mg/l DO for one-year-olds and 1.80 mg/l DO for two years olds. they concluded that two years old were more susceptible to hypoxic conditions than one-year-olds probably because of their productive condition since 96.7% of two years olds were ripe in contrast to only 7.6% of one-year-olds. Masato et al. (2017) exposed Atrina lischkeana from Kyushu, Japan, with an average length of 13.6 cm to <3.0 mg/l DO for 6 hours a day for 30 days. The animals moved upwards in the substratum under low oxygen levels and then downwards in aerobic conditions for 19 days but the response stopped after 20 days. No mortality was observed but the glycogen content of specimens decreased and Masato et al. (2017) suggested the specimen were exhausted by their movement relative to the sediment. Masato et al. (2017) also noted that prior studies had reported significant mortality of Atrina lischkeana exposed to 0.36 mg/l DO for 6 hours/day for 31 days and 50% mortality after exposure to 14.9% oxygen (ca 4.8 mg/l DO) continuously for 72 hours.

Sensitivity assessment. The limited evidence suggests that Atrina fragilis is likely to suffer significant mortality in hypoxic conditions (e.g. below 2 mg/l for one week) and severe mortality in anoxic conditions.  Therefore, resistance is assessed as 'Low'. Hence, resilience is probably 'Low' and sensitivity is assessed as 'High'.

Nutrient enrichment

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

Evidence

Pinnids are mainly found in sheltered oligotrophic (low nutrient) waters (Butler et al., 1993), and they filter continuously, presumably an adaptation to low food availability. A small population of Atrina fragilis was recorded near a sewage discharge in Dingle Harbour (Dan Minchin pers comm.). An increase in nutrients is likely to increase phytoplankton production in the short term, which may benefit larvae and juveniles. But excessive nutrient enrichment may lead to the development of algal blooms, and hypoxic conditions in the benthos (see deoxygenation above). However, no evidence of the direct or indirect effects of changes in nutrients (e.g. nitrogen or phosphates) on Atrina spp. was found. 

Organic enrichment

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

Evidence

Pinnids are mainly found in sheltered oligotrophic (low nutrient) waters (Butler et al., 1993), and they filter continuously, presumably an adaptation to low food availability. A small population of Atrina fragilis was recorded near a sewage discharge in Dingle Harbour (Dan Minchin pers comm.). Organic enrichment is likely to result in hypoxic sediment and an increase in opportunistic infauna, together with an increase in suspended sediments and siltation, which may be detrimental. However, Atrina fragilis is recorded from detritic bottoms with terrigenous ooze in the Adriatic (Fryganiotis et al., 2013).  Overall, the evidence was not adequate and no sensitivity assessment was made.

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

A change from sediment to hard rock substratum would result in loss of the habitat for this species.  Therefore, 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

The Pinnidae live embedded in soft substrata but with enough underlying gravel to provide attachment for their byssus threads (Yonge, 1953). Atrina fragilis has been recorded from a variety of sediment types e.g. muddy sands to clean sands, often mixed with gravels and shell (see habitat preferences above). For example, Fryganiotis et al. (2013) stated that this species was characteristic of 'detritic' bottoms, sandy bottoms and terrigenous ooze sediments, while Šimunović et al. (2001) reported that it was most abundant on sand-silt-clay sediments and clayey 'relict' sand. Howson et al. (2012) reported that the fan mussel bed in the sound of Canna occurred on mixed muddy sand with cobble, gravel, shell debris and occasional boulders but that one station with dense Atrina fragilis occurred on rippled sand with burrows. Furthermore, Stirling et al. (2016) reported that Atrina fragilis was recorded from 0.6% to 74% mud (with a mean of 32%) and 0.2%-62% gravel (with a mean of 12%) in the waters of west Scotland. Their model predicted the highest abundances in the range of 20-60% mud but approx. <20% gravel. In addition, this biotope is recorded from mud and mixed sediments (JNCC, 2022). Therefore, the characteristic Atrina fragilis is probably resistant to a change in one Folk class (see benchmark), and resistance is assessed as  'High'. Hence, resilience is 'High' and the species is assessed as 'Not sensitive' at the benchmark level.

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

Atrina fragilis lives buried in sediment, with about one-third to one-half of its shell length above the sediment surface, which, in adults, can be up to 10 -15 cm above the sediment surface. However, the removal of sediment to a depth of 30 cm is likely to remove the entire population of fan mussels in the affected area. Therefore, a resistance of 'None' is recorded. Resilience is probably 'Very low' so sensitivity 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

Atrina fragilis has a fragile shell, damaged easily by anchor impact, or trampling by bathers or fish predation. It is probably adapted to such damage as the mantle and ctendia can be withdrawn into the remainder of the shell, and the damaged edge of the shell can repair quickly, e.g. at ca 1 cm /day in Pinna carnea (Yonge, 1953; Solandt, 2003).  Atrina can burrow vertically but cannot 'right' itself if removed from the sediment and laid on its side (Yonge, 1953). Specimens removed from the sediment by a passing trawl, mooring chain etc. are unlikely to be able to reburrow.

Scallop dredging and demersal trawling have been implicated in the decline in populations of this species (Anon, 1999c; Hall-spencer et al., 1999; Solandt, 2003; Šimunović et al., 2001; Fryganiotis et al., 2013). Solandt (2003) noted anecdotal records where 'considerable fragments of Atrina shells were collected by scallop trawlers', and large individuals caught in the Celtic Sea in the 1970s with 'decks covered with the broken fragments of this species'. Solandt (2003) also reported anecdotal records from the diving community of considerable numbers of Atrina fragilis found in areas where scallop trawlers and dredgers cannot set gear.  Rapido trawling for scallops (a form of beam trawl) in the Gulf of Venice resulted in the removal of organisms from the top 2 cm of sediment and an 87% reduction in Atrina fragilis abundance in the trawl tracks. Some specimens were speared on the trawl teeth and pulled from the sediment (Hall-Spencer et al., 1999). Anon (1999c) suggested that the destruction of a population of Atrina fragilis off Glengad Head, Ireland after 1975 was caused by scallop dredging.  In the Adriatic queen scallop (Aequipecten opercularis) trawl fishery, Atrina fragilis incurred more damage as a result of the fishing and sorting process than any other species of bycatch (Pranovi et al., 2001). In the Adriatic, Fryganiotis et al. (2013; Fig 2) reported that the density of fan mussels in trawled areas (ca 0.03 individuals /km) was sparse compared to the areas in which bottom trawling was prohibited for 25 years (ca 5.5 individuals /km).  Furthermore, Stirling et al. (2016) used species distribution models, based on existing records, to examine the habitat preferences of Atrina fragilis and predict suitable habitat in the waters of the west coast of Scotland.  Stirling et al. (2016) identified depth and habitat complexity (bathymetric ruggedness) as the most important determinants of distribution followed by current speed, and substratum type (percentage of mud and gravel), while aspect had less importance. In particular, they suggested that habitat complexity, either natural or artificial, protected the substratum and, hence, adults from the effects of fishing activities. Stirling et al. (2016) noted that Atrina fragilis was most abundant in the Sound of Canna, which had the highest habitat complexity (ruggedness) in their study. The Sound of Canna is a deep-sided channel deepened by glaciation with a complex benthic profile including glacial moraines and deep water dredge disposal site (Stirling et al., 2016). 

Sensitivity assessment. The above evidence suggests that Atrina fragilis can survive low levels of abrasion e.g. trampling in the shallow sublittoral and possibly pots and creels that damage the exposed shell. However, any passing chains or fishing gear that could remove individuals, or objects placed on the substratum temporarily (e.g. legs of jack-up barges) are likely to cause mortality.  Therefore, a resistance of 'Low' is suggested. Resilience is probably 'Low' and sensitivity 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

Atrina fragilis has a fragile shell, damaged easily by anchor impact, or trampling by bathers of fish predation. It is probably adapted to such damage as the mantle and ctendia can be withdrawn into the remainder of the shell, and the damaged edge of the shell can repair quickly, e.g. at ca 1 cm /day in Pinna carnea (Yonge, 1953; Solandt, 2003).  Atrina can burrow vertically but cannot 'right' itself if removed from the sediment and laid on its side (Yonge, 1953). Specimens removed from the sediment by a passing trawl, mooring chain etc. are unlikely to be able to reburrow.

Scallop dredging and demersal trawling have been implicated in the decline in populations of this species (Anon, 1999c; Hall-spencer et al., 1999; Solandt, 2003; Šimunović et al., 2001; Fryganiotis et al., 2013). Solandt (2003) noted anecdotal records where 'considerable fragments of Atrina shells were collected by scallop trawlers', and large individuals caught in the Celtic Sea in the 1970s with 'decks covered with the broken fragments of this species'. Solandt (2003) also reported anecdotal records from the diving community of considerable numbers of Atrina fragilis found in areas where scallop trawlers and dredgers cannot set gear.  Rapido trawling for scallops (a form of beam trawl) in the Gulf of Venice resulted in the removal of organisms from the top 2 cm of sediment and an 87% reduction in Atrina fragilis abundance in the trawl tracks. Some specimens were speared on the trawl teeth and pulled from the sediment (Hall-Spencer et al. 1999). Anon (1999c) suggested that the destruction of a population of Atrina fragilis off Glengad Head, Ireland after 1975 was caused by scallop dredging.  In the Adriatic queen scallop (Aequipecten opercularis) trawl fishery, Atrina fragilis incurred more damage as a result of the fishing and sorting process than any other species of bycatch (Pranovi et al., 2001). In the Adriatic, Fryganiotis et al. (2013; Fig 2) reported that the density of fan mussels in trawled areas (ca 0.03 individuals /km) was sparse compared to the areas in which bottom trawling was prohibited for 25 years (ca 5.5 individuals /km).   Furthermore, Stirling et al. (2016) used species distribution models, based on existing records, to examine the habitat preferences of Atrina fragilis and predict suitable habitat in the waters of the west coast of Scotland.  Stirling et al. (2016) identified depth and habitat complexity (bathymetric ruggedness) as the most important determinants of distribution followed by current speed, and substratum type (percentage of mud and gravel), while aspect had less importance. In particular, they suggested that habitat complexity, either natural or artificial, protected the substratum and, hence, adults from the effects of fishing activities. Stirling et al. (2016) noted that Atrina fragilis was most abundant in the Sound of Canna, which had the highest habitat complexity (ruggedness) in their study. The Sound of Canna is a deep-sided channel deepened by glaciation with a complex benthic profile including glacial moraines and deep water dredge disposal site (Stirling et al., 2016). 

Sensitivity assessment. The above evidence suggests that Atrina fragilis can survive low levels of abrasion. However, penetrative gear such as beam trawls, Rapido trawls and scallop dredges are likely to cause severe mortality. Therefore, a resistance of 'None' is suggested. Resilience is probably 'Very low' and sensitivity 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

Pinnids are adapted to a sedimentary lifestyle and possess a unique ciliated waste canal for the removal of sediment from the mantle cavity (Yonge 1953). However, increased siltation will require increased metabolic demand on filtration and a likely decrease in growth and reproductive capacity. Pinna bicolor and Pinna nobilis occur in sheltered areas of low turbidity. However, juveniles settle in the boundary layer and grow rapidly to escape the high levels of sediment and it is likely that Pinnids are tolerant of suspended sediment. The absence of Pinna sp. from areas of severe sediment disturbance (Bulter et al. 1993) suggests that the populations in areas of high sediment availability will be adversely affected by increased siltation.

Thrush et al. (1999) demonstrated a decrease in the biochemical condition in Atrina zealandica with increasing sediment load in the Mahurangi Estuary, New Zealand. Ellis et al. (2002) examined the effects of the addition of sediment in laboratory experiments, at a range of turbidity treatments that represented the range of values (23-512 mg/l) experienced in the Mahurangi Estuary, where the normal background turbidity ranged from 12-90 mg/l but were much higher in storm associated resuspension of sediment or runoff from forestry. The initial addition of suspended sediment increased clearance rates, in the same way, that increased seston (phytoplankton) was found to increase filtration rates (Ellis et al., 2002; Safi et al., 2007). Clearance rates increased with increasing suspended sediment until a threshold of ca 120 FTU (Formazin Turbidity Unit) at which clearance rates declined (Ellis et al., 2002). Clearance rates continued to decrease over the duration of the experiment (12 days) in all of the sediment addition treatments. Negative effects on the condition of Atrina zelandica became apparent after only three days of exposure to increased suspended sediment levels, compared to controls with 'no' sediment added (Ellis et al., 2002).  In transplantation experiments, Ellis et al. (2002) found that Atrina transplanted to the area closest to the mouth of the estuary (lower suspended sediment flux) improved in condition over the three months of the experiment. But Atrina transplanted to upper estuary sites (with high suspended sediment flux, equivalent to 108 g dry weight of sediment per month in sediment traps) lost condition. No Atrina occurred naturally at this upper estuary site, which may represent the upper limit of its tolerance of suspended sediment. Atrina also lost condition at intermediate sites (e.g. at 49 g dry weight of sediment per month) (Ellis et al., 2002).  It may be that Atrina zelandica found in areas with naturally high sediment loading are adapted to cope better with increases in suspended sediment than those from areas with lower background sediment concentrations. Nonetheless, very large increases in suspended sediment are still likely to be detrimental to Atrina zelandica (Hewitt & Pilditch, 2004). Hewitt & Pilditch (2004) examined the response of feeding in Atrina zealandica to 0-500 mg/l for ca one day. Atrina was able to reject filtered particles (75-100%) but maintain high organic absorption efficiencies. However, they identified site-specific differences in response that they suggested were due to prior history of exposure to suspended sediments at each site (Hewitt & Pilditch, 2004). Arrieche et al. (2010) noted that juvenile Atrina maura withstood high seston levels of ca 900 mg/ l  for 10 consecutive days in their flume experiments. 

Sensitivity assessment. Atrina sp. are probably well adapted to a sedimentary habitat and the occasional resuspension of sediment due to storms, as they are able to cleanse themselves quickly. Short-term (i.e. 3 day) increases in suspended sediment, similar to that created by storms and storm runoff, are likely to result in a loss of condition but not mortality. However, an increase in turbidity from, for example, 'clear' to 'intermediate' (100-300 mg/l) or turbid (>300 mg/l) for a period of a year (see benchmark) may be detrimental. Therefore, a resistance of 'Medium' is recorded. Resilience is probably 'Low' and sensitivity is assessed as 'Medium'.

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

Atrina fragilis cannot burrow upwards through sediment (Yonge, 1953). However, one-third to one-half of the animal can protrude above the surface which, in adults, can be up to 10-15 cm above the sediment surface. Therefore, adult specimens may not be affected by smothering by 5 cm of fine sediment (see benchmark). Pinnids are adapted to a sedimentary lifestyle and exhibit a powerful exhalent current and a unique ciliated waste canal to remove sediment from the mantle cavity, as would be expected from occasional smothering due to storms (Yonge, 1953). Clearance of sediment from the mantle constitutes a metabolic cost that may reduce their reproductive ability (Butler et al., 1993). Individuals are likely to cleanse themselves relatively quickly. However, small juveniles may be smothered and resistance is assessed as 'Medium'.  Resilience is probably 'Low' and sensitivity is assessed as 'Medium'.

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

Atrina fragilis cannot burrow upwards through sediment (Yonge, 1953). One-third to one-half of the animal can protrude above the surface which, in adults, can be up to 10-15 cm above the sediment surface. Therefore, adult and juvenile specimens are likely to be smothered by 30 cm of fine sediment (see benchmark), which is likely to remain in sheltered conditions. Therefore, resistance is assessed as 'None'.  Resilience is probably 'Very low' and 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

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

Not relevant. Atrina fragilis probably reacts to localised vibration but is unlikely to react to the noise from passing vessels etc. 

Introduction of light or shading

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

Evidence

Not relevant. Atrina fragilis is a suspension feeder, feeding on phytoplankton. Artificial light or localised shading is unlikely to alter phytoplankton productivity to any significant level, especially in deep waters and/or where currents supply food to the fan mussel.

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. Physical and hydrographic barriers may limit the dispersal of larval stages or propagules. However, 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’ above.

Visual disturbance

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

Evidence

Not relevant. Atrina fragilis probably reacts to localised shading but is unlikely to react to the visual disturbance from passing vessels etc. 

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

Liu et al. (2011) identified five lineages of mtDNA in Atrina pectinata along the coast of China and one location in Japan. The linages corresponded to six morphotypes although intraspecies hybridization between lineages obscured the differences. However, no evidence of translocation, breeding or hybridization with other species 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

Crepidula sp. may have had some impact on near-shore populations of Atrina fragilis on the south coast of England (Dan Minchin pers comm.). But no further evidence was found. 

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

The Pinnids are parasitized by the pea crab (Pinnotheridae) (Yonge 1953). Butler et al. (1993) state that Pinna bicolor and Pinna nobilis harbour macroscopic commensals or parasites of unknown effect, although an unidentified parasitic microbe has been recorded as causing castration of Pinna nobilis. Any parasite is likely to reduce the condition of the host but no information on mortality rates (if any) was found.

Maeno et al. (2006) examined specimens of Atrina pectinata after the mass mortality (60-90%) of the pen shell in the fishing grounds of Ariake Bay, Japan between 2003 and 2004.  The authors concluded that novel virus-like particles found in the kidney and gill tissues of moribund specimens were the probable cause of mortality. Subsequent, experimental infection of Atrina lischkeana in the laboratory and the field (Ariake Bay, Japan) resulted in necrosis of kidney, gill and mantle tissue and 20% mortality in the laboratory but 80-100% mortality of infected specimens in the field Maeno et al. (2012; abstract only). Therefore, the authors concluded that the virus-like particles were the causal agent. 

Sensitivity assessment. The evidence from Japan suggests that Atrina is sensitive to the disease documented. However, no evidence of the disease outside Japan was found. Hence, an assessment of 'No evidence' is given for UK populations until further evidence is found. 

Removal of target species

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

Evidence

In Spain, pinnids may be collected for consumption, used as bait, or for use as souvenirs. In the Bay of Naples, the byssus threads were historically used for making glues. In the Pacific, declines in production have occurred as a result of exploitation in other species of pen shell (Cardoza-Velasco & Maeda-Martinez, 1997).  However, Atrina fragilis is not targeted by any commercial fishery in the UK.

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

Scallop dredging and demersal trawling have been implicated in the decline in populations of Atrina fragilis and other pinnids (Anon, 1999c; Hall-spencer et al., 1999; Solandt, 2003; Šimunović et al., 2001; Fryganiotis et al., 2013; Chavaz-Villabla et al., 2022). In the UK, Atrina fragilis was more common in scallop beds in the early 1900s than at present. Presumably trawling and dredging of these formerly populated regions is the reason for the decline of this species (Minchin pers. comm.). Dredging of a Pecten maximus bed off Glengad Head, Ireland, after 1975, removed many live specimens of Atrina fragilis in scallop dredges and the population of fan mussels is thought to have been destroyed by subsequent dredging (Anon 1999c). Solandt (2003) noted anecdotal records where 'considerable fragments of Atrina shells were collected by scallop trawlers', and large individuals caught in the Celtic Sea in the 1970s with 'decks covered with the broken fragments of this species'. Solandt (2003) also reported anecdotal records from the diving community of considerable numbers of Atrina fragilis found in areas where scallop trawlers and dredgers cannot set gear.

In the Adriatic queen scallop (Aequipecten opercularis) trawl fishery, Atrina fragilis incurred more damage as a result of the fishing and sorting process than any other species of bycatch (Pranovi et al., 2001). In the Adriatic, Fryganiotis et al. (2013; Fig 2) reported that the density of fan mussels in trawled areas (ca 0.03 individuals/km) was sparse compared to the areas in which bottom trawling was prohibited for 25 years (ca 5.5 individuals/km). Rapido trawling (a form of beam trawl) for scallops in the Gulf of Venice resulted in the removal of organisms from the top 2 cm of sediment and an 87% reduction in Atrina fragilis abundance in the trawl tracks. Some specimens were speared on the trawl teeth and pulled from the sediment (Hall-Spencer et al., 1999). Pinnids in the Mediterranean are associated with seagrass beds, the removal of which has been linked to the decline in pinnid populations (Richardson et al., 1999).

Sensitivity assessment. The above evidence suggests that Atrina fragilis is vulnerable to demersal fisheries. Therefore, a resistance of 'None' is suggested. Resilience is probably 'Very low' and sensitivity is assessed as 'High'.