Biodiversity & Conservation

SS.CMU._.BriAchi

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Species within the CMU.BriAchi biotope are infaunal and will be lost if the substratum is removed so the overall intolerance of the biotope has been recorded as high. Although some species are mobile e.g. Calocaris macandreae and Nephrops norvegicus, if disturbed they are likely to seek refuge within a burrow within the substratum and so are also likely to be removed. The characterizing species do not reach sexual maturity for several years and recovery has been assessed to be moderate (see additional information below).
Smothering
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The biotope will probably have a low intolerance to smothering by 5 cm of sediment because the characterizing species are all infaunal burrowers. There may be some energetic cost expended to either re-establish burrow openings in the case of Calocaris macandreae and Nephrops norvegicus, or to self-clean feeding apparatus though this is not likely to be significant. The biotope is likely to be more intolerant of smothering by viscous or impenetrable materials e.g. smothering by sediment of a coarser texture may affect burrowing and feeding. At the benchmark level, recovery of the community from smothering is assessed to be immediate.
Increase in suspended sediment
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Suspension feeders are not found within the biotope so clogging of feeding apparatus by suspended sediment is not a consideration. Brissopsis lyrifera, Amphiura chiajei, Calocaris macandreae and Turritella communis are burrowing infauna and non-selective surface and sub-surface deposit feeders. For most benthic deposit feeders, food is suggested to be a limiting factor for body and gonad growth, at least between events of sedimentation of fresh organic matter (Hargrave, 1980; Tenore, 1988). Consequently, an increase in the suspended matter settling out from the water column to the substratum may increase food availability. This suggests that an increase in siltation may be beneficial and the biotope is not considered to be sensitive.
Decrease in suspended sediment
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A decrease in the suspended sediment and hence siltation will reduce the flux of particulate material to the seabed. Since this includes organic matter the supply of food to the biotope would probably also be reduced. However, the benchmark states that this change would only occur for one month and therefore a decrease in siltation would be unlikely to cause a significant alteration to species composition. Therefore intolerance has been assessed to be low.
Desiccation
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The biotope only occurs in the circalittoral zone (below 10m) and is not subject to desiccation.
Increase in emergence regime
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The biotope only occurs in the circalittoral zone (below 10m) and is not likely to be subjected to a change in emergence regime.
Decrease in emergence regime
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The biotope only occurs in the circalittoral zone (below 10m) and is not likely to be subjected to a change in emergence regime.
Increase in water flow rate
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The presence of the biotope is determined by a low energy hydrodynamic regime facilitating the deposition of cohesive fine silts and clays. Following an increase in water flow rate only the surface sediments are likely to be winnowed away in a unidirectional flow. The lower substratum inhabited by mature specimens of Brissopsis lyrifera and Amphiura chiajei is likely to remain unchanged. However, the settlement of the planktonic larvae of these key species may be inhibited owing to re-suspension along with particulate matter. Consequently the viability of the population may be reduced. Furthermore the deposit feeding community may experience a reduction in food availability owing to reduced deposition of organic matter. Intolerance to increased water flow rate has been assessed to be intermediate. On return to prior conditions, specimens of the characterizing species will have remained and are likely to repopulate via successful larval settlement. However, attainment of a fully diverse community is likely to take several years and recovery has been assessed to be moderate (see additional information below).
Decrease in water flow rate
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The presence of the biotope is determined by a low energy hydrodynamic regime facilitating the deposition of fine silts and clays, hence the community is not likely to be directly intolerant of a decrease in water flow rate. Sediments may become muddier owing to increased settlement of particulate matter. However, as deposit feeders are the dominant trophic group such additional material may be utilizable as a food resource and the community may benefit indirectly.
Increase in temperature
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In shallower locations e.g. sea lochs, sedimentary biotopes typically experience seasonal changes in temperature of about 10°C (5-15°C) (Hughes, 1998b) and it is likely that the CMU.BriAchi community would be tolerant of a long term chronic temperature increase. For most offshore burrowing species, temperature changes in the water column are likely to buffered by the insulation offered by the substratum and the depth of overlying water. Furthermore, a temperature increase may enhance growth and fecundity. Muus (1981) showed that juvenile Amphiura filiformis are capable of much higher growth rates in experiments with temperatures between 12 and 17°C (unlimited food supply). Juvenile disc diameter increased from 0.5 to 3.0 mm in 28 weeks under these conditions compared to over 2 years in the North Sea. Mean summer temperatures of 14°C and an apparent abundant food supply may also account for the early rapid growth of Amphiura chiajei in Killary Harbour (Munday & Keegan, 1992). In Brissopsis lyrifera, processes such as mobility, sediment turnover and remineralization may increase (K. Hollertz, pers. comm., Hollertz & Duchêne, 2001). Hollertz & Duchê (2001) found that in Brissopsis lyrifera, the amount of reworked sediment due to burrowing almost doubled from 14 to 22 ml/l sediment per hour when the temperature increased from 7 to 13 °C. This temperature increase also saw the amount of ingested sediment increase from 0.02 to 0.08 g dry sediment per hour. However, increased water temperature may enhance microbial decomposition within the substratum and promote deoxygenation, to which Brissopsis lyrifera is intolerant. Owing to the fact that the biotope is subtidal, where wide and rapid variations in temperature, such as those experienced in the intertidal, are not common, the community is likely to be more intolerant of an acute temperature increase of 5°C and intolerance has been assessed to be intermediate. Recovery has been assessed to be high since members of the community are likely to remain to revitalize the population (see additional information below).
Decrease in temperature
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In shallower locations e.g. sea lochs, sedimentary biotopes typically experience seasonal changes in temperature of about 10°C (5-15°C) (Hughes, 1998b) and it is likely that the CMU.BriAchi community would be tolerant of a long term chronic temperature decrease. For most offshore burrowing species temperature changes in the water column are likely to buffered to some extent by the insulation offered by the substratum and the depth of overlying water. However, burrowing itself has been found to be significantly affected by temperature in Brissopsis lyrifera. Hollertz & Duchêne (2001) found that Brissopsis lyrifera reworked almost half the amount of sediment per hour at 7 °C compared to activity at 14 °C. Furthermore, Brissopsis lyrifera maintains a continuous contact with the overlying water column through the funnel (Hollertz, 2002). Also, the biotope community seems to be periodically affected by severe winters. During the winter of 1962-1963 a few dead Nephrops norvegicus were caught in the North Sea, although the majority were caught alive (Crisp, 1964). Mean densities of Amphiura chiajei in Killary Harbour, west coast of Ireland, decreased following months with the lowest recorded bottom temperatures, 4°C and 6°C, for February 1986 and January 1987 respectively. Intolerance of the acute change and depressed temperatures on the part of some of the older individuals probably led to their demise (Munday & Keegan, 1992). Low temperatures are also a limiting factor for breeding which occurs in the warmest months in the UK. Temperature tolerances of Brissopsis lyrifera are unknown but low water temperatures have caused mass mortalities of other similar echinoderms, such as Echinocardium cordatum. In the severe winter of 1962-63 masses of dead Echinocardium cordatum were observed in regions of the North Sea and English Channel, although it was reported that living specimens were obtained easily enough by digging (Crisp, 1964). Therefore, intolerance has been assessed to be intermediate as key species within the community appear to be periodically degraded by acute decreases in temperature. Recovery has been assessed to be high, as members of the community remain to revitalize the population.
Increase in turbidity
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The community is unlikely to be directly intolerant of the light attenuating effects of an increase in turbidity, however, for other related but indirect effects, see suspended sediment above. In the long term, increased turbidity may affect primary production by the microphytobenthos on the substratum surface depleting food availability. Furthermore, increased turbidity may hinder predation by visual predators such as Nephrops norvegicus, dab Limanda limanda, haddock Melanogrammus aeglefinus upon Amphiura chiajei, which provides an important link between the benthic and pelagic realms. There may be some increased energetic costs experienced by certain species, associated with increased turbidity, but effects are not likely to be significant and so intolerance has been assessed to be low. Recoverability is likely to be very high on return to conditions prior to the impact.
Decrease in turbidity
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The community is unlikely to be directly intolerant of increased light penetration of the water column caused by a decrease in turbidity. Greater light penetration of the water column may improve primary production by phytoplankton in the water column and contribute to secondary productivity via the production of detritus from which the community may benefit. For other related but indirect effects see decrease in suspended sediment above.
Increase in wave exposure
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The CMU.BriAchi biotope occurs offshore and in sheltered near shore habitats where wave exposure is negligible, so the biotope is probably very intolerant of increased wave exposure. However, the factor is only likely to affect the biotope where it occurs at depths of less than 60 m, as the effects of wave action are attenuated with depth. Wave action resulting from storms may disturb the surface sediment. McIntosh (1875) reported specimens of Amphiura chiajei thrown on to West Sands, St. Andrews Bay after storms. Over the duration of a year increased wave exposure is likely to cause the substratum character to drastically alter, as wave action would penetrate the substratum to a greater depth, and become outside the habitat preference of the species. The community would no longer occur at that location. Intolerance has therefore been assessed to be high. Once fine sediments have been removed it would take a very long time for a suitable substratum to reform so recovery has been assessed to be very low.
Decrease in wave exposure
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The CMU.BriAchi biotope occurs offshore and in sheltered near shore habitats where wave exposure is already negligible, so a reduction in wave exposure is not likely to have a direct impact upon the biotope community and intolerance has been assessed to be low.
Noise
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No information concerning noise reception in the characterizing species of this biotope was found, but it is likely that the community will be not sensitive to noise disturbance at the benchmark level.
Visual Presence
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Although some species within the community have visual perception e.g. Calocaris macandreae and Nephrops norvegicus, detecting the presence of boats or machinery, is likely to be beyond their visual acuity and the biotope community is assessed as not sensitive.
Abrasion & physical disturbance
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The CMU.BriAchi biotope can be affected by fishing activity in areas such as the northern Irish Sea, where the community may also contain Nephrops norvegicus (Mackie et al., 1995). In areas of the North Sea where heavy demersal fishing for Nephrops norvegicus occurs, populations of Brissopsis lyrifera are likely to be reduced owing to damage inflicted to the 'test' by the fishing gear. Broken tests may be seen on the seabed (E.I.S. Rees, M. Costello, pers comm. to Connor et al., 1997). Similar evidence has been reported for other heart urchins. For example, Houghton et al. (1971), Graham (1955), de Groot & Apeldoorn (1971) and Rauck (1988) refer to significant trawl-induced mortality of heart urchin Echinocardium cordatum. A substantial reduction in the numbers of the species due to physical damage from scallop dredging has been observed (Eleftheriou & Robertson, 1992). Bergman & van Santbrink (2000) suggested that Echinocardium cordatum was one of the most vulnerable species to trawling. Bradshaw et al. (2000) suggested that fragile species such a urchins (e.g. Spatangus purpureus and Echinus esculentus), suffered badly from impact with a passing scallop dredge. Overall, species with brittle, hard tests are regarded to be sensitive to impact with scallop dredges (Kaiser & Spencer, 1995; Bradshaw et al., 2000).

Brittlestars have fragile arms that are likely to be damaged by abrasion or physical disturbance. Amphiura chiajei burrows in the sediment and extends its arms across the sediment surface to feed. Ramsay et al., (1998) suggests that Amphiura species may be less susceptible to beam trawl damage than other species of echinoid or tube dwelling amphipods and polychaetes. Bergman & Hup (1992) for example, found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Bradshaw et al. (2002) noted that the brittlestars Ophiocomina nigra, Ophiura albida and Amphiura filiformis had increased in abundance in a long-term study of the effects of scallop dredging in the Irish Sea. Brittlestars can tolerate considerable damage to arms and even the disc without suffering mortality and are capable of disc and arm regeneration so their recovery is likely to be rapid.

Deeper burrowing crustaceans such as Calocaris macandreae may occasionally be displaced from burrow openings by towed gear (Atkinson, 1989). During long term monitoring of fishing disturbance on the Northumberland coast Frid et al. (1999) observed a decrease in the numbers of sedentary polychaetes, echinoid echinoderms and large (> 5 cm) brittlestars.

Therefore, while brittlestars may increase in abundance in the long term, the dominant heart urchin species is likely to be reduced in abundance and an intolerance of intermediate has been recorded. Recovery is likely to be high, as members of the community are likely to remain and be able to repopulate. Brissopsis lyrifera may not regenerate as well as the brittle star (K. Hollertz, pers. comm.).

Displacement
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Although not highly active Brissopsis lyriferaand Amphiura chiajei are burrowing infaunal species, as are Calocaris macandreae and Nephrops norvegicus. Following displacement to suitable sediments these species are likely to commence burrowing immediately provided that individuals are not damaged during displacement. The species will be exposed to predators for a short time so intolerance is assessed to be low.

Chemical Factors

Synthetic compound contamination
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Effects caused by synthetic chemicals have been reported for some of the individual species in the CMU.BriAchi biotope. Dahllöf et al. (1999) studied the long term effects of tri-n-butyl-tin (TBT) on the function of a marine sediment system. TBT spiked sediment was added to a sediment that already had a TBT background level of approximately 27 ng/g (83 pmol TBT per g) and contained the following fauna: Amphiura spp., Brissopsis lyrifera and several species of polychaete. Within two days of treatment with a TBT concentration above 13.7 µmol / m² all species except the polychaetes had crept up to the surface and after six weeks these fauna had started to decay. Thus contamination from TBT is likely to result in the death of some intolerant species such as brittle stars and heart urchins. Amphiura chiajei is also known to bioaccumulate PCBs, although direct effects of synthetic chemicals on this species are unknown. (Gunnarsson & Skold, 1999). However, Walsh et al., (1986) observed inhibition of arm regeneration in another brittlestar, Ophioderma brevispina, following exposure to TBT at levels between 10 ng/l and 100 ng/l. Loizeau & Menesguen (1993), found that 8-15% of the PCB burden in dab, Limanda limanda, from the Bay of Seine could be explained by ophiuroid consumption. Thus Amphiura communities may play an important role in the accumulation, remobilization and transfer of PCBs and other sediment associated contamination to higher trophic levels. The key species seem to be highly intolerant of some chemical pollutants and may be lost from the biotope. In their absence the biotope would not be recognized so intolerance has been assessed to be high. In the absence of synthetic chemical contaminants re-population of the biotope is likely to occur, but owing for the time for the community to reach maturity recovery is assessed to be moderate.
Heavy metal contamination
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Information concerning the effects of heavy metals on echinoderms is limited and no information specific to Brissopsis lyrifera and Amphiura chiajei was found. In Norwegian fjords Rygg (1985) found a relationship between species diversity in benthic fauna communities and sediment concentrations of heavy metals Cu, Pb and Zn. Cu in particular showed a strong negative correlation and the author suggested a cause-effect relationship. Those species not present at sites where Cu concentrations were greater than ten times the background level, such as Calocaris macandreae, Amphiura filiformis and the bivalve Nucula sulcata (also found in CMU.BriAchi), were assessed as non-tolerant species. Tolerant species were all polychaete worms. Polychaete worms are the dominant component of the biomass in the CMU.BriAchi biotope and thus may not be as sensitive as the characterizing species. Crompton (1997) reports that the concentrations above which mortality of crustaceans can occur is 0.01-0.1 mg/l for mercury, copper and cadmium, 0.1-1.0 mg/l for zinc, arsenic and nickel and 10 mg/l for lead and chromium. The biotope is considered to have an intermediate intolerance owing to the fact that heavy metal contamination of the sediments may change the faunal composition of the community and decrease overall species diversity. Some burrowing crustaceans, brittlestars and bivalves may disappear from the biotope and lead to an increasing dominance of polychaetes. In the absence of heavy metal contaminants re-population of the biotope is likely to occur, but owing for the time for the community to reach maturity recovery is assessed to be moderate.
Hydrocarbon contamination
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Amphiura chiajei was reported to be very intolerant of hydrocarbon contamination. Chronic sub-lethal effects were detected around the Beryl oil platform in the North Sea where the hydrocarbon content of the sediment was very low (<3 ppm total hydrocarbons in sediment), and Amphiura chiajei was excluded from areas nearer the platform with higher sediment hydrocarbon content (> 10 ppm) (Newton & McKenzie, 1998). As Amphiura chiajei is an important characterizing species within the biotope intolerance is assessed to be high, as in the absence of it the biotope would not be recognized. Brissopsis lyrifera is also likely to be intolerant of hydrocarbon pollution owing to exposure of the epidermis. Furthermore, Brissopsis lyrifera has a continuous water flow over the test so the exposure route through the epidermis may also be important (K. Hollertz, pers. comm., Hollertz, 2002). However, as a burrower and deposit feeder, ingestion of contaminated sediments is likely to be a more important route of exposure. A range of effects (mortalities, feeding/growth inhibition and embryological abnormalities) have been reported for other echinoderms following hydrocarbon exposure (reviewed by Suchanek, 1993). It is also likely that Crustacea such as Calocaris macandreae would be intolerant of hydrocarbons. The abundance of a similar species Callianassa subterranea was significantly reduced up to and over 1 km away from a site of oil drilling one year after drilling ceased (Daan et al., 1992). Therefore, intolerance has been assessed to be high. Re-population is likely following degradation of the contaminants but considered to be moderate (see additional information below).
Radionuclide contamination
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Investigations of bioturbation in radionuclide contaminated sediments on the Irish Sea floor near the Sellafield nuclear reprocessing plant found Nephrops norvegicus and Calocaris macandreae to be present (Hughes & Atkinson, 1997). There is insufficient information on the intolerance of Amphiura chiajei to radionuclides, although adult echinoderms, such as Ophiothrix fragilis, are known to be efficient concentrators of radionuclides (Hutchins et al., 1996). However, no information concerning the effects of such bioaccumulation was found.
Changes in nutrient levels
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Nutrient enrichment can enhance primary productivity in the water column and consequently generate organic detritus that falls to the sea bed. The characterizing species of the CMU.BriAchi biotope demonstrate a preference for substratum high in organic matter. Nilsson (1999) reported a positive response by Amphiura chiajei to increased organic enrichment (27 and 55 g/C/m², applied four times over eight weeks) demonstrable by an increase in arm tip regeneration rate. For benthic deposit feeders, food is suggested to be a limiting factor for body and gonad growth, at least between events of sedimentation of fresh organic matter (Hargrave, 1980; Tenore, 1988). Nilsson (1999) also found that Amphiura chiajei was able to utilize an increased input of organic matter for growth in conjunction with moderate hypoxia. Hollertz (1998) demonstrated increased surface deposit feeding activity for Brissopsis lyriferaafter the addition of organic matter and recorded an increase in growth. In Loch Sween, Scotland, where the organic content is about 5% and as high as 9% in some patches, the burrowing crustaceans Calocaris macandreae and Nephrops norvegicus are present in high densities (Atkinson, 1989). At the benchmark level, a 50% increase in nutrients as an annual average, the biotope community may benefit. In conditions of gross nutrient enrichment hypoxia becomes a factor of consideration, see oxygenation below.
Increase in salinity
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The biotope CMU.BriAchi is found within fully marine subtidal locations and it is highly unlikely that the biotope would experience conditions of hypersalinity and in this instance the factor is considered not relevant. However, it is likely that key components of the biotope community would be intolerant of an increase in salinity. For instance, echinoderms such as Brissopsis lyrifera and Amphiura chiajei are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate causing body fluid to decrease when individuals are exposed to higher salinity (Stickle & Diehl, 1987).
Decrease in salinity
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The biotope CMU.BriAchi is found within fully marine subtidal locations. However, it is likely that key components of the biotope community would be intolerant of a decrease in salinity. For instance, echinoderms such as Brissopsis lyrifera and Amphiura chiajei are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate causing body fluid to increase when individuals are exposed to lower salinity (Stickle & Diehl, 1987). Pagett (1979), examined the tolerance of Amphiura chiajei to brackish water (0.5-30 psu) in specimens taken from Loch Etive, Scotland. Loch Etive is a sea loch subject to periods of reduced salinities owing to heavy rain and fresh-water runoff. Pagett (1979) found that specimens nearer freshwater influxes were more tolerant of reduced salinities than those nearer the open sea. Amphiura chiajei taken from an area of 24 psu had an LD50 of > 21 days for a 70% dilution (17 psu) and an LD50 of 8.5 days for a 50% dilution (12 psu). In comparison, specimens taken from an area with salinity 28.9 psu, had an LD50 of > 12.5 days for a 70% dilution (20 psu) and an LD50 of 6 days for a 50% dilution (14 psu). As Amphiura chiajei is mobile and burrows it may be able to avoid changes in salinity outside its preference, e.g. burrowing may help Amphiura chiajei to withstand depressed salinities owing to the 'buffering' effect of the substratum. However, as some mortalities were recorded for decreases in salinity over a time period less than the benchmark level, it is likely that Amphiura chiajei and the other characterizing species would be highly intolerant of a decrease of one category from the MNCR salinity scale for one year. The characterizing species do not reach sexual maturity for several years and recovery is likely to be moderate (see additional information below).
Changes in oxygenation
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As infaunal burrowers the community lives in close association with hypoxic and even anoxic muddy substrata. In experiments, Amphiura chiajei exposed to decreasing oxygen levels only left its protected position in the sediment when oxygen levels fell below 0.54 mg l-1 (Rosenberg et al., 1991). This escape response increases its risk to predators. Mass mortality in a superficially similar species of ophiuroid, Amphiura filiformis from the south-east Kattegat was observed during severe hypoxic events (< 0.7 mg/l), while the abundance of Amphiura chiajei remained unchanged at the same site and time (Rosenberg & Loo, 1988). Nilsson (1999) maintained specimens of Amphiura chiajei in hypoxic conditions (1.8-2.2 mg/O2/l for eight weeks and recorded no deaths or witnessed specimens escaping to the surface. In moderately hypoxic conditions (1 mg/l) Nephrops norvegicuscompensates by increasing production of haemocyanin (Baden et al., 1990). In the laboratory this compensation lasted one week so at the level of the benchmark the species would not be killed. However, at levels of about 0.6 mg O2/l Nephrops died within four days. Catches of Nephrops norvegicus have been observed to be high in hypoxic conditions, probably because the animals are forced out of their burrows. Brissopsis lyrifera was reported as a species intolerant of hypoxia (Diaz & Rosenberg, 1995). It was recorded to leave its position within the substratum and lie exposed on the sediment surface in bottom waters with an oxygen concentration of 1 ml/O2/l (Baden et al., 1990). Thalassinidean mud-shrimps such as Calocaris macandreae are resistant to oxygen depletion. Anderson et al., (1991) reported oxygen availability within the burrow of Calocaris macandreae to be consistently severely low. It has a low oxygen consumption rate and responds to hypoxia by hyperventilation. However, at the benchmark level of 2 mg/O2/l for one week it is unlikely that the community composition would be greatly altered as many species are well adapted to conditions of hypoxia. However, as a key species, Brissopsis lyrifera, is especially intolerant of hypoxia and that the viability of some species may be affected if they are lying at the surface and exposed to predators or benthic trawls, intolerance has been recorded as high. On return to normoxia animals will rebury in to the substratum, but owing to the potential loss of the population of Brissopsis lyrifera, recoverability has been assessed to be moderate (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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The only major biological agent known to affect a species in this biotope is the dinoflagellate parasite, Hematodinium sp., now prevalent in Nephrops norvegicus populations from the west of Scotland, Irish Sea and North Sea. The Hematodinium parasite occurs in the blood and connective tissue spaces and appears to cause death in the host by blocking the delivery of oxygen to the host's tissues (Taylor et al., 1996). Heavily-infested animals become moribund, spend more time out of their burrows and are probably less able to evade capture by predators or fishing gear. However, the ecological consequences of this infestation are unknown but evidence to date suggests that the Nephrops stocks have not been seriously affected (Hughes, 1999b). The occurrence of the ascothoracidan parasite Ulophysema öresundense (Brattström) has been observed in the body cavity of Brissopsis lyrifera (Brattström, 1946). This parasite may cause sexual castration but no further information concerning the effect of this parasite on the population was found.
Introduction of non-native species
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There are no records of any non-native species invading the biotope and it is considered not to be relevant.
Extraction
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Neither Brissopsis lyrifera or Amphiura chiajei are targeted for collection or harvesting. However, Nephrops norvegicus, one of the species indicative of sensitivity, is the target of a large commercial fishery.

Findings from the western Irish Sea suggest that the structure of some Nephrops populations may render them vulnerable to over-exploitation (Hughes, 1998(b). During the spring and summer a gyre (circulating water mass) forms, which coincides with the period when Nephrops larvae are present in the plankton. The gyre retained the larvae in the vicinity of the parent population, rather than being carried off by currents into areas of unsuitable substratum (Hill et al., 1997; Hill et al., 1996). The retention of larvae by the gyre may be essential for the maintenance of the local Nephrops population and it is possible that over-exploitation of Nephrops in this area could lead to a self-perpetuating population decline owing to a reduction in recruitment.

In a study on the effects of otter trawling for Nephrops norvegicus on the benthos of locations in the Irish Sea and Scottish sea lochs, Ball et al., (2000) reported a reduction in the abundance of large-bodied and fragile organisms such as Brissopsis lyrifera and Amphiura chiajei and suggested that these species are particularly intolerant of trawling disturbance. An altered but stable community resulted, comprising of fewer species and reduced faunal diversity, consisting primarily of small polychaetes.
In areas of the North Sea where heavy demersal fishing for Nephrops norvegicus occurs, populations of Brissopsis lyrifera are likely to be reduced owing to damage inflicted to the 'test' by the fishing gear. Broken tests may be seen on the seabed (E.I.S. Rees, M. Costello, pers comm. to Connor et al., 1997). Similar evidence has been reported for other heart urchins. For example, Houghton et al. (1971), Graham (1955), de Groot & Apeldoorn (1971) and Rauck (1988) refer to significant trawl-induced mortality of heart urchin Echinocardium cordatum. A substantial reduction in the numbers of the species due to physical damage from scallop dredging has been observed (Eleftheriou & Robertson, 1992). Bergman & van Santbrink (2000) suggested that Echinocardium cordatum was one of the most vulnerable species to trawling. Bradshaw et al. (2000) suggested that fragile species such a urchins (e.g. Spatangus purpureus and Echinus esculentus), suffered badly from impact with a passing scallop dredge. Overall, species with brittle, hard tests are regarded to be sensitive to impact with scallop dredges (Kaiser & Spencer, 1995; Bradshaw et al., 2000).
Brittlestars have fragile arms that are likely to be damaged by abrasion or physical disturbance. Amphiura chiajei burrows in the sediment and extends its arms across the sediment surface to feed. Ramsay et al., (1998) suggests that Amphiura species may be less susceptible to beam trawl damage than other species of echinoid or tube dwelling amphipods and polychaetes. Bergman & Hup (1992) for example, found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Bradshaw et al. (2002) noted that the brittlestars Ophiocomina nigra, Ophiura albida and Amphiura filiformis had increased in abundance in a long-term study of the effects of scallop dredging in the Irish Sea. Brittlestars can tolerate considerable damage to arms and even the disc without suffering mortality and are capable of disc and arm regeneration so their recovery is likely to be rapid. Deeper burrowing crustaceans such as Calocaris macandreae may occasionally be displaced from burrow openings by towed gear (Atkinson, 1989). During long term monitoring of fishing disturbance on the Northumberland coast Frid et al., (1999) observed a decrease in the numbers of sedentary polychaetes, echinoid echinoderms and large (> 5 cm) brittlestars.

Therefore, while some authors have reported that brittlestars may increase in abundance in the long term, the dominant heart urchin species is likely to be reduced in abundance. Following the evidence of Ball et al. (2000), a high intolerance has been recorded. Recovery is likely to be moderate (see additional information), as members of the community are likely to remain and be able to repopulate. Brissopsis lyrifera may not regenerate as well as the brittle star (K. Hollertz, pers. comm.).

Additional information icon Additional information

Recoverability:
The biotope is likely to have a moderate capacity for recovery. The burrowing megafauna that characterize the biotope vary in their reproductive strategies and longevity. Brissopsis lyrifera is short lived (4 years) but is fecund and has shown clear evidence of successful and consecutive annual recruitment (Buchanan, 1967). Individuals become sexually mature in their forth year.

Amphiura chiajei is longer lived than Brissopsis lyrifera and reaches sexual maturity in its forth year, thus the population structure of these species will not reach maturity for at least this length of time. Once established, a cohort of Amphiura chiajei can dominate a population, even inhibiting its own consecutive recruitment, for up to 10 years. Time to reach sexual maturity is longer in Nephrops norvegicus, about 2.5 - 3 years and for the very long-lived Calocaris macandreae individuals off the coast of Northumberland did not become sexually mature until five years of age, and produced only two or three batches of eggs in their lifetime. In the biotope, polychaetes account for the vast proportion of the biomass, and these are likely to reproduce annually, be shorter lived and reach maturity much more rapidly.

Most of the characterizing species reproduce regularly but recruitment is often sporadic owing to interference competition with established adults of the same and other species. However, owing to the fact that the characterizing species take between 3 and 5 years to reach sexual maturity, it is likely that the time for the overall community to reach a fully diverse state will also be several years. It is likely that the low-energy hydrodynamic regime is an important factor in the maintenance of stable benthic populations in this biotope, as larvae are retained in the vicinity of the parent population.

This review can be cited as follows:

Budd, G.C. 2004. Brissopsis lyrifera and Amphiura chiajei in circalittoral mud. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 31/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=139&code=1997>