Biodiversity & Conservation

CR.MCR.EcCr.CarSp.Bri

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Ophiothrix fragilis and the other brittlestars that may be present in the biotope are epibenthic animals so substratum loss would result in their removal and hence mortality. Infaunal organisms and sessile species, such as Alcyonium digitatum and Urticina felina, would also be lost if substratum were removed. Although there are some mobile species in the biotope, such as the starfish Asterias rubens and Crossaster papposus, they are not very fast moving and so are also likely to be removed. Therefore, most species would be lost if substratum were removed and so the biotope is highly intolerant. Recovery within five years should be possible - see additional information below for rationale.
Smothering
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Dense populations of brittlestars do not persist in areas of excessive sedimentation, because high levels of sediment foul the brittlestars feeding apparatus (tube feet and arm spines), and ultimately suffocates them (Schäfer, 1962 cited in Aronson, 1992). Therefore, smothering by 5cm of sediment is likely to result in the death of most individuals. Aronson (1989) refers to the demise of Warner's (1971) Ophiothrix bed in Torbay, and tentatively attributes this to increased sedimentation caused by the localised dumping of construction materials. Other species in the biotope such as the soft coral Alcyonium digitatum and the anemone Metridium senile project above the substratum so may not be completely covered with sediment but feeding structures may become clogged. Infaunal organisms are not likely to be significantly affected. However, with the loss of brittlestars the biotope no longer exists so intolerance is assessed as high. For recovery see additional information.
Increase in suspended sediment
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Ophiothrix fragilis is a passive suspension feeder and a significant supply of suspended material is needed to meet the energetic costs of the great numbers of individuals in a brittlestar bed. Brittlestar beds occur in a variety of water flow regimes from sea lochs to more energetic coastal sites and so are likely to tolerate a variety of different suspended sediment concentrations. In the Dover Straits for example, the concentration of suspended particles changes with tidal cycles. Increases in suspension of inorganic particles may interfere with the feeding of this species (Aronson, 1992 cited in Hughes, 1998) and increase self-cleaning costs, particularly in non current-swept areas. Other species occurring in the biotope, such as hydroids, bryozoans and Alcyonium digitatum are similarly suspension feeders with the ability to self-clean. At the benchmark level of an increase of 100mg/l for a month will probably only have sublethal impacts rather than mortality of individuals and so intolerance of the biotope is considered to be low. Asterias rubens, a predator of brittlestars and possible structuring force in brittlestar beds, also has low intolerance to increases in suspended sediment. Overall species richness in the biotope is not likely to change. On return to pre-impact conditions normal feeding will resume and growth rates etc should rapidly return and so recoverability is recorded as very high.
Decrease in suspended sediment
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Ophiothrix fragilis, together with many of the other members of the biotope, is a passive suspension feeder and a significant supply of suspended material is needed to meet the energetic costs of the great numbers of individuals in a brittlestar bed. Brittlestar beds occur in a variety of water flow regimes from sea lochs to more energetic coastal sites and so are likely to tolerate a variety of different suspended sediment concentrations. In the Dover Straits for example, the concentration of suspended particles changes with tidal cycles. A longer term decrease in suspended sediment may reduce food supplies to brittlestar beds. However, Ophiothrix fragilis has a low respiration rate and can tolerate considerable loss of body mass during reproductive periods (Davoult et al., 1990) so restricted feeding may be tolerated and the intolerance of the biotope is recorded as low. Once normal feeding recommences it may take a short time for condition to be regained.
Desiccation
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The biotope only occurs in the circalittoral zone (below 15 m) and is not subject to desiccation.
Increase in emergence regime
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The biotope only occurs in the circalittoral zone (below 15 m) and is not subject to emergence.
Decrease in emergence regime
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The biotope only occurs in the circalittoral zone (below 15 m) and is not subject to emergence.
Increase in water flow rate
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The biotope is found in moderately strong and weak tidal streams. For example, in sea lochs, brittlestar beds experience only weak tidal streams, but on more open coastlines brittlestar beds are generally associated with higher-energy environments of moderate to strong current. In the Dover Strait, Ophiothrix beds experience current speeds of up to 3 knots (1.5m sec -1 during average spring tides (Davoult & Gounin, 1995b cited in Hughes, 1998). Similarly strong tidal flow rates were recorded in the Isle of Man i.e. 1.0 - 1.2 m/sec (Brun, 1969 in Hughes). In strong currents, the support provided by the interlinked arms of densely-packed brittlestars increases the stability of the beds and reduces the chance of individuals being swept away (Warner, 1971). However, although the flux of particles is greater increasing food supplies flow rates above 25 cm s-1 causes the arms of Ophiothrix fragilis to be brought down from being extended in the water column (Warner & Woodley, 1975; Hiscock, 1983). Thus, although brittlestar beds can tolerate increased water flow over tidal cycles a long term increase will probably prevent the population feeding and over a period of a year this is likely to cause the death of the population. A benchmark increase of two categories (see benchmark) could result in flow rates becoming very strong (>3m/sec) where individuals are likely to be washed away and the beds will disperse. Other species such as Asterias rubens and Echinus esculentus are also likely to be washed away. The overall impact on the biotope is a loss of brittlestars, and hence the biotope so intolerance is therefore, reported to be high. For recovery see additional information below.
Decrease in water flow rate
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The biotope is found in moderately strong and weak tidal streams. For example, in sea lochs brittlestar beds experience only weak tidal streams, but on more open coastlines they are generally associated with higher-energy environments. Davoult & Gounin (1995) found that water current speeds below 0.2m s-1 were optimal for suspension feeding, and that feeding activity ceased if velocity exceeded 0.3m s-1. These figures agree with those found by Warner (1971). Therefore, reduced water flow rates would probably enable the brittlestars to feed for longer although food supplies are likely to be reduced because resuspension of material from the sea bed is inhibited in slow flow regimes. Growth rates and fecundity may decline and hence the viability of the population may be reduced. If flow rates become negligible, over the period of a year the supply of food particles is not likely to be sufficient to maintain dense aggregations. Thus, beds will probably reduce in density or reduce in size so intolerance of the biotope is considered to be intermediate. Species diversity may also decline if food supplies are no longer adequate for the other suspension feeders, such as Alcyonium digitatum and Antedon bifida, in the biotope. For recovery see additional information below.
Increase in temperature
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Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa and Ophiocomina nigra from Norway to the Azores and Mediterranean. Other component species in the biotope also have a widespread distribution in the north east Atlantic. Within the British Isles the distribution of brittlestars is not limited by temperature, although individual species such as Ophiopholis aculeata and Ophiura robusta do show a latitudinal distribution pattern. Therefore, a long term chronic increase in temperature of 2 °C will probably have little effect on the species. In the Dutch Oosterschelde Estuary fluctuations in the abundance of Ophiothrix fragilis during the period 1979-1990 appeared to be driven by winter temperatures (Leewis et al., 1994). When winter temperatures increased in 1979-80 and 1987-88, populations of brittlestars increased enormously, the animals occupying 60 - 90 % of the available hard substratum in layers up to 5cm deep. Populations were reduced to less than 10 % following the cold winters in 1978-79, 1984-85 and 1985-86. Thus, increases in temperature may be beneficial to populations. However, short term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis. Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995). Intolerance of the biotope is reported to be low. See additional information below for recovery.
Decrease in temperature
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Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa and Ophiocomina nigra from Norway to the Azores and Mediterranean. Within the British Isles the distribution of brittlestars is not limited by temperature, although individual species such as Ophiopholis aculeata and Ophiura robusta do show a latitudinal distribution pattern. In the Oosterschelde Estuary, Ophiothrix fragilis was common in areas regularly experiencing winter temperatures down to 3°C, but was eliminated when temperatures fell to 0°C (Wolff, 1968 cited in Hughes, 1998). Therefore, a long term chronic decrease in temperature of 2 °C and a shorter term decrease of 5 °C will probably have little effect on the species and so intolerance of the biotope is reported to be low. However, in the Dutch Oosterschelde Estuary fluctuations in the abundance of Ophiothrix fragilis during the period 1979-1990 appeared to be driven by winter temperatures (Lewis et al., 1994). When winter temperatures decreased in 1978-79, 1984-85 and 1985-86 populations were reduced from 60 - 90 % coverage to less than 10 %. This may be because development of sexual maturity in Ophiothrix fragilis is dependent on temperature and day length. Short term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis. Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995).
Increase in turbidity
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The resultant light attenuation effects of increased turbidity will not affect brittlestar beds directly because brittlestars are suspension feeders and there are no macroalgae present in the biotope. However, an increase in turbidity in coastal waters may adversely affect phytoplankton production. Davoult & Gounin (1995) found that the growth rate of Ophiothrix in the Dover Strait was maximal in April/May, coinciding with the spring phytoplankton bloom. Therefore, increased turbidity may indirectly reduce growth rates of brittlestars because food supplies could be reduced. However, since phytoplankton may arrive from distant sources and brittlestars may also feed on organic detritus or invertebrate pray any effects are expected to be small and so the intolerance of the biotope to increased turbidity is assessed as low.
Decrease in turbidity
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The resultant light attenuation effects of increased turbidity will not affect brittlestar beds directly because brittlestars are suspension feeders and there are no macroalgae present in the biotope. However, a decrease in turbidity in coastal waters may affect phytoplankton production. Davoult & Gounin (1995) found that the growth rate of Ophiothrix in the Dover Strait was maximal in April/May, coinciding with the spring phytoplankton bloom. Therefore, decreased turbidity may indirectly increase growth rates of brittlestars because food supplies could be higher and so there may actually be a benefit to the biotope.
Increase in wave exposure
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Brittlestar beds are usually sheltered from strong wave action, but examples in moderately exposed situations are known (Ball et al., 1995 cited in Hughes). The interlocking of brittlestar arms probably enable beds to exist in areas of moderate wave exposure. However, areas of extreme wave energy probably prevent dense ophiuroid aggregations from forming (Aronson, 1992) and so the biotope is likely to disappear if wave exposure increases by two ranks on the wave exposure scale (see benchmark) and so intolerance is assessed as high. For recovery see additional information.
Decrease in wave exposure
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Brittlestar beds are usually sheltered from strong wave action so a decrease in wave exposure will not be damaging to the biotope. In sedimentary habitats a decrease in wave action may reduce resuspension of particles and hence food supplies. However, since beds normally occur in areas of moderately strong water currents which supply adequate food supplies the effects are likely to be negligible and the biotope is expected to have low intolerance to decreased wave exposure.
Noise
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There is little known about the effects of underwater sound on marine invertebrates. Although there are no records of brittlestars reacting to noise, sound vibrations may trigger some response. However, at the level of the benchmark the biotope is not likely to be sensitive to noise pollution. For example, brittlestar beds have been recorded from Kinsale Harbour on the south coast of Ireland where there is likely to be noise disturbance from passing boat traffic.
Visual Presence
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Ophiothrix fragilis and other brittlestars and starfish are likely to have poor facility for visual perception and consequently are probably not sensitive to visual disturbance. Movement of a hand near to Ophiothrix fragilis, for example, elicits no escape response (Skoeld, 1998). Although some other species, such as crabs and fish, may respond to visual disturbance such behaviour is not likely to have an impact on the nature and function of a brittlestar bed so the biotope is expected to be not sensitive to the factor.
Abrasion & physical disturbance
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Brittlestars have fragile arms that are likely to be damaged by abrasion. Brittlestars can tolerate considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration (Sköld, 1998). Fishermen tend to avoid brittlestar beds since the animals clog their nets (Jones et al., 2000). However, a passing scallop dredge is likely to remove, displace, or damage brittlestars caught in its path. Although several species of brittlestar were reported to increase in abundance in trawled areas (including Ophiocomina nigra), Bradshaw et al. (2002) noted that the relatively sessile Ophiothrix fragilis decreased in the long term in areas subject to scallop dredging. Overall, a proportion of the population is likely to be damaged or removed and an intolerance of intermediate has been recorded. Asterias rubens is also able to tolerate the loss of one or more arms. Although individuals can survive loss of one or more arms, the viability of a population with a high index of arm damage may be reduced as nutritional resources are used for repair and growth at the expense of gametogenesis. An average of 36%of individuals in five British brittlestar beds were regenerating arms (Aronson, 1989). Significant impacts in population density would be expected if such physical disturbance were repeated at regular intervals. Other species, particularly sessile fauna such as Alcyonium digitatum and Metridium senile may be sensitive to physical damage. Recoverability of the biotope will be high as remaining brittlestars re-orientate to fill in spaces and create new patches. However, component species may take some years to recolonize.
Displacement
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Although not highly active, Ophiothrix fragilis is a crawling epibenthic species. In removal experiments displaced Ophiothrix fragilis were seen to travel across the prevailing current, pausing and changing direction at intervals until other brittlestars were encountered (Broom, 1975). Starfish are able to right themselves if upturned in the process of displacement. Several other species in the biotope will also be tolerant of displacement. Adults of the large anemone Urticina felina, for example, can detach from the substratum and relocate although locomotive ability is very limited. Most burrowing infaunal species will be able to reburrow after displacement. Some attached species found in the biotope such as Alcyonium digitatum will die if displaced. However, the key characterizing species are not likely to be significantly affected so intolerance of the biotope is assessed as low and will resume normal activities when replaced on the substratum so recovery is immediate.

Chemical Factors

Synthetic compound contamination
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Echinoderms tend to be very sensitive to various types of marine pollution (Newton & McKenzie, 1995) but there is no more detailed information than this broad statement. In laboratory experiments Smith (1968) found the concentration of BP1002 (the detergent used in the Torrey Canyon oil spill clean-up) needed to kill the majority of Ophiocomina nigra was 5 ppm. Although there are no known examples of brittlestar beds being damaged by chemical pollutants such as pesticides or anti-parasite chemical used in aquaculture it is logical to suppose they would be adversely affected. However, in the absence of any evidence an assessment of intolerance is not possible.
Heavy metal contamination
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Adult echinoderms such as Ophiothrix fragilis are known to be efficient concentrators of heavy metals including those that are biologically active and toxic (Ag, Zn, Cd and Co) (Hutchins et al., 1996). There is no information available regarding the effects of this bioaccumulation. However, Gounin et al. (1995) studied the transfer of heavy metals (Fe, Mn, Pb, Cu, Cd) through Ophiothrix beds. They concluded that heavy metals ingested or absorbed by the animals transited rapidly through the body and were expelled in the faeces and did not appear to accumulate the metals in their tissues.
Hydrocarbon contamination
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Echinoderms tend to be very sensitive to various types of marine pollution (Newton & McKenzie, 1995). Adult Ophiothrix fragilis have documented intolerance to hydrocarbons (Newton & McKenzie, 1995). The sub-cuticular bacteria that are symbiotic with Ophiothrix fragilis are reduced in number following exposure to hydrocarbons. Exposure to 30,000 ppm oil reduces the bacterial load by 50% and brittle stars begin to die (Newton & McKenzie, 1995). The water-accumulated fraction of diesel oil has been found to be acutely toxic to Ophiothrix fragilis and Ophiocomina nigra although no field observations of beds being damaged by hydrocarbon pollution have been found. However, oil from spills would have to be dispersed deep into the water column to affect the biotope and since the biotope usually occurs in areas sheltered from wave action this is not very likely to occur. Intolerance of the biotope is reported to be low.
Radionuclide contamination
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Adult echinoderms such as Ophiothrix fragilis are known to be efficient concentrators of radionuclides (Hutchins et al., 1996). There is no information available regarding the effects of this bioaccumulation.
Changes in nutrient levels
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It appears that brittlestar beds are able to tolerate increased nutrient levels in the form of dissolved nutrients or particulate matter. For example, in the Bay of Brest in Brittany, Hily (1991) estimated that Ophiothrix beds with over 400 individuals m-2 can filter the equivalent of 30% of the total water volume of the bay daily. The inflow of nutrient-rich stream water into the bay leads to very high primary productivity, but eutrophication does not occur, apparently because of the removal of particulate matter by the benthic community of brittlestars. A dense aggregation of Ophiothrix and Ophiocomina appeared to be unaffected by the presence of a salmon farm within 100m (B. Ball pers. comm. in Hughes 1998). Since such farms often result in an increase in nutrients to the sea bed brittlestar beds appear to be able to tolerate some increase in nutrient levels (Hughes, 1998). Raymont (1950) recorded an increase in populations of Ophiocomina nigra following the addition of fertilisers to the waters of an enclosed basin of Loch Sween in Argyll. Therefore, intolerance of brittlestar beds is reported to be low. However, there may be indirect effects of nutrient pollution, deoxygenation and increased sedimentation, which may lead to the death of species including brittlestars. For recovery see additional information.
Increase in salinity
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Brittlestar beds are generally found in fully marine conditions. Echinoderms are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate. The inability of echinoderms to osmoregulate extracellularly causes body fluid volume to decrease when individuals are transferred to higher external salinity. Therefore, Ophiothrix fragilis and Asterias rubens are likely to lose water in increased salinities affecting their ability to maintain a hydrostatic skeleton and hence ability to move and to feed. Such an increase, whether a large short term increase or a longer term but smaller increase is likely to result in the death of most individuals and so the intolerance of the biotope is assessed as high. However, the biotope is a circalittoral community that is unlikely to experience increased salinity. See additional information below for recovery.
Decrease in salinity
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Brittlestar beds are generally found in fully marine conditions although Wolff, (1968) observed dense aggregations of the species occurring in normal salinities of 16 psu and even persisting down to 10 psu in the Oosterschelde Estuary. However, brackish water populations are likely to be locally adapted ecotypes or genotypes and populations normally existing in fully marine conditions are unlikely to be able to tolerate a rapid decrease in salinity and so intolerance of the biotope is considered to be high. The intolerance of Asterias rubens to decreasing salinity may be high. A sudden inflow of river water into an inshore coastal area caused mass mortality of the conspecific species Asterias vulgaris at Prince Edward Island, Canada (Smith, 1940, in Lawrence, 1995). Most other fauna in the biotope are marine species and so are also likely to be intolerant of a decrease in salinity. For recovery see additional information.
Changes in oxygenation
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Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l. During colder winter temperatures extreme hypoxia is known to cause mass mortality (Stachowitsch, 1984). However, Ophiothrix fragilis is known to have a low respiration rate (Migné & Davoult, 1997(b)) and some individuals may be able to tolerate oxygen levels of 2mg/l oxygen for a week and so intolerance of the biotope is set to intermediate.

Biological Factors

Introduction of microbial pathogens/parasites
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Brittle stars, such as Ophiothrix fragilis, have symbiotic subcuticular bacteria. The host-bacteria association can be perturbed by acute stress and changes in bacterial loading may be used as an indicator of sub-lethal stress (Newton & McKenzie, 1995). The dense aggregations of brittlestars probably provide the ideal conditions for the spread of diseases or parasites. Although no such infestations have been recorded for brittlestar beds there are several examples of echinoderm populations that have been dramatically reduced by sudden outbreaks of epidemic diseases. Therefore, although an intolerance rank of low is reported, epidemic disease does have the potential to significantly affect the biotope.
Introduction of non-native species
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There are no records of any non-native species invading the biotope that may compete with or predate upon brittlestars and so the biotope is assessed as not sensitive. However, as several species have become established in British waters there is always the potential for adverse effects to occur.
Extraction
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It is highly unlikely that any of the species indicative of sensitivity would be targeted for extraction. Fishermen tend to avoid brittlestar beds since the animals clog their nets (Jones et al., 2000). Although several species of brittlestar (including Ophiocomina nigra) were reported to have increased in abundance in trawled areas, Bradshaw et al. (2002) noted that the relatively sessile Ophiothrix fragilis decreased in the long term in areas subject to scallop dredging. The bed may contract in size as individual brittlestars move to re-establish contact with neighbours or the number of low density patches could increase. If water currents were very strong some animals may be washed away as the support provided by other individuals in dense aggregations decreases. However, provided conditions remain the same recovery is likely to be high, once extraction or fishing has stopped, as the bed re-establishes the high and low density patches normally seen with brittlestar beds.

Other species such as Asterias rubens and Alcyonium digitatum may also be intolerant to dredging / trawling although again their recovery is likely to be high.

Additional information icon Additional information

Recoverability
Breeding of the main bed forming brittlestar Ophiothrix fragilis occurs annually and there may be multiple recruitment phases (Davoult et al., 1990). Reproductive capability may be reached in 6-10 months depending on time of recruitment (Davoult et al., 1990) so recovery is likely to be high. However, lost populations may not always be replaced because settlement of larvae of Ophiothrix fragilis is highly dependent on hydrographic conditions and consequently may be unpredictable. In the strong water currents of the English Channel larvae can disperse up to 70-100 km and establish populations elsewhere. Therefore, if conditions change recruitment may fail and lost populations may not be replaced. For example, dense aggregations of Ophiothrix fragilis in the Plymouth area have not been seen to have recovered since their decline in the 1970's are it is suggested that cyclical changes in the oceanographic cycle affecting the western Channel resulted in increased predation pressure from Luidia ciliaris and also recruitment failure of Ophiothrix fragilis. If any adults remain aggregations may re-establish as individual brittlestars tend to crawl back and forth across water currents until a conspecific is found (Broom, 1975). Other species occurring in the bed recruit mostly from the plankton or are mobile so that their recovery would be rapid.

This review can be cited as follows:

Hill, J.M. 2001. Ophiothrix fragilis and/or Ophiocomina nigra beds on slightly tide-swept circalittoral rock or mixed substrata. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 31/07/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=278&code=2004>