Biodiversity & Conservation

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The majority of the species within the biotope are permanently attached to the substratum or are slow moving therefore substratum removal would result in loss of faunal populations so the intolerant of the biotope has been assessed to be high. Following the loss of adult populations recovery would be dependent upon re-colonization by larvae. Recoverability has been assessed to be moderate (see additional information below).
Smothering
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Although the biotope and those biotopes it represents, occur in fairly silty environments, sudden smothering by 5 cm of sediment is likely to result in the death of many species. Feeding structures of the sea-squirts and pores of the sponges would probably become clogged and suffocate the organisms (Bakus, 1968). Some of the mobile predators such as Marthasterias glacialis may be able to uncover themselves. However, many species are likely to die so intolerance has been assessed to be high. Furthermore in the sheltered conditions in which this biotope is found smothering material is not likely to be removed very rapidly. Following the loss of adult populations recovery would be dependent upon re-colonization by larvae, but recoverability has been assessed to be moderate (see additional information below).
Increase in suspended sediment
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Many of the species are suspension feeders and an increase in suspended sediment may cause interference and blockages, for example in sponge canals and pores. Whilst some species may have an adaptive mechanism to 'slough' off relatively small amounts of such material, an energetic cost would be involved and thus intolerance has been assessed to be low, as the viability of species may be reduced. At the benchmark level the factor operates for a period of one month, so recovery should be very rapid on return to prior conditions. Recoverability has therefore been assessed to be immediate.
Decrease in suspended sediment
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Many of the species are suspension feeders and a decrease in suspended sediment may reduce interference and blockages, for example of sponge canals and pores. However, the species in the biotope are reliant on suspended organic material as a food resource, consequently a reduction would have an effect upon the species viability. However for the duration of one month effects are unlikely to be significant and intolerance has been assessed to be low. Recovery has been assessed to be immediate as optimal feeding would commence on return to prior conditions.
Desiccation
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The biotope is entirely subtidal and will not be subject to desiccation.
Increase in emergence regime
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The biotope is entirely subtidal and will not be subject to emergence.
Decrease in emergence regime
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The biotope is entirely subtidal and will not be subject to emergence.
Increase in water flow rate
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The biotope consists mainly of species firmly attached to the substratum and which would be unlikely to be displaced by an increase in the strength of tidal streams at the benchmark level. However, many of the species in this biotope are active suspension feeders An increase in tidal flow rate to moderately strong or strong (0.5 -3 m / sec) may cause loss of posture and interfere with feeding mechanisms, particularly in the more delicate species like Ciona intestinalis and Hiscock (1983) illustrated how the inhalant siphons of Ascidia mentula closed when exposed to a flow of 0.3 m /s. Feeding activity would be reduced subsequently affecting species viability. In increased tidal flow and over a period of one year the biotope would probably begin to change to another, with species tolerant of limited wave action and higher tidal flow, the biotope would not be recognized and intolerance has therefore been assessed to be high. Recovery has been assessed to be moderate on return to prior conditions (see additional information below).

In the SCR.AmenCio.Met biotope the plumose anemone, Metridium senile may benefit from increased water flow rate. Hiscock (1983) described the reaction of Metridium senile to increasing flow rate (to 90 cm/s) in a flume. The anemones were stimulated to expand tentacles as flow increased and only withdrew them at flow rates in excess of 70 cm/s. They were not swept away. Whilst large Metridium senile thrive in tidal narrows where surface velocity may be in the region of 3-5 knots, they do not appear to occur in very strong tidal flows (exceeding 5 knots) such as in the Gulf of Corryvreckan or Strangford Lough Narrows. Increase in water flow rate is therefore likely to favour settlement and growth of Metridium senile especially because of increased food supply. However, above 5 knots, adverse effects including inability to feed and possible displacement may occur.
Decrease in water flow rate
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The characterizing species of the biotopes are active suspension feeders and whilst they create their own inhalant current to draw food to them, they are still in part, reliant on some water flow, even if very weak, to deliver food. Water flow rates at locations where the biotopes are found are typically weak or very weak. A further reduction may consequently have an effect on the community as a result of reduced food supply. Over the duration of one year loss of condition and reduced viability might be expected, intolerance has therefore been assessed to be low. Recoverability has been assessed to be very high on return to prior conditions as optimum feeding commences.
Increase in temperature
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Many of the species found within the biotopes represented by this review have distributions that extend to the south of the British Isles, suggesting that the communities would be tolerant of a long-term chronic increase in temperature of 2°C. There are shorter term cyclical temperature fluctuations operating in the north Atlantic (Maximov et al., 1972), such as those associated with the now named North Atlantic Oscillation (NAO), with a period of 7-8 years. A variety of benthic populations have been demonstrated to co-fluctuate in abundance accordingly (Gray & Christie, 1983), and such cyclic fluctuations are recorded from typical rocky circalittoral species such as Ciona intestinalis (Lundläv, 1985) and Ascidia mentula (Lundälv & Christie, 1986). For instance, northerly populations of Ciona intestinalis did not begin to reproduce until temperatures rose above 8°C. No evidence has been found of adverse effects of short-term temperature increases, therefore an assessment of not sensitive has been made but with very low confidence.
Decrease in temperature
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During the severe winter of 1962-63, although no significant mortality was noted, Crisp et al. (1964) found that many compound ascidians were retarded in renewal of the colony after 'winter budding'. In the Mediterranean, growth of Ciona intestinalis is optimal at between 15-20°C and most of the adult population dies below 10°C. During cold spells the population is maintained through survival of young individuals which are more cold tolerant. Furthermore, the species occur extensively in shallow water habitats of Swedish and Norwegian fjords where water temperatures fall to low levels (K. Hiscock, pers. comm.). Intolerance has therefore been assessed to be low. Recovery from surviving tissue is likely to be rapid and re-colonization will occur from annual recruitment from the plankton. Recoverability has therefore been assessed to be very high.
Increase in turbidity
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It is unlikely that the light attenuating effects of an increase in turbidity would be of direct significance to the biotopes, as the biotope is an animal dominated community, dependent on secondary production rather than light dependent photosynthesis. However, as active suspension feeders, sponges and ascidians would feed upon phytoplankton as well as suspended detritus and increased turbidity may consequently impact upon a food resource. Intolerance has been assessed to be low as viability of the community would probably be reduced for the duration of one year. Recovery has been assessed to be immediate on return to prior conditions as optimal feeding would resume.
Decrease in turbidity
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The biotope is an animal dominated community, dependent on secondary production rather than light dependent photosynthesis. Following a decrease in turbidity and increased water clarity, competition with encrusting red algae for the substratum may increase in the SCR.AmenCio.Met biotope. However, a slight change in the species composition in the biotope will not radically alter the nature of the community and an assessment of not sensitive has been made.
Increase in wave exposure
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Increased wave action would clear silt from vertical surfaces of the substratum, making rock available for settlement of species more characteristic of wave exposed conditions, for instance erect Bryozoa and barnacles. It seems most likely that individual sponges would contract during strong wave action but survive for short periods. Suberites carnosus can contract to about ¼ of its fully expanded size when disturbed. However, the stalk of fig-shaped massive Suberites carnosus breaks when bent through 20 degrees (when in contracted condition) which is likely if subjected to wave oscillation for an extended period (Ackers & Moss, 1985). Furthermore, if sponges remain contracted for a prolonged period owing to a disturbance, feeding opportunities will be minimised and reduced viability would result. Suberites carnosus is a key structural and characterizing species in the SCR.SubSoAs biotope, following its loss the biotope would not be recognized and intolerance has been assessed to be high. Recoverability has been assessed to be moderate (see additional information below).
Decrease in wave exposure
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The biotopes occur in locations which are already very or extremely wave sheltered. Whilst water movement is required to bring food to suspension feeding species in the biotope, tidal streams are probably more important than wave oscillation in doing so. Consequently, an assessment of a further decrease in wave exposure was not considered relevant.
Noise
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Species in the biotope are likely to have poor ability for detection of noise vibrations and as such are unlikely to be intolerant of noise
Visual Presence
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Species characteristic of the biotope are sessile invertebrates and are no capable of visual perception. Other species that frequent the biotope e.g. crabs and fish may be temporally disturbed but there would be no effect on the biotope.
Abrasion & physical disturbance
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Physical impact is likely to cause damage and mortality to exposed individuals but, because sponges such as Suberites carnosus may be able to regenerate from fragments it seems likely that damaged individuals may re-grow. Mobile fishing gear is likely to scrape individuals off the substratum and damage others. In the SCR.AmenCio.Met biotope Metridium senile is soft, flexible and can reform its attachment to the substratum, the species also habitually reproduces by basal laceration so will regrow. Many species of ascidian are soft and delicate so would probably be damaged and recovery would be reliant on recolonization by larvae. For example, Ciona intestinalis is a large ascidian, with a soft, retractile body. Physical disturbance by a passing scallop dredge is likely to cause physical damage and death. The light bulb tunicate is permanently attached to the substratum and is unable to move out of the way from abrasive objects. The body of the species is soft and delicate, so abrasion is likely to cause physical damage and possibly death. Overall, physical disturbance by mobile fishing gear is likely to remove a proportion of all groups within the community and attract scavengers to the community in the short term. Therefore, an intolerance of intermediate has been recorded. Large scale physical disturbance would be similar to substrate removal (see above).

The recolonization of epifauna on vertical rock walls was investigated by Sebens (1985, 1986). He reported that rapid colonizers such as encrusting corallines, encrusting bryozoans, amphipods, and tubeworms recolonized within 1-4 months. Ascidians such as Dendrodoa carnea, Molgula manhattensis and Aplidium spp. achieved significant cover in less than a year, and, together with Halichondria panicea, reached pre-clearance levels of cover after 2 years. A few individuals of Alcyonium digitatum and Metridium senile colonized within 4 years (Sebens, 1986). Large sponges and anemones would probably take longer to reach pre-clearance levels. Therefore, a recoverability of moderate has been recorded.

Displacement
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The many sponge and ascidian species within the biotope are sessile organisms attached to the substratum that cannot re-attach once removed, thus intolerance has been assessed to be high. Recoverability has been assessed to be moderate (see additional information below).
However, Wahl (1984) observed that Metridium senile detached from the substratum during the first week of deoxygenation in the Inner Flensburg Fjord and may drift away eventually to resettle. Metridium senile from the SCR.AmenCio.Met biotope therefore seems able to survive displacement from the substratum and intolerance may be lower, but, presumably the anemone may be damaged during the displacement in which case some repair may be needed.

Chemical Factors

Synthetic compound contamination
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The plumose anemone, Metridium senile is a major component of the SCR.AmenCio.Met biotope represented by this review. Mercier et al. (1998) exposed Metridium senile to tri-butyl tin contamination in surrounding water and in contaminated food. The species produced mucus 48 hours after exposure to contaminated seawater. TBT was metabolised but the species accumulated levels of butyl tins leading the authors to suggest that Metridium senile seemed vulnerable to TBT contamination. However, Mercier et al., (1998) did not indicate any mortality and, since Metridium senile is a major component of jetty pile communities immediately adjacent to large vessels coated with TBT antifouling paints, intolerance has been assessed to be low specifically to TBT. No other information has been found on effects of contaminants on other species of the biotopes, so confidence is very low.
Heavy metal contamination
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Several species of ascidian are known to accumulate high concentrations of trace metals. However, no information was found with reference to effects on specific species of the biotope and insufficient information has been recorded. However, Cole et al. (1999) stated that arsenic, mercury, lead, zinc and copper are very toxic to invertebrates in general, and that cadmium is considered toxic to them.
Hydrocarbon contamination
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Insufficient information.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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There is some evidence that increased levels of organic nutrients is of benefit to populations of Ciona intestinalis (Naranjo et al., 1996). Dissolved organic matter can also form a nutritional component for other species such as sponges and an assessment of not sensitive* has been made, as improved growth and fecundity may result form moderate nutrient enrichment.
Increase in salinity
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The biotope occurs in fully saline waters (Connor et al., 1997a) so an increase in salinity has been assessed not to be relevant.
Decrease in salinity
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Components of the biotope community are tolerant of variable salinities. For instance Ascidia mentula survives down to 20 psu, Ascidiella aspersa is tolerant of salinities down to 18 psu and is often common in estuaries, whilst Ciona intestinalis is tolerant of salinities as low as 11 psu (Fish & Fish, 1996). Shallow examples of the biotope may be subject to some reduction in salinity and, in view of the tolerance of at least some component species an intolerance assessment of low has been made. Recoverability has been assessed to be very high on return to prior conditions following a short term decline in salinity as species would remain in situ. Over a period of one year some component species may decline in abundance and recovery may therefore take longer (see additional information below).
Changes in oxygenation
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The biotope may be found in locations with restricted water renewal (tidal streams: weak or very weak) where oxygen concentrations may drop. Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg O2/l and probable adverse effects below 2 mg O2/l. As a consequence of deoxygenation characteristic species of the biotope would probably be affected in terms of mortality, diminished condition, feeding or reproductive capability. The biotope may become degraded so intolerance has been reported to be intermediate. For instance, Hiscock & Hoare (1975) studied the sublittoral ecology of Abereiddy Quarry, Wales. During the summer the quarry developed a marked thermocline at 12-14m below which the water was 4°C colder, had lower light transmittance and was deoxygenated with a high level of hydrogen sulphide. Stratification of the water body occurred between spring and autumn resulting in a sharp decline in the amount of dissolved oxygen present for respiration. The oxycline was suggested to be responsible for the drastic reduction in species diversity below depths of 10-12 m. During July 1973, water at 10 m was only 55% saturated with oxygen, and the maximum depth to which various species extended was suggested to indicate their tolerance to low oxygen concentrations. On the stone wall Suberites carnosus was found occasionally down to 10 m depth but was more frequent at 8 m depth. Recoverability of characteristic species of the biotope has been assessed to be moderate (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information.
Introduction of non-native species
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The ascidian,Styela clava, which occurs in the biotope, but is rare, is a non-native species originating from the Pacific. It was introduced into European waters, from the Mediterranean to SW Britain by shipping and was first recorded in Plymouth in 1953 (Carlisle, 1954). It is a hardy species capable of tolerating salinity changes and temperature fluctuations and has spread rapidly (Eno, 1997; Houghton & Millar, 1960). As Styela clava is a hardy species it is unlikely that it will disappear once established, however, Styela clava has not been observed to dominate areas but rather occurs amongst other species in the biotope, usually growing taller than surrounding biota. However, competition for food between individuals and with other species can occur and therefore intolerance has been assessed to be low as the viability of other species within the biotope may suffer. Recoverability has been assessed to be very high as species would not be lost from the biotope (see additional information below).
Extraction
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It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction and we have no evidence for the indirect effects of extraction of other species on this biotope.

Additional information icon Additional information

Recoverability:
Immediate colonization following a catastrophic disturbance would depend on the availability of planktonic larvae at the time and any subsequent succession to the re-establishment of the biotope. Work undertaken by Sebens (1986) suggests that many of the species characteristic of the biotope would be likely to re-colonize rapidly. For instance, the sponge Halichondria panicea and the colonial ascidian Aplidium reached pre-removal cover in just under two years following experimental removal whilst Metridium senile took four years to become re-established in the same experiment. However, for some characterizing species, such as species of Suberites or branching sponges, little is known of colonization or growth rates. Whilst the biotope may be re-established within five years, the presence of all characteristic species fully grown may take longer and recoverability has been assessed to be moderate.

This review can be cited as follows:

Budd, G.C. 2002. Suberites spp. and other sponges with solitary ascidians on very sheltered circalittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 30/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=94&code=1997>