The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

The species is permanently attached to the substratum so substratum loss will result in loss of the population. Therefore an intolerance of high has been reported. For recoverability, see additional information below.

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

The species is permanently attached to the substratum and is an active suspension feeder so that some clearance of smothering silt may occur. The species can extend its siphons to a small extent above silt. It can also most likely maintain a passage through the silt to the siphons. However, groups are likely to be covered by silt or other material and therefore be subject to hypoxia. Of greatest importance may therefore be the ability of Molgula manhattensis to live in the hypoxic conditions that might occur under silt. Sagasti et al. (2000) demonstrated that Molgula manhattensis can withstand episodes of hypoxia and so mortality is unlikely to occur. Intolerance is likely to be low. Recovery of condition is likely to be very high.

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Molgula manhattensis frequently occurs in habitats with high levels of suspended matter. Increased suspended sediment may potentially have some detrimental effects in clogging feeding filtration mechanisms, however, there are possible benefits from increased siltation (Naranjo et al. 1996). On resumption of normal energy expenditure and feeding, condition should be restored rapidly.

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Although there may be some reliance on the organic material associated with suspended silt for nutrition, the reduced need for energy expenditure to remove silt may be beneficial. On balance, the species is most likely tolerant.

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

The species occurs in the intertidal near to low water level and so is exposed to some desiccation. Nevertheless, it has a soft body and may be easily subject to drying-up. Exposure to desiccating influences for one hour will probably kill a proportion of the population. Therefore, an intolerance of intermediate has been recorded. For recoverability, see additional information below.

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

The species occurs in the intertidal near to low water level and so is exposed to some emergence. Nevertheless, it has a soft body and may be easily subject to drying-up. Exposure to desiccating influences as a result of increased emergence will probably kill a proportion of the population. Therefore, an intolerance of intermediate has been recorded. For recoverability, see additional information below.

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

As a predominantly sublittoral species, increase in emergence may benefit populations found on the lower shore by providing additional substratum for colonization.

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

As a general rule, ascidians require a reasonable water flow rate in order to ensure sufficient food availability. High water flow rates may also be detrimental to feeding ability and posture. Hiscock (1983) found that, for the solitary ascidian Ascidia mentula, siphons closed when the current velocity rose above about 15 cm/sec. It seems likely therefore that some reduction in feeding would occur with increased water flow rate although that would result in slower growth and loss of condition but not mortality. Intolerance has therefore been assessed as low. On resumption of normal energy expenditure and feeding, condition should be restored rapidly.

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

As a general rule, ascidians require a reasonable water flow rate in order to ensure sufficient food availability and oxygen supply. However, Molgula manhattensis is frequently found in areas with minimal water exchange and renewal such as harbours, marinas and docks. Intolerance has therefore been assessed as low. Sagasti et al. (2000) demonstrated that Molgula manhattensis can withstand episodes of hypoxia and so, even if stagnation occurs for short periods, mortality is unlikely to occur. Whilst food availability may be reduced in comparison with areas with higher flow rates, on resumption of normal energy expenditure and feeding, condition should be restored rapidly.

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

In the North Atlantic and in Pacific locations where Molgula manhattensis has developed populations, temperatures may be higher by several degrees than in Britain and Ireland. It is not therefore expected that increased temperatures at the level of the benchmark will adversely affect populations.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

The distribution of Molgula manhattensis in the North Atlantic extends to Maine and to northern Norway so that decrease in temperatures at the level of the benchmark is unlikely to adversely affect populations.

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Molgula manhattensis lives in harbours and the entrances to estuaries where turbidity may increase to high levels. In experiments aimed at identifying the effects of adding clay suspensions to water, Frank et al. (2000) showed the ability of Molgula manhattensis to increase clearance rates as concentration of particles increased. It is not expected that increase in turbidity at the level of the benchmark will adversely affect Molgula manhattensis.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Although there may be some reliance on the organic material associated with turbidity for nutrition, the reduced need for energy expenditure to clear silt may be beneficial and an intolerance of tolerant* has been recorded.

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

As a general rule, ascidians require a reasonable water flow rate in order to ensure sufficient food availability and oxygen supply. However, high water flow rates may be detrimental to feeding ability and posture. Hiscock (1983) found that, for the solitary ascidian Ascidia mentula, siphons closed when current velocity rose above about 15 cm/sec. It seems likely therefore that some reduction in feeding would occur with increased oscillatory water movement although that would result in slower growth and loss of condition but not mortality. On resumption of normal energy expenditure and feeding, condition should be restored rapidly. Although individuals are firmly attached, there is a possibility that, especially in closely packed colonies, wave action may displace large numbers. Intermediate intolerance but with low confidence is recorded. Recovery is likely to be very high (see additional information below).

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

As a general rule, ascidians require a reasonable water flow rate in order to ensure sufficient food availability and oxygen supply and maintain surfaces clean of silt. If decrease in wave action occurs where tidal flow continues to provide favourable conditions, the species may benefit because of reduction in the likelihood of displacement. However, Molgula manhattensis is frequently found in areas with minimal water exchange and renewal such as harbours, marinas and docks suggesting that decrease in wave exposure even in the absence of significant tidal currents would not be adverse. Sagasti et al. (2000) demonstrated that Molgula manhattensis can withstand episodes of hypoxia and so, even if stagnation occurs for short periods, mortality is unlikely to occur. Whilst food availability may be reduced by reduction in wave action, on resumption of normal energy expenditure and feeding, condition should be restored rapidly. Overall, bearing in mind that the favoured location for Molgula manhattensis is in wave sheltered habitats, the species might benefit from decrease in wave exposure.

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Tunicates are not known to have organs sensitive to noise.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Tunicates are not known to respond to visual presence.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Colonies are flexible and soft providing a buffer against external abrasion from such factors as a fishing pot landing on a colony. However, individuals and colonies may be scraped off the rock by an anchor or passing dredge Intolerance is therefore assessed as high. For recoverability, see additional information.

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

The colonies are attached permanently to the substratum and will not re-attach so that displacement, even if to a suitable habitat, would most likely result in mortality. An assessment of high intolerance is therefore made. For recoverability, see additional information below.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

Molgula manhattensis is most likely tolerant of synthetic chemical pesticides. Weis & Weis (1992) found that the ascidian was commonly present, although in small numbers, on wood treated with chromated arsenate. In mesocosm experiments, Flemer et al. (1995) studied the effect of the pesticide endosulfan and found that the average abundance of Molgula manhattensis increased with increasing concentration of the pesticide possibly as a result of reduced competition with more susceptible organisms. The high abundance of the species in harbours where levels of tributyl tin are or were likely to be high also suggests tolerance. Molgula manhattensis may benefit from tolerance to synthetic pollutants by occupying space that would have been colonized by less tolerant species. No evidence has been found for sublethal effects from which recovery would be likely to be rapid. Overall, an intolerance of low is suggested but with a low confidence.

Heavy metal contamination

Evidence

No information has been found.

Hydrocarbon contamination

Evidence

No information has been found.

Radionuclide contamination

Evidence

No information has been found.

Changes in nutrient levels

Evidence

No information has been found.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Van Name (1945), quoted in Kott (1985), noted that Molgula manhattensis occurred in salinities equivalent to 20 to 36 psu whilst Hartmeyer (1923), quoted in Tokioka & Kado (1972), recorded Molgula manhattensis in brackish (16-30 psu) water of the Belt Sea. Ascidians are mainly found in full salinity and it is not expected that increase in salinity will have an adverse effect except in the possibility of allowing other species to out-complete Molgula manhattensis.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Van Name (1945), quoted in Kott (1985), noted that Molgula manhattensis occurred in salinities equivalent to 20 to 36 psu whilst Hartmeyer (1923) quoted in Tokioka & Kado (1972) recorded Molgula manhattensis in brackish (16-30 psu) water of the Belt Sea. A fall in salinity from full to reduced would not therefore be expected to have an adverse effect. However, in situations where salinity is already variable or reduced, a further lowering is likely to result in mortality. Intolerance is indicated as intermediate but may be high. For recoverability, see additional information.

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

Sagasti et al. (2000) demonstrated that Molgula manhattensis can withstand episodes of hypoxia and so intolerance is likely to be low amounting to some loss in condition.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

Saffo & Davis (1982) describe how the renal sac of Molgula manhattensis consistently harbours a collection of fungus-like cells, "Nephromyces". In turn, Nephromyces is infected with bacteria (Saffo, 1990). Both fungus and bacteria act in a symbiotic way and not strictly as harmful pathogens. In the absence of information about other pathogens, 'not sensitive' is recorded.

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

There are no non-native species currently known to displace or adversely affect Molgula manhattensis although the stalked ascidian Styela clava may occur in similar habitats.

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

There is no known extraction of this species.

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

There are no species with which Molgula manhattensis is associated that may be extracted.