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information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

A sea squirt (Ascidiella scabra)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.

Researched byDr Keith Hiscock Refereed byThis information is not refereed.
Authority(Müller, 1776)
Other common names- Synonyms-

Summary

Description

Ascidiella scabra is a small solitary ascidian (usually <4cm long) with an ovate body and anterior siphons separated by a distance about one quarter the body length. The test is semi-transparent and usually tinged red.

Recorded distribution in Britain and Ireland

Present all around Britain and Ireland.

Global distribution

Present from the Faeroe Islands and Trondheimfjord in the north, occurring in the Kattegat and extending into the Mediterranean.

Habitat

Present attached to natural and artificial hard substrata. Also present attached to algae such as Fucus serratus and on kelp stipes. Lindsay and Thompson (1930) suggested a depth range of 5-300 m although records from fucoid algae indicate intertidal occurrence.

Depth range

+2-300 m

Identifying features

  • Solitary but may occur in tightly packed groups.
  • Body ovate or elliptical up to 4 cm long.
  • Siphons anterior and separated from each other by about one quarter the body length.
  • Test semi-transparent and usually tinged red.
  • The internal structure includes 30-110 tentacles, the number always exceeding the number of inner longitudinal vessels of the branchial sac.

    Additional information

    Specimens as large as 7.5 cm have been sampled from the Dogger Bank. Almost colourless examples can be found.

    Listed by

    - none -

    Further information sources

    Search on:

    NBN WoRMS

    Biology review

    Taxonomy

    PhylumChordataSea squirts, fish, reptiles, birds and mammals
    ClassAscidiaceaSea squirts
    OrderPhlebobranchia
    FamilyAscidiidae
    GenusAscidiella
    Authority(Müller, 1776)
    Recent Synonyms

    Biology

    Typical abundanceModerate density
    Male size range< 4cmMale size at maturity
    Female size rangeSmall-medium(3-10cm)Female size at maturity
    Growth formBullate / SaccateGrowth rate
    Body flexibilityMobility
    Characteristic feeding methodActive suspension feeder, Non-feeding
    Diet/food source
    Typically feeds onSuspended particles including phytoplankton
    Sociability Environmental positionEpibenthic
    DependencyIndependent.
    SupportsNone
    Is the species harmful?No

    Biology information

    Ascidiella scabra is usually about 2-3 cm in length although specimens from the Dogger Bank have been recorded at 7.5 cm in length (Lindsay & Thompson, 1930)

    Habitat preferences

    Physiographic preferencesOpen coast, Offshore seabed, Strait / sound, Sea loch / Sea lough, Ria / Voe, Estuary, Enclosed coast / Embayment
    Biological zone preferencesLower circalittoral, Lower infralittoral, Sublittoral fringe, Upper circalittoral, Upper infralittoral
    Substratum / habitat preferencesMacroalgae, Artificial (man-made), Bedrock, Biogenic reef, Cobbles, Large to very large boulders, Small boulders, Under boulders
    Tidal strength preferencesModerately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Weak < 1 knot (<0.5 m/sec.)
    Wave exposure preferencesExtremely sheltered, Moderately exposed, Sheltered, Very sheltered
    Salinity preferencesFull (30-40 psu), Variable (18-40 psu)
    Depth range+2-300 m
    Other preferencesNo text entered
    Migration PatternNon-migratory / resident

    Habitat Information

    -

    Life history

    Adult characteristics

    Reproductive type Reproductive frequency Annual protracted
    Fecundity (number of eggs) Generation time <1 year
    Age at maturity Not known. Probably <6months. Season March - Insufficient information
    Life span 2-5 years

    Larval characteristics

    Larval/propagule type - Larval/juvenile development
    Duration of larval stage 2-10 days Larval dispersal potential 1 km -10 km
    Larval settlement period

    Life history information

    Lindsay & Thompson (1930) noted the great fecundity of Ascidiella scabra and that eggs were produced (in the laboratory) from March onwards. Berrill (1950) notes that the species is oviparous, that the eggs are small (0.16 mm diameter) and sink in still water. Tadpole larvae emerge from eggs.

    Sensitivity reviewHow is sensitivity assessed?

    Physical pressures

     IntoleranceRecoverabilitySensitivityEvidence/Confidence
    High Very high Low High
    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.
    Low Immediate Not sensitive Moderate
    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.Ascidiella scabra also attaches to other erect biota and, in such situations, may escape smothering effects. Intolerance is likely to be low. Recovery of condition is likely to be very high.
    Low Immediate Not sensitive Moderate
    Ascidiella scabra frequently occurs in habitats with high levels of suspended matter. Robbins (1985b) undertook experiments to establish the possible effects of high inorganic particulate concentrations on Ascidiella scabra. He concluded that growth rate was likely to be reduced and mortality was possible in high levels of suspended sediment. Therefore an intolerance of low has been recorded. On resumption of normal conditions, energy expenditure and feeding should be restored rapidly.
    Tolerant Not relevant Not sensitive Moderate
    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.
    Intermediate Very high Low Moderate
    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.
    Intermediate Very high Low Moderate
    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.
    Tolerant* Not relevant Not sensitive* High
    As a predominantly sublittoral species, increase in emergence may benefit populations found on the lower shore by providing additional substratum for colonization.
    Low Immediate Not sensitive High
    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.
    Low Immediate Not sensitive Moderate
    As a general rule, ascidians require a reasonable water flow rate in order to ensure sufficient food availability and oxygen supply. However, ascidians are active suspension feeders and can thrive in conditions of very little flow. 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.
    Tolerant Not relevant Not sensitive High
    In the North Atlantic and Mediterranean where Ascidiella scabra occurs, 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.
    Tolerant Not relevant Not sensitive High
    Ascidiella scabra occurs north to Trondheim in Norway and the Faroe Islands, where temperatures may be lower by several degrees than in Britain and Ireland. Crisp (1964) indicates that no certain mortality was observed in ascidians following the severe 1962-63 winter. It is not expected therefore that decreased temperatures at the level of the benchmark will adversely affect populations.
    Tolerant Not relevant Not sensitive Moderate
    Ascidiella scabra lives in estuaries and other enclosed areas where turbidity may increase to high levels. It is not expected that increase in turbidity at the level of the benchmark will adversely affect Ascidiella scabra.
    Tolerant* Not relevant Not sensitive* Moderate
    Although there may be some reliance on the organic material associated with turbidity for nutrition, the reduced need for energy expenditure to clear any silt that may be causing turbidity may be beneficial and an intolerance of tolerant* has been recorded.
    Intermediate Very high Low Low
    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 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).
    Tolerant* Not relevant Not sensitive* High
    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. 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 Ascidiella scabra is in wave sheltered habitats, the species might benefit from decrease in wave exposure.
    Tolerant Not relevant Not sensitive High
    Tunicates are not known to have organs sensitive to noise.
    Tolerant Not relevant Not sensitive High
    Tunicates are not known to respond to visual presence.
    High Very high Low High
    Epifaunal species have been found to be particularly adversely affected by trawling or dredging activities, either due to direct damage or modification of the substratum (Jennings & Kaiser, 1998). However, some epifaunal species have been reported to exhibit increased abundances on high fishing effort areas, probably due to their ability to colonize and grow rapidly (Bradshaw et al., 2000). In a study of the long term effects of scallop dredging, Bradshaw et al. (2002) reported that Ascidiella species had become more abundant and suggested that they were probably able to survive by regeneration of damage and budding. Individuals are easily ripped from the substratum and are unlikely to re-attach and will die. Intolerance is therefore high. For recoverability, see additional information.
    High Very high Low High
    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.

    Chemical pressures

     IntoleranceRecoverabilitySensitivityEvidence/Confidence
    Intermediate Very high Low Low
    Ascidians may be intolerant of synthetic chemicals such as tri-butyl-tin anti-foulants. Rees et al. (2001), working in the Crouch estuary, observed that six ascidian species were recorded at one station in 1997 compared with only two at the same station in 1987, shortly following the banning of TBT in antifouling paints. Also, there was a marked increase in the abundance of ascidians especially Ascidiella aspersa and Ascidia conchilega in the estuary. No evidence has been found for sublethal effects from which recovery would be likely to be rapid. Overall, an intolerance of intermediate is suggested but with a low confidence.
    Heavy metal contamination
    No information Not relevant No information Not relevant
    No information has been found.
    Hydrocarbon contamination
    No information Not relevant No information Not relevant
    No information has been found.
    Radionuclide contamination
    No information Not relevant No information Not relevant
    No information has been found.
    Changes in nutrient levels
    No information Not relevant No information Not relevant
    No information has been found.
    Tolerant Not relevant Not sensitive Moderate
    Ascidiella scabra occurs in full salinity although it may be abundant in variable salinity or reduced salinity (for instance in the biotope ECR.HbowEud (Halichondria (Halichondria) bowerbanki, Eudendrium arbusculum and Eucratea loricata on reduced salinity tide-swept circalittoral mixed substrata). Therefore, it is not expected that increase in salinity will have an adverse effect except in the possibility of allowing other species to out-complete Ascidiella scabra.
    Intermediate Very high Low Moderate
    A fall in salinity from full to reduced would not be expected to have an adverse effect as Ascidiella scabra occurs in reduced salinity conditions. 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.
    Low Immediate Not sensitive Very low
    Ascidians are active suspension feeders that pump water. It seems likely that the effects of lowered oxygenation will be reduced as stagnation can be avoided. An intolerance of low is therefore suggested but with very low confidence. Recovery is likely to be immediate.

    Biological pressures

     IntoleranceRecoverabilitySensitivityEvidence/Confidence
    No information Not relevant No information Not relevant
    No information has been found.
    Tolerant Not relevant Not sensitive Moderate
    There are no non-native species currently known to displace or adversely affect Ascidiella scabra although the ascidian Perophora japonica may occur in similar habitats.
    Not relevant Not relevant Not relevant Not relevant
    There is no known extraction of this species.
    Not relevant Not relevant Not relevant Not relevant
    There are no species with which Ascidiella scabra is associated that may be extracted.

    Additional information

    Ascidiella scabra has a high fecundity and settles readily, probably for an extended period from spring to autumn. Svane (1988) describes it as "an annual ascidian" and demonstrated recruitment onto artificial and scraped natural substrata. Eggs and larvae are free-living for only a few hours and so recolonization would have to be from existing individuals no more than a few km away. It is also likely that Ascidiella scabra larvae are attracted by existing populations and settle near to adults (Svane et al., 1987) . Fast growth means that a dense cover could be established within about 2 months. However, if mortality and the consequent establishment of free space available occurs at a time when larvae are not being produced, other species may settle and dominate. Therefore a recoverability of 'very high' is for when larvae are available to settle. If another species colonizes and dominates the substratum, recolonization by Ascidiella scabra may take several years.

    Importance review

    Policy/legislation

    - no data -

    Status

    National (GB) importance-Global red list (IUCN) category-

    Non-native

    Native-
    Origin- Date Arrived-

    Importance information

    Ascidiella scabra is a fast colonizing species and may be a fouling organism.

    Bibliography

    1. Berrill, N.J., 1950. The Tunicata with an account of the British species. London: Ray Society.

    2. Bradshaw, C., Veale, L.O., Hill, A.S. & Brand, A.R., 2002. The role of scallop-dredge disturbance in long-term changes in Irish Sea benthic communities: a re-analysis of an historical dataset. Journal of Sea Research, 47, 161-184.

    3. Crisp, D.J. (ed.), 1964. The effects of the severe winter of 1962-63 on marine life in Britain. Journal of Animal Ecology, 33, 165-210.

    4. Hiscock, K., 1983. Water movement. In Sublittoral ecology. The ecology of shallow sublittoral benthos (ed. R. Earll & D.G. Erwin), pp. 58-96. Oxford: Clarendon Press.

    5. Lindsay, S.J. & Thompson, H. 1930. The determination of specific characters for the identification of certain ascidians. Journal of the Marine Biological Association of the United Kingdom, 17, 1-35.

    6. Millar, R.H., 1970. British Ascidians London: Academic Press.[Synopses of the British Fauna, no. 1.]

    7. National Biodiversity Network (NBN) Atlas website. Available from: http://www.nbnatlas.org.  Accessed 01 April 2017

    8. Rees, H.L., Waldock, R., Matthiessen, P. & Pendle, M.A., 2001. Improvements in the epifauna of the Crouch estuary (United Kingdom) following a decline in TBT concentrations. Marine Pollution Bulletin, 42, 137-144.

    9. Robbins, I.J. 1985b. Ascidian growth rate and survival at high inorganic particulate concentrations. Marine Pollution Bulletin, 16, 365-367.

    10. Svane, I, Havenhund, J.N. & Jorgensen, A.J., 1987. Effects of tissue extract of adults on metamorphosis in Ascidia mentula O.F. Mueller and Ascidiella scabra (O.F. Müller). Journal of Experimental Marine Biology and Ecology, 110, 171-181.

    11. Svane, I., 1988. Recruitment and development of epibioses on artificial and cleared substrata at two site in Gullmarsfjorden on the Swedish west coast. Ophelia, 29, 25-41.

    Citation

    This review can be cited as:

    Hiscock, K. 2006. Ascidiella scabra A sea squirt. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/species/detail/1823

    Last Updated: 24/04/2006