Type of fouling organism: Sea Squirt

Ascidians or sea squirts are sessile filter feeding animals commonly found in the subtidal. This group of fouling organisms can be colonial or unitary and are often of indefinite shape and the surrounding tunic can give an appearance that resembles sponges, bryozoans or cnidarians. All are hermaphroditic but usually unitary species release eggs into the sea and colonial species incubate embryos internally. The larval duration can vary greatly affecting dispersal potential.

Ascidiella scabra is a small solitary ascidian with an ovate body, and a semi-transparent test. The test is often tinged red with a smooth to slightly wrinkled texture. This species is usually less than 40 mm long with an average length in the range 20 - 25 mm.

Environment and Habitat

- Found attached to natural and artificial hard substrata.
- Also present attached to algae such as Fucus serratus and on kelp stipes.
- Often attached to other animals, ascidians, bryozoans and algae.
- Usually found in sheltered areas to depths of 300 m.
- Feeds on suspended particles including phytoplankton.
- High intolerance of substratum loss, abrasion and displacement.
- Intermediate intolerance of desiccation, increase in wave exposure and decreases in salinity.
- Low intolerance of changes in water flow rates, oxygenation, smothering and increases in suspended sediment.
- Tolerant of decrease in suspended sediments, changes in temperature, increases in turbidity and salinity.

Reproduction

- Reproduction type: Oviparous.
- Reproduction frequency: Annual.
- Age at maturity: < 6 months.
- Egg production: From March onwards.
- Larval duration: 2 - 10 days.
- Dispersal potential: 1 - 10 km.
- Lifespan: 2 - 5 years.

Communities

- Solitary, gregarious.
- Non-migratory.
- Found in dense aggregations.

Equipment

- Mussel rope culture, oyster and suspended pectinid culture.
- Fishnets, cages, pontoons, shellfish trays, tanks, and pipes.

Effects and Impacts

- Problematic for stock species as can compete for space and resources.
- Increases the weight of equipment particularly lines making them so heavy that mussels can slip off before they can be harvested.
- Increases labour and production costs as a result of cleaning and removal of biofouling.

Control/ Strategies and Management

Nets
- Onshore Net washing
- Coatings (Copper sulphate, fouling release coatings e.g. silicon)
- Mechanical cleaning of infrastructure (Disk cleaners)
- Air drying nets
Trays
- Manual cleaning (scrubbing and/or brushing)
- Low power washing
- High power washing
- Jet washing
- Air drying
- Lowering trays below photic zone during major spatfalls
- Biological Control (Sea urchins and periwinkles)
- Coatings (Copper sulphate, fouling release coatings e.g. silicon)
Shellfish
- Manual Cleaning
- Mechanical Cleaning
- Hot water 55oC for 5 seconds (Stock mortalities of ca 5% with this method)
- Dipping (Freshwater or chemical solution)
- Lowering lines below photic zone during major spatfalls
- Biological control (Sea urchins and periwinkles)
- Coatings (Copper sulphate, spiky coatings, fouling release coatings e.g. silicon)

Principles of Management

C Combat Settlement
P Protect Equipment and Stock
R Remove Biofouling

Distribution

- Common on all coasts from Norway to the Mediterranean.

References

Hiscock, K., 2004. Ascidiella scabra. A sea squirt. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 05/02/2007]. Available from: http://www.marlin.ac.uk/species/Ascidiellascabra.htm

Picton, B.E. & Morrow, C.C., 2005. [In] Encyclopedia of Marine Life of Britain and Ireland
http://www.habitas.org.uk/marinelife/species.asp?item=ZD1430

Janet Moore (2001) An Introduction to the Invertebrates. Cambridge University Press

Hayward P, Nelson-Smith T & Shields C (1996) Seashore of Britain and Northern Europe. HarperCollins Pubs.