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.

Ascidia mentula has an oval body and is attached to the substratum on one side. The test is thick, cartilaginous and translucent, with a pink colour in well-lit places and grey in silty areas or deeper water. The body attaches to surfaces mostly on its left side. The oral siphon is wide but short, with eight small pointed white lobes around it. The atrial siphon is halfway along the body. Size range is 50 - 180 mm.

Environment and Habitat

- Subtidal to depths of 200 m.
- Found attached to hard substratum.
- Feeds on phytoplankton and suspended particles.

Reproduction

- Reproductive type: hermaphrodite with external fertilisation.
- Reproductive frequency: Year round with a peak in Oct - Nov.
- Age at maturity: Insufficient information.
- Development: Planktonic.
- Larval duration: Up to 240 hours in laboratory conditions.
- Dispersal distance: ca 1.5 km .

Communities

- Solitary.

Equipment

- Found on other biofouling organisms.
- Stock species particularly shellfish.
- Fishnets, cages, pontoons, shellfish trays, tanks, pipes.

Effects and Impacts

- Problematic for stock species as can compete for space and resources.
- Can obstruct the opening of bivalve shells.
- Can reduce the value of shellfish.
- Increases the weight of equipment.
- 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

- Found throughout the north east Atlantic from western Norway down to the Mediterranean.

References

M.J. de Kluijver & S.S. Ingalsuo Tunicata Macrobenthos of the North Sea P2Ascidea mentula (Zoological Museum, University of Amsterdam)
http://nlbif.eti.uva.nl/bis/tunicata.php?selected=beschrijving&menuentry=soorten&id=13

Picton, B.E. & Morrow, C.C., 2005.P2 Ascidea mentula [In] Encyclopedia of Marine Life of Britain and Ireland
http://www.habitas.org.uk/marinelife/species.asp?item=ZD1500

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

Havenhand (1991) Fertilisation and the potential for dispersal of gametes and larvae in the solitary ascidian Ascidia mentula (Muller). Ophelia 33:1-15.

Svane (1984) Observations on the long-term population dynamics of the perennial ascidian, Ascidia mentula (Muller), on the Swedish west coast. Biological Bulletin 167:630-646.

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