Botrylloides leachi (Colonial sea squirt)


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.

Botrylloides leachi is a colonial ascidian, which forms flat, gelatinous sheets. The zooids are arranged in two compact meandering lines or parallel chains. This feature as well as less exhalent openings is what sets them apart from other Botrylloides species. Each individual zooid is twice as long as is wide and average length is 1.5 - 3 mm long. Their colour varies but is usually plain orange, pink or bright yellow.

Environment and Habitat

- Found from the lower shore down to the sublittoral.
- Common on brown algae in the lower shore and in shallow water on rock substratum and wave exposed areas.


- Reproductive type: Hermaphroditic.
- Reproductive frequency: Unknown.
- Development: Lecithotrophic.
- Larval duration: minutes - < day.
- Dispersal potential: 1 - 10 km.
- Life span: < 1 year.


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


- 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.
- 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

- Onshore Net washing
- Coatings (Copper sulphate, fouling release coatings e.g. silicon)
- Mechanical cleaning of infrastructure (Disk cleaners)
- Air drying nets
- 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)
- 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


- Found on north east Atlantic shores from northern Norway down into the Mediterranean.


Ager, O.E.D., 2002. Botrylloides leachi. A colonial sea squirt. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/05/2007]. Available from:

Picton, B.E. & Morrow, C.C., 2005. [In] Encyclopedia of Marine Life of Britain and Ireland

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.

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