Obelia longissima


Type of fouling organism: Hydroid

Hydroids are simple in structure, all are aquatic and most are marine. The life cycle includes both sessile polyps and freely floating medusae, with hydroids formed as the sessile asexual polyp stage with a reduced pelagic free floating sexual medusae or jellyfish stage. In many hydroids, the medusa stage is reduced and remains attached to the hydroid. Some species may even have a swimming polyp stage in the life cycle.

Found on plants and inert substrata, from within rock pools in the intertidal down to over 30 m in the subtidal. Colonies appear like featherlike projections from the surface to which they attach. Largest reported colonies can reach 350 mm in length, although more usually are 200 mm. Colony is typically single stemmed, with many long branches running off the main stem of similar length. The stem is brown-black in colour and may be forked in longer lived colonies.

Environment and Habitat

- Attach to algae and hard substrates found in the subtidal.
- High intolerance of substratum loss, desiccation and increase in temperature.
- Intermediate intolerance of changes in water flow, smothering, decreases in salinity, abrasion and decreases in wave exposure.
- Low intolerance of changes in oxygen, turbidity and increases in salinity.


- Reproduction type: Budding, separate sexes, vegetative.
- Reproduction frequency: Annual.
- Age at maturity: Variable with change between sessile hydroid and free-living medusoid stages in the life cycle.
- Fecundity: 10,000 - 100,000 eggs per individual.
- Development: Planktonic.
- Larval settlement time: Variable with life cycle change from sessile to free living. Medusoid stage lasts 7 - 30 days and the planula larvae last 5 - 21 days.
- Dispersal potential: > 10 km.
- Life span: Variable, potential to be very long lived.


- Forms lines or groups of raised feather like colonies.


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

Effects and Impacts

- Can reduce the value of shellfish.
- Can increase the weight equipment and of bivalves shell, interfering with normal biological function.
- Hydroids can compete with stock species for space and resource.
- 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


- Common throughout the north east Atlantic and Mediterranean.


Tyler-Walters, H., 2003. Obelia longissima. A hydroid. 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: species/Obelialongissima.htm

Obelia: Definition topic/obelia

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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