A colonial hydroid that has been recorded from only two locations in Northern Ireland, but is probably under-recorded. It is a species of shallow sublittoral bedrock, boulders, stones and shells, where there is a moderate to strong current.

In brief

• Found on Rathlin Island and the North Rocks off the outer Ards peninsular
• Bedrock, boulders, large stones and shells, in areas of tidal current
• Can be found throughout the year
• It is listed as a Species of Conservation Concern on account of its rarity, and because it is listed as a UK SoCC
• Main threats — none known to be active at present, but could be vulnerable to various types of coastal development.

Species description: A sparsely-branched colonial hydroid, whose branches emerge alternately up the stem from opposite sides. The colonies are typically from 60-150mm in height and are colourless or a dull greyish-white. The hydrothecae (chambers containing the individual polyps, or colony inhabitants) are arranged in opposite pairs on the stem and branches, and these are tubular and attached for about half their length — an important feature when comparing with other species (see below).

Life cycle: Colonies have reproductive polyps, called gonozooids, which produce medusae (tiny jellyfish-like animals). However, reproduction within a colony may be achieved asexually by budding off new polyps from the main stem.

Similar species: Abietinaria abietina is similar but the hydrothecae are not opposite. It is also worth checking against the various Diaphasia species.

How to see this species: This is a circalittoral species, and can be found where moderate to strong tidal streams run over bedrock, boulders and large stones, and the larger shells. It is probably under-recorded, but has been recorded so far from Castle Head Rathlin Island, and east of the North Rocks, Cloghy, in County Down. Elsewhere in Ireland it has been sparsely recorded on all coasts, for example the Garvan Isles off Malin Head, Sheephaven, Donegal Bay and on the Cork coast. A similar sparse distribution has been recorded around Britain. It is present throughout the year.

Current status: Recorded from two localities in Northern Ireland. The species has no legal protection.

Why is this species a priority in Northern Ireland? The species is scarce in Northern Ireland, and is listed as a UK Species of Conservation Concern – nationally scarce marine species.

Threats: There are no threats currently known to be affecting this species and its habitat, but it would be vulnerable to coastal developments and changes to the local current and sediment regimes at its sites.

Action:

• Maintain the two viable populations of the species
• Maintain the range of the species
• Establish appropriate management of known sites
• Monitor the population at the two sites.

Objectives: Records of new sites for this species, with details of habitat and population are always valuable. Send these to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257 or e-mail cedar.info [at] nmni.com. If you are a diver, and interested in becoming involved in recording marine life in Northern Ireland through the Seasearch project, contact Claire Goodwin at claire.goodwin [at] gmail.com or look at the web site www.seasearch.org.uk (Northern Ireland web pages).

Conservation: Survey work is being conducted throughout Northern Ireland waters (including the north coast) by Northern Ireland Environment Agency for this and other species and habitats generally.

Rathlin Island and its waters are designated as a Special Protection Area under the EU Habitats Directive, and it receives indirect protection at its County Down site through designation of the Outer Ards Peninsular as a Special Protection Area under the EU Birds Directive.

There is no Species Action Plan for Tamarisca tamarisca; however, the UK Habitat Action Plan for Sublittoral Sands and Gravels has the following relevant objectives:

• protect the extent of a representative range of sublittoral sand and gravel habitats and communities
• protect the quality of a representative range of sublittoral sand and gravel habitats and communities.

The Inshore Sublittoral Sediment UK HAP also has the following objectives:

• protecting sites of conservation importance from damage through contamination and physical disturbance or excessive use (for example, nutrient enrichment, dredging and development);p
• requiring EIAs for coastal developments to examine potential effects on nearshore areas
• monitoring any impact of dump sites on inshore sublittoral sediment habitats, communities and wildlife, and take action as appropriate
• developing and implementing strategies for the conservation and management of the wider marine environment at local, regional and national levels. For example, integrated Coastal Management Plans, water quality objectives, pollution control and avoidance measures. Species recovery and habitat restoration programmes should be included.

Distribution Map from NBN: Tamarisca tamarisca at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Tamarisca tamarisca at iNaturalist World Species Observations database.

WoRMS: Tamarisca tamarisca at World Register of Marine Species. Accepted name: Tamarisca tamarisca (Linnaeus, 1758). AphiaID: 117930.

Literature:

Hayward, P.J. and Ryland, J.S. (Ed.) (1990). The marine fauna of the British Isles and North-West Europe: 1. Introduction and protozoans to arthropods. Clarendon Press: Oxford, UK. ISBN 0-19-857356-1. 627 pp.

Picton, B.E. and Morrow, C.C. (2007). [In] Encyclopedia of Marine Life of Britain and Ireland.

Wilson, E. (2002). Tamarisca tamarisca. A hydroid. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/03/2008]. http://www.marlin.ac.uk/species/Tamariscatamarisca.htm.

Classification: Biota; Animalia; Cnidaria; Medusozoa; Hydrozoa; Hydroidolina; Leptothecata; Sertularioidea; Sertulariidae; Tamarisca

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Allen & Mellon Environmental Ltd. (2026). Tamarisca tamarisca. (Linnaeus, 1758). [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=D6890. Accessed on 2026-07-28