A dull brownish, worm-like animal whose precise taxonomic status remains a topic of some debate. In Northern Ireland it is known only from Strangford Lough, and even there, it has only been recorded irregularly.

In brief

• Occasionally recorded south of Portaferry, Strangford Lough
• Occurs in shelly gravel sediments at extreme low water levels of sheltered shores
• It probably can be found throughout the year, but the scarcity of records mean that this has yet to be confirmed
• Listed as a SoCC on account of its scarcity, lack of data, and the likelihood of Northern Ireland holding a major component of the Irish population
• Whether a decline has taken place, and what if any the causes might be, are not known at present.

Species description: This fragile species is cylindrical and worm-shaped. The body is divided into three parts: an acorn, or egg-shaped proboscis about 10mm in length, a short fleshy collar, and a long cylindrical trunk that forms the main part of the body, the whole animal growing up to 500mm in length. The forward end of this is perforated by pairs of gill openings, whilst the skin of the rear parts may be transparent enough to reveal ‘liver’ pouches attached to the gut. The skin is covered in cilia and mucous glands.

Life cycle: The sexes are separate, and external fertilization is achieved by spawning, whereby yolky eggs are formed. These develop into larvae that, until the very latest stages of development, do not feed, instead living off the food matter provided by the yolk.

Similar species: None.

How to see this species: In Northern Ireland, acorn worms have been found in shelly and muddy sand and gravel at the lowest levels of the shore normally only accessible by foot at spring tides. They live in U-shaped burrows in these sediments, and these can be spotted by shallow depressions about 5-10cm diameter on the surface. There may be a strong smell of iodine present. There is only one location in Northern Ireland, that listed by Edwards (1953) being ‘Big Rock’ to the south of Portaferry in Strangford Lough, though other records come from Granagh Bay nearby. Further afield in Ireland, a site is recorded at the entrance to Lough Foyle (map unclear) and about 20 miles south of County Cork. It has also been recorded in the English Channel, the Channel Islands, Scillies, and in the North Sea and Skaggerak.

Current status: The acorn worm is known only from one site in Northern Ireland, and there is a paucity of data or indication of trends of its conservation status. The species has no legal protection.

Why is this species a priority in Northern Ireland? Glossobalanus sarniensis is extremely scarce, and Northern Ireland’s Strangford Lough population may well hold half, or a significant component of, the total Irish population, but there is little information on this.

Threats: There is insufficient data to determine if the species is under threat or declining.

Action:

• Establish true conservation status, and monitor to determine trends
• Maintain the only Northern Ireland population of the species in viable condition
• Maintain the range of the species.

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.

Strangford Lough has been designated a Special Area for Conservation under the EU Habitats Directive, and its intertidal zone is designated an Area of Special Scientific Interest.

There is no Species Action Plan for the acorn worm, but the UK Habitat Action Plan for Littoral Sediments has the following objectives:

• Maintain the extent and quality of littoral sediment habitats in the UK, including the full diversity of communities. In the case of estuarine habitats, where there have been considerable losses and deterioration in the past, and where there is a future threat from sea level rise, work to enhance the extent and quality of these habitats in the UK.

Measures to be considered further include:

• protecting sites of conservation importance from damage through contamination, physical disturbance or excessive use (for example oil spills, shellfish dredging and marina/harbour development)
• promotion of the management of littoral sediment habitats within strategies (eg MAFF Shoreline Management Plans which permit the natural functioning of sediment systems)
• ensuring that EIAs for coastal developments, including those above high water, examine potential effects on intertidal and nearshore areas
• 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: Glossobalanus sarniensis at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Glossobalanus sarniensis at iNaturalist World Species Observations database.

WoRMS: Glossobalanus sarniensis at World Register of Marine Species. Accepted name: Glossobalanus sarniensis (Koehler, 1886). AphiaID: 137609.

Literature:

Edwards, C. (1953). Occurrence of the Enteropneust Glossobalanus sarniensis Koehler in Northern Ireland. Nature 172: p343.

Hayward, P.J. and Ryland, J.S. (1995). Handbook of the Marine Fauna of North West Europe. Oxford University Press, ISBN 0-19-854055-8.

van der Land, J. (2001). Hemichordata. In: Costello, M.J. et al. (Ed.) (2001). European register of marine species: a check-list of the marine species in Europe and a bibliography of guides to their identification. Collection Patrimoines Naturels, 50: pp. 336 (look up in IMIS).

Classification: Biota; Animalia; Hemichordata; Enteropneusta; Enteropneusta incertae sedis; Ptychoderidae; Glossobalanus

Allen & Mellon Environmental Ltd. (2026). Glossobalanus sarniensis. Koehler. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=ZC280. Accessed on 2026-07-28