An encrusting cushion-like sponge known only from two locations in Northern Ireland and thought to be declining, though data are lacking.

In brief

• Found on Rathlin Island and the narrows in Strangford Lough.
• Found on vertical rock faces and boulders, in areas of strong water movement.
• Can be found throughout the year.
• A species of conservation concern because of its scarcity, and data deficiency.
• Main threats are not known.

Species description: An irregularly shaped, relatively low-lying sponge coloured sandy, or grey-brown. It has an encrusting form, the cushion-like crust up to 7 cm in diameter, and about 1.5 cm thick though it becomes narrower at the margin. A somewhat shaggy or lumpy appearance is caused by cone-like projections on the surface and dark pores. The spicules (skeletal structures in the body) are pin-like, but curled.

Life cycle: Sponges have either separate sexes or are hermaphrodite. In the case of the latter, self-fertilisation is avoided by eggs and sperms maturing at different rates within the same individual, so when spawning takes place and sperms are released through exhalent openings, it is unlikely that the eggs will be fertilised. Alternatively, the eggs are retained within the sponge, and sperms drawn in from surrounding water. Fertilised eggs are released and develop into planktonic larvae. Asexual reproduction may also be achieved by budding, whereby a small unit of the sponge separates and develops as an independent entity.

Similar species: The species has caused some identification problems; no less than 20 synonyms have been proposed over the years.

How to see this species: In Northern Ireland this sponge can be found off Rathlin Island and on the Lee's Wreck in Strangford Lough, both locations characterised by strong water flow. Generally it is found on vertical rock surfaces, the undersides of boulders, and despite its liking for strong currents, can also occur on boulders located in areas of stones, gravel and other sediments. Elsewhere in Ireland the species occurs quite widely in the west from Tory Island, Donegal Bay, Achill Island, Kerry Head, Sherkin Island and Lough Hyne. In Britain it is found off Anglesey, Pembrokeshire, St Kilda, and Orkney.

Current status: Scarce, probably declining in Northern Ireland. Data deficient. Known from only two locations in Northern Ireland. The species has no legal protection. Northern Ireland is thought to be a stronghold for Britain and Ireland.

Why is this species a priority in Northern Ireland?

• Criterion 3: Rare or restricted range – rare breeding species confined to 5 hectads or less (based on expert judgement).
• Criterion 4: NI stronghold or endemic – both stronghold range/population and designation as ‘endemic’ subject to expert judgement.

Threats: Other than rarity itself, no specific threats have been documented.

Action: Records of new sites for this species, with details of habitat and population are always valuable. Observations with a photograph can be recorded using iNaturalist, find the MBDP-NI iNaturalist project here.

Alternatively, records can be sent to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257, e-mail cedar.records@nationalmuseumsni.org, or via the online portal.

Objectives:

• Maintain the number of viable populations.
• Maintain the range of this species.
• Monitor the population (or samples) at the two known Northern Ireland locations.
• Protect sites of conservation importance from damage through contamination and physical disturbance (for example, turbidity and towed fishing gear).
• Require EIAs for coastal developments to examine potential effects on intertidal and nearshore areas.
• Monitor any impact of dump sites on inshore sublittoral rock habitats, communities and wildlife, and take action as appropriate.
• Implement strategies for managing the coastal zone at local, regional and national levels.

Conservation:

• Rathlin Island is a Marine Protected Area with multiple designations.
• Strangford Lough is a Marine Protected Area with multiple designations.
• There is Habitat Action Plan for Tide-Swept Channels.

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

iNaturalist: Biemna variantia at iNaturalist World Species Observations database.

WoRMS: Biemna variantia at World Register of Marine Species. Accepted name: Biemna variantia (Bowerbank, 1858). AphiaID: 133205.

Literature:

Picton, B.E. and Goodwin, C.E. (2007). Sponge biodiversity of Rathlin Island, Northern Ireland. Jour. Mar. biol. Ass. UK 87: 1441-1458.

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

http://www.habitas.org.uk/marinelife/species.asp?item=C5830.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Biemnida; Biemnidae; Biemna

Previous species

Next species

Adapted from text by Allen & Mellon Environmental Ltd. (2026). Biemna variantia. (Bowerbank, 1858). [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C5830. Accessed on 2026-09-13