This is an encrusting sponge that has important populations on Rathlin Island and along the north east coast of Northern Ireland - elsewhere it is very rare. Currently, populations appear to be well established, but are vulnerable to habitat disruption.

In brief

• Found at a number of sites on Rathlin Island, Carnlough Bay and in The Maidens.
• Habitat is usually bedrock and stable boulders.
• Can be found throughout the year.
• Listed as a NIPS on account of the general rarity of the species and the importance of the Northern Ireland populations.
• No threats are currently documented, but the species would be highly vulnerable to habitat loss.

Species description: This is a thin encrusting sponge with an uneven surface with softer inner parts. In Northern Ireland it has usually been coloured yellow or pinkish brown/beige, but in Sweden it has been reported as brick red and this possibility should not be ruled out here, though no such specimens have yet been recorded. The oscules (inhalant/exhalant pores) are sometimes quite prominent on cone-like structures. Spicules (skeletal structures) are variously anchor-shaped or more symmetrical.

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. In this species 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: Plocamionida ambigua is similar in external appearance, and can only be distinguished by examination of the spicules.

How to see this species: The species has been recorded at a number of sites round Rathlin Island, notably Doon Point, off the White Cliffs and Farganlack Point, as well as in Carnlough Bay, these being on bedrock and stable boulders. It has also been recorded at the Maidens (B. Picton, pers. comm.), where it appears to be well established. There are no records in the Republic of Ireland, but elsewhere it has been recorded in the Sound of Mull, the Inner Hebrides, and in various locations in Sweden. It can be found throughout the year.

Current status: This species is regarded as rare, and Northern Ireland appears to hold the entire Irish population, at a number of sites on Rathlin and along the north-east coast. It has no legal protection.

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: There are no known threats at present, but the species would be vulnerable to the impacts of mobile fishing gear in the more level terrain.

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 viable populations of this species.
• Maintain the range of this species.
• Monitor known populations, or a sample of these.
• Ensure protection of relevant areas of the Maidens, where an important population is currently well established.

Conservation:

• Rathlin Island is a Marine Protected Area with multiple designations.
• There is Habitat Action Plan for Fragile Sponge and Anthozoan Communities on Rocky Habitats.
• The Maidens are a Marine Protected Area with multiple designations.

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

iNaturalist: Plocamiancora arndti at iNaturalist World Species Observations database.

WoRMS: Plocamiancora arndti at World Register of Marine Species. Accepted name: Plocamiancora arndti Alander, 1942. AphiaID: 133902.

Literature:

Alander, H. (1942). Sponges from the Swedish West Coast and adjacent waters. Unpublished PhD Thesis, University of Lund.

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=C7620.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Myxillidae; Plocamiancora

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Adapted from text by Allen & Mellon Environmental Ltd. (2026). Plocamiancora arndti. Alander, 1942. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C7620. Accessed on 2026-09-13