Hymedesmia rathlinia is a yellow encrusting sponge which is a member of the Rathlin Island circalittoral sponge communities. Elsewhere it is known from the Maidens (Larne), St John’s Point, Co. Donegal and several sites in SW Scotland.

In brief

• In Northern Ireland it is found on Rathlin Island and the Maidens, off Larne.
• Lives on steep or vertical rock faces in water of 25 m or greater depth in moderate to strong tidal streams.
• Not seasonal, this sponge probably remains in one place for many years.
• Northern Ireland is a stronghold for this species.
• Requires rock reef habitats in clean water with a minimum of turbidity.

Species description: Forms small patches, 2 cm to 5 cm in extent. These patches are irregular in shape and may be joined. Each patch has one or several surface structures consisting of a central oscule surrounded by a sieve-like intake area of pores; this structure can close on disturbance or if conditions are unfavourable to the sponge. Tissue sections (made in the standard way for sponges) are black and opaque. The spicule complement includes small acanthostyles which are quite characteristic in shape.

Life cycle: Reproduction is by swimming larvae which must find suitable substratum to settle on. Settlement in suitable water conditions and initial growth are probably when most mortality occurs due both to chance and to competition for space.

Similar species: There are a number of yellow encrusting sponges, but this species is probably identifiable with certainty in situ. It is the only species with compound pore-sieve oscular structures that has been seen to date in the area.

How to see this species: Found on steep rock faces in the circalittoral zone usually in places where there are many other encrusting sponges. Its bright yellow coloration makes this species conspicuous although it is small.

Current status: Rare. This species has been found regularly at Rathlin Island and the Maidens, off Larne. All records are from 2005 onwards. Elsewhere it has been found in SW Scotland and Pembrokeshire, Wales. Northern Ireland is a stronghold for this species.

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:

• The main threat to this species is degradation of water quality due to increased turbidity, pollution or nutrient enrichment.
• Inorganic silt clogs sponges making them unable to filter feed and such silt is artificially suspended into the water column by bottom-fishing activities.
• Some species of sponge can actively clean their surfaces to some extent and may become competitively able to displace other species if there is a shift in water quality.
• Polluted water may cause sponges to become diseased and may benefit some species at the expense of others by providing more bacteria and less plankton as food.
• Nutrient enrichment may have similar effects by changing the amounts of different types of plankton on which the sponges feed.

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 of the species
• Maintain the range of the species
• Establish appropriate management on appropriate historic sites

Conservation:

• Rathlin Island is a Marine Protected Area with multiple designations, where there has been a ban on bottom fishing since 2016.
• There is Habitat Action Plan for Fragile Sponge and Anthozoan Communities on Rocky Habitats.
• The Maidens are a Marine Protected Area with multiple designations.
• There is a Habitat Action Plan for Tide-swept Channels.

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

iNaturalist: Hymedesmia rathlinia at iNaturalist World Species Observations database.

WoRMS: Hymedesmia rathlinia at World Register of Marine Species. Accepted name: Hymedesmia (Hymedesmia) rathlinia Goodwin & Picton, 2009. AphiaID: 416422.

Literature:

Goodwin, C.E., Picton, B.E. (2009). Demosponges of the genus Hymedesmia (Poecilosclerida: Hymedesmidae) from Rathlin Island, Northern Ireland, with a description of six new species. Zoological Journal of the Linnean Society. 156: 896–912. doi: 10.1111/j.1096-3642.2008.00498.x.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Hymedesmiidae; Hymedesmia

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Picton, B.E., Keatley, L. & Morrow, C.C. (2026). Hymedesmia rathlinia. Goodwin & Picton, 2009. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C7394. Accessed on 2026-09-13