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

• Found on Rathlin Island and the Maidens, off Larne
• Lives on steep or vertical rockfaces in water of 25m 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, 2cm to 5cm 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: 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.

Why is this species a priority in Northern Ireland?

• Rare - confined to a small population of one or two sites in Northern Ireland with Northern Ireland being a stronghold (S) consisting of either:
• >50% Irish population or
• >20% UK population/range
• At least 20% of international population of species or well-recognised subspecies occurring in Northern Ireland

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:

• Maintain the number of viable populations of the species
• Maintain the range of the species
• Establish appropriate management on appropriate historic sites
• 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.records@nationalmuseumsni.org, or via the online portal. You can also record observations including a photograph using iNaturalist, find the MBDP-NI iNaturalist project here.

Conservation:

• Rathlin Island and its surrounding waters are a designated Marine Conservation Zone Area where there has been a ban on bottom fishing since 2016.

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