Hymedesmia cohesibacilla is a very thin grey encrusting sponge which is a member of the Rathlin Island circalittoral sponge communities. Elsewhere it is only known from SW Scotland and from a deep-water coral reef near Mingulay, 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 or Lophelia reef habitats in clean water with a minimum of turbidity
• Any other relevant point(s)

Species description: This species forms small patches typically 1cm to 10cm in diameter and less than 1mm thick. It is grey or colourless but often has diatoms in the surface tissues which create a brown hue when present. The entire surface is covered with regularly distributed circular pore-sieves which have a paler appearance than the surrounding tissue. These pore sieves can close if the sponge is disturbed or if the sponge is not actively pumping water. Positive identification requires examination of the spicules, with the shape of the chelae being distinctive in this case.

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 grey encrusting sponges, including other species of Hymedesmia with pore-sieves and similar appearance, in our area and this species is not identifiable with certainty in situ.

How to see this species: Found on steep rockfaces in the circalittoral zone usually in places where there are many other encrusting sponges. Its grey coloration makes this species inconspicuous and it has no clear features which can be used to identify it in situ. It requires microscopic examination of its spicules for confirmation.

Current status: This species has been found regularly at Rathlin Island and the Maidens, off Larne. There is a single record from a shoal 1.25nm WNW of Crinan Hotel, SW Scotland. All records are from 2005 onwards.

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

Objectives: Encourage sustainable use of the seas by avoiding consumption of fish and shellfish captured by bottom dredging and bottom trawling.

Conservation:

• Rathlin Island is a marine Special Area of Conservation
• The Maidens are a candidate SAC
• Discovered on Rathlin Island by the Rathlin Island sponge project in 2005

iNaturalist: Hymedesmia cohesibacilla at iNaturalist World Species Observations database.

WoRMS: Hymedesmia cohesibacilla at World Register of Marine Species. Accepted name: Hymedesmia (Hymedesmia) cohesibacilla Goodwin & Picton, 2009. AphiaID: 416418.

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.
Goodwin, C.E., Picton, B.E., Van Soest, R.W.M. (2011). Hymedesmia (Porifera: Demospongiae: Poecilosclerida) from Irish and Scottish cold-water coral reefs, with a description of five new species. Journal of the Marine Biological Association of the United Kingdom. 91(5): 979–997.

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

Bernard Picton (2026). Hymedesmia cohesibacilla. Goodwin & Picton, 2009. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C7393. Accessed on 2026-07-28