Tethya hibernica is a small spherical sponge, yellow or brown in colour, which is a member of the Rathlin Island circalittoral sponge communities. Elsewhere it is only known from the Maidens, near Larne.

In brief

• 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 or stable boulder habitats in clean water with a minimum of turbidity.

Species description: This sponge forms spherical bodies which are typically 10 mm in diameter and are anchored to the seabed at the base. The surface is covered with silt and there are a few oscular openings at the top. It is pale yellow to brown in colour. The spiculation includes radially arranged tylostyles and subsurface layers of asters, with very numerous smaller asters near the surface and fewer, much larger ones below.

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 is a bigger species of Tethya, Tethya citrina, in our area which is common. Small individuals would be difficult to distinguish from this species.

How to see this species: Found on sloping rock faces and sides of stable boulders in the circalittoral zone usually in places where there are many other encrusting sponges. This species is inconspicuous but can be found by looking for a small spherical sponge. It requires microscopic examination of its spicules for confirmation of identity. Northern Irish sites are all around Rathlin island or The Maidens.

Current status: Rare. This species has been found on a few occasions in one bay at Rathlin Island and the Maidens, off Larne. All records are from 2005 onwards. Elsewhere it has been recorded in Wales. Northern Ireland is considered to be a stronghold for this species as it has more than 50% of the records worldwide. Like many encrusting sponges this is partly due to the lack of sampling but the North Channel and Irish Sea do have sponge-rich habitats which are rare and not found elsewhere.

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. This species is contractile and can expand to feed when conditions are favourable.
• It is more vulnerable than most other sponges listed here as it prefers stable boulders to bedrock. These boulders can be disturbed by scallop dredging.
• 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
• 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 Protected Area with multiple designations.
• The Maidens are a Marine Protected Area with multiple designations.
• There is Habitat Action Plan for Fragile Sponge and Anthozoan Communities on Rocky Habitats.

iNaturalist: Tethya hibernica at iNaturalist World Species Observations database.

WoRMS: Tethya hibernica at World Register of Marine Species. Accepted name: Tethya hibernica Heim, Nickel, Picton & Brümmer, 2007. AphiaID: 246616.

Literature:

Picton, B.E. and Goodwin, C.E. (2007). Sponges of Rathlin Island. Journal of the Marine Biological Association of the United Kingdom. 87(6): 1441-1458.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Tethyida; Tethyidae; Tethya

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Picton, B.E., Keatley, L. & Morrow, C.C. (2026). Tethya hibernica. Heim, 2007. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C2134. Accessed on 2026-09-13