Tethya hibernica – a sponge
Tethya hibernica Heim, Nickel, Picton & Brümmer, 2007
Family: Tethyidae
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 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 or stable boulder habitats in clean water with a minimum of turbidity
Species description
This sponge forms spherical bodies which are typically 10mm 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.
Current status
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.
Why is this species a priority in Northern Ireland?
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.
- 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/Causes of decline
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.
Conservation of this species
Current action
- 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
Proposed objectives/actions
- Maintain the number of viable populations of the species
- Maintain the range of the species
- Establish appropriate management on appropriate historic sites
What you can do
Encourage sustainable use of the seas by avoiding consumption of fish and shellfish captured by bottom dredging and bottom trawling.
Further information
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.
Text written by:
Bernard Picton
iNaturalist: Species account : iNaturalist World Species Observations database