Antho granditoxa is a red encrusting sponge which is a member of the Rathlin Island circalittoral sponge communities. Elsewhere it is only known from a single record from the Scilly Isles, SW England and a single record from Sanda Island, North Channel, Scotland.

In brief

• Found on Rathlin Island.
• 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 habitats in clean water with a minimum of turbidity.

Species description: This species forms encrusting sheets typically 2 cm to 6 cm in extent. It has a featureless, slightly velvety, surface and is bright red in colour. It can be identified positively only by examination of the skeletal spicules, which include so-called “giant toxa”.

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 red encrusting sponges in our area and this species is not identifiable with certainty in situ.

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 red coloration makes this species conspicuous but it has no clear features which can be used to identify it in situ and requires microscopic examination of its spicules for confirmation. This species has been found several times at Rathlin Island. There is a single record from Toll’s Island, Scilly Isles and one from Sanda Island. All records are from 2005 onwards.

Current status: Rare - confined to a small population at one or two sites in Northern Ireland, with Northern Ireland being a stronghold for this species in Britain and Ireland. Discovered on Rathlin Island by the Rathlin Island sponge project in 2005.

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.
• 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.
• There is Habitat Action Plan for Fragile Sponge and Anthozoan Communities on Rocky Habitats.
• The Maidens are a Marine Protected Area with multiple designations.

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

iNaturalist: Antho granditoxa at iNaturalist World Species Observations database.

WoRMS: Antho granditoxa at World Register of Marine Species. Accepted name: Antho (Antho) granditoxa Picton & Goodwin, 2007. AphiaID: 255166.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Microcionidae; Ophlitaspongiinae; Antho

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Picton, B.E., Keatley, L. & Morrow, C.C. (2026). Antho granditoxa. Picton & Goodwin, 2007. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C8422. Accessed on 2026-09-13