Boreolithothamnion corallioides is a red seaweed and a coralline alga, that is, an alga which is calcified and hard in texture.

In brief

• Boreolithothamnion corallioides has been recorded from only a few sites in Northern Ireland.
• It is an unattached, branched, coralline alga.
• It is scarce in Northern Ireland.
• Threats include anchoring, harvesting, eutrophication and obstruction of water flow.

Species description: This maerl generally grows as unattached branched thalli up to 1.5 mm wide and can measure up to 5 cm in diameter. They may be sparse or dense, and are brittle. Attached forms have been found in the Britain. Conceptacles only known in attached plants.

Life cycle: Unknown.

Similar species: Could be confused with Boreolithothamnion glaciale or Phymatolithon calcareum however the detached branches of both species are quite hard and tough, rather than brittle.

How to see this species: Unattached plants occur in the sublittoral. It has been recorded from several sites including Red Bay, The Maidens and Strangford Lough. It is best looked for by SCUBA diving.

Current status: Nationally scarce. Data deficient.

Why is this species a priority in Northern Ireland?

• Criterion 1: Internationally listed species.
• Criterion 3: Rare or restricted range – rare breeding species confined to 5 hectads or less (based on expert judgement).

Threats:

• Commercial extraction for use as a soil conditioner on acidic ground, as an animal food additive, for the filtration of acid drinking water and in pharmaceutical and cosmetic products.
• Scallop dredging has been identified as the biggest impact on maerl beds of both maerl, by breaking and burying the thin layer of living maerl, and the associated species. Other types of mobile fishing gear are also likely to damage the living layer of maerl on top of the bed.
• Heavy anchors and mooring chains could cause considerable damage to maerl beds.
• Eutrophication, which causes smothering of the maerl by excess growth of other seaweeds and increased sedimentation.
• Finfish farms nutrient and chemical discharges that can affect the fauna associated with maerl beds may be affected.
• Mussel aquaculture could cause damage to maerl beds as the production of pseudo-faeces could smother the live maerl.
• Obstruction to water flow building of barrages, causeways and bridges are potential blockages to water flow, causing fine sediment particles to accumulate between the maerl fragments and smother the bed.

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:

• The status of Boreolithothamnion corallioides will be surveyed and monitored and appropriate conservation action undertaken if required.

Conservation:

• Strangford Lough is a Marine Protected Area with multiple designations.
• There is a Habitat Action Plan for Maerl Beds.
• Red Bay was designated as a Special Area of conservation.
• The Maidens are a Marine Protected Area with multiple designations.

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

iNaturalist: Boreolithothamnion corallioides at iNaturalist World Species Observations database.

WoRMS: Boreolithothamnion corallioides at World Register of Marine Species. Accepted name: Boreolithothamnion corallioides (P.Crouan & H.Crouan) V.Peña & P.W.Gabrielson, 2025. AphiaID: 1871796.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Corallinophycidae; Hapalidiales; Hapalidiaceae; Boreolithothamnion

Previous species

Next species

Picton, B.E., Keatley, L. & Morrow, C.C. (2026). Boreolithothamnion corallioides. P Crouan et H Crouan. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=ZM4600. Accessed on 2026-10-29