Phymatolithon calcareum is a red seaweed which, when dead contributes, with other similar algae, to maërl found below low tide in certain places. Phymatolithon calcareum is a coralline alga, that is, an alga which is calcified and hard in texture.

In brief

• Phymatolithon calcareum has been recorded from several sites in Northern Ireland; in both Counties Antrim and Down
• It is an unattached, branched, coralline alga, living or dead. It occurs in extensive deposits referred to as ‘maërl beds or banks’ below low-water mark.
• Maërl is found in Ireland, England, Scotland and in Brittany. It is a popular fertiliser in these days of organic gardening
• Phymatolithon calcareum is scarce in Northern Ireland.
• A potential threat is harvesting.

Species description: Phymatolithon calcareum is a small branched, unattached alga. It is hard in texture being encrusted or impregnated with calcium carbonate. It exists either as an unattached branching plant varying from a single branch to a subglobular or flattened branching system about 7cm in diameter. The colour is mauvish-brown but changing somewhat when dry.

Life cycle: It is assumed that the unattached plants in Northern Ireland (and in fact the rest of the British Isles) propagate vegetatively, possibly by animal activity. In other countries other stages in the life cycle have been found.

Similar species: Unattached plants of other species contribute to the maërl beds. The differences between the species are microscopic.

How to see this species: Unattached plants occur in the sublittoral maërl beds, mostly in sheltered areas. It has been recorded from several sites in County Antrim including Rathlin Island, Garron Point, Ballygalley Head and on the north side of Belfast Lough. In County Down it has been recorded in Strangford Lough and also near Stalka Rock and north of Greenore Point, both in Carlingford Lough. It is best looked for by scuba-diving.

Current status: Phymatolithon calcareum has been recorded at several sites in Northern Ireland from 1985 to 1996. Sites in County Antrim include Rathlin Island, Garron Point, Ballygalley Head and on the north side of Belfast Lough. In County Down it has been recorded in Strangford Lough and also near Stalka Rock and north of Greenore Point, both in Carlingford Lough.

Why is this species a priority in Northern Ireland?

• Scarce but widely distributed on western shores of the British Isles.

Threats: Maërl beds are a rich environment and an important habitat for a wide variety of marine animals and plants. Maërl can be harvested and it is a popular fertilizer. As Phymatolithon calcareum is a coralline alga and grows slowly, any harvesting will be a danger to the species and the other animals in the maërl beds.

Action:

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

Objectives: No relevant action.

Conservation:

• Rathlin Island is protected as a SAC and Strangford Lough as a Marine Nature Reserve
• Implementation of the Northern Ireland habitat action plan for Maërl Beds.

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

iNaturalist: Phymatolithon calcareum at iNaturalist World Species Observations database.

WoRMS: Phymatolithon calcareum at World Register of Marine Species. Accepted name: Phymatolithon calcareum (Pallas) W.H.Adey & D.L.McKibbin ex Woelkering & L.M.Irvine, 1986. AphiaID: 145199.

Literature:

Irvine, L.M. and Chamberlain, Y.M. (1994). Seaweeds of the British Isles. Volume 1. Rhodophyta Part 2B Corallinales, Hildenbrandiales. HMSO, London.

Morton, O. (1994). Marine Algae of Northern Ireland. Ulster Museum, Belfast.

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

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Osborne Morton (2026). Phymatolithon calcareum. (Pallas) Adey et McKibbin. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=ZM4910. Accessed on 2026-07-28