Maerl (Phymatolithon calcareum)

Classification (WoRMS)

Full classification as recorded by WoRMS: Kingdom: Plantae · Phylum: Rhodophyta · Class: Florideophyceae · Order: Hapalidiales · Family: Hapalidiaceae · Genus: Phymatolithon.

Accepted name: Phymatolithon calcareum · Authority: (Pallas) W.H.Adey & D.L.McKibbin ex Woelkering & L.M.Irvine, 1986 · Taxonomic status: accepted · AphiaID: 145199.

Synonyms and superseded combinations recorded for this taxon: Apora polymorpha Gunnerus, 1768; Corallium anglicum J.Ellis, 1768; Eleutherospora polymorpha Heydrich, 1900; Lithophyllum calcareum (Pallas) Foslie, 1898; Lithothamnion calcareum (Pallas) Areschoug, 1852; Lithothamnion calcareum f. attenuatum Foslie, 1897; Lithothamnion calcareum f. australe Foslie, 1895; Lithothamnion calcareum f. compressum (M'Calla ex Harvey) Foslie, 1897; Lithothamnion calcareum f. palmatifidum (Foslie) Foslie, 1900; Lithothamnion calcareum f. primiramosum Heydrich, 1911; Lithothamnion calcareum f. pseudoflabelligerum Cabioch, 1966; Lithothamnion calcareum f. pseudosubvalidum Cabioch, 1966; Lithothamnion calcareum f. simpliusculum Cabioch, 1966; Lithothamnion calcareum f. squarrulosum (Foslie) Foslie, 1900; Lithothamnion calcareum f. subsimplex (Batters) Foslie, 1900; Lithothamnion calcareum f. validum Foslie, 1900; Lithothamnion calcareum var. compressum (M'Calla ex Harvey) Me.Lemoine, 1913; Lithothamnion calcareum var. squarrulosum (Foslie) Me.Lemoine, 1913; Lithothamnion corallioides f. australe Foslie, 1895; Lithothamnion corallioides f. subsimplex Batters, 1892; Lithothamnion hamelii Me.Lemoine, 1931; Lithothamnion polymorphum Areschoug, 1852; Lithothamnion squarrulosum Foslie, 1899; Lithothamnion squarrulosum f. australe Foslie, 1899; Lithothamnion squarrulosum f. palmatifidum Foslie, 1899.

Source: World Register of Marine Species (WoRMS) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=145199 (CC BY 4.0). Accessed 2026-09-23.

Biology and life history (SeaLifeBase)

Measured values published by SeaLifeBase for Phymatolithon calcareum. Only fields the database actually holds for this species are shown; these are database values, not aquarium guidance.

Habitat zone: sessile.

Source: SeaLifeBase species summary (SpecCode 128367) — https://www.sealifebase.se/summary/Phymatolithon-calcareum.html (CC BY-NC 3.0 — non-commercial use only). Accessed 2026-09-23.

Distribution and occurrence records (GBIF)

5,216 georeferenced occurrence records for Phymatolithon calcareum are published through GBIF from 31 countries or marine areas. The map above plots 54 distinct locations from a sample of 3,000 records, omitting 47 inland records that carry an institution's coordinates rather than a collection site; it shows where the species has been observed and reported, not a modelled range.

Most records by country or area: United Kingdom of Great Britain and Northern Ireland (3,325); France (646); Spain (573); United States of America (236); Ireland (115).

The full country table, WoRMS marine regions and the sampling notes are on this species' Distribution page.

Source: GBIF Occurrence Search (taxonKey 2654944) — https://www.gbif.org/occurrence/search?taxon_key=2654944&has_coordinate=true (records CC0 / CC BY); WoRMS distribution records (MarineRegions) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=145199 (CC BY 4.0); Natural Earth 110m basemap — https://www.naturalearthdata.com/ (public domain). Accessed 2026-09-23.

Description and natural history (Wikipedia)

In Europe maerl beds occur throughout the Mediterranean, along most of the Atlantic coast from Portugal to Norway, and in the English Channel, Irish Sea and North Sea. The distribution of maerl is dependent on water movement, light and salinity concentration. Maerl beds occur in the photic zone, and can be found to around 30 m depth in the British Isles and up to 120 m deep in the Mediterranean. Maerl deposits can reach up to 10 m thick, but are usually much thinner; carbon dating has shown that they can be more than 5500 years old.

In the British Isles maerl is composed of three species of coralline algae growing loose in beds of fragmented nodules in the sub-littoral. The species generally involved are: Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum.

Maerl is dredged from the sea floor and crushed to form a powder. It is still harvested around the coasts of Brittany in France and Bantry Bay, Ireland, and is a popular fertilizer for organic gardening. It was also dredged off Falmouth, Cornwall, but this ceased in 2004.

Scientists investigated Falmouth maerl and found that L. corallioides predominated down to 6 m and P. calcareum from 6–10 m. A 2024 survey by researchers from the University of Exeter, commissioned by Cornwall Council's Blue Natural Capital Project, found that the Fal and Helford Estuary Special Area of Conservation supports an 880 hectare bed of maerl.

Chemical analysis of maerl showed that it contained 32.1% CaCO3 and 3.1% MgCO3 (dry weight).

The ecology of maerl habitats has received very little attention in contrast to other marine ecosystems such as kelp forests or sea grass beds. Maerl beds provide a complex habitat for a wide range of taxa with a variety of niches that support high associated invertebrate and algal biodiversity.

Maerl beds act as nursery areas for the juvenile stages of commercial species such as juvenile cod Gadus morhua, saithe Pollachius virens, Pollack Pollachius pollachius and juvenile scallops Aequipecten opercularis. Maerl beds offer physical refuge and protection from predation as well as productive feeding grounds but are easily damaged by dredging and towed fishing gear.

Maerl has no tolerance for desiccation.

Maerl has been extracted for centuries mainly for use as an agricultural fertilizer. The amount extracted increased in the late 20th century and in 2000, maerl was extracted at c. 5,000 tonnes per year in Ireland and c. 500,000 tonnes per year in France. Large scale maerl extraction over the past 40 years has removed and degraded maerl beds. In Cornwall, England, maerl has been extracted since the 1970s, but was banned in 2005 by Falmouth Harbour Commissioners.

An early reference to maerl was made by John Ray in 1690 who reported it from Falmouth. In Ireland, maerl is extracted from subfossil beds in Bantry Bay by Celtic Sea Minerals. The maerl-forming species Lithothamion corallioides and Phymatolithon calcareum are listed in Annex V of the EC Habitats Directive.

Used as a soil conditioner, it is dredged from the sea floor and crushed to a powder.

The slow growth of individual nodules and their accumulation in beds over a millennial timescale means that there is no possibility of maerl keeping up with dredging for this purpose. Maerl should be considered as a non-renewable resource, and readily available alternative products (e.g., garden lime) make modern day exploitation controversial.

Source: Wikipedia: Maerl — https://en.wikipedia.org/wiki/Maerl (CC BY-SA 4.0). Accessed 2026-09-23.

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