Biodiversity & Conservation

Phymatolithon calcareum maerl beds with hydroids and echinoderms in deeper infralittoral clean gravel or coarse sand



Image Sarah Fowler - Maerl bed in Loch Gairloch, Highland. Image width ca 2 m in foreground.
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Distribution map

SS.SMp.Mrl.Pcal.Nmix recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Ecological and functional relationships

The ecological relationships of maerl beds can be very complex. The maerl thalli provide considerable surface area to which both flora and fauna can attach. The maerl nodules themselves may be directly grazed by species like Tectura virginea. The surface film of microalgae and detritus can also be grazed. The loose structure permits water circulation and oxygenation to considerable depth. As a consequence of this loose structure, maerl provides shelter for an astonishing variety of fauna e.g. molluscs (Hall-Spencer, 1998) and amphipods (Grave De, 1999). The loose structure also permits animals to burrow to considerable depths (at least 60 cm) within the gravel.

Seasonal and longer term change

Cabioch (1969) suggested Phymatolithon calcareum may have phasic reproduction with peaks every six years. This may account for observed changes in the relative proportions of live Lithothamnion corallioides and Phymatolithon calcareum nodules in some maerl beds. Dominance cycles with periods of about thirty years have been recorded on some of the maerl beds of northern Brittany.

Habitat structure and complexity

The habitat of this biotope is extremely complex. The maerl nodules are frequently loose and mobile preventing colonization by many species. However, deep burrowing fauna (to 68 cm) are a notable feature of this biotope (Hall-Spencer & Atkinson, 1999). Some surveys record as few as 10 species in the biotope, primarily because the vast majority of species live below the maerl surface. Maerl in general is known as a particularly diverse habitat with over 150 macro algal species and 500 benthic faunal species recorded (Birkett et al., 1998(a)).


Maerl beds may contain dead as well as live nodules. Productivity will depend on the relative proportions of dead and alive nodules. Primary productivity may be less than in maerl biotopes found in shallower waters (e.g. IGS.Phy.R) where there are more epiphytic algae. Secondary production may be very high in situations where there are dense aggregations of consumers. The sea cucumber Neopentadactyla mixta can reach densities of up to 400 per square metre in loose gravels such as maerl (Smith and Keegan, 1984).

Recruitment processes

Recruitment of Phymatolithon calcareum is mainly through vegetative propagation. Although spore bearing individuals of Phymatolithon calcareum thalli have been found in the British Isles, the crustose individuals that would result from sexual reproduction have yet to be recorded in the British Isles. Recruitment may occur from distant populations that exhibit sexual reproduction and have crustose individuals (e.g. Brittany). Hall-Spencer (pers. comm.) has observed that colonization of new locations by maerl can be mediated by a 'rafting' process where maerl thalli are bound up with other sessile organisms that are displaced and carried by currents (e.g. Saccharina latissima holdfasts after storms).

Time for community to reach maturity

Phymatolithon calcareum is extremely slow growing (c. 1mm per year) (Potin et al., 1990 and Birkett et al., 1998a). Development of a new maerl bed would take a long time. Maerl beds are also extremely long lived with life-span of the habitat being 6000 years or more (Birkett et al., 1998a) Within the biotope, the community is dependent on the growth of a surface veneer of photosynthetically active maerl thalli.

Additional information

Although Phymatolithon calcareum has a patchy distribution around the British Isles, it is the most widespread maerl-forming species in European waters (BIOMAERL team, 1999). "Maerl is a 'living sediment'; it is slow to recover from disturbance due to infrequent recruitment and extremely slow growth rates (Hall-Spencer & Moore, 2000(a))". Although from outward appearances suspension feeders may appear to be dominant, Grall & Glemarec (1997) found that dominant trophic groups varied according to the assessment criteria used. In terms of species richness carnivores were most dominant, for abundance it was detritivores and for biomass it was surface deposit feeders. Detrital input is important in enclosed areas such as the Firth of Clyde and the Fal estuary.

This review can be cited as follows:

Jackson, A. 2006. Phymatolithon calcareum maerl beds with hydroids and echinoderms in deeper infralittoral clean gravel or coarse sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 29/11/2015]. Available from: <>