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.IGS.Mrl.Phy.HEc recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Species indicative of sensitivity

To assess the sensitivity of the biotope, the sensitivity of component species is reviewed. Those species that are considered to be particularly indicative of the sensitivity of the biotope, and for which research has been undertaken in detail are shown below (see selection criteria). The biology of other component species of the biotope is also taken into account wherever information is known to the researcher.

Species found especially in this biotope

  • Halymenia latifolia
  • Scinaia turgida
  • Gelidiella calcicola
  • Gelidium maggsiae
  • Cruoria cruoriaeformis
  • Tectura virginea
Community Importance Species name Common Name
Key structural Phymatolithon calcareum Maerl
Important characterizing Nemertesia ramosa A hydroid
Important characterizing Neopentadactyla mixta Gravel sea cucumber


The biotope IGS.Phy.Hec occurs at greater depths than other maerl biotopes and consequently has fewer algal species. The community within the maerl bed is dominated by infaunal molluscs (Hall-Spencer & Atkinson, 1998) although hydroids and echinoderms may be the most apparent on the surface. The biotope is partly named after these more obvious groups of organisms (hydroids and echinoderms). Two species typical of the biotope that represent these groups are Nemertesia ramosa and Neopentadactyla mixta and these species have been selected to represent the sensitivity of their relevant group. N.B. These two species are often but not necessarily always present in this biotope. Other similar species may be present in addition to or in place of these two species. In order to try and give an idea of the sensitivity of the biotope, Nemertesia ramosa and Neopentadactyla mixta have been used as species indicative of sensitivity for these obvious groups. Even if these species are not present or other species within the group (e.g. Nemertesia antennina, Ophiothrix fragilis are more faithful or abundant, the sensitivity assessments can give a broad impression of the sensitivity of the biotope. In undertaking an assessment of sensitivity of this biotope, account is taken of knowledge of the biology of all characterizing species in the biotope. However, the selected 'indicative species' are particularly important in undertaking the assessment because they have been subject to detailed research.

Additional information

  • Maerl biotopes are well recognised as having particularly rich and diverse communities. The MNCR survey recorded a maximum of 88 species but the BIOMAERL team (1999) recorded a maximum species richness of 490 at one Scottish site. From maerl biotopes in general, over 150 macroalgal species and 500 benthic faunal species have been recorded (Birkett et al., 1998a).
  • Species richness can vary considerably in maerl beds, even within the same geographical area. There are also seasonal changes in species richness although this applies particularly to epiphytic algae.
  • Maerl beds that are or have been dredged for scallops have modified species compositions, reduced species richness and abundance (Hall-Spencer & Moore, 2000).
  • There are several species of algae that are apparently restricted to calcareous habitats and may be characteristically found in maerl beds (e.g. Halymenia latifolia, Scinaia turgida, Gelidiella calcicola, Gelidium maggsiae & Cruoria cruoriaeformis)(Birkett et al., 1998(a)). Since the biotope occurs in deeper waters, then the number of algal species present will be reduced in comparison to other biotopes such as IGS.Phy.R.
  • Tectura virginea can be considered to be associated with maerl although is most common on encrusting coralline algal species. There are several species of mollusc that are common in maerl beds (e.g. Gibbula cineraria, Rissoa interrupta, Modiolarca tumida, Hinia incrassata, Tricolia pullus & Hiatella arctica) but these are also common in other habitats and probably either reflect the nature of the substratum or are widespread in lower shore and sublittoral environments
  • Neither the MNCR surveys (JNCC, 1999) nor Birkett et al., 1998(a) specifically record any species recorded from maerl beds as being rare or scarce. However, this is likely to be caused by non-recognition or under-recording of rare or scarce species.

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 28/11/2015]. Available from: <>