Biodiversity & Conservation

Lithothamnion glaciale maerl beds in tide-swept variable salinity infralittoral gravel



Image Christine Howson - Lithothamnion glaciale nodules amongst pebbles with the sunstar Crossaster papposus, Isle of Lewis. Image width ca 20 cm.
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Distribution map

SS.IGS.Mrl.Lgla 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
  • Cruoria cruoriaeformis
  • Tectura virginea
Community Importance Species name Common Name
Key structural Lithothamnion glaciale Maerl
Important functional Ophiothrix fragilis Common brittlestar
Important functional Psammechinus miliaris Green sea urchin


The biotope is named after the key structural species Lithothamnion glaciale. Two other species have been selected as being representative of this biotope (Ophiothrix fragilis, and Psammechinus miliaris ). Ophiothrix fragilis has a potentially important functional role in the biotope as a suspension feeder. Benthic suspension feeders such as Ophiothrix fragilis can occur in very high densities and can have a dominant role in the main nutrient exchanges in estuarine and coastal ecosystems (Dame, 1993 cited in Smaal, 1994; Lefebvre & Davoult, 1997). Suspension feeders are important in coastal ecosystems because they can remove large amounts of suspended particulate matter (Davoult & Gounin, 1995). Psammechinus miliaris is an active grazing omnivore that may have a functional role in modifying densities of other species. N.B. These three 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. Even if the three selected species are not present, or other similar species 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 recognised as having particularly rich and diverse communities.
  • The BIOMAERL team (1999) recorded a maximum species richness of 490 from a maerl bed at one Scottish site. From maerl biotopes in general, over 150 macroalgal species and 500 benthic faunal species have been recorded (Birkett et al., 1998(a)).
  • Species richness can vary considerably in maerl beds, even within the same geographical area. There are also potential 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, 2000a).
  • 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., 1998a). These species are found in maerl beds in general, whether or not they are found in conditions of reduced salinity is uncertain.Tectura virginea can be considered to be associated with maerl although it 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 and 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.(1998a) specifically record any species recorded from maerl beds as being rare or scarce. However, this is likely to be caused by under-recording or difficulties of identification of rare or scarce species.

This review can be cited as follows:

Jackson, A. 2006. Lithothamnion glaciale maerl beds in tide-swept variable salinity infralittoral gravel. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/11/2015]. Available from: <>