Although maerl beds are analagous in many ways to kelp forests and seagrass beds (BIOMAERL, in press), to our knowledge there have been no overall comparisons of the diversity of maerl fauna and flora with those in equivalent samples from other biotopes. However, the algal diversity on maerl in Galway Bay (Maggs, 1983a, 1983b) can be compared with algal diversity in photophilic algal communities in the Mediterranean (Coppejans, 1980). Similar methods and sample sizes were used in both studies, showing that Galway Bay maerl flora is as diverse (average of 60 species per 300 cm³ sample at 10 m depth) as these highly speciose Mediterranean communities (average of 70 species per 400 cm² sample).

The branching of the maerl thalli provides shelter for small plants and animals, and the communities in the maerl beds are much richer than those on gravel or shell bottoms of an equivalent granulometry (J. Cabioch, 1969). Biodiversity in maerl, particularly of the faunal elements, has rarely been compared with that of other sedimentary substrata and with rock. Bosence (1979) found that maerl banks had more abundant epifauna and boring infauna than other sediments in Mannin Bay, such as sand and gravel. However, the overall species richness in maerl, as judged from his tables of animals found in each substratum type, was lower than in muddy algal gravel and clean algal gravel, similar to that in fine sand, but greater than in mud communities. Earlier workers such as L. Cabioch (1968) were concerned that the maerl 'biocoenosis' (equivalent to biotope complex) might be only a form of the 'Venus fasciata biocoenosis'. Later multivariate analysis (e.g. O'Connor et al., 1993) showed that maerl faunas were a distinct assemblage that clustered with other sedimentary faunas.

There are several reports of mobile substrata (i.e. substrata that move at least occasionally, e.g. stones, shells, maerl) supporting a more diverse algal community than the adjacent solid substratum (Lieberman et al., 1979; Sears & Wilce, 1975). In Galway Bay, the diversity of the algal community of maerl beds was very high compared with that of the surrounding habitat (Maggs, 1983a). The rocky outcrops adjacent to these maerl beds were subject to heavy grazing pressure by Echinus esculentus and these rocks supported only 24 algal species per 0.09 m², of which 13 were epiphytic on the larger algae. Samples of a similar surface area collected on the nearby maerl beds contained a year-round average of 46 species (Maggs, 1983a).

Unstable sediments such as shell or maerl banks may act as a reservoir for weakly competitive algal species, living on the fringes of their distribution range (Waern, 1958, p. 332). Most studies have been of cobbles or shells. Waern (1958) examined algae growing on deposits of dead shells off the west coast of Sweden; Kain (1960) briefly described the algae growing on pebble and gravel bottoms off the Isle of Man; Sears & Wilce (1975) and Connor (1980) included shell bottoms in their study of algal communities in North America; Lieberman et al. (1979) reported on the ecology of seasonally devastated cobble substrata off Ghana.

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