The maerl beds of Brittany and of the Mediterranean have long been recognised as communities with a particularly high diversity of plant and animal species. In the British Isles, there may be somewhere in the range of 150-200 or more macroalgal species found on maerl, and perhaps over 500 benthic faunal species. Amongst the associated organisms are many rare species which are unusual, rare or poorly known: a recent report on maerl beds in the sound of Arisaig, Scotland by Davies & Hall-Spencer (1996) revealed four species of polychaete and an isopod species likely to be new to science, in addition to large numbers of the isopod Paramunna bilobata, previously described as a rare gravel-dwelling species.

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