There are numerous features of maerl that contribute to its value as a habitat for other marine species (Nunn 1992). It provides a surface to which other seaweeds can attach (Cabioch 1969; Adey & Adey 1973; Adey & McIntyre 1973). Other species then feed on these e.g. Aplysia punctata and rissoids molluscs. The maerl may also be directly grazed by species such as Tectura virginea. Maerl also provides attachment sites for animals such as Antedon bifida, hydroids and bryozoans. The loose structure of maerl provides shelter for small gastropods with infauna including many bivalves such as Mya truncata and Dosinia exoleta. Epifauna include small Crustacea (Farnham & Bishop 1985). The integrity of the maerl bed in turn requires at least some elements of the rich epiflora associated with it as a stabilizing feature. Jacquotte (1962) and Cabioch (1969) discussed the importance of various creeping species in stabilizing the maerl deposits by the formation of stolons and secondary attachments. The interactions with invertebrate grazers are also very important in keeping open substratum clear for settlement by algal and animal species.

Many coralline algae produce chemicals, which promote the settlement of the larvae of certain herbivorous invertebrates. The herbivores then graze off the epiphytic and often fast-growing algae, which might otherwise overgrow the coralline algae, competing for light and nutrients. The presence of herbivores associated with corallines can generate patchiness in the survival of dominant seaweeds.

Maerl has not been studied as a habitat for the juvenile stages of demersal and pelagic species. Divers visiting maerl beds have commented on the numbers of small individuals that can be seen. The open structure of a maerl bed would certainly provide a secure habitat for juveniles. In the west of Ireland, maerl deposits are known to act as nursery grounds for the black sea urchin Paracentrotus lividus.

Lithothamnion corallioides, Lithothamnion glaciale, Phymatolithon calcareum, Lithophyllum denudatum and L. fasciculatum

No information available.

Juvenile maerl plants grow as crusts on pebble or shell substrata. The erect branches formed by these crusts break off and give rise to free-living maerl thalli growing as nodules. Jacquotte (1962) found Halopithys incurvus to be more frequent in the winter and attributed seasonal changes in maerl bed epiflora in the Mediterranean to seasonal changes in illumination. Cabioch (1969) found a few crustose species to be more abundant in the winter. In Ireland, the species diversity of maerl beds has been shown to increase in the summer (Maggs 1983) and it was suggested that this was as a result of the greater stability of the biotope owing to the calmer weather normally experienced at this time of year. Long- and short-term changes in biodiversity have both been noted. As an illustration of the difficulties that may be encountered in monitoring the epiflora component of maerl beds, Maggs (1983) reported that during a 2-year-long sampling programme, nine conspicuous species disappeared from the maerl beds under investigation while a further three species appeared in the biotope.

Species composition of the maerl in a bed is known to cycle over a period of time (3-30 years). Maerl thalli are long lived; thalli of Lithophyllum dentatum have been estimated to be between 20-100 years old (Fazakerley, unpubl.).

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