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.
Keystone (structuring) species
Lithothamnion corallioides, Lithothamnion glaciale, Phymatolithon calcareum,
Lithophyllum denudatum and L. fasciculatum
Importance of habitat for other species
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.
Time for community to reach maturity
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.).