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68 Gianmarco Ingrosso et al.

the upper intertidal belt, mussel and oyster beds and bryozoan reefs, host a
great diversity of life, but they do not produce substantial bioconstructions
and are not covered here.

2.2.1 Lithophyllum byssoides Concretions/Trottoirs
Lithophyllum byssoides (Lamarck) Foslie is a coralline red alga forming adher-
ent crusts with a crinkled surface made of convoluted lamellae or finger-like
protrusions (Bressan et al., 2009; Verlaque, 2010). It is relatively common in
the western and central Mediterranean, but it does not occur in the eastern-
most parts of the basin (Laborel, 1987). L. byssoides is one of the few genuinely
intertidal Mediterranean algae; it occurs in a narrow vertical belt a few dec-
imetres wide, located just above the mean sea level (Fig. 2A), forming large
populations on rocky shores directly exposed to prevailing winds (Verlaque,
2010), covering either horizontal tidal notches (Antonioli et al., 2015)or
overhangs. Under these conditions, numerous thalli of L. byssoides coalesce
and form thick concretions that cover the surface of the rock, often protruding
from it. Well-developed concretions form step-like or overhanging rims,
which may protrude up to 2m from the colonized rocky surface. These rims
are called L. byssoides “encorbellements” (corbels), when they protrude only a
little, or “trottoirs” (sidewalks), when protruding greatly. At highly exposed
sites they may be found up to 50cm above the mean sea level, and as a result
they form the uppermost bioconstructions on Mediterranean coasts. Living
L. byssoides occurs only on the upper and outer surfaces of the rims; the
remaining parts consist of very hard, multilayered materials formed by
deposition processes that fill the interstices between the lamellae of the dead
alga with a hardened sedimentary matrix (Verlaque, 2010). The overgrowth
of notches gives the same aspect of the bioconstruction, but, in this case, the
algae encrust a step of geological origin, protruding from its edge. Other
algae are usually associated with a well-developed rim of L. byssoides (e.g. Chae-
tomorpha ligustica (K€utzing) K€utzing (¼Chaetomorpha mediterranea (K€utzing)
K€utzing), Palisada perforata (Bory de Saint-Vincent) Nam (¼Laurencia papillosa
(C. Agardh) Greville), Pterocladiella melanoidea (Schousboe ex Bornet) Santelices
(¼P. melanoidea (Schousboe ex Bornet) E.Y. Dawson), Lophosiphonia cristata
Falkenberg and Taenioma nanum (K€utzing)Papenfuss)(Bressan et al., 2009).
Thelowerpart of therim isprofusely colonized by sciaphilic algae and inver-
tebrates that are similar to those living in clefts and caves (Laborel, 1987;
Verlaque, 2010). Various types of bioeroders (sponges of the genus Cliona)
and boring species (molluscs of the genus Lithophaga, etc.) are also present,
reducing the stability of the rim by creating tunnels and cavities into the