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72 Gianmarco Ingrosso et al.

Mediterranean rocky bottoms from 15 to 130m depth, depending on water
transparency (Ballesteros, 2006; Martin et al., 2014). These build-ups are
mainly due to the activity of several encrusting calcareous algae (of the genera
Lithophyllum, Lithothamnion, Mesophyllum, Neogoniolithon and Peyssonnelia)
(Laborel, 1961; Laubier, 1966; Sartoretto et al., 1996) whose remains are
cemented by microbial carbonates. Several species and/or assemblages can
be associated with these biogenic red algal constructions (Boudouresque
et al., 2015), namely: forests of the seaweed Cystoseira zosteroides (Turner)
C. Agardh (Ballesteros et al., 2009); animal forests consisting of gorgonians,
e.g., Eunicella spp. and Paramuricea clavata (Risso,1826);large erectsponges such
as Axinella polypoides Schmidt, 1862; bryozoans such as Turbicellepora spp. and
Pentapora fascialis (Pallas, 1766); stands of crustose but not bioconstructing
algae; turf and bushy macroalgae; and small-sized sessile metazoans belonging
mainly to sponges, cnidarians, serpulid polychaetes, bryozoans and ascidians
(Boudouresque, 1973). The result is an extraordinary habitat with a great
biodiversity. Ballesteros (2006) made a first estimate of the number of species
associated with coralligenous formations, listing about 1670 species. However,
this is probably an underestimated number because the complex structure of
coralligenous assemblages and their highly diverse composition suggests that
they host more species than any other Mediterranean habitat.
Light is an important driver in determining the distribution and assem-
blages of benthic organisms, together with water movement, sediment rates,
substrate features and geographical area (Ballesteros, 2006). The main cor-
alligenous geomorphological structures along the Italian coasts are on cliffs
(Fig. 3A) and platforms (Fig. 3B). Coralligenous rims develop on vertical and
subvertical cliffs between 10 and 50+m depth, or surrounding the opening
of submarine caves, often forming structures of considerable thickness. Cor-
alligenous banks are flat structures developing over more or less horizontal
substrata, between 15 and 100+m depth. These platforms can be extensive
and develop on both rocky substrates (Laborel, 1987) and coastal debris
deposits, generally starting from the coalescence of rhodoliths together with
sand and gravel (Basso et al., 2007).
Coralligenous assemblages are widespread along the Italian coast
(Fig. 3C), with the exception of the sandy-muddy seabed between the
Po delta river and the Gargano Peninsula. Many mesophotic biogenic habitats
are scattered on the northern Adriatic continental shelf. These structures,
locally called trezze or tegnu `e, are similar to coralligenous assemblages and their
size ranges from a few to thousands square metres in the surface area, and
they rise up to 4m in height from the surrounding sedimentary bottom