Page 5 - Bioconstructions_2018

Mediterranean Bioconstructions Along the Italian Coast 65

used to differentiate limestones and dolomites from dolomitic calcarenites.
Carozzi (1961) subsequently used the term “bioconstruction” for a stroma-
toporoid reef of the Upper Devonian in Alberta (Canada).
Jones et al. (1994, 1997) further integrated the term “bioconstruction”
with the definition of “physical ecosystem engineers” for species that create,
modify or maintain habitats by causing physical changes in biotic and abiotic
materials or structures that, directly or indirectly, modulate the availability of
resources to other species.
Only low-diversity (mono- or oligotypic) bioconstructions can be found
in the Mediterranean, as in all temperate areas, whereas high-diversity (poly-
typic) reefs are typical of tropical seas (Bianchi, 2002; Cocito, 2004; Sheppard
et al., 2017). In some Mediterranean reefs the engineer species are either one
(as in Cladocora reefs) or two (as in the Dendropoma–Neogoniolithon intertidal
reefs), but rich species assemblages contribute to the construction of some
coralligenous reefs.
Bianchi (2002) proposed two processes leading to biogenic reef forma-
tion: gregarism, in which individuals derived from different larvae or spores
settle side by side, as in polychaete worms and vermetid molluscs, or
modularity/coloniality, based on asexual reproduction (vegetative multipli-
cation), as in cnidarians. Clonal organisms grow bigger than individual ones
( Jackson, 1977), and their biogenic reefs are formed by aggregations of
clonal formations, i.e., by the gregarism of modular organisms. The most
conspicuous biogenic reefs are produced by the activity of skeleton-
producing organisms such as calcareous algae, oysters and corals. Other
biogenic reefs may be produced by sand-binding and cementing activities
of some organisms, as in the case of sabellariid worm reefs or stromatolites
formed by cyanobacteria. Calcareous sediments deriving from algae, mollusc
shells, echinoderms spines, sponge spicules and other skeletal debris often fill
the spaces within the reef framework and become consolidated in various
ways. Sponges, zoanthids and certain colonial ascidians can bind reef
materials together, becoming a sort of connective tissue. Organisms that
grow as calcareous sheets can act as biological cementing agents, building
permanent bonds that cause the strong adhesion of loose calcareous sedi-
ments to the reef frame. Beneath the living reef, the material has a marble-like
consistency. The mechanisms that produce such diagenetic cement are
numerous and partially unclear, but the process is thought to include also
a biological component derived from microbial activity or from organic
compounds produced by reef organisms (Macintyre and Marshall, 1988;
Scoffin, 1992).