Mediterranean seascape shaped by Coralligenous build-ups.

In the framework of Mediterranean marine benthic zonation, coralligenous (C) is a circalittoral biocenotic complex forming a new solid substrate, mainly produced by the overgrowth and accumulation of calcareous encrusting algae in dim light conditions, and consisting of tridimensional biogenic build-ups. As other benthic bio-constructions, it contributes to seascape formation through geological times, causing geomorphological changes of the seafloor. In the Mediterranean Sea it forms large structures that may be up to 4 m high and greater than 50 m in lateral continuity. Marine bionomists consider the substrate of C to be a key factor in distinguishing the C d'horizon inferieur de la roche littorale, always on a pre-existing hard substrate, from the C de plateau. Originally, the latter was indicated as a biogenic framework developed from the coalescence of rhodoliths; then it was generally indicated as forming a new solid substrate on an originally mobile substrate. However, given the diculty to investigate the type of substrate of C, some authors suggested that some C de plateau frameworks could have grown on submarine rocky outcrops. C produces various morphotypes on the seafloor, not exhaustively categorized yet. Several terms are used to dene C morphotypes, but their application through dierent geological and environmental settings still remains inconsistent. Only two general categories are reported: 1) banks { at over more or less horizontal substrata, and 2) rims { structures on submarine vertical clis or surrounding the opening of submarine caves. Apulian C is known in literature since decades. Large areas of the Apulian continental shelf have been investigated by acoustic methods from the coastline down to 100 m water depth, in the framework of the BIOMAP project and of a commercial survey. Collected remote data (Multibeam echosounder, Side Scan Sonar and Chirp/Sub Bottom Proler) have been ground-truthed by ROV and camera inspections. Seafloor mapping techniques allow obtaining large-scale, high-resolution seafloor images for the description of the seascape, and the distribution and extent of benthic habitats. We identied a combination of distinct C morphological expressions, at the scale of meso- and macro-habitat, based on distinctive and ground-truthed C morpho-acoustic facies in which the biogenic frameworks (i.e. coralligenous) prevail as sole biocommunity on the seafloor or associated with other type of habitats. We detected coralligenous morpho-acoustic facies as 0.2 up to 4 m topographic reliefs with steep anks and a rigid inhomogeneous biogenic framework, characterized by medium to strong Side Scan Sonar backscatter and a variable plan-view geometry. Finally, we propose a new descriptive, objective categorization based on the extrapolation of geometric parameters, such height and Shape Index, to dene C morphotypes on sub-horizontal, not only mobile, seafloor. These morphotypes are: 1) tabular banks, 2) discrete reliefs, or 3) hybrid banks.