ABSTRACT Sea floor mapping along the Apulia continental shelf (Italy) verified the abundance of autochthonou red algae build-ups, mapped as coralligenous habitats (CHs), in a water depth range of 5-100 m. In general, CHs were found to develop three dimensional structures, with a rigid framework and to represent an important geomorpho- logical and sedimentological element on the Mediterranean shelf. Here, we provide the first geomorphological description of CHs (thus poorly categorized) using acoustic data obtained from Side Scan Sonar (SSS) and MultiBeam (MB) echosounder surveys, ground-truthed using a ROV and underwater camera. In SSS mosaics, CHs generally yielded intermediate to high backscatter in response to a rigid cavernous framework. According to the various shapes and the lateral continuity that coralligenous build-ups dis- played in explored locations, two distinct textures were determined to be present. Various geomorphological expressions of CHs were noted within our dataset and in images obtained from MB bathymetry. We determined that coralligenous build-ups are typically represented by positive-relief structures that vary from isolated blocks (randomly scattered on a generally flat mobile soft bottom) to a field of blocks (adjacent or even coalescent), and/to ridge with several metres of lateral continuity. In most cases, CHs occurred on flat mobile soft bottom, thus representing an example of coralligenous de plateau. Our results characterize for the first time the CHs through seafloor mapping techniques, which demonstrated to represent an instrumental tool for their geomorphological characterization. KEY WORDS: Coralligenous, geomorphology, habitat map- ping, Mediterranean Sea. INTRODUCTION Modern autochthonous red algae build-ups, corallige- nous build-ups, are a well-known feature of the temperate Mediterranean Sea (FELDMANN, 1937; PÉRÈS & PICARD, 1964; LABOREL, 1961; LAUBIER, 1966; PÉRÈS, 1967; HONG, 1982; BOSENCE, 1983, 1985; SARTORETTO, 1994; SARTORETTO et alii, 1996; DI GERONIMO et alii, 2002; BALLESTEROS, 2006; BEN MUSTAPHA et alii, 2003; CASEL- LATO & STEFANON, 2008; AGNESI et alii, 2009). Due to their high level of biological diversity (PÉRÈS &PICARD, 1964; BOSENCE, 1983; BALATA et alii, 2005; BALLESTEROS, 2006), they are listed in the EC Habitat Directive. In the framework of Mediterranean marine benthic zonation, coralligenous habitat (CHs) are indicative of a circalittoral biocoenosis consisting of a three dimensional biogenic build-up that forms a solid substrate primarily dominated by coralline algae (PÉRÈS & PICARD, 1964; BELLAN-SANTINI et alii, 1994; BRESSAN et alii, 2001). Due to their three-dimensional structure, CHs often play a primary role in the geomorphological development of the continental shelf. CHs develop on both hard and soft bottoms, in 4 to 160 m of water depth (wd) (SARÀ, 1966; FREDJ, 1964; LABOREL, 1961, 1987; SARTORETTO, 1994; BALLESTEROS, 2006) and adapt to low levels of nutrients and tempera- ture from 10 to 23°C in waters with moderate hydro- dynamics (BOSENCE, 1983; BALLESTEROS, 2006). The extreme variability of their bathymetrical distribution is partially governed by the light-dependency of participat- ing benthic organisms (BALLESTEROS, 1992; MARTÌ et alii, 2004), a finding particularly true for macroalgae, adapted to grow at dim-light conditions (LITTLER et alii, 1986; VADAS & STENECK, 1988). Architecture and morphology are primarily controlled by biological carbonate productivity that responds to cli- mate, oceanography, physiography, changes in accommo- dation space and terrigenous supply (SCHLAGER, 1991, 1993; BETZLER et alii, 1997). Modern Mediterranean coralligenous build-ups are characterized by large structures that may be up to 4 m high and greater than 50 m in lateral continuity (BOSENCE, 1983, 1985). Coralligenous build-ups vary in shape and dimension: nevertheless, their geomorphological expression have not been exhaustively categorized. Various definitions for characterizing CHs structures, reflecting different con- structional morphologies, have been reported in the scien- tific literature. For examples, CHs have been identified as coralligenous de plateau (BOSENCE, 1985), columnar crustose coralline algal build-ups (DI GERONIMO et alii, 2001a,b, 2002), and algal reefs (BOSENCE, 1983) or banks (BALLESTEROS, 2006). Definitions are generally based on scuba diver’s visual observations or direct sampling. From a geomorphological point of view, the following main morphologies have been distinguished: 1) banks-flat frameworks with thickness ranging from 0.5 to 4 m mainly built over more or less horizontal substrata, and 2) rims-structures on submarine vertical cliffs or sur- rounding the opening of submarine caves, generally located in shallower waters than banks (PÉRÈS & PICARD, 1964; LABOREL, 1987; BALLESTEROS, 2006). If rims develop on a hard original substrate, banks are generally reported as a consequence of the coalescence of rhodoliths (PÉRÈS & PICARD, 1952), indicated as a coralligenous de plateau (PÉRÈS & PICARD, 1964). However these frame- (*) Università di Milano-Bicocca, Dipartimento di Scienze dell’Ambiente, del Territorio e di Scienze della Terra. Piazza della Scienza, 4 - 20126 Milano, Italy. Coralligenous habitat in the Mediterranean Sea: a geomorphological description from remote data VALENTINA BRACCHI (*), ALESSANDRA SAVINI (*), FABIO MARCHESE (*), SERENA PALAMARA (*), DANIELA BASSO (*) & CESARE CORSELLI (*) 251-R1 – BRACCHI ET ALII Ital. J. Geosci. (Boll. Soc. Geol. It.), Vol. 134, No. 1 (2015), pp. 00-00, 8 figs. (doi: 10.3301/IJG.2014.16) © Società Geologica Italiana, Roma 2015 Queste bozze, corrette debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, 5 – 00185 ROMA