"Trezze" are hot-spots of biodiversity rich in microenvironments and ecological gradients, compared to the monotonous context of the Northern Adriatic seabed made up of mud interspersed with areas of medium-fine sands rich in organogenic detritus. With reference to Annex I of the Directive, the rocky outcrops of the northern Adriatic can be considered as reef habitats characterized by biogenic and/or geogenic concretions. The biological communities of the Bardelli trezza fall within the definition of coralligenous, as proposed in the RAC/SPA meeting (2006, Tunisia), in the Action Plan for coralligenous conservation (UNEPMAP- RAC/SPA, 2008) and in Ballesteros (2006), where the organogenic contribution of calcareous algae become evident. Among the most important bio-builders there are the calcareous algae of the genera Lithophyllum, Lithothamnion, Mesophyllum, Neogoniolithon and Peyssonnelia, the madreporari Cladocora and Astroides, the briozoo Myriapora and the serpulid polychaetes Serpula and Pomatoceros. The contribution of calcareous algae is significant compared to the Veneto outcrops. The high biodiversity of rocky outcrops also varies due to the composition of the surrounding substrates (coastal terrigenous muds, well-calibrated fine sands, coarse sands). Elements that characterize the maërl, found in the Action Plan for the Conservation of Coralligenous and other Mediterranean bio-concretions (UNEP-MAP-RAC/SPA, 2008) have been found on the seabed surrounding the San Pietro and Bardelli trezze. The richness of the populations and the presence of ecological gradients, based on the variety of types, orientation and elevation of the substrates as well as on the hydrological characteristics of the area, represent a valuable element that assumes considerable importance, taking into account the relative uniformity of the north-western coastal funds of the Northern Adriatic. In addition to the presence of phytozoobenthic species found only in these sites, these bioconstructions play a fundamental role in the reproduction and development of juvenile stages and represent nuclei of attraction and protection for numerous demersal and pelagic fish species. These outcrops are the breeding grounds for Blue shark (Prionace glauca) and other shark species such as Small-spotted catshark (Scyliorhinus canicula), Nursehound (Scyliorhinus stellaris), and Common smooth-hound (Mustelus mustelus). The environmental value and the high biodiversity place the bioconstructions of the Upper Adriatic area to the attention of the scientific world as well as of numerous categories of stakeholders, as these are sites of high interest for divers and fishermen.