AN “OASIS” OF BIODIVERSITY IN DEEP WATERS: THE USE OF REMOTED OPERATED VEHICLES (ROVs) AS A TOOL TO ASSESS THE ECOLOGICAL ROLE OF ATRINA FRAGILIS (PENNANT, 1777).

Most marine environments still lack comprehensive habitat assessments, despite they are subjected to increasing threats. Therefore, the development of new technologies, like cost-effective Remotely operated vehicles (ROVs) have been providing an opportunity to carry out ecological surveys, solving some constraints related to the restricted accessibility to some environments (i.e dive time and depth). Moreover, this method permits to obtain a huge quantity of images and video, creates permanent records, facilitates resamplings (RIEGL, 2001; LAM, 2006) and hence is widely used for long-term monitoring activities, in particular for protected areas and threatened species, like the fan mussel Atrina fragilis (PENNANT, 1777) (SOLANDT, 2003). This Atlantic-Mediterranean species is the second largest bivalve living in the Adriatic Sea, where inhabits deep pelitic sandy bottoms (25-50 m depth), often severely affected by trawling, especially rapido trawling (HALL-SPENCER and FROGLIA, 1999). A. fragilis plays an important ecological role as ecosystem engineer, providing a hard substrate for sessile species and fishery targets, thus representing an “oasis” of biodiversity in a predominantly sedimentary biotope (HALL- SPENCER and FROGLIA, 1999). Currently, little is known about the ecology of A. fragilis and, as far as we know, there are no data for the Mediterranean Sea on the epibiontic community that lives on its shell. Therefore, this work is intended to be a preliminary study to describe the associated fauna inhabiting the shell of A. fragilis. Images of 40 live specimens of the fan mussel were randomly selected after the analysis of videos and HD images obtained during a four-year (2018-2021) monitoring plan with ROVs in an area of the northern Adriatic Sea. The study area, with a mean depth of 29 m, was characterised by pelitic-sandy sediments with scattered biogenic outcrops locally known as “trezze or tegnùe”. The assessment of associated fauna was performed on a single valve, considering both sessile and vagile organisms, taxonomic identification was carried out at the lowest possible level. In addition, the number of species and percent coverage were estimated. A total of about 36 taxa were identified, most of which were sessile (encrusting or massive sponges, ascidians, cnidarians and bryozoans). As regard the vagile fauna, few species were observed, but in high abundances. From initial observations, it appeared that only a few specimens of A. fragilis were found living in proximity to the outcrops, while aggregations were observed far from them. Moreover, specimens that lived farther away and isolated from the tegnùe, were more colonized than the nearest ones. These results confirm the hypothesis of the importance of the ecological role of A. fragilis as an ecosystem engineer in sedimentary bottoms. In addition, Remotely Operated Vehicles (ROVs) have proved to be a useful non-destructive tool for this first attempt to describe the fauna associated with threatened species such as the fan mussel A. fragilis, that inhabit deep bottoms, that are normally difficult to reach and study.