| Summary: | Many sessile and tube-dwelling polychaetes can act as ecosystem engineers, influencing the physical–chemical and biological characteristics of their habitats, increasing structural complexity. Thus, they are considered structuring species. In summer of 2021, in southern Sicily (Ionian Sea), benthic assemblages dominated by Ampharetidae <i>Amage adspersa</i> were discovered via an ROV survey at a depth range between 166 and 236 m on muddy horizontal seafloor. Large aggregations of this species (up to 297.2 tubes m<sup>−2</sup>), whose tubes are formed from <i>Posidonia oceanica</i> debris, occurred alternately with tube-free areas. The area was characterized by the sporadic presence of vulnerable sea pens <i>Funiculina quadrangularis</i> (up to 0.08 col. m<sup>−2</sup>) and <i>Virgularia mirabilis</i> (up to 0.16 col. m<sup>−2</sup>), and it was possible to detect signs of trawling as well the presence of marine litter (up to 24.0 items 100 m<sup>−2</sup>). The habitat description, distribution, and density of the tubes of <i>A. adspersa</i> were assessed via imaging analysis. In addition, morphological diagnostic analyses were carried out on some sampled specimens and on their tubes. The acquired data shed new light on how polychaetes can exploit the dead tissues of <i>P. oceanica</i>, contributing to highlight interactions between benthic fauna and seagrass detritus in the marine environment and their ecological role in enhancing the spatial heterogeneity of soft areas of the Mediterranean seafloor.
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