| Summary: | The Mediterranean Sea harbors more than 17,000 eukaryotic marine species, with several ecosystems recognized as biodiversity hotspots, such as <i>Posidonia oceanica</i> meadows. Recent research indicates that benthic mats formed by the fleshy red alga <i>Phyllophora crispa</i> are also associated with high species richness. Among key groups found in these mats are sessile polychaetes, which live as epiphytes on the red algae thalli. Knowledge of abundance, species richness, and spatial variation of polychaetes associated with these habitats is still scarce. We carried out a comparative assessment focusing on serpulid polychaetes within samples from <i>P. crispa</i> mats and neighboring <i>P. oceanica</i> meadows at six different sampling sites around Giglio Island (Tyrrhenian Sea, Italy). A total of 17 serpulid taxa were identified. The abundance of serpulids (5665 individuals m<sup>−2</sup> of <i>P. crispa</i> mat) were similar to neighboring <i>P. oceanica</i> meadows (2304 individuals m<sup>−2</sup> leaves and 5890 individuals m<sup>−2</sup> shoots). The number of serpulid taxa was significantly higher in <i>P. crispa</i> mats (average 6.63 ± 1.32 taxa) compared to <i>P. oceanica</i> beds (average 1.56 ± 0.63 and 1.84 ± 1.04 taxa in leaves and shoots, respectively). Within habitat type, there were no significant differences in species richness between sites. The most abundant species found was <i>Josephella marenzelleri</i> (61% of individuals), while <i>Vermiliopsis</i> spp. and <i>Bathyvermilia</i> sp. were exclusively found in <i>P. crispa</i> samples. Our results highlight that <i>P. crispa</i> mats host an exceptional diversity and that these habitats should be included in conservation strategies. Further research should focus on the significance of other important taxonomic groups within these mats and evaluate the distribution of <i>P. crispa</i> in different regions of the Mediterranean Sea.
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