Assessment of Non-Sessile Invertebrates Associated with Mats of the Red Alga Phyllophora crispa at Giglio Island, Mediterranean Sea

The Mediterranean Sea hosts highly diverse habitats such as Posidonia oceanica meadows, coralligenous communities, and gorgonian forests. Stressors including warming, eutrophication, pollution, and overfishing are driving shifts towards algae-dominated systems, often with reduced biodiversity. Among these, recent research surprisingly revealed that the mat-forming red alga Phyllophora crispa, which overgrows seagrass and gorgonian habitats, supports high sessile invertebrate diversity. However, little is known about its associated non-sessile fauna. This study thus investigated non-sessile invertebrates in P. crispa using a newly designed appropriate sampling technique at two study sites around Giglio Island (Italy), Fenaio, and Secca II (distance ca. 600 m from each other). Across all samples, 5464 organisms were identified, mostly to family level. We recorded 169 non-sessile taxa, including 96 families, 41 copepod morphotypes, 21 ostracod morphotypes, and 11 unclassified taxa. The dominant phyla were Arthropoda (67%), Mollusca (14%), Annelida (9%), and Nematoda (5%). The most abundant families were Calliopiidae (Amphipoda), Leptognathiidae (Malacostraca), and Mytilidae (Bivalvia). Of the 169 taxa, 128 occurred at both sites, while 20 were unique to Fenaio and 21 to Secca II, suggesting high connectivity likely linked to mobility. Organism abundances ranged from 1315 to 5759 individuals per m2 seafloor. Diversity indices were as follows: Shannon 1.5–3.4, Simpson 0.6–1.0, and Pielou 0.6–0.9. These values are similar or even exceed previously reported values for sessile invertebrates (Shannon 2.2–2.5). Notably, P. crispa supported diversity levels higher than those reported for seagrass meadows (Shannon 2.0–2.1) and even tropical coral reefs (2.0). Our study thus confirms P. crispa as a biodiversity hotspot and suggests that these algae mats should be considered in biodiversity conservation strategies.