From Epibiosis to Parasitism? Host Associations and Overgrowth Dynamics of Alcyonium coralloides on Mediterranean Gorgonians

Alcyonium coralloides has traditionally been classified as an opportunistic epibiont of exposed gorgonian skeletons. However, the ecological nature of its interaction remains poorly understood. Using Marine Strategy Framework Directive (MSFD) monitoring data (2015–2025) across 310 ROV transects (30–100 m depth) along the Ligurian coast (NW Mediterranean Sea) and long-term photographic series of infested colonies from the Tavolara–Punta Coda Cavallo MPA (Sardinia, NW Mediterranean Sea), we investigated its distribution and host associations in the NW Mediterranean. A. coralloides occurred in ~50% of the surveyed gorgonian forests, significantly increasing with host density. Infestation rates were markedly higher on Eunicella cavolini (10.1 ± 4.2%) than on Paramuricea clavata, E. verrucosa, and Leptogorgia sarmentosa (1.5–1.9%). While host necrosis facilitates initial settlement, time-series data revealed rapid overgrowth rates (30–55 cm yr−1) along host branches. Direct contact zones between the coenenchimes of the epibiont and that of the host exhibited portions of denuded host skeletons, hypertrophic tissue borders, and reduced host polyp expansion adjacent to the advancing parasitic margin. These results indicate that the ecological interaction extends beyond simple opportunistic epibiosis. We propose that A. coralloides represents an intermediate stage along a continuum between epibiosis and active spatial competition, shaping community dynamics within Mediterranean animal forests.