On the substrate adhesion of the black coral Antipathella subpinnata (Ellis & Solander, 1786) and other habitat-forming anthozoans

Adhesions to the substrate are essential for anthozoans that develop large arborescent colonies, yet adult adhesion structures are difficult to study, and larval anchorages remain challenging to obtain. This study exploited the ability of the Mediterranean black coral Antipathella subpinnata to form new anchorages from adult fragments upon contact with a surface. The objectives were to assess the adhesive capacity of A. subpinnata, evaluate the role of biomineralogy, and determine whether other Mediterranean habitat-forming anthozoans exhibit comparable abilities. Long-term photographic monitoring was conducted on fragments of five species (one black coral and four gorgonians) under three experimental conditions, and the number, growth, and morphology of adhesions were analysed using histology, optical microscopy, and scanning electron microscopy. All species displayed morphological changes upon contact with the substrate, but only Paramuricea clavata and A. subpinnata developed well-formed adhesions. P. clavata showed specific responses, including a rigid membrane, coenenchyma flattening, lateral displacement of polyps, sclerite expulsion, and thread-like tentacles. Its adhesions exhibited limited development, expanding on average to 34% of their initial area, and the skeleton never reached the substratum. In contrast, A. subpinnata frequently produced permanent anchorages consisting of a basal chitinous lamina covered with spines. These adhesions underwent substantial morphological changes, expanding on average to 73% of their initial area. This study offers new insights into early adhesion mechanisms in arborescent anthozoans, suggesting greater skeletal plasticity in antipatharians than in gorgonians. It also documents, for the first time, the growth rates of newly formed anthozoan adhesions and a distinct mode of granular skeletal deposition in black corals. Biomineralogical preferences remain unresolved.