Echoes of life: unveiling the cryptic legacy of Megabalanus azoricus dead shells on epibiont assemblages

Barnacles provide a unique substrate for a diverse array of species. This paper focuses on the epibionts inhabiting the giant barnacle Megabalanus azoricus, exploring their diversity, interactions with host, and the ecological implications in bioengineering. This species is a valuable and vital socioeconomic resource, endemic of the Macaronesian region, and OSPAR species, hence with some degree of international concern. Empty barnacles were collected in different locations on São Miguel Island, Azores, at two depths (surface and 10 m): one in the south (Vila Franca do Campo) and two in the north (Ponta do Cintrão and Fenais da Ajuda). Epibionts were identified to the lowest possible taxonomic category and barnacle height, base, and opening diameters were recorded. One hundred eighteen different taxa were observed, of which 30 were algae, mainly red, and the most predominant were polychaetes, arthropods, and sipunculids, with a significant prevalence of polychaetes and arthropods at the north. Arthropods and polychaetes were more abundant in the shallower samples, while sipunculids were only found in the deeper ones. Only two barnacles did not present juveniles of M. azoricus on their walls, with this species representing approximately 21% of all epibionts. The empty shells of M. azoricus are of key importance for a high diversity and quantity of epibionts, especially when compared with other basibionts, and also vital for the establishment and recruitment of the younger barnacles. However, this may be impaired by its increasing socioeconomic importance as a marine delicacy for locals and tourists, threatening its sustainability and ecological role within the coastal ecosystems.