Growth and skeletal structure of the parasitic zoantharian Savalia savaglia (Bertoloni, 1819)

The gold coral Savalia savaglia parasitizes gorgonians in coralligenous habitats, causing tissue necrosis in its hosts and overgrowing their skeletons with its coenenchyme. While previous studies have mainly focused on its distribution, biology, ecology, and conservation, limited information is available about its skeletal architecture and growth patterns. Photographic observations of the same specimens over time revealed that S. savaglia grows rapidly during early colonization, slowing once the host is completely overgrown. In the Tavolara Channel (NE Tyrrhenian Sea), large, old dead specimens—radiocarbon-dated to approximately 800 years—resulted from the coalescence of several colonies, highlighting their evolution over a long timescale. Colonies merge when trunks or apical branches come into contact, forming polygonal meshes. Scanning Electron Microscopy (SEM) analysis revealed a laminar skeletal structure covered with small blunt spines, resembling those observed in antipatharians. These spines cement adjacent layers, increasing the skeleton’s resistance to currents and affecting its susceptibility to mechanical damage. These findings support early descriptions by H. Lacaze-Duthiers in the 18th century. Chemical analysis revealed the presence of 7% chitin in the skeleton, along with a high protein content.