Competitive overgrowth of the zoantharian Savalia savaglia by the sponge Crambe crambe in the Mediterranean Sea

Abstract Savalia savaglia is one of the few zoantharians capable to grow with a parasitic habitus, infesting damaged gorgonians, such as Eunicella spp., Paramuricea sp., Leptogorgia spp., and antipatharians. It causes tissue necrosis in its hosts and progressively overgrows their skeleton with its own coenenchyme. While previous studies have mainly focused on its distribution, biology, ecology, and conservation, little information is available about its growth patterns and ecological interaction with other benthic organisms. A photographic monitoring of the same S. savaglia specimen over time (2018 and 2025) was carried out within the C zone of the Tavolara—Punta Coda Cavallo Marine Protected Area (NE Sardinia, Tyrrhenian Sea). The image analysis revealed a progressively overgrown of this colony by the encrusting orange sponge Crambe crambe observed. After seven years, the covered portion increased of about 185 cm2 y -1 . The case here described represents the first documented instance of S. savaglia losing a competitive interaction with another benthic organism, indicating that the latter is not always a winner. Probably, the sponge’s aptitude to spatial competition and environmental tolerance facilitates its expansion at the expense of S. savaglia, particularly when the latter’s colony integrity is already compromised or exhibits reduced resilience to disturbance.