Beyond darkness: environmental drivers of a cave-restricted sponge’s distribution in Brazil

Context Freshwater sponges of the order Spongillida have colonised a variety of environments, including subterranean habitats, where records remain scarce. Aims This study examined the environmental factors influencing the distribution of the cave-restricted sponge Racekiela cavernicola, listed as Vulnerable in the most recent Official List of Threatened Brazilian Fauna, within the extensive semi-arid cave system in Brazil. We also sought to discuss aspects of the species’ life cycle. Methods Data on sponge abundance, water physicochemical properties and environmental physical characteristics were collected along the cave during two sampling periods, corresponding to the rainy and dry seasons. Additionally, historical records of species occurrence and laboratory-based experiments were analysed to elucidate its life cycle. Key results A spatio-temporal variation in species abundance was observed, primarily associated with fluctuations in water temperature. Furthermore, this study provided the first insights into its life cycle. Conclusion R. cavernicola retains gemmules and filtering structures throughout the year, with peak abundance recorded during the dry season, and its population dynamics are strongly influenced by water physicochemical parameters. Implications Continuous monitoring of R. cavernicola populations is essential for assessing potential environmental impacts and ensuring the ecological integrity and long-term conservation of the Brejões Cave system.