Occurrence, distribution, and linear progression of a coral-killing sponge Haliclona (Chalinula) nematifera on coral reefs in the Nusa Penida Marine Protected Area, Indonesia

Coral reefs worldwide face increasing threats from climate change, overfishing, and other human pressures. These stressors are often associated with pollution and habitat degradation, which may facilitate biological invasions, including coral-killing sponges that can overgrow and kill live coral tissue, altering reef structure and function. This study investigates the occurrence and distribution of Haliclona (Chalinula) nematifera in the Nusa Penida Marine Protected Area (MPA), Indonesia, a tourism destination experiencing growing pressures from urbanization and nutrient pollution. Field surveys, photographic analysis, and linear progression monitoring were conducted across multiple reef sites to assess sponge occurrence frequency, coral host range, and linear progression rates on coral hosts. The sponge was recorded at six of the 12 surveyed sites, occurring primarily in the northern and eastern regions, with limited presence at one western site and complete absence from all surveyed southern sites. These occurrences were predominantly observed at reefs situated near developed coastlines and areas influenced by land-based inputs such as wastewater discharge, sediment runoff, and coastal construction. Targeted surveys and long-term monitoring consistently showed higher occurrence frequencies in the northern and eastern regions. H. nematifera was observed on 45 coral species belonging to 23 coral genera, as well as on dead coral, rock, sponges, corallimorphs, zoanthids, and artificial substrates, with no consistent depth-related pattern detected. Linear progression rates varied significantly, with the sponge proliferating fastest on Echinopora (1.15 ± 0.55 mm day⁻¹), approximately 1.8 times faster than on Porites (0.63 ± 0.44 mm day⁻¹). The capacity of H. nematifera to spread across diverse hosts and habitats signals a broader conservation concern, underscoring the importance of incorporating invasive species management into broader conservation strategies.