Coral-killing and competitive sponges in Nusa Dua, Bali, Indonesia

Coral-killing sponge (CKS) is one of the threats faced by coral reefs. Studies have shown increases in the population of CKS and other competitive sponges across the Indo-Pacific region in recent years. CKS has been observed to negatively impact coral populations, leading to a decrease in coral cover and productivity. Additionally, competitive sponges limit substrate availability for corals. In Bali, data on the distribution and abundance of CKS and competitive sponges is lacking. Studies on the identification and distribution patterns of these sponges are important as an initial step in preventing outbreaks and managing coral reefs sustainably. This study aims to identify the types, distribution, and abundance of CKS and competitive sponges at Samuh Beach, Nusa Dua, Bali. Data collection was carried out using the Underwater Photo Transect (UPT) method on six transects along 50 m at two different depths, i.e. shallow (5 and 7 m); and deep (10 and 12 m). The observed sponges in contact with corals were photographed and sponge samples were kept in DNA shield for molecular analysis using CO1 or 28S gene, and in 70 % ethanol for spicule observation. The UPT data were processed using the Coral Point Count with Excel extension (CPCE) software and followed by calculating the number of coral colonies interacting with CKS and competitive sponges. This study identified two types of CKS—Mycale sp. and Neopetrosia sp.— as well as one competitive encrusting sponge, Lamellodysidea herbacea. Interactions were predominantly observed with corals from the genera Porites, Pocillopora, and Favites across all Sub-sites. Generally, we found higher number of individuals per m² of Mycale sp. at shallow depths, while L. herbacea at deeper ones. Research on the biological aspects of these sponges i.e. phototrophic symbiont and allelochemicals is crucial, and recommendation for sponge monitoring, would be important for further management.