Reef fish assemblages impacted by sponges overgrowing corals

Abstract

Numerous stressors are driving long-term declines in coral cover on coral reefs, significantly impacting reef fish biodiversity and community structure. The benthic space made available by declining coral cover is often occupied by increases in other organisms, including macroalgae, encrusting ascidians, and sponges. Using a combined observational and experimental approach, we examined whether encrusting sponges impact fish communities as they replace corals on degraded reefs. First, fish assemblages were compared across 36 quadrats (3 × 3 m) with sponge cover ranging from 0% to 60%, primarily comprised of the species Lamellodysidea cf. chlorea and Lamellodysidea sp. Second, we quantified fish responses to the experimental removal of sponges from 12 additional quadrats with high sponge cover (>20%). High sponge cover (>20%) was associated with a significant reduction in fish abundance (39% fewer individuals) and species richness (26% fewer species) compared to areas with lower sponge coverage. While these differences showed some correlation with reduced substratum complexity, they were independent of coral cover and depth. Distinct fish communities were associated with high and low sponge coverage. High-sponge areas were dominated by larger species, such as the surgeonfish Ctenochaetus tominiensis, damselfishes Neoglyphidodon nigroris and Amblyglyphidodon leucogaster, and snapper Lutjanus biguttatus. In contrast, areas with less than 20% sponge cover primarily hosted smaller cryptobenthic and coral-associated fish species. Interestingly, experimental sponge removal did not affect overall fish abundance or species richness after 100 days, as both removal and control sites showed similar temporal increases. However, significant differences in fish community composition emerged within just 7 days post-removal. High-sponge areas retained distinct assemblages, containing 42 exclusive species (38% of total), including characteristic residents like Ctenochaetus binotatus, N. nigroris, and L. biguttatus. Conversely, sponge removal zones attracted 18 unique species (21%), predominantly generalist and rubble-associated fishes such as Pomacentrus aurifrons and Apogon neotes, revealing rapid but transient shifts in community structure following habitat modification. These results indicate that encrusting sponges exert a substantial influence on local fish assemblages. The differences between the observational and experimental results likely reflect both long-term and short-term effects of encrusting sponges on coral reef fish communities.