Threat of Fast-Growing Corallimopharian to Health of Corals Reef at Bawe and Changuu Islands, Zanzibar

Abstract

Coral reefs globally face degradation from anthropogenic stressors, often shifting toward dominance by non-reef building organisms like Corallimorpharia. This study investigated 0distribution and competitive dynamics on the reefs of Bawe and Changuu Islands, Zanzibar, which experience differing human pressures. Benthic surveys using line intercept transects were conducted across reef zones in December 2023 and January 2024. Results showed hard coral dominance at both sites (Bawe: 62.1%; Changuu: 58.3%), with Porites as the predominant genus. Corallimorpharia cover was statistically similar between sites (Bawe: 12.4%; Changuu: 12.1%; p=0.45), with four species recorded: Discosoma rhodostoma, D. nummiforme, D. unguja, and Ricordea yuma. Spatial adjacency modeling revealed Corallimorpharia as disproportionately aggressive competitors, engaging in 34-37% of competitive encounters despite moderate cover. Direct contact with Corallimorpharia caused visible coral tissue mortality and bleaching. Correlation analysis identified Physogyra and Plerogyra as particularly vulnerable genera. Changuu exhibited significantly higher macroalgae cover (p = 0.0109), adding competitive pressure absent at Bawe. Although Corallimorpharia do not currently dominate these reefs, their persistent presence and competitive impacts on scleractinian corals suggest increasing pressure on reef-building communities. Management of land-based pollution and long-term monitoring are essential to prevent potential coral–Corallimorpharia phase shifts.