Host Mortality and Morphology Influence Crustose Coralline Algal-Associated Invertebrate Diversity in Moorea, French Polynesia

Foundation species, like crustose coralline algae (CCA), are important to structuring reefs and supporting high biodiversity. However, increasing anthropogenic disturbance may cause CCA mortality and impair their role as habitat. Our understanding of the factors driving CCA-associated epifaunal assemblages, including host partial mortality, is currently limited. This study examined the importance of CCA partial mortality, flow environment, and morphology (e.g., interstitial volume, branch density, and thallus rugosity) in structuring epifaunal assemblages within the branch-forming CCA species, Lithophyllum kotschyanum, on a back reef in Mo’orea, French Polynesia. A total of 1,879 invertebrates were removed from 52 L. kotschyanum thalli, the majority of which were crustaceans (accounting for 78.4% of the total). Regardless of the flow environment, thalli with high levels of partial mortality supported assemblages with similar abundances and significantly higher taxa richness than entirely live thalli. Assemblage abundance and richness increased with interstitial volume (available living space) and lower branch density, which were morphological traits associated with higher partial mortality of algal thalli. These results suggest that the physical structure provided by a branch-forming algal foundation species has the potential to sustain reef biodiversity even after host mortality, but its persistence will likely depend on erosion and disturbance rates over time.