Beyond Seagrass Density: Holothuroid Occurrence across Fine-scale Habitat Mosaics in Reef Flats of Tomini Bay, Indonesia

Abstract

Seagrass-associated reef flats are spatially heterogeneous, yet holothuroid occurrence is often interpreted primarily in relation to seagrass density. We examined fine-scale seagrass–holothuroid co-occurrence in Tomini Bay, Indonesia, using three stations, three 1×1 m quadrats per station, and 25 microplots per quadrat. The reconstructed sampling frame comprised 225 microplots and 450 plot-time records. Holothuroids were recorded in 37 co-presence microplots, while 33 microplots contained seagrass only and no holothuroid-only microplots were recorded. Among seagrass-present microplots, seagrass shoot-density distributions provided no evidence of a difference between seagrass-only and co-presence classes (Mann–Whitney U = 664, p = 0.533; rank-biserial effect size = 0.088). Pooled holothuroid observations were numerically greatest at ST1, whereas seagrass shoot density, seagrass richness, and holothuroid richness were numerically greatest at ST3. Field-observed substrate-class frequencies showed descriptive variation among stations. Exploratory Spearman correlations and PCA across nine quadrats summarized combined biological, environmental, and substrate-class gradients; PC1 and PC2 accounted for 35.13% and 20.50% of variation, respectively (55.63% cumulative). These patterns are consistent with a fine-scale habitat-mosaic interpretation rather than a shoot-density-only interpretation, but they do not isolate causal habitat effects. The study provides baseline ecological information for habitat assessment in Tomini Bay.