Composition and structure of tropical intertidal hard coral communities on natural and man-made habitats

Living scleractinian corals form a narrow but often conspicuous band of marine life along tropical intertidal shores worldwide but they have generally been considered as outliers of mainstream coral communities and are thus poorly characterized. This study examined coral communities at three intertidal habitats, i.e., reef flats, sloping seawalls, and vertical seawalls, in Singapore, looking at species diversity, abundances, growth forms, and colony sizes. A total of 35 coral species were recorded, of which the majority (51%) were typically of massive growth form. Reef flats had the highest number of species (30), followed by sloping seawalls (21). Species on vertical seawalls (11) were a subset of the two other habitats. Colonies were dominantly massive (72%) and sparsely distributed with average colony densities between 0.05 and 0.4 colonies/m 2 . Species Porites lobata-lutea complex was widespread and most abundant, comprising 21–30% of colonies in each habitat. Six other species common across all habitats were Dipsastraea speciosa , Favites abdita , Goniastrea retiformis , Platygyra verweyi , Platygyra pini , and Platygyra sinensis. Of these, only colony sizes of G. retiformis (mean ± SE, reef flats 30 ± 4 cm, sloping seawalls 9.5 ± 1.7 cm, vertical seawalls 11.7 ± 1.4 cm) and Porites lobata-lutea complex (41.8 ± 8.3 cm, 26.8 ± 6.3 cm, 12.3 ± 1.6 cm) showed significant differences ( p < 0.05) amongst habitats. Relative abundances on the reef flats correlated moderately with those on the sloping seawalls (Pearson’s ρ = 0.6) and vertical seawalls (Pearson’s ρ = 0.7), respectively. Multivariate analyses showed that habitat origin (natural vs man-made) and surface rugosity (heterogeneous vs. homogeneous) were factors that significantly ( p < 0.05) differentiated intertidal coral communities. Nevertheless, sloping seawall communities bore higher resemblance to those on natural reef flats than to man-made vertical seawalls in species and growth form richness and also coral densities. These findings highlight interesting opportunities for incorporating coral-friendly designs into existing man-made sloping coastal structures to encourage the growth of coral communities in the intertidal zone.