Corals have built reefs on the benthos for millennia, becoming an essential element in marine ecosystems. Climate change and human impact, however, are favoring the invasion of non-calcifying benthic algae and reducing coral coverage. Corals rely on energy derived from photosynthesis and heterotrophic feeding, which depends on their surface area, to defend their outer perimeter. But the relation between geometric properties of corals and the outcome of competitive coral-algal interactions is not well known. To address this, 50 coral colonies interacting with algae were sampled in the Caribbean island of Curaçao. 3D and 2D digital models of corals were reconstructed to measure their surface area, perimeter, and polyp sizes. A box counting algorithm was applied to calculate their fractal dimension. The perimeter and surface dimensions were statistically non-fractal, but differences in the mean surface fractal dimension captured relevant features in the structure of corals. The mean fractal dimension and surface area were negatively correlated with the percentage of losing perimeter and positively correlated with the percentage of winning perimeter. The combination of coral perimeter, mean surface fractal dimension, and coral species explained 19% of the variability of losing regions, while the surface area, perimeter, and perimeter-to-surface area ratio explained 27% of the variability of winning regions. Corals with surface fractal dimensions smaller than two and small perimeters displayed the highest percentage of losing perimeter, while corals with large surface areas and low perimeter-to-surface ratios displayed the largest percentage of winning perimeter. This study confirms the importance of fractal surface dimension, surface area, and perimeter of corals in coral-algal interactions. In combination with non-geometrical measurements such as microbial composition, this approach could facilitate environmental conservation and restoration efforts on coral reefs.
In tropical ecosystems, autotroph organisms are continuously competing for space, with some plant species benefiting from disturbances such as fire, grazing, or bioturbation that clear habitat (Pulsford et al. 2016). These disturbances can open up layers of vegetation, thereby promoting colonization of opportunistic species that would have been competitively inferior without disturbance (Castorani et al. 2018). Opportunistic fast-growing species also include often invasive species that are therefore also likely to increase in dominance after disturbance (Altman and Whitlatch 2007). In seagrass meadows in the southern Caribbean, we observed that the marine invasive plant Halophila stipulacea uses bioturbation mounds, created by burrowing infauna such as sea cucumbers and shrimp (see Suchanek 1983), to colonize new habitats (Figure 1a, b). On Bonaire and Curaçao, in habitats with ~100% native Thalassia testudinum cover, invasive H. stipulacea often at first only occurred on bioturbation mounds that smothered native T. testudinum seagrass, likely due to fragmentation and subsequent settlement (Smulders et al. 2017). These observations suggest that bioturbation mounds serve as starting points for further invasion (Fig. 1c).