Priority effects, environmental filtering and neutral coexistence explain large- to small-scale distribution of invasive sun corals in the SW Atlantic

Two sun coral species, Tubastraea tagusensis and T. coccinea, have successfully colonized reef habitats along the Southwest Atlantic. However, their invasive biology has been largely addressed without considering species-specific distribution patterns. Here, we assessed the distribution and abundance of Tubastraea spp. at vertical rocky reef sites within a number of islands along 120 km of coastline off the northern coast of Sao Paulo State, Brazil, to (1) investigate possible mechanisms underlying the invasion dynamics in the region, (2) test species-specific distributions according to a key environmental filter (depth), and (3) examine within-patch patterns to assess whether competition, niche-based or neutral processes are best candidates to modulate local species coexistence. Sun corals were found in the great majority of the studied locations, and the probability of finding them at any given reef site was estimated to be 0.54. There was substantial species segregation across locations, consistent with primary priority effects. Within locations, results suggest environmental filtering, with T. coccinea apparently advantaged in more hydrodynamic environments just below the surf zone. At sun coral patches with extensive co-occurrence of T. tagusensis and T. coccinea, the presence of each species can be, remarkably, modeled as an independent event, suggesting neutral coexistence. The spread of sun corals is an ongoing and increasingly invasive process that may be explained by the enemy-release hypothesis and the lack of negative interactions between Tubastraea species. The stochastic nature of small-scale distributions sets an additional challenge to predict (and thus control) sun coral invasion.