Sun coral invasion of shallow rocky reefs: effects on mobile invertebrate assemblages in Southeastern Brazil

Abstract

Invasive engineering species impact local biodiversity and ecosystem services as they often change habitat complexity while displacing native species, ultimately altering fundamental processes such as secondary production and the energy flow through trophic levels. The sun corals Tubastraea coccinea and T. tagusensis have successfully invaded reef habitats along the Brazilian coast, drastically reducing the diversity of benthic fouling invertebrates and macroalgae at places where colonies had taken large fractions of the available space. Yet, there is no consistent information on the effects of this invasion on assemblages of mobile invertebrates. We sampled shallow vertical reef areas at Buzios Island, SP, Brazil, and compared univariate and multivariate attributes of these assemblages at different levels of sun-coral cover (no cover, half and full cover), at two sites separated by a few km, and two areas within sites separated by several tens of meters. Consistent declines of overall abundance and biomass with increasing coral cover were found at one sampled site, while richness responded only to small-scale heterogeneity with no sun-coral effects. Changes in assemblage structure were area-specific, but similarity analyses most often grouped no coral and half coral cover, leaving aside full coral plots. Tanaids, ostracods and harpacticoid copepods were among the most important groups where sun corals were absent or covering only half of the reef habitat. However, these groups became almost absent in coral saturated habitats. Polychaetes also contributed substantially to dissimilarities, but effects on this group were less clear. Overall, results suggest a negative tipping point between partial to nearly full coral cover, especially at sites where physically complex macroalgae, capable to retain sediments and hence the invertebrates therein, are displaced by the establishment and growth of sun-coral colonies. As important prey for reef fishes, the collapse of small crustacean populations may alter whole-reef ecosystem functioning and negatively impact local fisheries.