Interactions between invaders: facilitation of boring bivalves leads to reduced adhesive strength of sun corals

Sun corals (Tubastraea spp.) have invaded the SW Atlantic coast in the 1980s, saturating vertical walls at heavily impacted areas. More recently, the boring bivalve Leiosolenus aristatus, another invasive species in the SW Atlantic, was found inhabiting sun corals. Here we show that Tubastraea tagusensis is the main coral host for L. aristatus at an extensively invaded island in Southeastern Brazil. Bivalve biomass adjusted to colony volume was similar between invasive T. tagusensis and native corals. However, when adjusted to colony basal area per reef space, bivalve density was exceptionally higher in sun corals, especially on vertical substrates or those with negative orientation. Therefore, sun corals constitute main doorways for borer bivalves at most invaded reefs. Moreover, and owing to the phaceloid colony morphology of T. tagusensis, boring bivalves reduce, on average, the contact area between sun corals and the substrate in 9.6% (SD = 9.9), ranging from 1 to 44%. Regardless of sampling site and reef inclination, the decrease of such surface contact area causes a drop of sun-coral adhesive strength (from 36 to 22 kgf), that can be described by a general exponential decay function, and explain the accumulation of coral debris at the bottom of invaded reefs. While possibly ceding space for native species in the reef community, dislodgment through bivalve infestation may eventually reduce intraspecific competition among sun-coral colonies and favor resettlement in alternative reef habitats, ultimately contributing to the ongoing invasive process.