Age-dependent effects of mutualists on predator resistance in coral transplants: implications for reef restoration

Coral transplantation is a key tool for reef restoration, yet high post-transplant mortality (particularly during early stages) continues to limit its effectiveness. Predation by corallivorous sea stars is a major cause of transplant loss, but the role of mutualistic crabs (Trapezia spp. and Tetralia spp.) in mediating age- and size-dependent resistance remains poorly understood. Here, we experimentally tested whether the protective function of symbiotic crabs depends on coral colony age, size, and symbiont community development, and whether delaying transplantation until functional mutualisms are established enhances survival. Nursery reared colonies of Pocillopora verrucosa and Acropora latistella 0-, 3-, 6-, and 12-month-old were transplanted onto reef bioherms in Tre Island (Nha Trang Bay, Vietnam) and exposed to natural predation by Culcita novaeguineae and Acanthaster planci. We quantified transplant mortality, predator attacks, symbiont crab presence, species composition and body size, and documented defensive behaviors in situ. Mortality was strongly age- and size-dependent in P. verrucosa: freshly collected fragments and 3-month-old colonies suffered high losses, whereas 6- and 12-month-old colonies exhibited low mortality. This shift coincided with the near-universal presence of well-developed male-female Trapezia pairs whose size increased significantly with host age and whose defensive efficiency scaled positively with crabs’ body size. We identified a critical colony volume threshold (~0.3–0.7 dm³), typically reached after 3–6 months of nursery growth, beyond which crab-mediated defense effectively prevented sea star predation. In contrast, A. latistella showed high mortality across all age classes due to predation pressure, reflecting comparatively weaker protective capacity of Tetralia crabs, although some size-dependent resistance emerged under lower predation intensity. Our results demonstrate that coral transplant survival depends not only on colony size but also on the establishment of functional mutualistic interactions. We recommend delaying transplantation until colonies exceed the identified age and size threshold and host mature functional symbiont crab pairs. Integrating symbiont establishment into nursery protocols can significantly reduce predation driven mortality and substantially improve the efficiency and ecological significance of coral reef restoration efforts.