Adult crabs are known to play critical roles in the survival of their adult coral hosts, but little is known of the mutualism between juvenile crabs (≤0.5 cm) and their juvenile hosts. Field and laboratory experiments both demonstrated that the presence of juvenile crabs of the genus Trapezia in young host Pocillopora corals (2 to 3 cm diameter) increased coral survival by 32% and reduced consumption by the corallivorous seastar Acanthaster planci. These experiments also showed that juvenile Trapezia were not effective at deterring predation by another common predatory seastar, Culcita novaeguineae. Finally, our work highlights that the defensive ability of symbiotic crabs may be genus-specific, as juvenile Tetralia spp. crabs, obligate symbionts of Acropora spp., displayed no protection against either A. planci or C. novaeguineae.