Coral Guard substrates accelerate growth and fragment fusion for coral restoration

Abstract Coral reefs are declining globally, creating an urgent need for scalable technologies that improve the efficiency and effectiveness of active reef restoration. Coral reef restoration is increasingly constrained by algal overgrowth, which suppresses coral growth, survival, and restoration efficiency. Building on the recently developed Coral Guard platform, we evaluate its restoration performance under long-term in situ coral nursery conditions and introduce Fusion Guard Tiles, a geometry-optimized Coral Guard design that accelerates microfragment fusion. We evaluated Coral Guard Plugs using the branching coral Stylophora pistillata under complementary ex situ conditions and Fusion Guard Tiles using the massive reef-building coral Porites evermanni in in situ coral nurseries. In P. evermanni , Fusion Guard Tiles increased lateral tissue growth 2.6-fold and three-dimensional tissue surface area growth by >2.7-fold after 6 months compared with conventional substrates. After 12 months, colony height and volume were approximately 4.0-fold and 2.2-fold greater, respectively, while complete fragment fusion occurred only on Fusion Guard Tiles. In S. pistillata , Coral Guard Plugs increased lateral tissue growth by ∼1.7-fold. Microcomputed tomography revealed 10–13% higher skeletal density in both species. In P. evermanni , Fusion Guard Tiles also increased symbiont density by 70% and tissue protein content by ∼2.5-fold relative to controls. Together, these findings demonstrate that Coral Guard substrates suppress algal competition while accelerating coral growth, skeletal development, and microfragment fusion, providing a scalable, low-maintenance technology to enhance coral nursery productivity and reef restoration.