Cylindrical, pylon-like structures with helix recesses enhance coral larval recruitment

The decline of coral reefs requires scalable restoration strategies that enhance natural recovery processes such as coral larval recruitment. Previous work has shown that 3D-printed clay domes with helix recesses significantly increase settlement and survival of coral recruits compared to control designs without recesses. To examine the broader applicability of these findings for engineering applications, we tested whether the helix microhabitat design could be transferred to space-efficient pylon-like, cylindrical geometries and evaluated its performance across different materials and fabrication methods (cast concrete vs. 3D-printed clay). In a field experiment in Kāneʻohe Bay, Hawaiʻi, coral recruitment was monitored over six months on four cylindrical rod types (varying in diameter and material) and two conical superdome modules, featuring recesses based on the previously optimized helix profile. Cylindrical rods had similar recruitment densities to those on superdome designs, indicating a successful transfer of the design. Most recruits settled inside the recesses across all structures, while outer surfaces consistently supported fewer recruits. Settlement was initially higher on concrete modules, but this difference disappeared over time. The large clay rods, which had wider recesses, maintained the highest recruit densities after six months, suggesting enhanced post-settlement survival. Compared to natural reefs, planar recruit densities on the structures were ∼300 times higher. These results indicate that helix design is functionally robust across geometries and materials, with recess size influencing post-settlement outcomes. Cylindrical structures with integrated helix recesses represent a practical, low-cost method that can be implemented in coastal engineering to enhance coral settlement.