Directing coral larval settlement in coral aquaculture for reef restoration.

Optimising coral early life stages, particularly larval settlement, remains a challenge for coral aquaculture. Settlement typically relies on biologically conditioned substrates, such as crustose coralline algae (CCA) or microbial films, which are difficult to scale for mass-production. To overcome this bottleneck, we evaluated chemical and physical alternatives that enable spatial control of directing larval settlement. Larval responses from 14 coral species were tested against known and potential chemical inducers, including neurotransmitters and neuropeptides. Next, we investigated the efficacy of a set of known chemical inducers and microtopography to direct larval settlement. The neuropeptide Hym-248 was the most effective, inducing settlement in seven acroporid species. Embedding soluble inducers (CCA extract and Hym-248) in agar hydrogels within ceramic cubes successfully induced and directed settlement of Acropora kenti. Similarly, CCA-conditioned ceramic protrusions, with and without microtopographic pores, achieved > 99% settlement on or adjacent to protrusions. These findings demonstrate that immobilised chemical cues can precisely localise larval settlement on unconditioned substrates and that conditioning only small, discrete 3-D printed features is as effective as conditioning entire tiles. This approach substantially reduces aquarium space and resource requirements, offering a scalable, cost-effective pathway to enhance the efficiency of sexually propagated coral aquaculture in reef restoration.