Composite Substrates Reveal Inorganic Material Cues for Coral Larval Settlement

The widespread loss of stony reef-building coral populations has been compounded by the low settlement and survival of coral juveniles. To rebuild coral communities, restoration practitioners have developed workflows to settle vulnerable coral larvae in the laboratory and outplant settled juveniles back to natural and artificial reefs. These workflows often make use of natural biochemical settlement cues, which are presented to swimming larvae to induce settlement. This paper establishes the potential for inorganic cues to complement these known biochemical effects. Settlement substrates were fabricated from calcium carbonate, a material present naturally on reefs, and modified with additives including sands, glasses, and alkaline earth carbonates. Experiments with larvae of two Caribbean coral species revealed additive-specific settlement preferences that were independent of bulk surface properties such as mean roughness and wettability. Instead, analyses of the substrates suggest that settling coral larvae can detect localized topographical features more than an order of magnitude smaller than their body width and can sense and positively respond to soluble inorganic minerals such as silica (SiO₂) and strontianite (SrCO₃). These findings open a new area of research in coral reef restoration, in which composite substrates can be designed with a combination of natural organic and inorganic additives to increase larval settlement and perhaps also improve post-settlement growth, mineralization, and defense.