ABSTRACT: 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.
Elkhorn coral (Acropora palmata) populations provide important ecological functions on shallow Caribbean reefs, many of which were lost when a disease reduced their abundance by more than 95% beginning in the mid-1970s. Since then, a lack of significant recovery has prompted rehabilitation initiatives throughout the Caribbean. Here, we report the first successful outplanting and long-term survival of A. palmatasettlers reared from gametes collected in the field. A. palmata larvae were settled on clay substrates (substrate units) and either outplanted on the reef two weeks after settlement or kept in a land-based nursery. After 2.5 years, the survival rate of A. palmata settlers outplanted two weeks after settlement was 6.8 times higher (3.4%) than that of settlers kept in a land-based nursery (0.5%). Furthermore, 32% of the substrate units on the reef still harbored one or more well-developed recruit compared to 3% for substrate units kept in the nursery. In addition to increasing survival, outplanting A. palmata settlers shortly after settlement reduced the costs to produce at least one 2.5-year-old A. palmataindividual from $325 to $13 USD. Thus, this study not only highlights the first successful long-term rearing of this critically endangered coral species, but also shows that early outplanting of sexually reared coral settlers can be more cost-effective than the traditional approach of nursery rearing for restoration efforts aimed at rehabilitating coral populations.