Comprehensive Assessment of Chemical and Microbial Inducers for Coral Larval Settlement Across Diverse Coral Species

HuiMing Sun^1,2Qingsong Yang^1,3*Junde Dong^1,4Jie Li¹Chang Chen^1,3Xiaoyu Tang¹Ying Zhang¹Juan Ling^1*

• ¹Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences (CAS), Guangzhou, China
• ²University of Chinese Academy of Sciences, Beijing, Beijing, China
• ³Xisha Marine Environmental National Observation and Research Station, Sansha, China
• ⁴Sanya National Marine Ecosystem Research Station, Hainan Tropical Marine Biology Research Station, South China Sea Institute of Oceanology, Sanya, China

Coral restoration initiatives increasingly prioritize larval settlement and post-settlement survival. However, the species-specific effectiveness of settlement inducers remains underexplored. This study systematically evaluated three settlement inducers—Crustose coralline algae (CCA), Chemical (CaCl2), and Microbial (Metabacillus sp. cB07)—across seven coral species, including brooders and broadcast spawners. Coral larvae were exposed to gradient concentrations of inducers to gain an optimal inducer concentration and treatment duration and evaluate their effects on larvae survivorship, metamorphosis, and settlement. Subsequently, for both planula larvae and recruits treated by optimized settlement-inducing procedures, metabolic rates and morphology, post-settlement survival, and growth indicators were quantified. Results revealed that the optimal concentration and treatment duration of CaCl2 (10–60 mmol/L) and strain cB07 (3 × 106–3 × 107 cfu/mL) varied significantly across different coral species. Both CaCl2 and cB07 demonstrated broad-spectrum induction activity comparable to CCA, among the seven tested coral species. Compared with the control (up to 13.3% within 7 days), CCA showed the highest settlement rates (43.3%–93.3%) within 1–2 days, while CaCl2 exhibited moderate effectiveness (23.3%–60.3%) within 0.5–4 days, and cB07 displayed comparable efficacy (26.7%–60.0%) within 2–4 days. The tested inducers exhibited distinct biological effects: CaCl2 accelerated metamorphosis but reduced survival rates in sensitive species, whereas cB07 delayed metamorphosis and demonstrated higher toxicity, which was associated with significantly suppressed respiratory rates. In the post-settlement phase, recruits induced by CCA and CaCl2 showed higher survival and calcification rates compared to those induced by cB07. These findings highlight the importance of customizing inducer protocols based on the unique needs and life histories of different coral species. By integrating physiological and ecological perspectives, this study provides actionable guidelines for optimizing coral restoration strategies amidst escalating anthropogenic and climatic threats.