Abstract: Mesophotic coral reefs, distributed at depths of approximately 30 to 150 meters, account for about 80% of coral reef habitats. They are less affected by natural disturbances such as thermal stress and cyclonic mixing, as well as human activities, resulting in a relatively stable habit for marine life. These reefs are considered potential "refuges" for shallow-water coral species and play a crucial role in maintaining the stable reef foundation and the ecological resilience of islands and reefs. This unique environment also represents an untapped reservoir of novel species, genes, natural products, and marine drugs. Limited by technical methods and equipment, China's exploration in this field is still in its early stages. With the continuous advancement of the national maritime strategy, oceanographic research is progressively extending into more remote and deeper waters. From 2022 to 2025, funded by National Science & Technology Fundamental Resources Investigation Program "Survey of Biodiversity and Special Habitats of Deep-water Coral Reefs in the Xisha Islands," the South China Sea Institute of Oceanology, Chinese Academy of Sciences, in collaboration with other leading institutions/universities, carried the first systematic investigation of mesophotic coral reefs in the Xisha Islands. The investigation included a total of four voyages, spanning over 60 days and involving 67 participants. The study aims to deepen the understanding of mesophotic biodiversity, explore the relationship between marine life and their habitats, and analyze biological adaptations and distribution patterns. The findings from this investigation will provide fundamental data and scientific support for ecological conservation, economic development, and national defense strategies in the South China Sea islands and reefs.

Key words: Xisha Islands, Mesophotic Zone, Coral Reefs, Comprehensive Scientific Survey