The coral settlement-sphere: microenvironments that shape reef restoration in the Anthropocene

Abstract

Accelerating losses of reef-building corals are driving steep declines in coral recruitment, imperiling the biodiversity and long-term resilience of reef ecosystems worldwide. Here, we review the mechanisms underlying coral recruitment success through a critical – yet largely overlooked – microenvironmental lens relevant to the scale of microscopic coral larvae. We introduce the concept of the “settlement-sphere”, which encompasses the physical, biochemical, and microbial microhabitat properties that govern successful coral larval recruitment. We then examine how anthropogenic stressors erode this settlement-sphere by diminishing essential chemical cues, reducing microhabitat complexity, and altering key physico-chemical conditions (e.g., hypoxia), ultimately leading to recruitment failure. Finally, we argue that advancing a collective understanding of the settlement-sphere is essential for developing actionable strategies to restore degraded coral reef systems. Such strategies include augmenting reef microhabitats with chemical cues and advanced materials that promote coral recruitment, thereby supporting genetic diversity, enhancing resilience, and enabling the restocking of coral populations.