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One of the main challenges in coral reef conservation and restoration is the identification of coral populations resilient under global warming. Seascape genomics is a powerful tool to uncover genetic markers potentially involved in heat tolerance among large populations without prior information on phenotypes. Here, we aimed to confirm the role of several candidate heat associated loci identified using this method in driving the phenotypic response of Acropora millepora from New Caledonia to thermal stress. We subjected 7 colonies to a long-term ex-situ heat stress assay (4°C above the Maximum Monthly Mean) and investigated their physiological response along with their Symbiodiniaceae communities and genotypes. Despite having similar thermal histories and associated symbionts, these conspecific individuals differed greatly in their tolerance to heat stress. More importantly, the clustering of individuals based on their alleles at the candidate loci was able to resolve most of the phenotypic variation in heat tolerance. Colonies harboring a higher proportion of beneficial alleles sustained on average lower mortality, higher Symbiodiniaceae/chlorophyll concentrations and photosynthetic efficiency under prolonged heat stress. Together these results support the relevance of seascape genomics to reveal candidate loci for heat adaptation in corals and develop predictive models of heat tolerance.

Outbreaks of the corallivorous starfish Acanthaster cf. solaris (COTS) are not fully understood, especially in the shallowest areas behind the reef crests where limited COTS impacts are often reported. Here, we investigated the outbreak occurrence and compared post-outbreak habitat impacts in reef flats vs. upper fore reefs at 13 sites in New Caledonia. In this Pacific territory so far considered marginally affected by COTS, we documented wide occurrences of aggregations, combining massive population increases (maximum 605 COTS per swim, abundances >100 COTS recorded in 46% of the sites) with limited spatiotemporal boundaries. Abundance levels met the highest values in the literature, but were restricted to relatively small, isolated mid-shelf reefs. Typical residence time was short (< two years), with at least one site where COTS seemingly disappeared within seven months. The spread of COTS population and subsequent impacts appeared self-contained, and never affected the reef flats (0.5–1.5 m). The transition to severely affected fore reefs was abrupt, with live coral cover divided by nine and morphological diversity halved within a very few meters (3–5 m). We suggest that these differential impacts likely result from harsh environmental conditions making shallow, coral-rich areas less suitable for COTS, thereby pushing predation pressure further downwards.