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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.

The authors are particularly grateful to Richard Farman, director ofthe Aquarium des Lagons in Noumea, for hosting our experiments. Wealso wish to express our warmest thanks to the aquarium staff for theirlogistical support, in particular Sylvain Govan and Jeff Dubosc.; International audience; Outbreaks of the coral-eating crown-of-thorns starfish Acanthaster spp. (COTS) have become to be amongst the most severe threats to coral reefs worldwide. Although most research has focused on COTS early development, it remains unclear how COTS populations will keep pace with changing ocean conditions. Since reproduction is a key process contributing to outbreaks, we investigated the reproductive success of adult COTS acclimated for 3-4 months to different treatment combinations of ambient conditions, ocean warming (+2 • C) and acidification (− 0.35 pH). Our results suggest that the optimal breeding season in New Caledonia is concentrated around the end of the calendar year, when water temperature reaches >26 • C. We found negative effects of temperature on egg metrics, fertilisation success, and GSI, conflicting with previously documented effects of temperature on echinoderm reproductive outputs. Fertilisation success dropped drastically (more than threefold) with elevated temperature during the late breeding season. In contrast, we detected no effects of near-future acidification conditions on fertilisation success nor GSI. This is the first time that COTS reproduction is compared among individuals acclimated to different conditions of warming and acidification. Our results highlight the importance of accounting for adult exposure to better understand how COTS reproduction may be impacted in the face of global change.

Giant clams have been over-exploited throughout their wide geographical range in the Indo-Pacific. Marine reserves could be one tool to potentially safeguard broodstock, but their effect on giant clam abundance and the differential benefits of lagoon reserves compared to barrier reef reserves is still unclear. We assessed abundances of four species of giant clams across 20 barrier reef and 30 lagoon reef sites across 600 km of coastline in New Caledonia using stratified underwater visual censuses. Tridacna maxima was the most common species. T. derasa was significantly more abundant in marine reserves than at sites open to fishing, but there was no effect of marine reserves on total abundance pooled across the four species. T. squamosa was significantly more abundant on barrier reefs, while Hippopus hippopus was found solely on lagoon reefs. National data from fisher logbooks shows a marked reduction in commercial catches of giant clam following implementation of daily catch limits of 5 giant clams per vessel per trip. Despite small daily bag limits of 2 giant clams per recreational fisher per trip, the recreational catch appears to be significant and much greater than the commercial catch. The utility of marine reserves on giant clams appears to be species specific, and might be diminished for the group as a whole by the concomitant protection of their natural predators. Both lagoon and barrier reef sites should be included in reserve networks. Our data suggest that daily boat/trip limits are an effective output control, at least for commercial fishers, but might not safeguard giant clams if recreational fishers are numerous. Further fishery restrictions on the less common species might be needed to ensure their persistence in the Indo-Pacific.