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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.