Proximity and current alignment drive fertilisation success in a broadcast-spawning coral (Acropora cf. hyacinthus)

As populations decline under climate change and other stressors, survivors become increasingly isolated. Allee effects in such fragmented populations may arise during reproduction, particularly for sessile organisms such as corals, reducing fertilisation potential and increasing the probability of recruitment failure. However, the critical distances between corals needed to maintain viable populations remains uncertain. We experimentally investigated fertilisation patterns in the broadcast spawning coral Acropora cf. hyacinthus using a manipulated patch, spacing colonies at increasing distances downstream from a central spawning aggregation. Field samples showed a steep decline in fertilisation with increasing intercolonial distance and genetic paternity analysis indicated most parents were located within 3 m of each other. Colony position relative to the current direction also influenced fertilisation success, with 81% of sequenced progeny sired by upstream colonies. Simulations projecting natural population distributions onto a virtual grid, incorporating experimental results, indicated that populations remained well-mixed at typical adult densities. Yet at reduced densities approaching 0.01 colonies m^-2, reproductive isolation and patchy breeding units emerged. These findings demonstrate that colony isolation substantially impairs reproductive connectivity, with additional influence from the spatial arrangement of colonies. Conservation strategies should prioritise maintaining colony density and spatial connectivity to support viable coral populations on degraded reefs.