Patch size drives settlement success and spatial distribution of coral larvae under space limitation

Space availability is a key factor linked to the settlement success of marine invertebrates. Settlement space on coral reefs is predicted to become increasingly fragmented and occupied by competitors under future disturbance regimes, yet how this impacts coral settlement remains largely unknown. We test the effects of space limitation on larval settlement in three common Indo-Pacific corals (Acropora valida, Acropora digitifera and Anacropora spinosa) by manipulating substrate area while maintaining a constant larval supply. Settlement success was highly variable among coral species, with reduced space leading to an up to four-fold increase in settlement of A. valida larvae, a two-fold decrease in settlement of An. spinosa larvae and no significant effect for A. digitifera. All species displayed similar spatial settlement patterns, whereby larvae settled gregariously irrespective of how much space was available. At the same time, settlers were found to increasingly occur in aggregates (in direct contact with each other) as space decreased. We propose that increased settler aggregations, coupled with settlement intensification for some species, facilitates the formation of chimeras as space becomes limiting. In colonial organisms, the formation of aggregates and particularly chimeric individuals may offset the negative effects of increased competition for space by allowing settlers to rapidly exceed size-escape thresholds, thereby increasing the likelihood of survival.