What is the optimal density of larval seeding in Acropora corals?

Density of coral larvae can influence settlement, post-settlement colony abundance and coral cover in larval restoration

“…To test the effect of larval density on settlement rates and colony persistence, single replicates of 30 larval densities ranging between 10-5,000 were selected for investigation. Previous testing of densities with 500-1,000 larvae per tile indicated settlement rates can range between 10-20%, consistent with 10-15% settlement observed in other in situ settlement studies 21 . We therefore chose ~5,000 larvae as the upper density limit in this study, assuming this would result in a maximum of ~1,000 initial settlers on the highest density tile (~3.3 settlers/cm 2 ), which would quickly be subject to intraspecific competition 22 .…”

“…However, space can be a limited resource for both initial settlement and subsequent growth of the sessile juvenile coral colonies 22 and density-dependent relationships between settlement and recruitment have been observed in early life-history stages of Acropora species, an important early-colonising reef-building group 12,17 . Previous studies have reported highest rates of post-settlement mortality occurring in the highest settler densities investigated after one month 12,21 and between five weeks to 30 weeks 17 . However, the effect of a wide range of initial larval densities on settlement through to longer-term post-settlement survival and recruitment has not been investigated.…”

“…While this occurs, determining the optimal densities of coral larvae to supply to degraded reefs to maximise successful recruitment will assist effective distribution of the reared larvae. Positive relationships have been observed between the abundance of coral larvae supplied and numbers of initial settlers in larval densities examined to date 12,16,21 . However, space can be a limited resource for both initial settlement and subsequent growth of the sessile juvenile coral colonies 22 and density-dependent relationships between settlement and recruitment have been observed in early life-history stages of Acropora species, an important early-colonising reef-building group 12,17 .…”

“…Mortality of coral spat within six months of settlement is naturally very high 18,23,24 , therefore improving survival through this period is a key focus of coral reef restoration research 11,25 . Post-settlement mortality can result from predation, competition, overgrowth, limited energy and sedimentation 10,17,18,21,26,27 , with complex ecological trade-offs operating in response to multiple biophysical interactions throughout early coral ontogeny 15 . Structural interventions, such as cages to shelter newly settled recruits 11,25,28 , can increase post-settlement survival to varying degrees by reducing predation and sedimentation, although initial benefits may be lost over time as macroalgal growth within structural shelters can reduce the growth and survival of coral recruits 15,29,30 .…”

“…However, where available space in these preferred positions is limited, ecological trade-offs between aggregated and solitary settlement may influence larval substratum selection. When coral larvae settle in aggregations, there are risks of increased competition for food resources (including for photosymbiont microalgae), disease and overgrowth by neighbours 21,37 . However, there is also potential for close neighbours to fuse in early life-history stages, which can lead to improved survival and rapid growth compared to non-fused colonies 38,39 .…”