Optimization of coral micro-fragment cultivation in recirculating aquaculture systems: Comparative effects of heterotrophic feeding and trace element supplementation on <i>Acropora hyacinthus</i> and <i>Acropora muricata</i>

Optimizing nutritional strategies for coral propagation in land-based Recirculating Aquaculture Systems (RAS) is critical for reef restoration. However, the interaction between colony morphology and particulate heterotrophy remains poorly understood, leading to inconsistent survival and growth outcomes in nurseries. This study evaluated the influence of particulate organic feeding (Reef-Roids) and trace element supplementation on two morphologically distinct species Acropora hyacinthus (tabular) and Acropora muricata (branching) across three size classes (1, 2, and 3 cm) over 88 days. The results identified a “morphological filter” effect: while heterotrophy is often beneficial, it induced a significant nutritional paradox in the tabular morphology. A. hyacinthus exhibited extreme sensitivity to organic loading, with the “Food” treatment resulting in a tenfold increase in mortality risk (Hazard Ratio = 10.22, p &lt; 0.001) compared to the control. In contrast, the branching architecture of A. muricata facilitated superior resilience to organic entrapment, maintaining stable survival and leveraging heterotrophy for enhanced growth. Across both species, the 1 cm size class consistently achieved the highest specific growth rate (SGR), peaking at 0.89 ± 0.37%.day -1 for A. hyacinthus under oligotrophic conditions. Trace element supplementation was found to significantly mitigate mortality in tabular corals under high-stress organic loading but could not fully compensate for growth inhibition caused by bio-fouling. These findings emphasize that coral mariculture success is morphology-dependent, requiring stratified management: oligotrophic tracks for tabular species and nutrient-enhanced regimes for branching forms to maximize RAS production efficiency.