Co-culture with grazers can improve survival and growth of multiple coral species

Abstract High post-settlement mortality in ex situ sexually propagated coral recruits remains a significant bottleneck to production of corals for reef restoration, scientific experiments and the ornamental trade. Co-culture with grazing herbivores represents a potentially cost-effective method to reduce recruit competition with algae, thus improving the survival and yield of corals. Here we examined the effectiveness of co-culture of coral recruits of Acropora millepora, A. tenuis, A. secale, Porites lobata and Platygyra daedalea with herbivorous gastropods, Calthalotia strigata and Turbo haynesi, and herbivorous juveniles of the Asteroid Acanthaster cf. solaris. Coral recruits (200 per species) were settled onto individual aragonite plugs, with 10 plugs per species introduced to twenty 50 L experimental tanks. Each tank was randomly assigned to one of five grazing treatments; (1) Thalotia, 30 C. strigata per tank, (2) Turbo, 30 T. haynesi per tank, (3) CoTS, 30 A. cf. solaris per tank, (4) Mix, 10 C. strigata, 10 T. haynesi and 20 A. cf. solaris per tank, and (5) Control, no grazers added. We found that over a 2-month period single species co-culture with C. strigata significantly improved the survival of A. millepora, A. tenuis, A. secale and P. lobata, whilst P. daedalea benefitted most from co-culture with T. haynesi. Over 6-months, C. strigata produced the highest survival in A. millepora (51.6% ± 5.24%), A. tenuis (46.6% ± 3.16%) and A. secale (38.5% ± 3.48%), but T. haynesi co-culture resulted in higher survival in P. lobata (67.3% ± 3.76%) and P. daedalea (100% ± 0%). These results were all significantly higher than the survival of corals in controls, which after 6 months averaged ~1.87% for the Acropora species, 43.0% ± 4.79% and 57.6% ± 8.60% for P. lobata and P. daedalea respectively. Increase in basal surface area of recruits was varied, with the highest relative increases in recruit basal disk area occurring in Thalotia treatments for A. millepora and A. secale, in Turbo for A. tenuis, and in the Control tanks for P. lobata and P. daedalea. This study provides strong evidence that co-culture with C. strigata can improve ex situ sexual coral propagation, with minimal additional intervention from aquarists. The large increase (23.6× higher) in survival of ecologically and economically important coral species such as A. millepora demonstrates the potential of co-culture to enhance ex situ coral propagation.