The early acquisition of symbiotic algae benefits larval survival and juvenile growth in the coral Acropora tenuis.

Larval dispersal and postsettlement survival of corals play significant roles in the maintenance of coral populations. Most corals acquire their symbiotic algae (Symbiodiniaceae) from the environment in each generation (horizontal transmission). For horizontal transmitters, the quick establishment of symbiosis is important for their survival, since the photosynthetic activity of symbiotic algae provides energy. However, recent studies have indicated that oxidative stress resulting from photosynthesis might also harm coral larvae. Therefore, it remains unclear whether symbionts contribute energy sources along with intrinsic lipids from eggs and assist in settlement/metamorphosis in early life stages. In the present study, we show that symbiotic algae contribute supplemental energy and are also associated with settlement. Furthermore, although juveniles acquired symbiotic algae after settlement, the acquisition of symbiotic algae in the larval stages caused higher growth (number of polyps and size) and low mortality in the juvenile stage. Our data suggest that symbiotic larvae potentially have longer dispersal periods due to their lower lipid consumption rates, which make them better able to retain buoyancy and motility, increasing the ability of symbiotic larvae to settle in favored locations compared with aposymbiotic larvae. Moreover, postsettlement juveniles may continue to benefit from symbiotic relationships formed during the larval stage. Overall, these findings reveal that the effects of symbiotic algae on Acropora tenuis coral larvae are beneficial, particularly under normal seawater temperature conditions.