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For corals that establish symbioses with dinoflagellate Symbiodinium spp. at the lar- val stage or later through horizontal transmission, the ecological significance of the early uptake of algal symbionts remains unknown. It has been hypothesized that early uptake of symbionts is an advantage for long-distance dispersal. Here, we tested the hypothesis that early acquisition of symbionts enhances post-settlement survival. We used a cultured strain of clade A Symbiodinium that was isolated from wild Acropora spat as the algal symbiont. Symbiotic and aposymbiotic Acropora larvae were prepared in the laboratory and settled on experimental plates in the field. The survival of settlers was monitored for 15 mo. Our results showed that more larval-stage set- tlers harbouring symbionts survived than those without, even when there was no difference in the initial density of settled larvae. We analysed the Symbiodinium clades harboured by the corals at 1 mo after settlement, and found that clade A was less abundant in the corals that grew from aposymbiotic larvae than in those that developed from symbiotic larvae. There was also a marked difference in coral survival between aposymbiotic and symbiotic larvae over this period. The higher survival rate of 'early uptake' corals was more pronounced on shaded plates. These results suggest that the early uptake of specific symbionts enhances post-settlement survival in dark places such as reef crevices, which are sites commonly settled by coral larvae.

Outbreaks of the crown-of-thorns sea star (Acanthaster planci) are likely to be strongly associated with drastic changes in larval survival influenced by food availability. However, no quantitative or qualitative data are available on the distribution of A. planci larvae in the field nor on the environmental factors that influence their survivorship. Here we use a DNA barcoding approach to describe the distribution of A. planci larvae in Sekisei Lagoon, Ryukyu Archipelago, Japan after conducting three days of high-intensity sampling. High densities (53.3 individuals/m3) of A. planci larvae were found outside of Yonara Channel, which is the largest reef channel in this lagoon. Surprisingly, most (94%) of the aggregated larvae were advanced-stage brachiolaria. Considering that it takes several days to develop to this stage, this result demonstrates that A. planci larvae were floating for some time and maintaining a high-density population. However, this dense larval cloud disappeared immediately after a typhoon. No spatial correlation was found between larval density and either nutrient or chlorophyll a concentrations, suggesting that A. planci larvae do not necessarily aggregate in nutrient-rich water. These data suggest that some high-density populations of late developmental stage A. planci larvae were produced under a low phytoplankton concentration and could potentially trigger an adult outbreak. Consequently, our data suggest that adult outbreaks may not necessarily be triggered by food availability alone.

The plasticity of allocation between life history traits and of sex allocation in the sessile colonial coral Goniastrea aspera was examined by field survey and transplant experiment at two zones (lower intertidal and reef edge) of a fringing reef at Sesoko Island, Okinawa. A field survey showed that colony density, colony size, and polyp size were greater at the lower intertidal zone than at the reef edge zone. Mortality of transplanted fragments was higher at the reef edge zone than at the lower intertidal zone. These results suggest that the environment at the lower intertidal zone was more favorable for G. aspera. There was, however, no significant difference in the growth rate and reproductive effort of the transplanted fragments between the two zones. In contrast, sex allocation to testis was higher at the lower intertidal zone than at the reef edge zone in the transplanted fragments. Both spawning of gametes and brooding of planulae are known in G. aspera, and natural colonies at the lower intertidal zone brooded more planulae than those at the reef edge zone. Therefore, it is suggested that G. aspera produced more sperm at the habitat where number of brooded planulae was larger. It is considered that, in order to increase the possibility of cross-fertilization, G. aspera colonies exhibited the plasticity to allocate more resources to sperm where number of brooded planulae was larger.

Coral reef ecosystems are based on coral–zooxanthellae symbiosis. During the initiation of symbiosis, majority of corals acquire their own zooxanthellae (specifically from the dinoflagellate genus Symbiodinium) from surrounding environments. The mechanisms underlying the initial establishment of symbiosis have attracted much interest, and numerous field and laboratory experiments have been conducted to elucidate this establishment. However, it is still unclear whether the host corals selectively or randomly acquire their symbionts from surrounding environments. To address this issue, we initially compared genetic compositions of Symbiodinium within naturally settled about 2-week-old Acropora coral juveniles (recruits) and those in the adjacent seawater as the potential symbiont source. We then performed infection tests using several types of Symbiodinium culture strains and apo-symbiotic (does not have Symbiodinium cells yet) Acropora coral larvae. Our field observations indicated apparent preference toward specific Symbiodinium genotypes (A1 and D1-4) within the recruits, despite a rich abundance of other Symbiodinium in the environmental population pool. Laboratory experiments were in accordance with this field observation: Symbiodinium strains of type A1 and D1-4 showed higher infection rates for Acropora larvae than other genotype strains, even when supplied at lower cell densities. Subsequent attraction tests revealed that three Symbiodinium strains were attracted toward Acropora larvae, and within them, only A1 and D1-4 strains were acquired by the larvae. Another three strains did not intrinsically approach to the larvae. These findings suggest the initial establishment of corals–Symbiodinium symbiosis is not random, and the infection mechanism appeared to comprise two steps: initial attraction step and subsequent selective uptake by the coral.

The initial factors that cause a decline in the survival of in situ settled corals remain poorly understood. In this study, we demonstrated through field experiments that the design of artificial grid plates may influence the initial survival of Acropora corals, with narrower grids being the most effective. In fact, grid plates with a 2.5-cm mesh presented the highest recorded survival rate (14%) at 6 months after settlement (representing approximately 50 corals per 0.25 m2 of plate). This is the first study where such high survival rates, matching those of cultures under aquarium conditions, were obtained in the field without using additional protective measures, such as guard nets against fish grazing after seeding. Therefore, our results provide a foundation for establishing new and effective coral restoration techniques for larval seeding, in parallel to clarifying the details of the early life stages of reef-building corals.