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.

For corals that establish symbioses with dinoflagellate Symbiodinium spp. at the larval 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 settlers 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.

The effect of colony size and age on growth and sexual reproduction was examined by taking small-sized fragments from large mature donor colonies of Goniastrea aspera and Favites chinensis. Both species are massive colonial corals in the family Faviidae. Monopolyp and oligopolyp (5 to 8 polyps) fragments were established. The fragments were cultured in running seawater aquaria for 22 mo, and growth and gamete production were measured. In both species, monopolyp fragments had higher growth rates than oligopolyp fragments. F. chinensis had higher growth rates than G. aspera in both fragment sizes. Polyps tended to bud when there was open space, and smaller colonies had higher growth rates, probably because the polyps of smaller colonies had more open space. In G. aspera, both the monopolyp and oligopolyp fragments produced gametes, whereas in F. chinensis, neither type of fragment produced gametes. These results suggest that sexual maturity is determined by colony age in G. aspera and by colony size in F. chinensis. Our results also suggest that polyps are integrated at the colony level. Polyp integration activity in a colony may be determined by the age of the colony in G. aspera and by the size of the colony in F. chinensis, likely maximizing colony fitness.

Abstract: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/m 3 ) 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.

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