Ecological and Evolutionary Connectivity of Reef Corals in Subtropical Eastern Australia: Implications for the persistence of high-latitude coral populations

Annika ME Noreen

Coral reefs worldwide are threatened by a variety of pressures, and climate change models predict increasing levels of chronic stress as well as acute disturbances for most reefs. In addition to these global threats, populations on remote reefs are also predicted to be vulnerable due to their isolation from potential source reefs, usually low population size and associated increased extinction risk. Critical for the future of these reefs is dispersal and gene flow between populations (connectivity). This study investigated connectivity, population subdivision, and genetic diversity for three scleractinian reef coral species in discontinuous high-latitude nearshore coral communities and offshore reefs in Eastern Australia, with comparisons to the Great Barrier Reef (GBR) for two species. Two related brooding reef corals, Seriatopora hystrix and Pocillopora damicornis, and the common subtropical broadcast spawning coral Acropora solitaryensis, were used in this study to compare connectivity variability in the subtropics based on differences in reproductive mode and geographic range limits. Microsatellite markers were used to determine ecological dispersal and historical isolation in all three species and mitochondrial sequence data were used to infer deeper evolutionary data in P. damicornis. Not all species occurred at all locations, hence between three and eight reefs or coral communities were sampled for each species. Additionally, scleractinian species diversity was compiled for the offshore high-latitude reefs to provide information on stable, common species as well as locally rare species. This study detected high variability in gene flow among different species on spatial scales ranging from ~1,270 km. Pocillopora damicornis displayed higher gene flow than Seriatopora hystrix over the majority of temporal and spatial scales. Both species are larval brooders with relatively short mean larval settlement and competency periods, however, some P. damicornis planulae have extended larval competency periods (>100 days) that may provide some explanation for the higher gene flow in this species. The broadcast spawning coral species, Acropora solitaryensis, had levels of genetic subdivision similar to the brooding species over similar spatial scales. Although there were occasionally sharply contrasting gene flow patterns among the species, dispersal pathways were sometimes consistent. One consistent gene flow pattern was lower differentiation in the highest-latitude nearshore coral communities (the Solitary Islands, 13-30 km), compared to significantly higher differentiation over shorter distances at the offshore reefs (~2-11 km). Historical isolation was evident between most subtropical regions, locations, and sites. All pairwise FST values between nearshore and offshore reefs (>630 km distant) were significant at p≤0.01-0.001, indicating historical isolation. Additionally, despite the relatively closer distances between the offshore reefs (<235 >km), all pairwise FST values between Lord Howe Island, Elizabeth Reef and Middleton Reef were significant at p≤0.001. On a larger geographic scale, connectivity was low from the Great Barrier Reef to both subtropical nearshore coral communities and offshore reefs. However, the presence of historical or evolutionary gene flow between the GBR and subtropical reefs is supported by microsatellite data for all three species, and mtDNA in P. damicornis. A single unique mtDNA haplotype for P. damicornis was detected in the GBR, Flinders Reef and Middleton Reef samples as well as in three of the four Elizabeth Reef samples. There were two other unique mtDNA haplotypes detected; one unique haplotype at the Solitary Islands, and another haplotype at Lord Howe Island and Elizabeth Reef. Genetic diversity at studied loci was moderate to high for all three species. All species had a rare private allele in at least one subtropical population, indicating probable adequate population sizes for the maintenance of genetic diversity. The existing populations do not appear to exhibit detrimental genetic effects of small population sizes, and thus are unlikely to have an increased extinction risk. The results also suggest that many high-latitude populations of S. hystrix and P. damicornis are supplemented by long-distance migrants from the GBR. Locally rare species and recent new species records at high-latitude offshore reefs provides additional support for successful long-distance dispersal. Thus, the implications for high-latitude reefs of Eastern Australia are favorable, despite climate change predictions and relatively demographically closed populations (i.e. locally extirpated subtropical populations would likely recover only over an extended time frame). In summary, populations of these three reef coral species at subtropical reefs are not genetically depauperate, most high-latitude reefs are likely supplemented by immigrants over an evolutionary time frame.

Ecological and Evolutionary Connectivity of Reef Corals in Subtropical Eastern Australia: Implications for the persistence of high-latitude coral populations

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Ecological and Evolutionary Connectivity of Reef Corals in Subtropical Eastern Australia: Implications for the persistence of high-latitude coral populations

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