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Genetic population patterns in coral reefs are important for understanding biodiversity and forecasting recovery following disturbance. Studying gene flow between small and disrupted marine ecosystems, such as the reefs along the Singapore Strait, is especially crucial due to their fragile nature. Here we sampled the ubiquitous coral species complex Pachyseris speciosa from seven sites in Singapore and applied genotyping-by-sequencing (nextRAD) for a fine-scale population genomic assessment. Our results confirm the existence of two clearly distinct, possibly cryptic, lineages that occur sympatrically at each of the seven sampling sites and are related to the "blue" and "green" lineages found in other parts of the Indo-Pacific. The closely related lineages show contrasting patterns characterised by panmixia and substantial (but unexplained) substructuring, respectively. Analysis of barriers and corridors to gene flow reveals limited connectivity between the two largest clusters of reefs at the eastern and western sectors of Singapore's southern coastal waters. Our results extend the geographic range of the two recently uncovered P. speciosa species, further confirming their status as distinct and widespread species. More broadly, our findings highlight how conservation and management strategies may maximise connectivity among Singapore's urbanised reef ecosystems. [ABSTRACT FROM AUTHOR]

Thermal stress drives the bleaching of reef corals, during which the endosymbiotic relationship between Symbiodiniaceae microalgae and the host breaks down. The endosymbiont communities are known to shift in response to environmental disturbances, but how they respond within and between colonies during and following bleaching events remains unclear. In 2016, a major global-scale bleaching event hit countless tropical reefs. Here, we investigate the relative abundances of Cladocopium LaJeunesse & H.J.Jeong, 2018 and Durusdinium LaJeunesse, 2018 within and among Pachyseris speciosa colonies in equatorial Singapore that are known to host both these Symbiodiniaceae clades. Bleached and unbleached tissues from bleaching colonies, as well as healthy colonies, during and following the bleaching event were sampled and analyzed for comparison. The nuclear ribosomal internal transcribed spacer (ITS) regions were separately amplified and quantified using a SYBR Green-based quantitative polymerase chain reaction (qPCR) method and Illumina high-throughput sequencing. We found Cladocopium to be highly abundant relative to Durusdinium. The relative abundance of Durusdinium, known to be thermally tolerant, was highest in post-bleaching healthy colonies, while bleached and unbleached tissues from bleaching colonies as well as tissue from healthy colonies during the event had depressed proportions of Durusdinium. Given the importance of Durusdinium for thermal tolerance and stress response, it is surprising that bleached tissue showed limited change over healthy tissue during the bleaching event. Moreover, colonies were invariably dominated by Cladocopium during bleaching, but a minority of colonies were Durusdinium-dominant during non-bleaching times. The detailed characterization of Symbiodiniaceae in specific colonies during stress and recovery will provide insights into this crucial symbiosis, with implications for their responses during major bleaching events. [ABSTRACT FROM AUTHOR]