Population genetic structure of a major reef-building coral species Acropora downingi in northeastern Arabian Peninsula

Current seawater temperatures around the northeastern Arabian Peninsula resemble future global forecasts as temperatures > 35 °C are commonly observed in summer. To provide a more fundamental aim of understanding the structure of wild populations in extreme environmental conditions, we conducted a population genetic study of a widespread, regional endemic table coral species, Acropora downingi, across the northeastern Arabian Peninsula. A total of 63 samples were collected in the southern Arabian/Persian Gulf (Abu Dhabi and Qatar) and the Sea of Oman (northeastern Oman). Using RAD-seq techniques, we described the population structure of A. downingi across the study area. Pairwise G’st and distance-based analyses using neutral markers displayed two distinct genetic clusters: one represented by Arabian/Persian Gulf individuals, and the other by Sea of Oman individuals. Nevertheless, a model-based method applied to the genetic data suggested a panmictic population encompassing both seas. Hypotheses to explain the distinctiveness of phylogeographic subregions in the northeastern Arabian Peninsula rely on either (1) bottleneck events due to successive mass coral bleaching, (2) recent founder effect, (3) ecological speciation due to the large spatial gradients in physical conditions, or (4) the combination of seascape features, ocean circulation and larval traits. Neutral markers indicated a slightly structured population of A. downingi, which exclude the ecological speciation hypothesis. Future studies across a broader range of organisms are required to furnish evidence for existing hypotheses explaining a population structure observed in the study area. Though this is the most thermally tolerant acroporid species worldwide, A. downingi corals in the Arabian/Persian Gulf have undergone major mortality events over the past three decades. Therefore, the present genetic study has important implications for understanding patterns and processes of differentiation in this group, whose populations may be pushed to extinction as the Arabian/Persian Gulf warms.

Variability of SST and ILD in the Arabian Sea and Sea of Oman in Association with the Monsoon Cycle

Sea surface temperature (SST) and isothermal layer depth (ILD) are important oceanic parameters and could play a significant role in understanding the upper thermal structure as well as improve the predictive capability of monsoons in the tropical oceans. In a disparate departure from the past research, the present study investigates the seasonal variability of SST and ILD in association with the monsoon cycle in the Arabian Sea and Sea of Oman regions by examination of Argo datasets for 2016-17. In this study, the ILD climatology is determined from temperature profiles provided by the Argo floats based on a threshold technique T z ≥ SST − 1 ° C to investigate the region of stronger and weaker monsoon wind forcing. For SST, values of temperature are used nearest to the sea surface (depth z ≤ 5 m). The region is split into four distinct zones for an accurate description of the monsoon cycle: the south Arabian Sea, the central Arabian Sea, the north Arabian Sea, and the Sea of Oman. It is observed that summer monsoon is more pronounced in the south-central basin of the Arabian Sea, where ILD is deepening (>100 m in September 2016) mainly due to stronger wind forcing in this region. On the contrary, the Sea of Oman region is displayed with smaller ILD amplitude (<10 m in June 2016) with larger SST, meaning that this region is weakly influenced by the summer monsoon. The seasonal relationship established between ILD variability and monsoon cycle for 2016-17 shows that ILD could be a useful indicator for predicting summer monsoon in the Arabian Sea regions. Our analysis results indicate that the dynamics for SST variability are different in these regions and are influenced either by large-scale atmospheric forcing, such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), or by the effects of mesoscale variations occurring along the Oman-Arabian coast.