Rising ocean temperatures are pushing reef-building corals beyond their temperature optima (Topt), resulting in reduced physiological performances and increased risk of bleaching. Identifying refugia with thermally resistant corals and understanding their thermal adaptation strategy is therefore urgent to guide conservation actions. The Gulf of Aqaba (GoA, northern Red Sea) is considered a climate refuge, hosting corals that may originate from populations selected for thermal resistance in the warmer waters of the Gulf of Tadjoura (GoT, entrance to the Red Sea and 2000 km south of the GoA). To better understand the thermal adaptation strategy of GoA corals, we compared the temperature optima (Topt) of six common reef-building coral species from the GoA and the GoT by measuring oxygen production and consumption rates as well as photophysiological performance (i.e. chlorophyll fluorescence) in response to a short heat stress. Most species displayed similar Topt between the two locations, highlighting an exceptional continuity in their respective physiological performances across such a large latitudinal range, supporting the GoA refuge theory. Stylophora pistillata showed a significantly lower Topt in the GoA, which may suggest an ongoing population-level selection (i.e. adaptation) to the cooler waters of the GoA and subsequent loss of thermal resistance. Interestingly, all Topt were significantly above the local maximum monthly mean seawater temperatures in the GoA (27.1°C) and close or below in the GoT (30.9°C), indicating that GoA corals, unlike those in the GoT, may survive ocean warming in the next few decades. Finally, Acropora muricata and Porites lobata displayed higher photophysiological performance than most species, which may translate to dominance in local reef communities under future thermal scenarios. Overall, this study is the first to compare the Topt of common reef-building coral species over such a latitudinal range and provides insights into their thermal adaptation in the Red Sea.
The vertical thermohaline and biogeochemical structures of the upper layer (0-200 m) were studied in the Gulf of Tadjourah using high-resolution hydrographic data collected in July-August 2013, September 2013 and February 2014. During summer, the superficial layer consisted of the mixed layer (ML) extending to a depth of about 20-30 m followed by the thermocline located between 30 and 50 m depth. The ML was thicker in the west and the southeast where the thermal gradient and chlorophyll a concentrations were particularly high. During September, this stratification persisted but the ML became warmer and saltier and the thermocline moved slightly deeper. In February, the ML extended to about 120 m, and the thermocline was less pronounced. A comparison of the directly measured currents to the wind induced Ekman current and to geostrophic velocity profiles revealed that the thermohaline and the biogeochemical features in summer were related to the southwest monsoon (SWM). The SWM drives surface water from the Gulf of Tadjourah to the Gulf of Aden and thus induces westward intrusion of the high salinity thermocline water from the Gulf of Aden; this near surface flow mixes surface waters in the extreme west of the Gulf of Tajourah. In contrast, the northeast monsoon (NEM), predominant in winter, brings cold water toward the Gulf of Tadjourah and thickens the ML through convective mixing. Our study shows that the SWM plays a crucial role in the stratification of the water column during summer but bathymetry influences its effects. The bowl-shape of the basin and its elongated slope in the west enhance the upwelling in this area where negative sea surface temperature anomalies and high chlorophyll a concentrations were observed. Y. Moussa Omar et al.
We observed groupers with an unusual color pattern in Ghoubet-Al-Kharab Bay, Djibouti, on two occasions in 2014 and 2020. It matches the field observation of a juvenile Cephalopholis formosa in India in 1980. If this is the case, our observation represents a major range extension for C. formosa. Alternatively, based on the intermediate coloration of the specimens and sightings, we propose that they may be hybrids of C. formosa and C. oligosticta. We present two possible scenarios that may have facilitated hybridization: rarity and overlapping ranges in Djibouti with self-recruitment or rarity and overlapping ranges in another location (e.g., Socotra or the Arabian Sea) with long-distance larval dispersal to Djibouti. This hybridization is possible given the genetic similarity between the two putative parent species and because similar hybridization cases have been recorded within this genus and family elsewhere. However, both of these scenarios would require a range extension for one or both parent species as they are not previously known to overlap. Nevertheless, further field observations and genetic studies are required to verify the proposed identification of the putative hybrid and test the presented scenarios.
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