Phylogeographic analysis suggests a recent population Bottleneck in the rare Red Sea Tridacna squamosina

Giant clams are an important ecological component of coral reefs in the Red Sea, as they enhance the reef’s productivity and provide habitat that can increase diversity. Three species of giant clams, namely Tridacna maxima, T. squamosa, and T. squamosina have been described within the Red Sea. However, due to its scarcity, information about the distribution and ecology of T. squamosina in the Saudi Arabian Red Sea is still lacking. This study used DNA barcoding to confirm the identity of the rare T. squamosina in the Farasan Banks. Six mtCOI fragments (500 bp) of T. squamosina were successfully amplified using the SQUA-primers for the first time. We used our data along with 18 reference sequences (16S) from the online database to assess the genetic diversity and population structure of T. squamosina. Low genetic diversity among the T. squamosina populations inferred from the 16S sequences implies a recent bottleneck for this species, which is supported by their historically higher diversity based on the coalescent-based estimator. Given the small population abundance and limited genetic variation of T. squamosina, it may warrant immediate local protections such as biobanking and fertility preservation programs as well as effective integrated coastal zone management plans.info:eu-repo/semantics/publishedVersio

The small giant clam, Tridacna maxima exhibits minimal population genetic structure in the Red sea and genetic differentiation from the Gulf of Aden

The Red Sea serves as a natural laboratory to investigate mechanisms of genetic differentiation and population dynamics of reef organisms due to its high species endemism. Giant clams, important yet understudied coral reef engineering species, are ideal candidates for such study in this region. This paper presents the first population genetics study of giant clams covering the entire East coast of the Red Sea. Our study aimed to investigate the population structure of the small giant clam, Tridacna maxima, based on 501-bp fragment of the cytochrome c oxidase I gene from 194 individuals (126 new sequences from this study plus 68 sequences from GenBank), collected from 14 locations in the Red Sea and Gulf of Aden (RSGA). For the genetic analysis, each sampling site was treated as a population. T. maxima showed high genetic diversity, with high gene flow in almost all sampling sites. The insignificant global #ST-value of 0.02 (p > 0.05) suggests the presence of one large, panmictic population across a wide range of temperature and salinity gradients in the RSGA. Despite this, the population in Djibouti was genetically differentiated from the other 11 populations in the Red Sea, suggesting a connectivity break between the Red Sea and the Gulf of Aden. These results could be explained by the oceanographic features facilitating wide larval transport inside the Red Sea, and creating a dispersal barrier to the Gulf of Aden. Besides larval dispersal by currents, apparent successful establishment following dispersal is probably facilitated by the mode and time of reproduction as well as the ability of T. maxima to achieve high fitness in the highly variable environmental conditions of the Red Sea.King Abdullah University of Science & Technology: BAS/1/1071-01-01info:eu-repo/semantics/publishedVersio

Differential thermal tolerance between algae and corals may trigger the proliferation of algae in coral reefs

Marine heatwaves can lead to rapid changes in entire communities, including in the case of shallow coral reefs the potential overgrowth of algae. Here we tested experimentally the differential thermal tolerance between algae and coral species from the Red Sea through the measurement of thermal performance curves and the assessment of thermal limits. Differences across functional groups (algae vs corals) were apparent for two key thermal performance metrics. First, two reef-associated algae species (Halimeda tuna and Turbinaria ornata,) had higher lethal thermal limits than two coral species (Pocillopora verrucosa and Stylophora pistillata) conferring those species of algae with a clear advantage during heatwaves by surpassing the thermal threshold of coral survival. Second, the coral species had generally greater deactivation energies for net and gross primary production rates compared to the algae species, indicating greater thermal sensitivity in corals once the optimum temperature is exceeded. Our field surveys in the Red Sea reefs before and after the marine heatwave of 2015 show a change in benthic cover mainly in the southern reefs, where there was a decrease in coral cover and a concomitant increase in algae abundance, mainly turf algae. Our laboratory and field observations indicate that a proliferation of algae might be expected on Red Sea coral reefs with future ocean warming

In situ Skeletal Growth Rates of the Solitary Cold-Water Coral Tethocyathus endesa From the Chilean Fjord Region

Cold-water corals (CWC) can be found throughout a wide range of latitudes (79°N–78°S). Since they lack the photosymbiosis known for most of their tropical counterparts, they may thrive below the euphotic zone. Consequently, their growth predominantly depends on the prevalent environmental conditions, such as general food availability, seawater chemistry, currents, and temperature. Most CWC communities live in regions that will face CaCO3 undersaturation by the end of the century and are thus predicted to be threatened by ocean acidification (OA). This scenario is especially true for species inhabiting the Chilean fjord system, where present-day carbonate water chemistry already reaches values predicted for the end of the century. To understand the effect of the prevailing environmental conditions on the biomineralization of the CWC Tethocyathus endesa, a solitary scleractinian widely distributed in the Chilean Comau Fjord, a 12-month in situ experiment was conducted. The in situ skeletal growth of the test corals was assessed at two sites using the buoyant weight method. Sites were chosen to cover the naturally present carbonate chemistry gradient, with pH levels ranging between 7.90 ± 0.01 (mean ± SD) and 7.70 ± 0.02, and an aragonite saturation (Ωarag) between 1.47 ± 0.03 and 0.98 ± 0.05. The findings of this study provide one of the first in situ growth assessments of a solitary CWC species, with a skeletal mass increase of 46 ± 28 mg per year and individual, at a rate of 0.03 ± 0.02% day. They also indicate that, although the local seawater chemistry can be assumed to be unfavorable for calcification, growth rates of T. endesa are comparable to other cold-water scleractinians in less corrosive waters (e.g., Lophelia pertusa in the Mediterranean Sea)

Abundance survey data on Tridacna spp. in the eastern Red Sea

Abundance survey data on Tridacna spp. (Tridacna maxima and Tridacna squamosa) giant clams in the eastern Saudi Arabian Red Sea. Surveys cover a latitudinal gradient from 29-18 Degree, 7 different depths (0.5, 1.5, 3, 5, 8, 11, and 15m), windward and/or leeward sides of reefs, and different distances to the shore

Abundance, primary production rates and net calcification rates of Tridacna maxima giant clams at two reefs in the Central Red Sea

Abundance data of Tridacna maxima giant clams at 6 different depths (0.5, 1.5, 3, 5 , 8 and 11m) at two reefs (Station 1 - 22.303833 N, 39.048278 E) and Station 2 - 20.753764 N, 39.442561 E ) in the Central Red Sea. Primary production in Tridacna maxima giant clams was assessed during the three different incident light incubations (561, 959, and 1061 μmol quanta m-2 s-1). Oxygen (μmol L-1) content in the incubation chambers was automatically logged (miniDOT, Precision Measurement Engineering, Inc., USA) in 15 min intervals over the 3 h incubation period. Net photosynthesis (NPP) was calculated from the variation of oxygen concentration over time and normalized for clam mantle surface area (μmol O2 cm-2 h-1). Dark respiration rates (R), also given in μmol O2 cm-2 h-1, were used to calculate gross primary production (GPP) as: GPP = NPP +R. Net calcification rates of the giant clam Tridacna maxima were determined under 7 different light levels (1061,959, 561, 530, 358, 244, and 197 μmol quanta m-2 s-1) and in the dark. At the start, after 3 h and after 6 h of each incubation, seawater was sampled from each experimental aquaria in gas tight 100mL borosilicate bottles (Schott Duran, Germany) and poisoned with mercury chloride, following Dickson et al. (2011). Each sample was analysed for TA by open-cell titration with an AS-ALK2 titrator (Apollo SciTech,USA) using certified seawater reference material (Andrew Dickson, Scripps Institution of Oceanography). During the incubations at moderate light levels (530, 358, 244,and 197 μmol quanta m-2 s-1), additional samples for dissolved inorganic carbon (DIC) were analysed using an ASC3 infrared DIC analyser (Apollo SciTech, USA). Further components of the carbonate system were calculated with R package Seacarb (Lavigne and Gattuso, 2013) using first and second carbonate system dissociation constants of Millero (2010) as well as the dissociations of HF and HSO-4 (Dickson, 1990; Dickson and Goyet, 1994), respectively. Net calcification (G in μmol CaCO3 h-1) was estimated from changes in total alkalinity (TA) using the alkalinity anomaly technique (Smith and Key, 1975), where delta TA is the variation of TA during the time (t ) of the incubations and the factor 2 accounts for a decrease in TA by two equivalents per CaCO3 precipitated (Zeebe and Wolf-Gladrow, 2001). Calcification rates were expressed relative to either mantle surface area (cm2) or tissue dry mass (g). For mantle surface area, the power relationship between standard length in centimetres (L) and mantle area (cm2) (Jantzen et al., 2008) was used to calculate the mantle surface in cm2. For tissue dry mass (DM in gram) of 5 clams, all clams were dissected and their biomass was determined subsequently to the incubation experiment. Clams were opened by cutting the adductor muscle with a scalpel, and the mantle and other tissues were separated from the shells and dried at 60 Degree Celsius for 24 to 48 h to determine tissue DM to the nearest 0.01 g