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Giant clams are ecologically and economically relevant reef inhabitants that host photosynthetic dinoflagellates inside tubules located mainly in their colorful outer mantle. This study examined the effects of exposure to darkness for 30 days and the subsequent 11 days of recovery under a normal photoperiod on the outer mantle of the fluted giant clam Tridacna squamosa. Changes in the abundance of iridocytes and symbionts were assessed by fluorescence microscopy. Light microscopy was applied to quantify symbionts isolated from the outer mantle, while chlorophyll was extracted and analyzed by spectroscopy. The transcript levels of the host’s vacuolar-type H+-ATPase subunit A (ATP6V1A) and symbionts’ form II ribulose-1,5-bisphosphate carboxylase/oxygenase (Zoox-rbcII) were determined by quantitative real-time PCR (qPCR) and used as proxies for iridocyte abundance and phototrophic potential, respectively. The protein abundance of ATP6V1A and Zoox-RBCII was quantified by western blotting. After exposure to darkness for 30 days, the outer mantle of T. squamosa individuals lost the distinct multiple color patterns, and the gene and protein expression levels of ATP6V1A/ATP6V1A and Zoox-rbcII/Zoox-RBCII decreased dramatically. Microscopy assessment confirmed the reduction in iridocyte and symbiont populations, and the chlorophyll level also decreased considerably. However, just 11 days after returning to a normal light:dark regimen, the quantity of coccoid dinoflagellates and the expression of Zoox-rbcII/Zoox-RBCII in the outer mantle increased significantly to levels higher than those of the individuals prior to exposure to darkness (control), while the chlorophyll content returned to the control level. Additionally, the outer mantle regained most of its coloration with partial recovery of the iridocyte population. These results are relevant not only for understanding the phenomena of light deprivation and symbiont loss in dinoflagellate-associated reef organisms, but also signify that the giant clam-coccoid dinoflagellate holobiont is phototrophically plastic and particularly tolerant to bleaching.

Giant clam marine culture is commonly influenced by a wide variety of pathogenic bacteria including Vibrio species, resulting in high losses. The current study was organized to investigate the bacterial infections, in the Hurghada area, between the culture of Tridacna maxima and Tridacna gigas in indoor aquaria and offshore cages. Eighty samples of T. maxima and T. gigas in both indoor aquaria and offshore cages were collected for bacteriological analysis. Samples were taken from gills, gut, mantels, and gonads of Tridacna. The results showed reduced motion followed by loss of attractive color pattern with 25 % and 20 % mortality rate among indoor cultured T. maxima and T. gigas, respectively. About forty-six bacterial isolates were identified, using phenotypic and biochemical testing. Most of those isolates were Vibrio logei, V. harveyi, and unidentified Vibrio. The pathogenicity and virulence of V. logei seem high with a 30% mortality rate. The current study would provide useful information for better management of bacterial infections in T. maxima and T. gigas.

The photosymbiosis of tropical giant clams (subfamily Tridacninae) with unicellular algae (Symbiodiniaceae) restricts their distribution to the sunlit, shallow waters of the euphotic zone where organisms are additionally exposed to potentially damaging levels of solar UV radiation. Metabolic and physiological responses of Red Sea Tridacna maxima clams, including net calcification and primary production, as well as valvometry (i.e., shell gaping behavior) were assessed when exposed to simulated high radiation levels received at 3 and 5 m underwater. The two levels of radiation included exposure treatments to photosynthetically active radiation (PAR; 400–700 nm) alone and to both, PAR and ultraviolet-B radiation (UV-B; 280–315 nm). The valvometry data obtained using flexible magnetic sensors indicated that specimens under PAR + UV-B exposure significantly reduced the proportion of their exposed mantle area, a potential photo-protective mechanism which, however, reduces the overall amount of PAR received by the algal symbionts. Consequently, specimens under PAR + UV-B displayed a slight, although non-significant, reduction in primary production rates but no signs of additional oxidative stress, changes in symbiont densities, chlorophyll content, or levels of mycosporine-like amino acids. Net calcification rates of T. maxima were not affected by exposure to UV-B; however, calcification was positively correlated with incident PAR levels. UV-B exposure changes the valvometry, reducing the exposed mantle area which consequently diminishes the available PAR for the photosymbionts. Still, T. maxima maintains high rates of primary production and net calcification, even under high levels of UV-B. This provides experimental support for a recently described, effective UV-defensive mechanism in Tridacninae, in which the photonic cooperation of the associated algal symbionts and giant clam iridocytes is assumed to establish optimal conditions for the photosynthetic performance of the clams’ symbionts.

The present investigation focuses on population genetic structure analysis of the endangered giant clam species Tridacna maxima across part of the Red Sea, with the main aim of assessing the influence of postulated potential barriers to gene flow (i.e., particular oceanographic features and marked environmental heterogeneity) on genetic connectivity among populations of this poorly dispersive bivalve species. For this purpose, a total of 44 specimens of T. maxima were collected from five sampling locations along the Saudi Arabian coast and examined for genetic variability at the considerably variable mitochondrial gene cytochrome c oxidase I (COI). Our results revealed lack of population subdivision and phylogeographic structure across the surveyed geographic spectrum, suggesting that neither the short pelagic larval dispersal nor the various postulated barriers to gene flow in the Red Sea can trigger the onset of marked genetic differentiation in T. maxima. Furthermore, the discerned shallow COI haplotype genealogy (exhibiting high haplotype diversity and low nucleotide diversity), associated with recent demographic and spatial expansion events, can be considered as residual effect of a recent evolutionary history of the species in the Red Sea.

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates, however, the diversity of the giant clams, as well as the diversity and distribution of Symbiodiniaceae in different Tridacnine species remain relatively poorly studied in the South China Sea. In this study, a total of 100 giant clams belonging to small type’s giant clams, Tridacna maxima, T. crocea, and T. noae, were collected from Hainan and Xisha Islands. Based on mtDNA cytochrome c oxidase subunit 1 gene (COI) and 16S rRNA fragments, T. maxima and T. crocea showed a closer phylogenetic relationship than T. noae. All the three species of giant clams hosted Symbiodiniaceae including genera Symbiodinium (formerly Clade A) and Cladocopium (formerly Clade C). Geographically, symbionts in Cladocopium are restricted to Xisha Islands, probably because Cladocopium prefers to inhabit in waters with higher mean temperatures. The endosymbiont specificity among the three giant clam species was also detected. T. noae and T. crocea are found to harbor Symbiodinium preferentially, compared with Cladocopium. These results could provide important information to understand various endosymbionts occurring in giant clams in the South China Sea.

The molluscan 28S ribosomal RNA (rRNA) naturally cleaves into two fragments of similar size as 18S rRNA. This phenomenon, known as the hidden break, has been described in many other protostome species. Here, we characterize the 28S rRNA hidden break region in seven giant clam species, Tridacna gigas, T. derasa, T. maxima, T. noae, T. crocea, Hippopus hippopus and H. porcellanus. We confirm that the break consistently occurs in giant clams and is observed in all tissues and developmental stages. The region of the giant clam 28S rRNA break is located at a position homologous to the well-characterized 28S rRNA break in insects. High AU content and a stem–loop secondary structure in the region may contribute to fragmentation of the 28S rRNA molecule. An awareness of the existence of the hidden break in molluscs, such as the giant clams, which are emerging systems for understanding how environmental change affects life in the ocean, will facilitate progress of RNA sequencing-based analyses that currently rely on standard RNA profiles as a measure of sample integrity.

The mantle color’s pattern of giant clams exhibit a rich polymorphism through four iridocytes according to the RGB principle display. The boring giant clam (Tridacna crocea) expresses brilliant mantle coloration in colors including blue, green, purple, gold, and orange. To evaluate the genetic laws driving these mantle color’s pattern, complete diallel crosses between two color strains, the blue strain (only blue iridocyte) and the yellow-green strain (yellow and green iridocytes), were conducted at an experimental station in Sanya, Hainan Island Province. Two intra-strain crosses (BB and YY) and two reciprocal inter-strain crosses (BY and YB) were carried out successfully three times using single to single mating. High fertilization rates and D larvae rate were observed in all the experimental groups, suggesting that there was no sperm-egg recognition barrier between the two’s strains. The size of the reciprocal hybrids were larger than the two pure strains during the grow out stage with a degree of heterosis. Additionally, the hybrids have a strong larval metamorphosis ability and a high survival rate when compared to the two pure strains. At one year, the mantle color’s pattern of pure strains is 100% stably inheritable, while the reciprocal hybrids both exhibited colorful patterns in blue, yellow, and green co-coloring, suggesting that there is a genetic recombination of mantle colors at the stable expression period. These results provided a basis to the theory of mantle color formation and their genetic separation law, and also suggests a practice on the genetic breeding of giant clams for aquaculture in coral reef area.

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates. While clams are economically important for fisheries, Symbiodiniaceae are responsible for most of the primary production in coral reefs. This review addresses key issues regarding the giant clam–Symbiodiniaceae holobiont: diversity and distribution; functional traits; evolution and coevolution; and consequences of climate change. Findings show that holobiont distribution is partitioned by host species and geography. So far, giant clams have been reported in association with only 30 Symbiodiniaceae phylotypes, all within genera Symbiodinium, Cladocopium and Durusdinium. Species and functional diversities are highest in the Coral Triangle. The association evolved in the Eocene–Oligocene transition; there is evidence for coevolution, including host organs developed for housing symbionts, change in host habitat and multiple symbiont phylotypes found exclusively in giant clams. Clam bleaching events have been recorded throughout the Indo-Pacific. These clams may be particularly vulnerable, as they rarely associate with stress-tolerant symbionts and are heavily targeted by fishing. The Great Barrier Reef is particularly at risk, as clams in that area do not associate with thermally tolerant, low-irradiance-adapted and opportunistic symbiont phylotypes. Further reduction in giant clam populations may serve as an important indicator of the increasing decline of coral reef biodiversity.

The increasing frequency and intensity of precipitation attributed to global climate change, in combination with sewage disposal , agricultural runoff , and other anthropogenic sources , could lead to a reduction in the salinity levels of shallow water reefs. Given that many reef animals including the giant clam exhibit a bipartite life cycle wherein fertilization and larval development take place in the water column prior to settlement, tridacnine early life stages are susceptible to reduced salinity levels. To investigate the impacts of reduced salinities on the early life stages of the giant clam, we exposed the gametes , embryos, and larvae of Tridacna gigas to salinities of 18, 22, 26, 30, and 34 ppt. Fertilization occurred in all treatments but was significantly reduced at 18 ppt. Lower salinity conditions resulted in delayed embryonic development , with fewer trochophore and veliger larvae at 18, 22, and 26 ppt. No larvae survived more than two days of exposure at 18 ppt and four days at 22 or 26 ppt. Salinity did not have a significant effect on pediveliger larvae settlement rate or shell length. This study shows that reduced salinity affects both the fertilization success and survival of developing tridacnine larvae, suggesting that intensified precipitation events could further inhibit the survival of giant clams on the reef.

Giant clams provide and support valuable functions to coral reefs, as well as represent a sustainable resource for traditional fisheries throughout the Indo‐Pacific region. The Ryukyu Archipelago (southern Japan) is known to be the northern latitudinal limits of giant clam distribution, but there is only limited information in the literature regarding species diversity, status, and distribution in this region. In this study, we report findings from a rapid survey in 2016, the first of its kind for the Ryukyu Islands, to determine species distribution and abundance of giant clams (tridacnines) around Okinawa‐jima Island. Results indicate the presence of four species with an overall density of 5.03 per 100 m², from most to least abundant: Tridacna crocea, Tridacna maxima, Tridacna squamosa, and Tridacna noae. The previously reported species Tridacna gigas and Hippopus hippopus were both absent from the surveys. The densities and distributions of tridacnines varied among species and sites, which are likely attributable to efforts in replenishing and protecting stocks of selected species. The most abundant species, T. crocea, is an important fishery species in Okinawa, and has been widely cultured and restocked to augment depleted populations. In comparison, restocking efforts for less popular species, such as T. squamosa, have been limited, and their current sizes and numbers suggest recruitment constraints. Given the importance of the giant clam fishery in this region and the current declines of various species (except T. crocea), further regulations should focus on the protection of larger and mature clams that function as broodstock necessary to maintain spawning and natural recruitment.

The giant clam, Tridacna maxima were reared in cages in the sea; measurements of the shell's length, height, width and buoyant weight were taken under water to compare the growth rates of different size groups of T. maxima. The results showed that the specific growth rates were significantly high in the smallest size group and decreased with increasing clam size (P

The capacity for inter-specific hybridisation to yield improvements in animal production has long been realised in the aquaculture sector. However, the extension of hybridisation to improve production of giant clams (Tridacninae) has received little research attention. Aquaculture production of giant clams is directed primarily at aquarium markets, and hybridisation offers an opportunity to diversify available products. Here we report on the successful inter-specific hybridisation of Tridacna maxima and T. noae, and assess hybrid performance during hatchery culture. Both maternal and paternal hybrid crosses of T. maxima/T. noae yielded competent larvae. Fertilisation success was significantly lower (40.7 ± 3.2%) for the T. maxima♀/T. noae♂ cross compared to the T. noae ♀/T. maxima♂ cross (89.3 ± 1.8%; χ2=154.03, P & 0.01). Despite the disproportionate fertilisation success, the two hybrids had similar survival curves (survival=2.03×days-2.08, r2=0.67, P=0.13) for the first 30 days of culture. Antero-posterior shell growth (APM) was superior in the T. maxima♀/T. noae♂ hybrid (APM=10(2.158+0.019×days)) when compared to the T. noae♀/T. maxima♂ hybrid (APM=10(2.169+0.016×days)) over the same time frame (r2=0.92, P & 0.001). Both survival and growth of the hybrids exceeded that reported in the literature for pure-strain hatchery culture of giant clam species. This suggests capacity for hybridisationto contribute to giant clam aquaculture production. The application of hybridisation to supply aquarium markets and necessary considerations for manipulating stock genetics are discussed in view of these results.

Giant clams are one of the most important animals in coral reef ecosystem, and its growth and reproduction are being threatened by heat stress due to global warming. In the present study, the symbiont density, the crucial enzyme activities and the transcriptome were investigated in the outer mantle of giant clam Tridacna crocea after the acute exposure of high temperature. The density of symbiotic zooxanthellae decreased significantly during 12-24 h, with the minimum level (7.75 × 105 cell cm-2, p < 0.05) at 12 h after heat stress. The activities of superoxide dismutase in the heat stress group was significantly lower than that in the control group at 24 h after heat stress, while no significant change in the activities of catalase was observed during the entire stress process. The activation level of caspase3 began to increase significantly at 12 h (1.22-fold, p < 0.05), and reached the highest level at 24 h (1.38-fold, p < 0.05) after heat stress. Six paired-end libraries were sequenced in two groups, including the heat stress and control group at 12 h after heat stress. Through the assembling of 187,116,632 paired-end reads with lengths of 2 × 150 bp, a total of 26,676 genes were obtained which derived from giant clam. Bioinformatics analysis revealed 47 significantly upregulated and 88 significantly downregulated genes at 12 h after the treatment. There were 12 overrepresented GO terms for significantly upregulated genes, mostly related to unfolded protein binding and ATP binding, whereas no GO term was overrepresented for significantly downregulated genes. These results collectively suggest high temperature could induce excessive oxidative stress through the repressed antioxidant ability, the apoptosis activated by the unfolded protein response, and further the collapse of the symbiosis between host and symbiont, which has been threatening the growth and reproduction of the giant clam T. crocea.

Giant clams Tridacna crocea harbor symbiotic zooxanthellae in tubular systems and display high growth rate when exposed to light. Light/dark alternation plays an important role in growth performance and physiological change of giant clam–zooxanthellae symbiosis in natural ecosystems. In this study, photosynthetic performance, antioxidant enzymes, transcriptome, and expression of eight illumination-related genes were investigated in the mantle of giant clam at different treatment times of 12-h light/12-h dark (15,000 lx). (1) The effective photochemical quantum yield of PSII (Y(II)) was high in the early morning, but significantly decreased to a low level at 6 h of light treatment (P

Giant clams can grow to large sizes despite living in oligotrophic waters of the tropical Indo-Pacific as they maintain a mutualistic relationship with symbiotic dinoflagellates (zooxanthellae) and receive photosynthate from them. The phototrophic dinoflagellates live extracellularly inside a tubular system located mainly in the colorful outer mantle and have no access to the ambient seawater. Hence, the clam host needs to absorb exogenous inorganic carbon (Ci), nitrogen (N) and phosphorus (P), and supply them to the symbionts. As photosynthesizing symbionts need more nutrients in light than in the dark, the uptake rates of these exogenous nutrients by the host must increase during illumination, implying that the host’s transporters involved need to be regulated by some kind of light-responsive mechanisms. Furthermore, the growth and development of the host can also be augmented by light, because of the photosynthate donated by the photosynthesizing symbionts. Consequently, giant clams display many light-dependent phenomena related to phototrophy, antioxidative defense, biomineralization, as well as absorption of exogenous Ci, N, and P. These phenomena may involve collaborations among enzymes and transporters in several organs of the host, whereby the gene and protein expression levels of these biocatalysts are up- or down-regulated during illumination. This review aims to examine the molecular mechanisms of light-dependent physiological phenomena that occur in intact giant clam-dinoflagellate associations, and to highlight the differences between giant clams and scleractinian corals in those regards. As the population of giant clams in nature are dwindling due to climate change and anthropogenic activities, a good understanding of their light-dependent processes may generate new ideas to improve their growth and survival under rapidly changing environmental conditions.

Giant clams (Subfamily Tridacninae), are important members of Indo-Pacific coral reefs, playing multiple roles in the framework of these communities. Although they are prominent species in Red Sea reefs, data on their distribution and densities in the region are scarce. The present study provides the first large-scale survey of Red Sea Tridacna spp. densities, where we examined a large proportion of the Saudi Arabian Red Sea coast (1,300 km; from 18° to 29°N). Overall, Tridacninae were found at densities of 0.19 ± 0.43 individuals m–2 (±SD). Out of the total 4,002 observed clams, the majority (89%) were Tridacna maxima, with 0.17 ± 0.37 individuals m–2, while only 11% were Tridacna squamosa clams with 0.02 ± 0.07 individuals m–2. We also report on a few (total 6) Tridacna squamosina specimens, found at a single reef. We identified different geographical parameters (i.e., latitude and distance to shore) and local environmental factors (i.e., depth and reef zone) as the main drivers for local Tridacna spp. densities. Our results show that the drivers influencing the densities of Red Sea giant clams are complex due to their co-occurrence and that this complexity might explain the high variation in Tridacninae abundances across the Indo-Pacific, but also within a given reef. We also estimate that giant clam calcification likely contributes to an average of 0.7%, but potentially up to 9%, of the overall mean calcium carbonate budget of Red Sea coral reef communities.

International audience; Giant clams are conspicuous bivalves inhabiting Indo-Pacific reefs. Since Rosewater’s seminal paper in 1965, the number of giant clam species described or resurrected has exactly doubled. The increased number of species reported and accessibility to genetic material of rare or uncommon species therefore call for a reappraisal of the phylogenetic relationships within the Tridacninae subfamily. Here, we aim to reconstruct the evolutionary relationships among all 12 extant species by performing a comprehensive phylogenetic analysis of mitochondrial genome and nuclear 18S rRNA data from a combination of genome skimming, Sanger sequencing and previously published sequences. Comparing the mitogenomes among Tridacninae species, we report two new findings: (1) the T. crocea mitogenome length obtained here (18,266 bp) is shorter than previously known, and (2) the mitochondrial gene order in T. crocea and T. squamosa differs from the other species. Our phylogeny based on a concatenated 16-gene dataset (15 mitochondrial markers and nuclear 18S rRNA) reveals highly supported relationships within and between the three subgenera, Tridacna (Tridacna), Tridacna (Persikima) and Tridacna (Chametrachea). Overall, the inclusion of new molecular markers greatly improves the confidence and support for the subfamily’s phylogeny. The availability of this comprehensive phylogenetic dataset serves as the foremost baseline of Tridacninae relationships to support future studies examining giant clam systematics, ecology and conservation.

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/1/1071-01-01 info:eu-repo/semantics/publishedVersion

Farming of marine organisms (mariculture) represented 36% of global aquaculture, with molluscs representing 58.8% in live weight. Mollusc populations in some locations are, however, threatened by degradation of the ecosystems and/or over-fishing. This threat is increasingly being addressed through Restorative Shellfish Mariculture (RSM), as opposed to mariculture alone. There is no general consensus in the literature on what can and cannot be considered RSM. While maximization of benefits other than provisioning services is often considered a prerequisite, in other cases the maximization of fisheries yields is prioritised. Here we define RSM as the farming of marine shellfish, implying some form of intervention during the species life cycle, in order to address negative socio-ecological issues arising from the unsustainable use of marine ecosystems, independent of the final ownership regime of the resource. Strategies for developing RSM were reviewed and classified along a gradient from the most conservation-oriented (e.g. habitat restoration, reintroduction of locally extinct endangered species), to the most fisheries-oriented (including some forms of fisheries enhancement), and classified as Non-hatchery Dependent or Hatchery Dependent strategies. We reviewed the targeted species and strategies implemented across 584 individual projects developed in the last decades in North America, Europe, Asia, Oceania and South America. We found that some 48 species, including 34 bivalves and 15 gastropods were targets of RSM in 34 countries. US projects accounted for ca. three quarters of the total (N=438), with Philippines, Japan and Australia also being home to a large number. More than 90% of the projects involved five species, namely the eastern oyster (Crassostrea virginica, N=379), the giant clam (Tridacna gigas, N=65), the Olympia oyster (Ostrea lurida, N=25), the bay scallop (Argopecten irradians, N=25) and the hard clam (Mercenaria mercenaria,N=15). Of the RSM projects, 51% used Non-hatchery dependent methods, mostly habitat restoration providing substrata for settlement, whilesome 49% involved hatcheries. Some three percent of the projects combined both methods. This review provides an overview of the breadth and depth and aims of RSM globally, develops a broad definition of the activity, and proposes a structure for classifying RSM.

Iridocytes, containing multiple stacks of proteinaceous platelets and crystalized guanine, alternating with thin cytoplasm sheets, are specialized cells that act as multilayer nano-reflectors. Convergence evolution led to their arising across a broad range of organisms, including giant clams of the Tridacninae subfamily - the only sessile and photosymbiotic organism, among animals known to possess iridocytes. Through the interference of light with their nanoscale architecture, iridocytes generate ‘structural colors’, which are reported to serve different purposes, from intra-species communication to camouflage. In giant clams, iridocytes were previously reported to promote a lateral - and forward scattering of photosynthetically productive radiation (PAR) into the clam tissue, as well as the back reflection of non-productive wavelengths. Hence, they are assumed to promote an increased efficiency in the use of available solar energy, while simultaneously preventing photodamage of the algal symbionts. We report the use of guanine crystals within Tridacna maxima giant clam iridocytes as a basis for photonic cooperation between the bivalve host and their photosynthetic symbionts. Our results suggest that, in addition to the previously described scattering processes, iridocytes absorb potentially damaging UV radiation (UVR) and, through successive emission, emit light at longer wavelengths, which is then absorbed by the photosynthetic pigments of the algal symbionts. Consequently, both, host and algal symbionts are sheltered from (potentially) damaging UVR, while the available solar energy within the PAR spectrum increases, thereby potentially enhancing photosynthetic and calcification rates in this large bivalve. Further, our results suggest that this photonic cooperation could be responsible for the broad repertoire of colors that characterizes the highly diverse mantle patterns found in T. maxima.

Giant clams harbor three genera of symbiotic dinoflagellates (Symbiodinium, Cladocopium, and Durusdinium) as extracellular symbionts (zooxanthellae). While symbiotic dinoflagellates can synthesize amino acids to benefit the host, they are nitrogen-deficient. Hence, the host must supply them with nitrogen including urea, which can be degraded to ammonia and carbon dioxide by urease (URE). Here, we report three complete coding cDNA sequences of URE, one for each genus of dinoflagellate, obtained from the colorful outer mantle of the giant clam, Tridacna squamosa. The outer mantle had higher transcript level of Tridacna squamosa zooxanthellae URE (TSZURE) than the whitish inner mantle, foot muscle, hepatopancreas, and ctenidium. TSZURE was immunolocalized strongly and atypically in the plastid, moderately in the cytoplasm, and weakly in the cell wall and plasma membrane of symbiotic dinoflagellates. In the outer mantle, illumination upregulated the protein abundance of TSZURE, which could enhance urea degradation in photosynthesizing dinoflagellates. The urea-nitrogen released could then augment synthesis of amino acids to be shared with the host for its general needs. Illumination also enhanced gene and protein expression levels of TSZURE/TSZURE in the inner mantle and foot muscle, which contain only small quantities of symbiotic dinoflagellate, have no iridocyte, and lack direct exposure to light. With low phototrophic potential, dinoflagellates in the inner mantle and foot muscle might need to absorb carbohydrates in order to assimilate the urea-nitrogen into amino acids. Amino acids donated by dinoflagellates to the inner mantle and the foot muscle could be used especially for synthesis of organic matrix needed for light-enhanced shell formation and muscle protein, respectively.

Giant clams provide valuable functions to the coral reefs and traditional fisheries in the South China Sea (SCS), but many populations face escalating threats of overexploitation and habitat loss. To provide critical biodiversity data for development of protection and sustainable utilisation strategies, we examine the status of wild tridacnine population at Dongsha Atoll, the largest northernmost atoll of the SCS. This study also examines the genetic patterns of Tridacna maxima and Tridacna noae, based on partial mitochondrial COI gene sequences. We found four species at Dongsha Atoll with an overall density of 3.14 per 100 m2. Over-harvesting may have depleted populations of T. squamosa and Hippopus hippopus, and they may no longer be reproductively viable. Populations of T. maxima and T. noae appear to be thriving and replenished by recruits, but species showed low levels of mitochondrial genetic diversity that could reduce its adaptability and may become further impacted by exploitation and global warming. Haplotype networks for T. maxima and T. noae showed strong significant population structuring consistent with contemporary geographic boundaries: Dongsha Atoll, Taiwan and Philippines—likely established by the intrusion of the Kuroshio Current. Given the contrasting species status and genetic diversity, conservation strategies need to be tailored to increase success of these efforts.

Giant clams harbor extracellular symbiotic zooxanthellae in a tubular system that pervades mainly the fleshy and colorful outer mantle. During insolation, the symbiotic zooxanthellae conduct photosynthesis and donate photosynthates to the host clam. The host could encounter hyperoxia because photosynthesizing zooxanthellae also release O2. Hence, the host tissues, particularly the outer mantle, would need to prevent plausible oxidative damages caused by the increased production of reactive oxygen species (ROS), including superoxide radical (O 2 •− ), during insolation. Superoxide dismutases (SODs) are enzymes which scavenge intracellular O 2 •− . The complete cDNA coding sequence of a Manganese SOD (MnSOD) had been cloned from the outer mantle of the fluted giant clam, Tridacna squamosa. It comprised 678 bp, encoding for a 226 amino acid polypeptide of 24.9 kDa, with a mitochondrial targeting sequence. Its host origin was confirmed through phenogramic and sequence similarity analyses. After 12 h of light exposure, the transcript level and protein abundance of MnSOD/MnSOD increased significantly in the outer mantle as compared with the control kept in darkness for 12 h, which was unrelated to circadian rhythm. In light, the outer mantle could have augmented metabolic rate with increased mitochondrial ROS production due to increases in the supply of substrates to the zooxanthellae for photosynthesis and the receipt of O2 and photosynthates from the symbionts. Therefore, the light-dependable expression of MnSOD/MnSOD in the outer mantle of T. squamosa could be an antioxidative response acquired by the host to detoxify excess mitochondrial O 2 •− and prevent oxidative damage attributable to its symbiotic association with autotrophic zooxanthellae.

This study provides a first evaluation of different hatchery culture methods for the giant clam Tridacna noae. Three culture methods (intensive, semi-intensive, and extensive) using two tank designs (60 L and 277 L) were successful in producing 30-day old T. noae juveniles. There was no statistically significant interaction between tank design and culture method when considering cumulative survival (P = 0.89) or antero-posterior growth (P = 0.20), and the two tank designs did not influence cumulative survival (P = 0.78) or antero-posterior growth (P = 0.81). Differences in cumulative survival among the extensive (0.027 ± 0.015%), semi-intensive (0.005 ± 0.002%), and intensive (0.004 ± 0.001%) culture methods were non-significant (P = 0.11). The mean antero-posterior measurements at 30 days post-fertilisation among intensive (583.57 ± 12.71 μm), semi-intensive (530.92 ± 11.02 μm), and extensive methods (541.67 ± 10.95 μm), were also non-significant (P = 0.05). Further analysis of survival among different developmental stages identified significantly greater survival of T. noae between the pediveliger and juvenile development stages within the extensive culture method. Our results show that extensive culture methods can achieve comparable hatchery production of T. noae to semi-intensive and intensive culture methods, which require additional managerial, infrastructural, and nutritional inputs.

To validate the usability of the giant clam shell as a recorder of short-term environmental changes such as typhoons, we collected a live Tridacna maxima from Okinotori Island, Japan, on 15 June 2006. Growth increment thickness, stable isotope ratio (O-18(shell), C-13(shell)), and the barium/calcium ratio (Ba/Ca) in the T.maxima shell sample were measured and compared to Okinotori Island instrumental environmental data. In the outer layer of the shell sample, there were 3656 growth increments per year, as estimated by the O-18(shell) profile compared with sea surface temperature. The growth increments in the specimen were formed daily, and thus, we can determine the date of the sampling points of O-18(shell), C-13(shell) and the Ba/Ca ratio by counting growth increments. After typhoon approach, there is a decrease in increment thickness and some disturbed growth increments. The positive peaks in the shell Ba/Ca ratio and O-18(shell) corresponded to lower sea surface temperature caused by typhoons. These results indicated that the microstructural and geochemical record in Tridacna maxima shells could be useful for detecting past typhoon events.

The boring giant clam (Tridacna crocea) is the smallest species of the Tridacna genus, but they strongly express mantle color polymorphism. These color morphs represent potential genetic resources for aquaculture to supply the aquarium market, and the evaluation of their genetic and production characteristics is important. Wild yellow-green clams were consistently larger than blue clams in both the Huangyan Island and Zhaoshu Island populations in the Southern China Sea. To determine the relationship between mantle color and phenotypic traits, blue, yellow-green and control strains (various colors) were constructed and cultured for 1.5 years at an experimental station in Sanya, Hainan Island Province. All three strains had a high fertilization rate, but the survival to D larvae of the blue strain was significantly lower than the other two strains. Larval survival rate was over 85 % for all three strains, but the metamorphosis rate of the control strain was significantly lower than that of the other two strains. During the grow-out stage, the survival rate of the control strain from day 120 to day 480 was highest and that of the blue strain was lowest of the three groups. After metamorphosis, growth divergence of the three strains varied until day 120. Consistent growth trends appeared from day 120 to day 180, with the yellow-green strain largest, followed by the control strain. The process of mantle coloration was divided into three stages: the non-coloring period (day 0–60), the coloring period (day 60–180), and the stable expression period (day 180 onward). The yellow-green and blue mantle colors are genetically stable. This study provides a base for analysis of mantle color heredity and the genetic breeding of giant clams, and has great potential applications in the aquaculture industry, aquarium market and the coral reef tourism.

The giant clam Tridacna noae is a recently resurrected taxon distinguished from Tridacna maxima on the basis of reproductive isolation, mitochondrial DNA differences, and mantle morphology and ornamentation; however, the morphological characteristics used to distinguish the two species are not consistent with genetic assignment throughout their zone of overlap. This has led to the suggestion that hybridization occurs and constituted a need to reexamine the reproductive isolation of T. noae and T. maxima. This study provides evidence that sympatric populations of T. maxima and T. noae in the center of their range overlap and can hybridize and produce juveniles. Breeding trials using T. maxima and T. noae for both maternal and paternal hybrid crosses were conducted to compare larval and early juvenile development. The fertilization success and survival of both hybrid crosses suggest potential for T. maxima and T. noae hybrids to exist in nature. On this basis, the previously reported reproductive isolation between T. maxima and T. noae may not be apposite across their zone of overlap. Future genetic, population demographic, and ecological studies should consider the possibility of hybridization between T. maxima and T. noae.

Giant clams are in symbiotic relationship with the photosynthesizing zooxanthellae and dwell in the shallow coral reefs of the Indo-Pacific region where they may be exposed to fluctuating salinitie...

Territorial farmer damselfishes are facilitators that selectively farm turf algae in degraded reef patches. The effect of damselfish on algae and corals has been widely studied but the ability of the damselfish to establish farms on other substrates is less understood. In the giant clam ocean nursery in Bolinao, northwestern Philippines, farmer damselfishes, particularly Stegastes nigricans, Dischistodus prosopotaenia, and D. chrysopoecilus, make use of giant clam shells as substrate for turf algae farms. To facilitate growth of the algal crop, damselfishes reduce shading and create more space for cultivation by trimming the overhanging mantles of Tridacna gigas within their colonies. This form of habitat modification occurs within weeks, with affected clams exhibiting mantle lesions and turf algae growth on their shells. These observations suggest that damselfishes can potentially extend their farmed turf algae patches in a manner that, although not typical of natural reef conditions, may be facilitated by the availability of giant clam shells as substrate. The versatility of damselfishes in establishing turf algae farms on a different substrate and their ability to condition the habitat through active modification of other reef biota emphasize the potential of the farmer damselfish to significantly influence reef community structure and biodiversity.

Giant clam population has been decreased in a few years. Resource management requires information from various aspects, such as ecological, population, and other aspects. This study was aimed at assessing the genetic profile of Tridacna giant clam in Wakatobi National Park waters using Cytochrome oxidase subunit I (COI) genetic marker. Sample collection was conducted around the three main islands, i.e., Wangi-wangi, Kaledupa, and Tomia. Genetic analysis using COI gene may contribute in identifying giant clams up to the species level and showed the relationship among species. The research found 41 specific nucleotide sites for the clams . T. crocea, T. squamosa and T. maxima had 2, 15 and 24 sites, respectively. COI gene as a biological marker was able to separate groups of giant clam by species. Nucleotide variation of T. crocea from Wakatobi was the highest among other locations, so it could be used as a genetic source for translocation and domestication. Keywords: c ytochrome ox i dase subunit I , specific nucleotide , Tridacna, Wakatobi National Park

The biodiversity of the Red Sea remains relatively understudied, particularly for invertebrate taxa. Documenting present patterns of biodiversity is essential for better understanding Red Sea reef ecosystems and how these ecosystems may be impacted by stressors (such as fishing and climate change). Several species of giant clams (genus Tridacna) are reported from the Red Sea, although the majority of research effort has occurred in the Gulf of Aqaba. We investigated the genetic diversity (16S rDNA) of the Tridacna species found in the central Saudi Arabian Red Sea. We also investigated the genetic diversity (ITS rDNA) of symbiotic dinoflagellates Symbiodinium associated with these clams. Samples were collected from nine reefs on a cross-shelf gradient near Thuwal, Saudi Arabia. Two species, T. squamosa and T. maxima, were recorded, with the latter being the most abundant. Tridacna squamosina, a species recently reported in the northern Red Sea, was not found, suggesting that this species is not present or is very rare in our study region. All tridacnids sampled were found to harbor Symbiodinium grouped in Clade A, considered an opportunistic, heat-tolerant symbiont group in anemones and corals. The consistent association with Clade A Symbiodinium in central Red Sea tridacnids may reflect the consequence of adaptation to the relatively extreme conditions of the Red Sea. This study contributes to an ever-growing catalog of Red Sea biodiversity and serves as important baseline information for a region experiencing dynamic pressures.

The giant clam, Tridacna gigas, is an important faunal component of reef ecosystems of the Indo-Pacific region. In addition to its ecological role, shells of this bivalve species are useful bioarchives for past climate and environmental reconstructions. However, the biomineralization processes involved in shell aragonite deposition are insufficiently understood. Here, we present a study of the shell microstructure of modern specimens from Palm Island, Great Barrier Reef (GBR), Australia, and Huon Peninsula, Papua New Guinea (PNG), using a combination of petrography, scanning electron microscopy, electron backscatter diffraction, Raman spectroscopy and stable carbon isotope ratios. Daily growth increments were recognizable in all specimens through ontogeny, and counting these growth lines provides a robust specimen age estimate. For the internal layers, paired increments of organized aragonitic needles and compact, oblong crystals were recognized in a specimen from PNG, whereas specimens from GBR were composed of shield-like crystals that were not definable at the microscale. The combination of nutrient availability, rainfall and solar irradiance are likely to be the most significant factors controlling shell growth and may explain the observed differences in microstructure. The external layer, identical in all specimens, was composed of dendritic microstructure that is significantly enriched in 13C compared to the internal layer, suggesting different metabolic controls on layer deposition. We propose that the mineralization of the internal and external layers is independent from each other and associated with the activity of specific mantles. Future studies using T. gigas shells as bioarchives should consider the microstructure as it reflects the environment in which the individual lived and the differences in mineralization pathways of internal and external layers.

Restocking of giant clams, particularly Tridacna gigas, aims to reestablish breeding populations that have long been decimated by overexploitation. In Camiguin, southern Philippines, T. gigas juveniles that were restocked in 2001 have reached reproductive maturity and recruits have been observed on the reefs. To help document T. gigas recruits in Camiguin, the Cantaan Centennial Multi-purpose Cooperative (CCMPC) initiated a program to encourage local fishers to report any sightings of giant clams, in the surrounding reefs. The fishers would mark the location of any giant clam recruits they encountered during their regular fishing trips using makeshift buoys. Personnel from CCMP would then visit the marked sites to collect GPS data. The giant clams would later be relocated to an ocean-based nursery at the CCMP marine protected area (MPA) to ensure their protection. A total of 26 T. gigas recruits with a mean shell length of 17.8 ± 1.5 cm were found 0.2 km–2.3 km away from the restocking site between 2015 and 2017. Twenty-three of these individuals were reported by fishers while 3 were encountered during field surveys. It is highly probable that these T. gigas recruits are the progeny of the restocked individuals because there are no local populations of wild T. gigas in the area and no artificial spawning events had been conducted at the site. These results suggest that the reefs near Camiguin are suitable for giant clam recruitment and indicate that the recruitment envelope of T. gigas can extend up to several kilometers. Importantly, these findings highlight the value of a strong commitment by local organizations and collaboration with fishers as citizen scientists to enhance the sustainability of giant clam conservation efforts.

Veliger larvae of giant clams are demonstrable planktotrophs and provision of a particulate food source is generally accepted to improve hatchery production. However, the appropriateness of continuing to administer particulate food after larvae settle and develop as pediveligers is unclear. This study evaluated whether larval (age and size) and microalgal (species and cell density) factors influenced the capacity for pediveligers of Tridacna noae to ingest and digest cells from commercially available microalgae concentrates. Pediveligers at 6 and 10-days post-fertilisation were offered one of four microalgae species (Isochrysis sp., Pavlova sp., Tetraselmis sp., Thalassiosira weissflogii) at two cell densities (4000 and 8000 cells mL−1) and subsequently examined using epifluorescence microscopy. The appropriateness of continuing to administer microalgae as a food source was found to diminish as pediveligers approached metamorphosis, with negligible ingestion occurring at 10-days post-fertilisation. Earlier in development, at 6-days post-fertilisation, ingestion was significantly influenced by larval size, microalgae species, and microalgae cell density. Considering capacity for both ingestion and digestion, the Isochrysis sp. and Tetraselmis sp. microalgae concentrates had the greatest potential as a particulate food source for pediveligers of T. noae. The results of this study provide a basis for further refinement of giant clam hatchery procedures utilising microalgae concentrates as a larval food source.

In an effort to develop molecular tools for giant clam species identification, this study reveals the usefulness of a multiplex species-specific PCR for the reliable, rapid and low-cost identification of five giant clam species found in the South China Sea: Hippopus hippopus, Tridacna maxima, Tridacna derasa, Tridacna squamosa and Tridacna noae. We used the mitochondrial COI gene for marker development. Five reverse primers and a common forward primer were designed to generate five different size species-specific PCR fragments. These six primers were combined in a multiplex PCR to enable identification of five giant clam species in a single reaction. This molecular approach provides a simple and rapid identification of the giant clam species from the South China Sea, thus increasing the efficiency and quality of giant clam stock management, protecton and forensic identification by reducing the risk of wrong species identification.

This paper reports on embryonic and larval development of the giant clam Tridacna noae. Spawning was induced by serotonin injection into the gonad. Unfertilized eggs had a mean (±SE) diameter of 101.14 ± 0.47 µm and spermatozoa heads were 8.92 ± 0.09 µmlong. Embryonic development had progressed to the 8-cell and 16-cell stages by 3 h postfertilization and to the 32-cell stage by 4 h postfertilization. Rotating gastrulae accounted for 82% of developing embryos at 9 h postfertilization. Trochophore larvae accounted for 54% of embryos at 16 h postfertilization, and 98% of larvae at 20 h postfertilization. Straighthinged (D-stage) veligers comprised 74% of developing larvae at 22 h postfertilization with mean (±SE) anteroposterior measurement (APM) of 146.32 ± 0.58 µm, dorsoventral measurement (DVM) of 118.12 ± 0.74 µm, and hinge length of 77.46 ± 0.73 µmat 24 h postfertilization. At 96 h postfertilization, 74% of veligers had settled but had retained the velum without showing development of the foot...

Giant clams (tridacnid shellfishes) contain threatened species and represent important fishery resources. They establish a symbiotic relationship with zooxanthellae (Symbiodinium) by receiving photosynthetic products from these symbiotic algae. Symbiodinium are divided into various phylotypes (A–I); each physiological character attributed to a certain phylotype may be different and may affect infectivity with clams. However, lack of information related to Symbiodinium diversities in giant clams causes difficulties in the culture of these clams, especially during the artificial infection process of Symbiodinium sources to clam larvae, which leads to an extremely low success rate in establishing infection. In this report, we have developed a multiplex PCR method, suitable for analyzing Symbiodinium phylotypes in giant clams. We have designed new specific PCR primer sets for three phylotypes (A, C and D) that give different product sizes and thus are distinguishable from each other on an electrophoresis gel. The method is reliable, highly sensitive and most practicable for aquaculture using one PCR reaction. We also performed a trial case of the method for extracted Symbiodinium DNA from clams and revealed an unexpected heterogeneity of phylotypes even though the clams tested were kept in the same pond.

We report these first in situ observations of the Noah’s giant clams, Tridacna noae from Christmas Island. On two separate occasions during diving in Christmas Island, we found five unique individuals of T. noae, which were first identified by their mantle ornamentation patterns. These preliminary observations expand geographic range westwards from South China Sea to East Indian Ocean, and are compared with known information on current T. noae geographic distribution.

Giant clams are able to grow in oligotrophic tropical marine environments with the aid of photosynthates from its symbiotic zooxanthellae. Therefore, light intensity affects the survival and growth of giant clams, and is one of the most important environmental factors in giant clam cultures. In this study, two size classes of Tridacna crocea (small: ~50 mm shell length, large: ~85 mm shell length) were cultured at three light intensities (5000, 10,000, and 15,000 lx) for 16 weeks, to evaluate the effects of light intensity on the survival, growth performance and biochemical composition of different size clams. T. crocea under 15,000 lx light intensity showed a significantly higher survival rate and relative growth rate (RGR) (P

Giant clams have been over-exploited throughout their wide geographical range in the Indo-Pacific. Marine reserves could be one tool to potentially safeguard broodstock, but their effect on giant clam abundance and the differential benefits of lagoon reserves compared to barrier reef reserves is still unclear. We assessed abundances of four species of giant clams across 20 barrier reef and 30 lagoon reef sites across 600 km of coastline in New Caledonia using stratified underwater visual censuses. Tridacna maxima was the most common species. T. derasa was significantly more abundant in marine reserves than at sites open to fishing, but there was no effect of marine reserves on total abundance pooled across the four species. T. squamosa was significantly more abundant on barrier reefs, while Hippopus hippopus was found solely on lagoon reefs. National data from fisher logbooks shows a marked reduction in commercial catches of giant clam following implementation of daily catch limits of 5 giant clams per vessel per trip. Despite small daily bag limits of 2 giant clams per recreational fisher per trip, the recreational catch appears to be significant and much greater than the commercial catch. The utility of marine reserves on giant clams appears to be species specific, and might be diminished for the group as a whole by the concomitant protection of their natural predators. Both lagoon and barrier reef sites should be included in reserve networks. Our data suggest that daily boat/trip limits are an effective output control, at least for commercial fishers, but might not safeguard giant clams if recreational fishers are numerous. Further fishery restrictions on the less common species might be needed to ensure their persistence in the Indo-Pacific.

Inbreeding depression is common, especially in hermaphroditic animals, and causes a decline in biological fitness. To evaluate the effects of inbreeding on giant clams, two by two factorial cross experiments between individuals were conducted on three species, Tridacna derasa, T. squamosa, T. crocea. Thus, self-fertilized progeny (f = 0.5) and cross-fertilized progeny (f = 0) were produced for each species. The effects of inbreeding depression on fertilization rate, hatching rate, progeny growth and survival were slight for all three species. However, heavy inbreeding depression was observed on the larval metamorphosis rates of T. squamosa and T. crocea, and very few self-fertilized juveniles from these two species were successfully obtained. Surprisingly, inbreeding depression of performance traits was not found in T. derasa. Our results show that medium (T. squamosa) and small type (T. crocea) giant clam are prone to serious inbreeding depression (δ>0.99) at the larval metamorphic stage, while little inbreeding depression occurred in the large type giant clam (T. derasa). Two-factor ANOVA of survival rates for the whole experimental period showed that progeny survival of medium and small type giant clams was always affected by the mating strategy. Almost no inbreeding depression on growth traits was found these species, which may be due to the nutritional strategy of these autotrophic species. Our study proposes a theory regarding the magnitude of inbreeding depression in different giant clam species based on the evolution of the wild populations and provides a practical account of a mating strategy suitable for seed production for giant clam aquaculture.

The boring giant clam, Tridacna crocea, is distributed along coasts in the South China Sea, and has formed several geographic populations. These populations represent potentially different genetic resources for aquaculture, and the evaluation of their genetic and production characteristics is important. To determine these performance characteristics, complete diallel crosses between two geographical populations with about 600 km distance, Zhaoshu Island and Huangyan Island, were conducted using pooled gametes at an experimental station in Sanya, Hainan Island Province. Two intra-population crosses (ZZ and HH) and two reciprocal inter-population crosses (ZH and HZ) were carried out successfully three times. High fertilization rates and D larvae rate were observed in all the experimental groups, suggesting that there was no sperm-egg recognition barrier between the two geographic populations. Reciprocal hybrids had a higher survival rate than the pure populations, but with the exception of metamorphosis rate there were no significant differences between the inter- and intra-population crosses. Growth heterosis was evident in both the spat and adult stages, and was mainly influenced by egg origin and mating strategy in the adult stage. In view of the growth differences between the two pure populations' progeny, the Huangyan Island population's progeny was more suitable for giant clam aquaculture than the Zhaoshu Island population's progeny. Our results demonstrate that crossbreeding between the Huangyan Island and Zhaoshu Island populations of boring giant clams can produce considerable heterosis, which has promising prospects for application in the giant clam aquaculture industry in the South China Sea.

Interspecific hybridization has been used as an important tool for genetic manipulation in mollusks, with the aim to produce a new species with desirable traits from both the parental species. A two by two factorial cross between sympatric giant clam species, the fluted giant clam, Tridacna squamosa (S) and the boring clam, Tridacan crocea (C), yielded hybrids that were fast growing (characteristic of the S) and had brightly colored mantles (characteristic of C). Reciprocal hybrids (SC and CS) were obtained by the acceptable level of fertilization and hatching rate compared to these of two parental species, and the survival rate of the hybrids was significantly higher than that of the parental species. Yearling hybrids exhibited fully developed gonads and normal gametogenesis when sexually mature. Relative to CC, CS and SC also showed significant growth advantages in shell length and wet weight. The shell morphology of the hybrids was mainly dictated by maternal effect, while mantle coloration was dictated by paternal effect for SC progeny and parental effect for CS progeny. Our results revealed that reciprocal hybrids exhibited great growth heterosis and survival advantage (compared to C), as well as better mantle colors. Thus, these hybrids have great potential for application in the aquarium market and aquaculture industries across the globe.

The present study is an attempt to assess the effects of contamination of several sites in the Red Sea coasts of Saudi Arabia using bivalves as a biomonitoring tool. Oxidative stress biomarkers (including reduced glutathione level (GSH), glutathione-S-transferase activity (GST), Malondialdehyde level (MDA) and Catalase activity (CAT)), neurotoxicity acetylcholinesterase activity (ACHE), and genotoxicity micronucleus rate (MN) were measured in three distinct tissues - digestive glands, gills and mantle - of specimens of the giant clam Tridacna maxima, collected from five sites in Saudi Arabian Red Sea coast (Al-Khuraybah, Al-Wajh, Yanbu, Rabigh and Thuwal). Our results demonstrated that T. maxima showed differential biomarker responses according to the nature of pollutants and human activity that affect the coast. This study can be considered as the first one using biomarkers to assess the state of the Red Sea coast in Saudi Arabia which must be followed by periodic studies for surveillance of aquatic pollution.

Epifluorescence microscopy has numerous applications in molluscan research. Methods rely on distinguishing target fluorescent signals from natural fluorescence, termed autofluorescence, of a specimen. Limited research has been conducted on the autofluorescence of marine bivalves, which hinders development of epifluorescence microscopy techniques in this sector. We examined autofluorescence in larvae of the giant clam Tridacna noae to identify challenges and opportunities in applying epifluorescence microscopy to the study of giant clams. Under blue-light excitation (λex = 460–490 nm) three sources of autofluorescence were identified in larvae of T. noae: (1) green autofluorescence of the gut region; (2) green autofluorescence of the ‘dark bodies’ anatomical structure; and (3) red autofluorescence of ingested zooxanthellae, Symbiodinium species. Each of these sources of autofluorescence varied in prevalence and relative intensity over the course of larval development. The multiple sources and varied wavelengths of the observed autofluorescence presents challenges for applying fluorescent labelling techniques. However, our study identifies epifluorescence microscopy as a novel means of examining larval ontogeny and the uptake of zooxanthellaein giant clams.

Coral reefs are diverse ecosystems relying on the presence of dinoflagellates (genus Symbiodinium), that are found in symbiotic association with multiple phyla and performing the majority of primary production. However, coral reefs are currently threatened by climate change and the increase in seawater temperature, which causes the bleaching phenomenon. While bleaching has been well documented for adult host organisms, it is still poorly understood in larval stages. We offered Symbiodinium types within clades A–F to the larvae of Mussismilia hispida (scleractinian coral), Berghia stephanieae (nudibranch) and Tridacna crocea (giant clam) and manipulated the temperature to 26, 29 or 32 °C. Samples were taken at 0, 12, 24, 48 and 72 h post-temperature increase, chlorophyll-a (chl-a) was extracted and its content measured in a fluorometer. Symbiodinium type, temperature and time all influenced chl-a content. M. hispida larvae displayed a bleaching threshold at 29 °C; larvae containing Symbiodinium A–F all bleached at 32 °C, but with significantly lower bleaching in larvae associated with type A1. B. stephanieae digested the symbionts; while chl-a content decreased over time equally for all clades, it is not possible to determine if it is related to bleaching. T. crocea larvae at 29 °C bleached for all symbiont types, except for A1. At 32 °C, all types were bleached, but type A1 bleached significantly less. These findings show that type A1 seems to be more thermo-tolerant in larvae of the tested species. This may be related to the fact that strains within this clade are homologous to both M. hispida and T. crocea, as they are found within these adult host’ tissues. Therefore, symbiont type may have an important role in invertebrate larvae development and present relevant implications for recruitment.

The effectiveness of Marine Protected Areas (MPA) to manage natural resources has been undermined in small insular lagoons due to massive mortalities triggered by climatic events that have hit some lagoons but not others. To minimize the future risk of ineffective management efforts, it has previously been argued that management should focus on a multi-island conservation target (regional scale), rather than on individual lagoons (local scale). However, it is unclear how a MPA network designed to meet objectives at a regional scale would impact on the management of resources at the local scale. In particular, it is necessary to understand if a regional plan might incidentally maintain conservation objectives at the local scale, without disproportionately affecting, or relying on particular islands. This study used the population of the giant clam ( Tridacna maxima ) in a fishery context to explore the distributions of conservation features and socio-economic costs for regional networks (computed within a set of islands), compared to individual islands. Designing a MPA network at regional scale led to unbalanced representation of conservation features among atolls and incidentally missed the targeted level of protection for conservation features at local scale. Moreover, the regional network generated inequitable costs for fishermen between islands, which is likely to lead to poor perceived equity. This study suggests that perceived equity and the representation of local conservation objectives will be major factors to consider, if the French Polynesian authorities follow the path of implementing MPAs in each atoll for a regional-scale resource management plan.