Clam Populations Research Articles

Effects of high water temperature on physiology, survival, and resistance to high temperature air-exposure in the Manila clam Ruditapes philippinarum

Ruditapes philippinarum is a typical burrowing shellfish, living in the intertidal zone. In natural conditions, the mortality of R. philippinarum is most affected by high water temperatures, high temperature air-exposure, and other environmental stresses. In this study, the mortality rates of the two populations of R. philippinarum under high water temperature stress were recorded, and catalase (CAT), superoxide dismutase (SOD) and total antioxidant capacity (T-AOC) antioxidant enzyme activities in the hepatopancreas were analyzed. The results showed that the survival times of cultured clams were longer than those of wild clams after acute high temperature stress. CAT, SOD, and T-AOC activities increased after acute high water temperature and high temperature air-exposure stress. These antioxidant enzyme activities gradually decreased to their initial levels after 2 days of recovery from these high temperature stresses. Based on these experimental results, we found that the cultured clam population had better heat and high temperature air-exposure resistances than the wild clams. CAT, SOD, and T-AOC enzymes play an important role in the antioxidant processes of R. philippinarum in response to high water temperature and high temperature air-exposure. This study provided a theoretical basis for the development of healthy aquacultural practices for these shellfish.

Rebuilding A Collapsed Bivalve Population, Restoring Seagrass Meadows, and Eradicating Harmful Algal Blooms In A Temperate Lagoon Using Spawner Sanctuaries

During the past century, bivalve populations across the globe have collapsed, resulting in negative ecosystem consequences due to their outsized impact on shallow estuaries. In response, there has been strong interest in the restoration of marine bivalve populations. Here, we present a decade-long restoration effort that sought to rebuild a collapsed (99.5% reduction in harvest) and recruitment-limited population of hard clams (Mercenaria mercenaria) in Shinnecock Bay, NY, USA, using spawner sanctuaries: no-harvest zones where adults were planted at high densities (~27 m-2). Between 2012 to 2019, more than 3.2 million clams were planted in 64 discrete sanctuary plots (~1,850 m2each) located in zones with maximal larval retention and sediment and seawater characteristics that would maximize the conditioning and spawning of adults. Hydrodynamic models, quantification of hard clam larvae, and spatial recruitment patterns demonstrated larvae spawned within sanctuaries were transported to regions where clam densities significantly increased 18-fold over seven years (2015-2021;p<0.001) and harvests significantly increased more than 16-fold over nine years (2012-2021;p<0.0001). Increases in populations and harvests were caused by smaller clams recruited within the time frame of the creation of spawner sanctuaries. Higher clam densities caused biological filtration times of the bay to decrease from up to three months at the start of the project to as low as 10 days in 2021. Concurrently, concentrations of the harmful brown tide alga,Aureococcus anophagefferens, and chlorophyllasignificantly decreased (p<0.005) while water clarity and the extent of seagrass beds significantly increased (p<0.05). Increases in clam landings and improvements in water quality were not observed in adjacent lagoonal estuaries where restoration did not occur. Given these outcomes and the global need for rebuilding marine life, the implementation of spawner sanctuaries using the criteria set forth herein may be a promising approach for restoring hard clam and other bivalve populations in estuaries elsewhere.

Effects of larval trematode parasitism on the reproductive capacity of Manila clam Ruditapes philippinarum in a sandy-mud tidal flat on the west coast of Korea

Gonad intrusion of the larval trematodes often results in substantial damage to the reproductive capacity of some economically important marine bivalves. Such detrimental impacts of the larval trematode infection are observed in Manila clam Ruditapes philippinarum in the coastal Yellow Sea, although the effects on the host’s reproduction are poorly understood. Accordingly, this study attempts to understand the impacts of parasitism on clam reproduction using histology. Of the 1,255 adult clams collected in early June from a tidal flat on the west coast of Korea, 68 individuals (5.4%) were infected by three different larval trematodes, including Cercaria tapidis, Cercaria pectinata, and Parvatrema duboisi. Histology revealed that the uninfected clams were in the late developing or ripe stage, exhibiting fully developed gametes in the expended follicles. In contrast, clams infected by the larval trematode showed retarded gonad maturation. Some of those clams remained sexually indifferent (13/68 infected clams) or early developing (9/68 infected clams). We categorized the infection level into uninfected (0), light (1), moderate (2), heavy (3), and very heavy (4), according to the area of sporocysts occupying the histology section. The image analysis indicated that approximately 50% of the infected clams were in “heavy” and “very heavy” conditions, and they occupied 16.7% and 26.9% of the total cross-section area, respectively. The nonparametric Kruskal–Wallis’s test and the post-hoc Tukey’s test indicated that the condition index (CI) of clams in heavy and very heavy infection were significantly lower than uninfected clams. In conclusion, the larval trematodes infected about 5.4% of the clam population in the tidal flat on the west coast of Korea, and we estimated that 32.4% of the infected clams might fail to reproduce during a spawning period due to the severe infection.

Abstract PR017: Progression and regression dynamics of bivalve transmissible neoplasia in the soft-shell clam (Mya arenaria) after both natural and experimental exposure

Abstract In nature, cancer is not typically contagious – cancer lives and dies within a host. However, there are three known forms of clonal cancer lineages, in which the cancer cells themselves spread throughout populations by “jumping” from one animal to another. In mammals, there are two species known to be affected by these transmissible cancers – Tasmanian devils (Tasmanian Devil Facial Tumor Disease) and dogs (Canine Transmissible Venereal Tumor). In bivalves, transmissible cancers are found in at least six different species around the world, in which there are at least seven independently derived clonal neoplastic lineages. Mya arenaria (the soft-shell clam) is the first of these bivalve species confirmed to be affected by a Bivalve Transmissible Neoplasia (BTN). BTN was originally described as disseminated neoplasia when first documented in the 1970s. This disease is characterized by high numbers of rounded, polyploid, non-adherent neoplastic cells in the circulatory fluid (hemolymph) of animals that later disseminate into tissues during the final stages of disease, often proving fatal. In this emerging field, there is still much that is unknown regarding the dynamics of disease progression in vivo. I sought to define these dynamics for the first time. I analyzed hemolymph samples from both naturally infected and experimentally infected soft-shell clams, collected from Maine, USA, over the course of disease progression. Every two weeks, hemolymph was drawn from each animal, and the DNA was analyzed using a cancer-specific qPCR assay, which quantifies the copy number of a cancer-specific target (Steamer) and a control target (N1N2). I used the two values to determine the fraction of neoplastic cells found in the hemolymph of each animal at every two-week timepoint. I have observed three main disease outcomes for both naturally and experimentally infected animals - 1) progression to death; 2) regression; and 3) long-term non-progression. My observation of significant regression and non-progression in clams exposed to BTN contrasts earlier reports suggesting near complete lethality of the disease. This finding may suggest that clam populations have evolved resistance to this cancer. Interestingly, I additionally observed a consistent long latent period after injection in the experimentally infected animals where neoplastic cells were undetectable in the hemolymph for a period of 16-20 weeks before progression. Based on my findings of this latent period, where no cancer is detectable in the hemolymph of infected animals, I hypothesize that the cells may be engrafting into a solid tissue rather than solely dividing and circulating in the hemolymph itself as originally hypothesized. These findings are a significant starting point in understanding how BTN can transmit both within individuals and throughout populations, potentially providing remarkable insight into cancer evolution and host resistance. Citation Format: Rachael M. Giersch, Marisa A. Yonemitsu, Samuel F.M. Hart, Michael J. Metzger. Progression and regression dynamics of bivalve transmissible neoplasia in the soft-shell clam (Mya arenaria) after both natural and experimental exposure [abstract]. In: Proceedings of the AACR Special Conference on the Evolutionary Dynamics in Carcinogenesis and Response to Therapy; 2022 Mar 14-17. Philadelphia (PA): AACR; Cancer Res 2022;82(10 Suppl):Abstract nr PR017.

Abstract A003: Genome evolution of a transmissible cancer in the soft-shell clam (Mya arenaria)

Abstract A small number of transmissible cancer lineages exist in which cancer cells metastasize repeatedly to new hosts and live past the death of their original hosts. Transmissible cancers have been observed in the dog, Tasmanian devil, and most recently in numerous bivalves species, including one lineage that has spread through the soft-shell clam (Mya arenaria) population along the east coast of North America. We assembled a highly contiguous 1.2 Gb soft-shell clam reference genome and characterized the genome evolution of this transmissible cancer lineage. Our analyses confirm that all cancer samples descended from a single founder clam, but cluster in two geographically separated sub-lineages. In both sub-lineages, we observe high somatic mutation density, widespread copy number gain, structural rearrangement and transposable element activity, all indicative an unstable cancer genome. We also discovered a novel mutational signature generating mutations at a stable rate, predicting the cancer to be 344 to 877 years old and indicating it spread undetected long before it was first observed in the 1970s. Citation Format: Samuel F.M. Hart, Rachael M. Giersch, Marisa A. Yonemitsu, Brian Beal, Stephen P. Goff, Michael J. Metzger. Genome evolution of a transmissible cancer in the soft-shell clam (Mya arenaria) [abstract]. In: Proceedings of the AACR Special Conference on the Evolutionary Dynamics in Carcinogenesis and Response to Therapy; 2022 Mar 14-17. Philadelphia (PA): AACR; Cancer Res 2022;82(10 Suppl):Abstract nr A003.

Kelp (Saccharina latissima) Mitigates Coastal Ocean Acidification and Increases the Growth of North Atlantic Bivalves in Lab Experiments and on an Oyster Farm

Coastal zones can be focal points of acidification where the influx of atmospheric CO2 can be compounded by additional sources of acidity that may collectively impair calcifying organisms. While the photosynthetic action of macrophytes may buffer against coastal ocean acidification, such activity has not been well-studied, particularly among aquacultured seaweeds. Here, we report on field and laboratory experiments performed with North Atlantic populations of juvenile hard clams (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), and blue mussels (Mytilus edulis) grown with and without increased CO2 and with and without North Atlantic kelp (Saccharina latissima) over a range of aquaculture densities (0.3 – 2 g L-1). In all laboratory experiments, exposure to elevated pCO2 (>1,800 µatm) resulted in significantly reduced shell- and/or tissue-based growth rates of bivalves relative to control conditions. This impairment was fully mitigated when bivalves were exposed to the same acidification source but also co-cultured with kelp. Saturation states of aragonite were transformed from undersaturated to saturated in the acidification treatments with kelp present, while the acidification treatments remained undersaturated. In a field experiment, oysters grown near aquacultured kelp were exposed to higher pH waters and experienced significantly faster shell and tissue based growth rates compared to individuals grown at sites away from kelp. Collectively, these results suggest that photosynthesis by S. latissima grown at densities associated with aquaculture increased pH and decreased pCO2, fostering a carbonate chemistry regime that maximized the growth of juvenile bivalves. As S. latissima has been shown to benefit from increased CO2, growing bivalves and kelp together under current or future acidification scenarios may be a synergistically beneficial integrated, multi-trophic aquaculture approach.

Effect of Predator Exclusion on Softshell Clam (Mya arenaria L.) Recruitment

Despite estimates of very high (>99.9%) pre-settlement mortality, the extremely large numbers of eggs released into the plankton means that there are abundant post-larval softshell clams (Mya arenaria) available to settle and populate intertidal flats. Many seemingly suitable flats remain devoid of clams, presumably due to post-settlement predation. Predator exclusion/clam recruitment boxes (wooden frames covered top and bottom with a fine plastic screening) were set on a mudflat in Salem Harbor, Salem, MA from April to November 2019. Clam larvae were able to settle through the screen into the boxes but large (>1.9 mm) predators such as European green crabs (Carcinus maenas) and milky ribbon worms (Cerebratulus lacteus) were excluded. At the end of the study, four of five boxes contained between 121 and 290 juvenile softshell clams ranging in size from 7 mm to 32 mm shell length. A bimodal frequency distribution of sizes likely illustrates an early summer (June) set represented by a larger size grouping followed by a second late-summer set represented by a smaller size grouping. Small green crabs (9–46 mm carapace width) found in all five boxes must have grown from extremely small stage 1 crabs that were able to settle or crawl through the screening as recently settled individuals. Resulting empty clam shells could be paired to yield the equivalent of additional clams, making the total number per box between 147 and 417 individuals. No clams were found in samples outside of the boxes, indicating that the absence of a clam population on the mudflat is due to post-settlement predation and not pre-settlement mortality.

Population status and genetic diversity of two endangered giant clams (Tridacna squamosa and Tridacna maxima) on the fringing reefs of Perhentian Islands, Malaysia

Abstract With the increasing demand for giant clams in the ornamental trade as well as habitat destruction of coral reefs, giant clam populations have been threatened across the Indo‐Pacific. This study documents the population status and genetic diversity of giant clams in Perhentian Islands Marine Park (PMP), a Marine Protected Area (MPA) on the east coast of Peninsular Malaysia, plus Rhu Island, an adjacent island outside the MPA. Of the 13 reef sites surveyed across an area of 11,200 m2, two giant clam species were recorded: Tridacna squamosa and Tridacna maxima, with average densities of 1.5 ± 2.2 and 5.2 ± 6.0 ind. 100 m−2, respectively. The size‐class survey revealed a higher number of T. maxima recruits (88 recruits) as compared to T. squamosa (only three recruits), suggesting a disparity in recruitment in the area. The genetic diversity of T. squamosa (n = 83) and T. maxima (n = 104) was explored using the mitochondrial cytochrome c oxidase I (COI) and 16S rRNA gene markers. Interestingly, a higher genetic diversity was detected in COI than 16S for both species. No significant genetic differentiation was detected between the populations of PMP and Rhu Island, while a low but significant genetic structure was detected in both species across the sites of PMP (COI datasets, AMOVA, T. squamosa, FCT = 0.14, P < 0.05; T. maxima, FCT = 0.11, P < 0.05). In general, the results of this study revealed healthy giant clam populations in PMP, but the decline warrants urgent attention to integrating conservation strategies such as restoration programmes in conjunction with a sustainable giant clam fishery. Given the relatively high genetic diversity of T. maxima at Rhu Island, expansion of the current MPA is needed for better conservation coverage.

Fluted giant clam (Tridacna squamosa) restocking experiment in an urban turbid reef environment

Abstract Giant clam populations in Singapore are endangered due to historical exploitation, habitat loss, and sediment pollution. Transplanting cultured giant clams onto reefs is a potential conservation strategy. This study examined the growth and survival of three initial size classes (50.0–60.0 mm, 60.1–70.0 mm, >70 mm) of cultured juvenile fluted giant clams, Tridacna squamosa, transplanted to three reef sites under two caging treatments (caged and uncaged). After 145 days, there was no significant difference in clam survival within the size classes and within caging treatments across reef sites, even though environmental conditions varied greatly between sites. However, there was a significant effect of initial size class on T. squamosa survival, with higher survival with increasing size class (50.0–60.0 mm: 11.1%; 60.1–70.0 mm: 34.1%; >70.0 mm: 46.9%). Clams in caged treatments had significantly lower survival (16.7%) than uncaged clams (43.1%), likely due to biofouling atop caged treatments reducing light availability. Growth rates in the caged treatment (2.2 ± SD 1.8 mm month−1) were also lower than those in the uncaged treatment (3.9 ± SD 2.2 mm month−1). By optimizing transplant procedures for Singapore's turbid reef conditions, T. squamosa restocking efforts could play an important role in boosting local population numbers to facilitate natural recovery.

Enhanced understanding of physicochemical constraints on Corbicula japonica habitat in Lake Shinji assisted by machine learning

The catch of Corbicula japonica is one of the top three in Japan's inland water fisheries. Most of the Japanese fishing stock of this bivalve comes from Lake Shinji. Since the 1980's the catch of this species declined drastically with strong inter-annual variation. New recruitment and poor growth of the clams were thought to be the main reasons for this decline. This bivalve has a poor transportation ability after the settlement. Therefore, it is also important to know their suitable habit that is affected by multi interacting parameters of water qualities and sediments. In order to get a better understanding of their habitat, we used machine learning models to test abiotic limiting factors. The database that we used in our study included 337 sampling stations with 7 physicochemical variables and the Corbicula japonica counting which was divided into two size categories: small (shell length < 4 mm) and large (shell length ≥ 4 mm). Due to their low self-transportation ability, their survival is primarily influenced by the site environment, such as water quality and sediment conditions. To extract physicochemical thresholds that directly impact these clam populations, we applied a coupled methodology based on a random forest model and partial dependence plots.We highlighted that the use of three different populations groups for each size category, allowed us to significantly increase the accuracy of our model to predict groups. Moreover, our results show that the three most influential predictors were, in order of importance, the water depth, ignition loss of the bottom sediments and silt clay (diameter ≤ 0.063 mm) content of the bottom sediments; with, respectively, the following physicochemical limitations for the preferable habitat: 4 m, 10% and 45%. These limitations are based on the thresholds that we extracted from the coupling between a Random Forest model and the partial dependence plots. Our findings emphasize that this coupled methodology has the advantage of being able to manage the zero values of population and give graphical interpretation of the limiting environmental factors. Consequently, the results that we presented here clearly demonstrate a further step towards comprehension of the inter-annual variations in the fishery resources of Corbicula japonica after accounting for the impact of new recruitment. In addition, this kind of numerical tool could help the local government to manage the clam's population by restoring and conserving the water quality and sediment conditions of the lake.

Fitness‐related traits are maximized in recently introduced, slow‐growing populations of a global invasive clam

AbstractMany species are shifting their ranges and being forced to rapidly respond to novel stressful environmental conditions. Colonizing individuals experience strong selective forces that favor the expression of life history traits notably affecting dispersal and reproductive rates in newly invaded habitats. Limited information is currently available on trait variation within the invasive range despite being critical for understanding ecological and evolutionary factors that drive the process of range expansion of invasive species. Here we evaluated life history shifts in the widely introduced Asian clam Corbicula cf. C. fluminea within its invaded range. Through an exhaustive literature search, we obtained data for 17 invasive populations of this clam from different ecosystems worldwide. We tested the relationship between population and individual parameters relevant to the process of range expansion. Our main results are that recently introduced populations of clams were characterized by (a) low density and low rate of population increase, (b) earlier reproduction in slow‐growing populations of clams, and (c) no effect of density on population increase. All populations of the Asian clam analyzed in this study, which are fixed for one genotype (the lineage called Form A/R), experienced different selective environments in the introduced range. These findings support the perspective that adaptive phenotypic plasticity favored the expression of traits that maximize fitness in recently established populations, which faced stronger r‐selective forces relative to long‐established ones. We discuss the role of plasticity in facilitating rapid adaptation and in increasing the likelihood of populations to overcome difficulties associated with low densities and low population increase in newly invaded areas.

Comparative transcriptome analysis to reveal the genes and pathways associated with adaptation strategies in two different populations of Manila clam (Ruditapes philippinarum) under acute temperature challenge

Increasing temperature is causing rapid changes in oceanic environments. Understanding the responses of marine animals to such changes remains a major challenge, yet it is key to predicting future biodiversity loss or gain. In this study, we investigated molecular mechanisms of adaptation to acute temperature change in low and high latitude populations of the Manila clam Ruditapes philippinarum. Using transcriptome analysis, we studied temperature-responsive genes and pathways in clams cultured under acute temperature stress (−1°C and 28°C) and identified an extensive set of genes and pathways that respond to acute temperature stress. Physiological changes at the molecular level were compared to explore potential differences in the response of clams and their adaptation to temperature change and to uncover the underlying mechanisms of resilience or sensitivity to stress conditions in the two clam populations. The enrichment of genes involved in protein processing in the endoplasmic reticulum was integral to the adaptation of clams to stressful conditions. Furthermore, phagosome-related genes and pathways were significantly enriched under acute high temperature stress. The upregulation of xenobiotic metabolism by cytochrome P450 showed a population-specific response in the low-latitude clams under both heat and cold stress but not in the high-latitude clams, whereas genes encoding peroxisomes were upregulated in high-latitude clams under heat stress but not in low-latitude clams. This study provides useful information related to the response of R. philippinarum to temperature stress, which could be helpful for improving the temperature stress resistance of these clams in the aquaculture industry. Our results may also help predict future biodiversity trends under temperature change scenarios.

Spatial and annual distribution of age structure in the Tunisian populations of European clams Venerupis decussata

Spatial and annual distribution of age structure in the Tunisian populations of European clams Venerupis decussata

Spatio-Temporal Variation of Elemental Contamination and Health of Mya arenaria Clam in the Saguenay–St. Lawrence Marine Park

The impacts of pollution and long-term effects of local clam populations are misunderstood in estuaries. The purpose of this study was to follow inorganic contamination in tissues, changes of physiological health indicators, such as condition factor (CF), growth index (GI), resistance in air emersion and dehydration rate, for 5 years in Mya arenaria clams. The sampling scheme comprised one reference site, two sites impacted by human activity (thereafter polluted) and one site recognized as a Saint-Lawrence Estuary (SLE) beluga whale feeding area without known pollution source (Baie Sainte-Marguerite (BSM)). This study revealed that the elemental contamination profiles in clams were increased but differed between the polluted and BSM compared to the reference site. At polluted sites, clams were contaminated by Ag (2.4-fold of reference site), Mn (2.5-fold) and V (6.3-fold). With respect to BSM, clams were mainly contaminated by Ce (2.5-fold), Co (2-fold), Ga (2-fold), La (2.8-fold), Hg (2.5-fold), Ni (2.2-fold), Sm (2-fold) and V (20-fold). This contamination profile suggests sources of pollution from particulate combustion products of gasoline/diesel, crude oil and urban inputs of pollution. The CF, GI and air survival time were all reduced in clams at the polluted sites, while only the CF and dehydration rates were decreased and increased, respectively, at BSM. Long-term analysis revealed that CF and GI tended to decrease over time with episodes of strong amplitude changes and became more resilient to air survival time. In conclusion, the long-term contamination of clams towards metals and elements could compromise the health status of local clam populations. The increased contamination of clams at BSM could represent a risk to the endangered SLE beluga whale population.

Unravelling the role of local and large-scale factors in structuring sandy beach populations: the wedge clam Donax hanleyanus

Changes in the structure and dynamics of sandy beach macrofaunal populations result from the simultaneous action of local and regional factors acting synergistically. This is particularly noticeable in transitional interfaces between freshwater and marine ecosystems, where large-scale gradients affect local beach morphodynamics and resident fauna. The wedge clam Donax hanleyanus occurs on beaches with contrasting morphodynamics along the salinity gradient generated by the Rio de la Plata estuary. We conducted sampling surveys to determine a fine-grained comprehensive coverage of the species’ distribution in Uruguay to assess the concurrent effects of large-scale variations in salinity and local beach morphodynamics on wedge clam populations along 400 km (16 sandy beaches) over 2 yr. The main factor controlling wedge clam occurrence was salinity, overriding the effects of local habitat features. On beaches where the species was present, total abundance was higher on oceanic dissipative shores, which also harbored a higher abundance and relative representation of recruits. Recruitment was almost lacking in reflective beaches. However, biomass of adults with larger body sizes prevailed on reflective beaches, suggesting a scaling of abundance to body size and potential density-dependent effects. In this metapopulation, estuarine beaches are sinks, whereas oceanic beaches act as source habitats. We concluded that large-scale gradients generated by salinity variations translated into local population patterns (distribution, abundance and population structure) and processes (recruitment), which were secondarily driven by local morphodynamics. The relative importance of pre- and post-settlement processes across the morphodynamic spectrum from reflective to dissipative beaches has yet to be elucidated.

Spatial Variation of Soft Shell Clam Biochemical Health Status-Impacts on Age Pigments, Glucose and Melatonin Status

Clams are sessile organisms feeding at the sediment/water interface making them particularly at risk to urban/anthropogenic pollution. The purpose of this study was therefore to examine the health impacts of wild clam populations at 2 harbors in the Saint-Lawrence Estuary and on site in Saguenay Fjord (Québec, Canada) a site known for Beluga whale high residency area. Health status was examined at tissue levels for selected metallic elements, oxidative stress, energy metabolism and circadian rhythms (serotonin and melatonin levels). The data revealed that clams collected at the fjord site (Beluga residency) were the most contaminated by Ag, Al, Cd, Cr, Co, Cu Fe, Mn, Hg, Ni Pb, and Zn. The clams were physiologically older (age related pigments) showed signs of oxidative stress (peroxidase activity and lipid peroxidation) at most of the polluted sites and were strongly correlated to most of the above elements/metals. There was an increase in glucose, esterase and ketone bodies indicative of lipid mobilization for energy. Moreover, glucose levels were significantly correlated with known hyperglycemic metals such as Cd and Pb. Based on melatonin to serotonin ratio, clams at the most polluted site in Saguenay fjord and one marina site had elevated ratio suggesting altered circadian rhythm and were seemingly out of phase to the normal tidal schedule in this area.

Molecular and microscopic identification of Eomarteilia granula infection in Manila clam Ruditapes philippinarum off the south coast of Korea.

A report on the new species Eomarteilia (=Marteilia) granula infecting Manila clam Ruditapes philippinarum from Japan in 2014 suggests the possibility of E. granula infecting other Manila clam populations in the Northwest Pacific region, including Korea. In this study, we report the first infections by E. granula in Manila clams off the south coast of Korea. Histology revealed Marteilia-like plasmodia in the digestive tubule epithelia. Tissue imprints demonstrated that each parasite sporangium enclosed 4 spores and transmission electron microscopy (TEM) revealed ultrastructure of primary cells enclosing secondary cells, which contained spores. Mature spores consisted of 3 sporoplasms: outermost, intermediate, and innermost. The outermost sporoplasm showed a peripheral electron-dense monolayer characteristic of E. granula. The 18S rDNA amplified from the Marteilia-like parasite yielded 1784-bp PCR amplicon sequences which were 99.8% similar to that of E. granula previously reported (as M. granula) from Japan. In the molecular phylogenetic analysis, the novel Marteilia-like organism formed a well-supported clade with E. granula. Accordingly, we concluded that the novel Marteilia-like parasite that we found infecting some Korean Manila clams is Eomarteilia granula. Field surveys revealed that the infection was limited to clams of the south coast of Korea, with the prevalence ranging from 3.3 to 5.0%.

Evaluating the effect of nuclear inclusion X (NIX) infections on Pacific razor clam populations.

Nuclear inclusion X (NIX), the etiological agent of bacterial gill disease in Pacific razor clams Siliqua patula, was associated with host mortality events in coastal Washington State, USA, during the mid-1980s. Ongoing observations of truncated razor clam size distributions in Kalaloch Beach, Washington, raised concerns that NIX continues to impact populations. We conducted a series of spatial and longitudinal NIX surveillances, examined archived razor clam gill tissue, and used population estimates from stock assessments to test whether (1) the prevalence and intensity of NIX infections is higher at Kalaloch Beach relative to nearby beaches, (2) infected gill tissue has features consistent with historical descriptions of NIX-associated histopathology, and (3) annual clam survival is inversely related to NIX infection prevalence and intensity. NIX prevalence exceeded 85% at all sampled locations, and infection intensity was the highest at Kalaloch Beach by 0.9-2.6 orders of magnitude. Kalaloch Beach clams revealed histopathology consistent with previous NIX epidemics, including enlarged and/or rupturing branchial epithelial cells, branchial necrosis, and high hemocyte densities. Estimated annual survival was 22% at Kalaloch Beach, and ranged between 57 and 99% at other study sites. NIX infection intensity (via quantitative PCR) was not significantly correlated with annual survival; however, annual survival was lowest at Kalaloch Beach, where infection intensities were highest, suggesting that clams can tolerate infections up to a lethal threshold. Collectively these data support the hypothesis that high NIX intensities are associated with host mortality. NIX-associated mortality appears to be more pronounced at Kalaloch Beach relative to other Washington beaches.

Population structure of giant clams (subfamily: Tridacninae) across Palau: Implications for conservation

Abstract Giant clams (subfamily: Tridacninae) are an important food and economic resource for the Republic of Palau and the greater Indo‐Pacific region. However, giant clam diversity and distribution data for Palau are out of date. This study reports the species diversity and distribution of giant clams across the Palauan archipelago (total survey area of 3,3002 m) from data collected between 2015–2017. This is the first documented finding of Tridacna noae in Palau, however, it was rare. Only four individuals were found at only two locations. Results show that Palau is home to a diverse and abundant population of giant clams. Tridacna crocea (an important food and economic resource) exhibited the highest abundance, with an average of 20.0 ± 2.9 individuals per 50 m2. Tridacna maxima and Tridacna squamosa were ranked next in abundance. In contrast, T. noae, Tridacna derasa, Tridacna gigas, and Hippopus hippopus were found in low numbers or only found in few locations. The density of all recruits, juveniles, and adult T. crocea and T. maxima did not differ significantly between conservation areas and open fishing sites, which suggests the possibility that ecological factors such as habitat loss, acute weather events, or changes in sea surface temperatures could be impacting replenishment or recruitment. However, one protected area was found to have a higher abundance of T. crocea recruits and adults, which suggests that protection from fishing may have increased recruitment rates and lowered mortality rates at this site. Taken as a whole, clam populations in Palau remain reasonably abundant and healthy. As demand for giant clams continues to rise in Palau and the region, local regulations should focus on sustainable fishing practices by establishing size limits, species bans, and long‐term monitoring plans to maintain the diverse populations of giant clams found there.

Assessment of spatiotemporal variability of giant clam populations (Cardiidae:Tridacna) from 11 years of monitoring at Koh Tao, Thailand

ABSTRACTGiant clams (Tridacninae) are an ecologically important species in coral reef habitats across the Indo-Pacific. Numerous examples of giant clam population declines of varying degrees of severity have been documented since the 1970s. These have been attributed to several reasons, such as overexploitation in regional fisheries and ornamental trades, extreme weather events and anomalous marine warming events leading to bleaching. In Thailand, this has led to extensive conservation efforts, such as legal protections and population restocking. Despite these strong measures, to date no long-term studies have been conducted on giant clam populations in Thai waters. We provide results from 11 years (2009–2019) of giant clam population monitoring, at Koh Tao, an island with a well-documented history of coral reef-associated stressors as well as conservation efforts. Surveys were conducted across two depth ranges at 18 reef sites around the island, revealing contrasting trends. Our findings indicate a significant population decline of Tridacna crocea from coral reefs in the 6–8 m depth range, from 1.41 (±0.47) individuals/100 m2 in 2010 to 0.59 (±0.17) individuals/100 m2 in 2019, with, however, no significant change in T. squamosa populations at this depth range. Data from the 3–5 m depth range indicate no significant change in the T. crocea population over the years, but a population increase of T. squamosa from 0.78 (±0.18) individuals/100 m2 in 2009 to 2.07 (±0.38) individuals/100 m2 in 2019. Abundance estimates from these sites indicate extensive heterogeneity in giant clam populations around the island, and highlight the importance of sufficient spatial resolution in identifying population trends.