Adult Clams Research Articles

Investigation of Toxicity of the Combined Exposure of Microplastics and Arsenic (III) on Clams

Introduction: Microplastics is a new type of global pollutant that can absorb pollutants in the environment and enter the food chain. Arsenic (As) is a kind of heavy metal element, and its pollution to the environment has been concerned. Nowadays, the threat of both to Marine ecology is gradually increasing, and people are paying more attention to the harm of microplastics and heavy metal pollution to human health. Method: The aim of this paper is to study the adsorption of arsenic by microplastics and the toxic effects of the combination of the two on clams. The toxicity effects of microplastics and As (III) on clams were determined by joint toxicity test. The fatality rates caused by microplastics polypropylene (PP), polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA) with concentrations between 10 mg/L and 500 mg/L adsorbed with As (III) in first 48 h were 0-30.0%, 0-10.0%, 0-30.0%, 0-15.0% and 0-50.0% (larval clams); 0-35.0%, 0-25.0%, 0-30.0%, 0-50.0% and 0-15.0% (adult clams), respectively. Results: Compared with the group of As (III) alone, the fatality rates caused by PP, PE, PS, PVC and PMMA after 48 h increased dramatically to 80%, 62%, 40%, 60% and 70% (larval clams); 85%, 72%, 40%, 72% and 65% (adult clams), respectively. Conclusion: The death rate of clams was proportional to the input of microplastic particles containing As (III), that is, the higher the concentration used, the higher the fatality rates and the faster the death rates of clams.

The survival and growth of adult and juvenile hard clam Meretrix meretrix exposed to variable estuarine salinities in aquarium settings

Salinity fluctuations in estuaries can significantly impact marine organisms, leading to economic losses through mass mortalities of commercially important species. The effects of varying estuarine salinities on the survival, daily mortality, and growth of adult and juvenile hard clam Meretrix meretrix under controlled aquarium conditions were investigated. Adults (shell length ~45 mm) and juveniles (shell length ~30 mm) were collected from an 18 ppt estuarine habitat in Chowfaldondi, southeast Bangladesh, and exposed to six salinity levels (10, 15, 20, 25, 30, and 35 ppt) with three replications at ambient temperature. Adults were monitored for 51 days and juveniles for 23 days. Survival was significantly higher (≥80%) at 15–30 ppt for adults and 10–20 ppt for juveniles, with the lowest survival at 35 ppt. Analysis of co-variance showed that salinity had a greater effect on survival than the combined salinity*size model (p<0.05). While adult clams exhibited no mortality in first 15 days, juvenile mortality (~10%) began on day two across all salinities with a dose-dependent mortality exceeding 50–80% in 25–35 ppt. Adults were more tolerant to high salinities while juveniles to low salinities. Shell length and width did not vary significantly across salinities, likely due to insufficient plankton in the supplied seawater. This study has implications for potential site selection for coastal clam aquaculture along the Bay of Bengal. The sudden changes and combined effects of multi-stressors on bivalves can be assessed for better understanding how near future climate change can impact bivalve fishery and blue economy. Res. Agric. Livest. Fish. Vol. 11, No. 2, August 2024: 239-254

Reproductive conditioning, spawning induction and early development of the estuarine Caribbean clam, Polymesoda arctata (Deshayes, 1854), under hatchery conditions

Polymesoda arctata is a Caribbean estuarine clam commercially important that currently is considered threatened. The reproductive conditioning, spawning and early development were evaluated under laboratory conditions using organisms collected in the lagoon system of Ciénaga Grande de Santa Marta (Colombian Caribbean) between December 2020 and May 2021. Two reproductive conditioning experiments were conducted over 7 weeks, examining the frequency of mature animals in the natural environment (controls) and in the laboratory under two water temperatures (26 and 29 °C) and different salinity regimes (constant at 5 and 15 ppt, increasing from 5 to 15 ppt, and decreasing from 15 to 5 ppt). One spawning experiment was done, comparing the spawning response of adult clams exposed to various stimuli (changes in water temperature and salinity, flow of seawater irradiated with UV light, addition of a high concentration of food and sperm to the water, as well as injections of serotonin and KCl into the foot and/or adductor muscle). Higher frequencies of mature clams were observed after 2 to 5 weeks of reproductive conditioning at a low temperature (26 °C) and under a constant salinity regime (at 5 or 15 ppt). Higher percentages of spawning and post-stimulation survival as well as spawning response speed were observed in specimens injected in the foot with a low concentration of serotonin (0.4 mL at 20 μM). The time of spawning response onset was shorter in males than in females. The fecundity increased with the live weight of the females, showing an average value of 3.4 × 106 oocytes per female. The early development of the estuarine clam from oocyte to juvenile stage was described in detail. The growth rates of larvae and spat were relatively high but their survival values were low, showing the need for additional research on the larval and post-larval rearing.

Resistant calcification responses of Arctica islandica clams under ocean acidification conditions

Ocean acidification (OA) directly impacts marine calcifying organisms including ecologically and commercially important shellfish species such as Arctica islandica (A. islandica). To test whether documented growth resilience of A. islandica to OA is a general response across ages and populations or a function of adaptation to local habitat, we cultured juvenile and adult clams collected from an environment with little pH variation under four pH levels (7.5, 7.7, 7.9, and 8.1) for three months and integrated our understanding with relevant literature. The average shell growth over the experiment among all (69) individuals was 57 ± 55 μm, and there were no statistically significant differences in growth among pH treatments, including the control treatment, despite the general growth rate differences between juveniles and adults. Our results show that A. islandica can maintain its shell growth even in aragonite undersaturated (Ω < 1) conditions (0.65 and 0.83 for pH 7.5 and 7.7 treatments, respectively), supporting the hypothesis that resistance to OA conditions is likely a generalized response across populations. Although the present results show A. islandica can maintain their shell growth under short-term OA, long-term impacts of OA on A. islandica shell growth and other physical parameters including shell density and microstructure are still needed to better assess the sustainability of A. islandica in a more acidified future and to provide guidance on managing this important shellfish stock.

Molecular cloning and expression analysis of CDK genes during selective and wild populations of juvenile Meretrix meretrix

The saltwater hard clam Meretrix meretrix is an important commercially bivalve in China. Therefore, identification of key genes involved in growth regulation are the primary prerequisites for further development of phenotype-selective breeding of the clams and understanding molecular mechanisms of growth regulation. In the present study, we cloned the full-length cDNA sequences of the growth-related genes cyclin-dependent kinase 1 and 7 in M. meretrix (MmCDK1 and MmCDK7) and then investigated their gene expression characteristics and associations with growth traits. The full-length cDNA of MmCDK1 was 1623 base pairs (bp), with a 900 bp open reading frame (ORF) encoding 299 amino acids. The full-length MmCDK7 cDNA was 1296 bp, with a 1017 bp ORF encoding 338 amino acids. In adult clams, expression levels of MmCDK1 and MmCDK7 were significantly higher in the gonad than other tissues (p < 0.05). In the eight developmental stages of M. meretrix, MmCDK1 and MmCDK7 had the highest expression levels in the multicellular stage, followed by oosperm, blastula stage and gastrulae stage. The BLAST searches showed that MmCDK1 has high similarity to the cyclin-dependent kinases from Crassostrea gigas (89.26%), and the protein structure of MmCDK7 was similar to the Mizuhopecten yessoensis (75.00%). In addition, the growth trait (shell length) of clams was tested in different cultivation conditions. A 2 × 2 layout was designed including four treatment groups were abbreviated as LHFH (high light and high food), LHFL (high light and low food), LLFH (low light and high food) and LLFL (low light and low food). The shell length of the selective and wild clams in the LHFH group was higher than those in other groups on the 75th day. Moreover, the shell length and MmCDK7 expression levels of the selective clams was higher in the LHFH, LLFH and LHFL groups than the wild clams on the 75th day, significantly (p < 0.05). However, the shell length and relative expression levels of MmCDK7 were no significant differences between the selective clams and the wild clams in the LLFL group (p > 0.05). MmCDK1 expression levels of the selective clams were higher than the wild clams in four treatment groups on the 75th day, significantly (p < 0.05). Further analysis showed that there was a significant linear correlation between shell length and relative expression levels of MmCDKs genes (MmCDK1 and MmCDK7). These results suggest that MmCDK1 and MmCDK7 are key growth-related genes, and they may have potential value in selective breeding to improve growth performance of M. meretrix.

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.

First report and molecular analysis of population stability of the invasive Gulf wedge clam, Rangia cuneata (G.B. Sowerby I, 1832) in the Pomerian Bay (Southern Baltic Sea)

Native to the Gulf of Mexico, Gulf wedge clam (Rangia cuneata) has been successfully invading Baltic Sea since first introduction with ballast waters in 2010. In this study, we report new occurrence sites in the Pomeranian Bay and in the brackish water of the Szczecin Lagoon. We aimed to describe the structure and stability of the new population of Gulf wedge clam using ecological indicators (i.e. condition index, length-at-age) and a molecular approach targeting first cytochrome oxidase subunit (COX1) fragments. The study showed an age structure dominated by adult clams (aged 4+), rather than the often reported juveniles. We found that the condition of the R. cuneata population in the Pomeranian Bay is characteristic of the expansion model after bottleneck. Genetic analyses indicated low nucleotide (π = 0.00387 ± 0.00012) and haplotype (h = 0.79 ± 0.018) diversity. A haplotype network inferred by a median-joining method showed lack of haplotype H6 (Chesapeake Bay, NW Atlantic) and numerous individuals with haplotype H1 (Gulf of Mexico). Demographic parameters (non-significant Tajima’s D = 2.058, Fu’s Fs = 2.298, Fu and Li’s D* = 1.195, and significant Fu and Li’s F test = 1.757) suggested that the population underwent a moderate population bottleneck, while analysis of mismatch distribution showed a bimodal shape which may imply population admixture as a consequence of “allelic surfing”. Moreover, we concluded that R. cuneata has successfully invaded the basin of the LNG terminal, although our study proved non-linear growth of this species.

Oxidative stress response to hydrogen peroxide exposure of Mytilus galloprovincialis and Ruditapes philippinarum: Reduced embryogenesis success and altered biochemical response of sentinel marine bivalve species

The elevated concentration of hydrogen peroxide (H2O2) due to natural and anthropogenic causes is a significant bane to marine and coastal organisms, as it has a prolonged half-life and can inject across lipid bilayers and interfere with aquatic redox processes. The present assessment aimed to determine the acute effect of H2O2 on larval development of the Mediterranean mussel Mytilus galloprovincialis and to observe the oxidative stress (OS) responses, including genotoxicity and neurotoxicity in the clam Ruditapes philippinarum. Two experiments were performed, in which (a) mussel embryos were exposed to the consecutive dilution (0.0, 0.05, 0.5, 5.0, 50.0 and 500.00 μM) of H2O2 for 48 h and (b) adult clams were exposed to similar concentrations - of this reactive oxygen species (ROS) for 21 days. Incremental H2O2 significantly retarded the growth of mussel larvae; the percentage of malformed larvae increased with increasing toxicant concentration (p < 0.01, and r = 0.884). H2O2 also induced oxidative stress (OS) in clams, indicated by the activation/inactivation of antioxidant enzymes (GST, GPx, and GR) with a significant rise in lipid peroxidation (LPO) in the digestive gland tissues of the clams (p < 0.05). No genotoxicity or neurotoxicity on R. philippinarum were evident after chronic exposure. The battery of responses (organismal and cellular parameters) showed embryotoxicity and sublethal effects on bivalves, which confirmed their suitability for assessing the effect of H2O2 mediated OS responses in the marine environment.

Dynamic Regulation of Ions and Amino Acids in Adult Asian Hard Clams Meretrix lusoria Upon Hyperosmotic Salinity

The dynamic regulation of ions and amino acids in the gills and mantle of the Asian hard clam, Meretrix lusoria, following the exposure to a hyperosmotic environment was hitherto unclear. The present study revealed that the osmolality as well as the Na+ and Cl– concentrations in the hemolymph were significantly increased 3 h after transferring the clams from an environment with the salinity of their natural habitat (brackish water; BW; 20‰) to one with hyperosmotic salinity (seawater; 35‰). In addition, we found that the specific activities of Na+/K+-ATPase, a key enzyme that plays a significant role in cell osmoregulation, in the gills and mantle of clams were significantly increased at 72 and 12 h post-transfer, respectively, during acclimation to hyperosmotic salinity. Similarly, the contents of free amino acids (FAAs) such as taurine, alanine, and glycine were significantly elevated during hyperosmotic salinity acclimation. Previous research indicates that taurine is the most abundant FAA in the gills and mantles of Asian hard clams and that the taurine transporter (TAUT) plays an important role in taurine accumulation. The present study showed that TAUT mRNA and protein expression were significantly and transiently increased in the mantle of Asian hard clams following exposure to seawater; although the expression of TAUT mRNA in the gills of Asian hard clams was also transiently stimulated by exposure to hyperosmotic salinity, the relative TAUT protein abundance decreased only at later stages. Accordingly, the findings of this study improve our understanding of the dynamic processes of ion and amino acid regulation in the peripheral tissues of bivalves under hyperosmotic stress.

Two fibrinogen-related proteins (FREPs) in the razor clam (Sinonovacula constricta) with a broad recognition spectrum and bacteria agglutination activity

Fibrinogen-related proteins (FREPs) that contain only the fibrinogen-related domain are likely involved in pathogen recognition. In this study, we identified two FREPs from the razor clam (Sinonovacula constricta), called ScFREP-1 and ScFREP-2, and investigated their roles in the immune response. Both ScFREP-1 and ScFREP-2 contained a fibrinogen-related domain at the C-terminal. ScFREP-1 and ScFREP-2 mRNAs were detected in all adult clam tissues tested, with the highest expression levels in the gill and mantle, respectively. Their expression levels were significantly upregulated after microbe infection. Recombinant ScFREPs could bind Gram-positive and Gram-negative bacteria as well as some pathogen-associated molecular patterns (PAMPs), and they could agglutinate those bacteria. These results showed that ScFREPs functioned as potential pattern recognition receptors to mediate immune response by recognizing PAMPs and agglutinating invasive microbes.

Galaxolide and tonalide modulate neuroendocrine activity in marine species from two taxonomic groups

Galaxolide (HHCB) and tonalide (AHTN) are polycyclic musk compounds (PMCs) used in household and personal care products that have been included on the list as emerging contaminants of environmental concern due to their ubiquity in aquatic and terrestrial environments. There still exists a dearth of information on the neurotoxicity and endocrine disrupting effects of these contaminants, especially for marine and estuarine species. Here, we assessed the neuroendocrine effects of HHCB and AHTN using adult clams, Ruditapes philippinarum, and yolk-sac larvae of sheepshead minnow, Cyprinodon variegatus. The clams were treated with concentrations (0.005–50 μg/L) of each compound for 21 days. Meanwhile, sheepshead minnow larvae were exposed to 0.5, 5 and 50 μg/L of HHCB and AHTN for 3 days. Enzyme activities related to neurotoxicity (acetylcholinesterase - AChE), neuroendocrine function (cyclooxygenase - COX), and energy reserves (total lipids - TL) were assessed in R. philippinarum. Gene expression levels of cyp19 and vtg1 were measured in C. variegatus using qPCR. Our results indicated induction of AChE and COX in the clams exposed to HHCB while AHTN exposure significantly inhibited AChE and COX. Gene expression of cyp19 and vtg1 in yolk-sac C. variegatus larvae exposed to 50 μg/L AHTN was significantly downregulated versus the control. The results of this study demonstrate that HHCB and AHTN might pose neurotoxic and endocrine disrupting effects in coastal ecosystems.

Growth inhibitory effect of dissolved organic matter from fresh free-floating Ulva spp. fronds on asari clam Ruditapes philippinarum

Green tides caused by Ulvales group macroalgae have been reported from many eutrophic waters around the world. The asari clam Ruditapes philippinarum is an important fisheries resource in Japan and co-exists in coastal waters with Ulvales macroalgae. This study examined a possible impact of fresh Ulva fronds on the growth of juvenile, adult, and spat asari clams by conducting rearing experiments. Significant inhibition of juvenile and spat growth, but not adult growth, was observed when fresh Ulva fronds were floated on rearing water at 42.7 g/l (or 0.68 g/cm2), a density per area corresponding to the vegetation at Ajina flat on June 2006. This growth inhibition was experimentally observed at a level of 1.8 mg C/l (or a 0.47 mg C/l increase against blank rearing water) of dissolved organic matter (DOM) released from Ulva fronds. On the other hand, coastal waters with Ulva blooms contained higher levels of DOM (mean ± SD) at 2.3 ± 0.8 mg C/l (or 1.6 mg C/l higher than neighboring waters with sparse Ulva vegetation), suggesting that similar inhibition of asari clam growth may be occurring in the natural water environment with prevalent Ulva blooms.

No experimental evidence for vector-free, long-range, upstream dispersal of adult Asian clams [Corbicula fluminea (M\xfcller, 1774)

The Asian clam (Corbicula fluminea) is one of the rapidly spreading, very successful aquatic invasive species, which has become established widely in many parts of the world. Its spread is assumed to be by both passive and active dispersal. However, the importance of active pedal movement in dispersal is hardly known. Since there was no direct evidence of this phenomenon, field observations were combined with laboratory experiments to find out if the clams move upstream actively, and how this is affected by the quality of the substrate, the density of the clams, and the water velocity. Field observations were conducted at a small watercourse with no waterborne transport. Experiments were done in an indoor artificial stream system, where the distances moved by adult clams were measured via digital image analysis. Substrate grain size, starting density of clams, and water velocity significantly affected clam movement. Fine grain sediment and slow flow velocity both facilitated spread, while there was no clear pattern of density-dependent dispersal. Also, we found no clear preference for either upstream or downstream movement. The maximum distance moved in the lab experiments predicts no more than 0.15 km/y active pedal movement in an upstream direction, while our field observations detected a much faster (0.5–11 km/y) upstream movement, which might be explained by passive dispersal, such as via human transport and ecto- or endozoochory. Overall, it seems that active movement of the species cannot read to long-distance migration.

Effects of biological behaviors of benthic bivalve (Corbicula japonica) on its passive transport

The bivalve Corbicula japonica is a popular food in Japan. The abundance of the clam in a lake changes over time. Clam transport is assumed to be passive even after the clam’s settlement. But it is not clear whether clams can be considered like sand particles, or whether biological features of the clams affect their movement. Features of clams that differ from sediment particles and may affect movement include secretion of mucous threads, use of the foot byssus, burrowing, as well as the clam’s shape. The aim of the work was to determine whether these biological features modified the interaction with the hydrodynamics which would affect the transport of juvenile and adult clams within a lake. We performed a series of laboratory experiments to measure the critical shear stresses of clams for transport. The mucous threads substantially decreased critical shear stress and in contrast, the byssus significantly increased critical shear stress. We then performed field measurements in Lake Shinji and evaluated the possibilities of clam transport depending on these biological behaviors. Thus, the evaluated shear stresses for clams will be applied into a clam dispersal and transport simulation model, which can be useful to understand the strategy of clams’ survivals.

RNAi-mediated knock-down of the dopamine beta-hydroxylase gene changes growth of razor clams

Dopamine beta-hydroxylase (DβH) plays an essential role in the synthesis of catecholamines (CA) in neuroendocrine networks. In the razor clam, Sinonovacula constricta a novel gene for DβH (ScDβH-α) was identified that belongs to the copper type II ascorbate-dependent monooxygenase family. Expression analysis revealed ScDβH-α gene transcripts were abundant in the liver and expressed throughout development. Knock-down of ScDβH-α in adult clams using siRNA caused a reduction in the growth rate compared to control clams. Reduced growth was associated with strong down-regulation of gene transcripts for the growth-related factors, platelet derived growth factors A (PDGF-A) (P<0.001) 24h after ScDβH-α knock-down, vascular endothelial growth factor (VEGF1) (P<0.001) and platelet derived growth factor B (PDGF-B-2) (P<0.001) 24h and 48h after ScDβH-α knock-down and transforming growth factor beta (TGF-β1) (P<0.001) 48h and 72h after ScDβH-α knock-down. Taken together the results suggest that the novel ScDβH-α gene through its role in CA synthesis is involved in growth regulation in the razor clam and possibly other bivalves.

Are Juvenile Manila Clam Ruditapes philippinarum Free from Perkinsus olseni Infection in Korean Waters?

As a suspension feeder, Manila clam Ruditapes philippinarum (A. Adams and Reeve 1850) plays a crucial role in the coastal soft bottom ecosystem in the temperate region, linking the benthic primary production to the upper trophic level. Manila clam density on tidal flats on the west coast of Korea has been declining for the past decades, and infection by the protozoan parasite Perkinsus olseni (Lester and Davis 1981) is one of the major causes for the decline. Recent studies carried out in Japan revealed that P. olseni induces mortalities of the juveniles in their natural habitats, which may lead to the recruitment failure and subsequent decline in the clam population. In this study, we surveyed P. olseni infection in juvenile Manila clam occurring on two tidal flats on the Taean coast. Ray’s fluid thioglycollate medium assay (RFTM) revealed that P. olseni infection was not limited to the adult clams, and the juvenile and small-sized clams are also infected by P. olseni. As young as four-month-old juveniles from Jugyo tidal flat were infected by P. olseni, with the prevalence (i.e., percentage of the infected individuals) of 75.0% and the intensity of 7.77 × 105 cells g−1 wet tissue weight (WT). The adult Manila clams (SL > 30 mm) from Jugyo tidal flat showed a prevalence of 96.0%, and the intensity as 5.80 × 10 cells g−1 WT. The observed infection prevalence and intensity of the juvenile are somewhat comparable to those of the adult clams, suggesting that a high level of P. olseni infection in the juveniles may lead to mortality and a long term decline in the clam population density.

Characteristic changes in the population dynamics of asari (Manila) clam Ruditapes philippinarum in a period of stock decrease on the Banzu intertidal flat, Tokyo Bay

In Tokyo Bay, harvesting of asari (Manila) clam Ruditapes philippinarum has decreased since the early 2000s. In the present study, based on the abundance of sampled clams in four sites from 1998 to 2015 in the Banzu intertidal area, the authors discussed changes in the processes of population dynamics of asari clam which are possibly related to harvest decrease in Tokyo Bay. The findings are summarized as following two points. Firstly, the recent decline in the harvest of asari clams in Tokyo Bay may be caused by the mortality of benthic juveniles. The spatial density of juvenile clams at recruitment showed a varied increase during the study period. In contrast, from the recruitment to adult stages, the mortality coefficient z and von Bertalanffy growth function k consistently increased and decreased, respectively. Moreover, decreasing trends in the spatial density of adult clams were apparent in the concurrent period. Thus, the decrease of adult clams was suggested to be due to poor survival in juvenile clams after recruitment. Secondly, the depletion of the autumn cohort and spatial relocation of the large juveniles were apparent after 2009. The autumn cohort significantly decreased in size-frequency composition, despite the increased abundance and size of juvenile clams at recruitment after 2009. Referring to recent reports, predation by fish may be one of the processes that depleted the abundance of the autumn cohort. Additionally, the density of year-class cohorts increased for 2–4 months from their first appearance to the completion of recruitment, which was likely due to the spatial immigration of large juveniles into the study area. The physical stress of sea water movement caused by increasing strong wind may have encouraged the spatial transport of the juveniles. The depletion of the autumn cohort and spatial immigration of the large juveniles were temporally overwrapped with the period of an outbreak and continuing infestation of the parasitic sea spider Nymphonella tapetis after 2007. This observation can hypothesize that poor burrowing activity due to the parasitic infestation allowed the clams to suffer predation or passive transport. However, above changes in population dynamics after the late 2000s cannot be solely connected to decline of asari harvest, because that decrease in the harvest of asari clams in Tokyo Bay began in the early 2000s; there remain factors other than this one that may have acted on the stock decline after the early 2000s in Tokyo Bay.

Identification of a dopamine receptor in Sinonovacula constricta and its antioxidant responses

The dopamine (DA) D2 receptor is a member of the G protein-coupled receptors of organisms and plays an important role in immune system regulation. The presence of DA receptors has been widely reported in vertebrates, but few studies have been conducted in shellfish. Here, we identified a novel DA-D2 receptor gene, ScDopR2-1, in the razor clam Sinonovacula constricta. ScDopR2-1 belongs to the family of G protein-coupled receptors, containing seven hydrophobic transmembrane domains, along with 16 predicted N-glycosylation sites and 69 phosphorylation sites. A longer third intracellular loop and a shorter C-terminus in ScDopR2-1 are characteristic features of D2 receptors. ScDopR2-1 is widely expressed in tissues from adult clams, showing high expression in siphon and foot tissues. Furthermore, in response to Vibrio anguillarum challenge, ScDopR2-1 expression levels are significantly increased in liver tissue. Moreover, changes in the activities of catalase (CAT) and superoxide dismutase (SOD) also indicate that the organism causes an immune response. In summary, the results indicate that ScDopR2-1 plays a pivotal role in antioxidant responses in S. constricta.

Application of a fish finder system to spatial distribution mapping for the razor clam Solen gordonis: Case study from coastal waters, northwestern Kyushu, Japan

Effective spatial management planning for sedentary fishery resources requires accurate spatial distribution mapping of a focal stock including both fished and associated unfished populations. This study focuses on (sub-)adult razor clams (Solen gordonis), with shell lengths ≥25 mm, in Sasebo Bay and Omura Bay, northwestern Kyushu, Japan. They are commercially harvested, but little is known about their spatial distribution pattern. The objective was to estimate the spatial distribution and configuration of clam beds existing within an area of ca. 12 km2. For this, clam and sediment sampling by scuba-equipped divers, acoustic bathymetric surveys using a recreational-grade fish finder (single-beam acoustic) system, and presence–absence modelling using generalized linear models were performed. For the collected clams, catch per unit effort (CPUE; number of clams collected/h) was calculated and used as a proxy for clam individual density. The results showed that the presence–absence variation in the spatial distribution of (sub-)adult clams was associated with the bottom sediment type. (Sub-)adult clams occurred frequently in the sediments with higher gravel (particles with diameters ≥2 mm, mainly shell fragments) and lower mud (diameters <0.063 mm) contents than the sediments of the absence sites. Acoustic signals of the fish finder system were backscattered more intensely from gravel-sand beds (i.e., typical of the presence sites) than from muddy sand beds (i.e., absence sites). Using the relationships among the presence–absence variation of (sub-)adult clams, sediment type, and backscatter strength, spatial distribution of clams was predicted with high accuracy. The presence probability of (sub-)adults (= x; continuous values from 0 to 1) predicted from the results was significantly regressed on CPUE (= y) [i.e., y = exp(−0.7984 + 4.9350x) − 0.05]. Using this relationship, a map for the relative abundance of (sub-)adult clams was obtained. This map depicted several clam beds including both exploited and unexploited ones. The latter beds virtually conform to no-take zones in the razor clam fishery.

Identification of BMP2 and BMP7 genes and association of their SNPs with growth traits in the hard clam (Meretrix meretrix)

The hard clam (Meretrix meretrix) is a commercially and ecologically important benthic mollusk in South and Southeast Asia. Growth is one of the most important and complex traits affecting clam breeding. Therefore, identification of key growth factor genes and their association with growth traits are the primary prerequisites for in-depth study of the mechanisms of growth regulation and molecular marker-assisted selection of the hard clam. In the present study, we generated the full-length cDNA sequences of the growth-related genes BMP2 and BMP7 in M. meretrix (MmBMP2 and MmBMP7) and then investigated their gene expression patterns and associations with growth traits. The full-length cDNA of MmBMP2 was 2641 base pairs (bp), with a 1395 bp open reading frame (ORF) encoding 464 amino acids. The full-length MmBMP7 cDNA was 3161 bp, with a 1257 bp ORF encoding 418 amino acids. In adult clams, expression of MmBMP2 and MmBMP7 were significantly higher in the mantle than in the other five tested tissues (p < 0.05). Among the seven developmental stages of M. meretrix, expression of MmBMP2 was the highest in juveniles (p < 0.05), and expression of MmBMP7 was very low in 4-cell embryo and eyespot larva stages (p < 0.05). Two hundred and sixty-three individuals from two populations were screened for MmBMP2 and MmBMP7 polymorphisms, and single nucleotide polymorphisms (SNPs) associated with growth traits in MmBMP2 was found in both populations. Subsequent analysis revealed that two SNPs in MmBMP2 were shared between the populations. One was A726G, for which individuals with genotypes AG had significantly higher growth traits than those with genotype AA (p < 0.05). The other was A732T, for which individuals with genotype AT had significantly higher growth traits than those with genotype AA (p < 0.05). These results suggest that BMP2 markers may have potential value in molecular selective breeding to improve clam growth.