Comparative physiology of young and old cohorts of bay scallop Argopecten irradians irradians (Lamarck): mortality,growth, and oxygen consumption

The bay scallop Argopecten irradians (Lamarck) undergoes rapid population decline in its second year of life. Pre-(1st-yr) and postreproductive (2nd-yr) bay scallops were held in cages at two sites on Long Island, New York, U.S. Survival, growth, and metabolic rates of the two cohorts were compared monthly throughout the autumn and winter. Second-year scallops, the harvestable crop of the year, maintained a positive energy balance until late November–December. Both age classes experienced similar relative tissue weight losses during overwintering (9–11% at the site where milder environmental conditions prevailed, and 24–25% at the more stressful site). Ambient water temperature explained a significant proportion (93%) of the seasonal variation in the rate of oxygen consumption. Thus the northern bay scallop A.i. irradians shows a limited ability to acclimatise oxygen consumption to seasonal temperature changes over the range of 1–23°C. A significant increase in oxygen uptake was associated with increased gametogenic activity of young scallops in May. Metabolic rate at this time was 50% higher than that predicted based on temperature, providing an estimate of the metabolic cost of reproduction in this species. The weight-normalized oxygen uptake rate of senescent scallops was significantly lower than that of young scallops. Mass natural mortality of the older cohort occurred during the winter, before the onset of a second gametogenesis; only 50% of the population survived beyond late January in 1985. Mortality was delayed by 2.5 months during a similar experiment conducted in 1986. Results of this study suggest that senescent mortality of New York bay scallop populations is not directly linked to the energy drain of a second reproductive event following overwintering stress.     

Acquisition of thermotolerance in bay scallops, Argopecten irradians irradians, via differential induction of heat shock proteins

Many cells and organisms are rendered transiently resistant to lethal heat shock by short exposure to sublethal temperatures. This induced thermotolerance is thought to be related to increased amounts of heat shock proteins (HSPs) which, as molecular chaperones, protect cells from stress-induced damage. As part of a study on bivalve stress and thermotolerance, work was undertaken to examine the effects of sublethal heat shock on stress tolerance of juveniles of the northern bay scallop, Argopecten irradians irradians, in association with changes in the levels of cytoplasmic HSP70 and 40. Juvenile bay scallops heat-shocked at a sublethal temperature of 32 °C survived an otherwise lethal heat treatment at 35 °C for at least 7 days. As determined by ELISA, acquisition of induced thermotolerance closely paralleled HSP70 accumulation, whereas HSP40 accrual appeared less closely associated with thermotolerance. Quantification of scallop HSPs following lethal heat treatment, with or without conditioning, suggested a causal role for HSP70 in stress tolerance, with HSP40 contributing to a lesser, but significant extent. Overall, this study demonstrated that sublethal heat shock promotes survival of A. irradians irradians juveniles upon thermal stress and the results support the hypothesis that HSPs have a role in this induced thermotolerance. Exploitation of the induced thermotolerance response shows promise as a means to improve survival of bay scallops in commercial culture.     

Development of a Rapid and Efficient Method for Non-Lethal DNA Sampling and Genotyping in Scallops

Non-lethal DNA sampling has long appealed to researchers studying population and conservation genetics, as it does not necessitate removing individuals permanently from their natural environment or destroying valuable samples. However, such an approach has not yet been well established in bivalves. In this study, we demonstrate that the gill represents a good source of tissue for non-lethal sampling in scallops. Removal of a few gill filaments caused no noticeable behavioral abnormalities or increased mortality rates in Zhikong scallop (Chlamys farreri) during a three-month period of observation. To facilitate rapid gill-based DNA extraction, six methods (MA-MF) were designed and evaluated, each requiring less than one hour of processing time. The optimal method was identified as MF, in terms of maintaining DNA integrity and genotyping accuracy. Further optimization of MF method by orthogonal experimental design suggested that the utilization of gills could be limited to 2 mg of sample, which is sufficient for performing up to 20,000 PCR reactions. We also demonstrate the excellent cross-species utility of MF in two additional scallop species, Yesso scallop (Patinopecten yessoensis) and bay scallop (Argopecten irradians). Taken together, our study provides a rapid and efficient approach for applying non-lethal DNA sampling in bivalve species, which would serve as a valuable tool for maintaining bivalve populations and conservation genetics, as well as in breeding studies.     

Heat shock protein responses in thermally stressed bay scallops, Argopecten irradians, and sea scallops, Placopecten magellanicus

The effects of thermal stress on the induction of heat shock proteins (HSPs) were examined in northern bay scallops, Argopecten irradians irradians, a relatively heat tolerant estuarine species, and sea scallops, Placopecten magellanicus, a species residing in cooler, deeper water. Polyclonal antibodies used in this work for analysis of inducible HSP70 and HSP40 only recognized proteins of 72 and 40 kDa respectively from the mantles of both scallop species. Additionally, HSP quantification using the antibody to HSP70 was equally effective by either immunoprobing of western blots or ELISA, demonstrating that either approach could be successfully employed for analysis of thermal response in scallops. Sea scallop HSP70 and HSP40 did not change when animals were heat-shocked for 3 h by raising the temperature from 10 °C to 20 °C; however, a 24 h treatment of the same magnitude elicited a significant response. Conversely, bay scallops displayed rapid and prolonged HSP70 and HSP40 responses during the recovery period following a 3 h heat shock from 20 °C to 30 °C. Temperature reduction from 20 °C to 3 °C for 3 h also caused significant HSP70 and HSP40 increases in bay scallops; this represents the first time cold shock was shown to induce HSP synthesis in bivalve mollusks. The onset of the HSP40 response was more rapid than for HSP70, occurring at the end of the cold shock itself prior to transfer to a recovery temperature. Both proteins responded maximally during recovery at control temperature. HSP responses of sea and bay scallops to thermal stress may be related to their habitat in the natural environment and they suggest a differential capacity for adaptation to temperature change. This is an important consideration in assessing the response of these scallops to different culture conditions.     

Genomic Characterization of Interspecific Hybrids between the Scallops Argopecten purpuratus and A. irradians irradians

The Peruvian scallop (Argopecten purpuratus) has been introduced to China and has successfully been hybridized with the bay scallop (A. irradians irradians). The F1 hybrids of these two scallops exhibited a large increase in production traits and some other interesting new characteristics. To understand the genetic basis of this heterosis, nuclear gene and partial mtDNA sequences, and genomic in situ hybridization (GISH) were employed to analyze the genomic organization of the hybrids. Amplification of the ribosomal DNA internal transcribed spacer (ITS) showed that the parental ITS sequences were present in all the hybrid individuals, illustrating that the hybrid offspring inherited nuclear DNA from both parents. Sequence analyses of the ITS region further confirmed that the hybrids harbored alleles from their parents; some recombinant variants were also detected, which revealed some alterations in the nuclear genetic material of the hybrids. The analysis of mitochondrial 16S rDNA showed that the hybrids possessed sequences that were identical to the 16S rDNA of the female parents, proving a matrilineal inheritance of mitochondrial genes in scallops. In addition, GISH clearly discriminated between the parental chromosomes and indicated a combination of haploid genomes of duplex parents in the hybrids. The genetic analyses in our study illustrated that the F1 hybrids inherited nuclear material from both parents and cytoplasmic genetic material maternally, and some variations occurred in the genome, which might contribute to a further understanding of crossbreeding and heterosis in scallop species.     

The role of hydrodynamics in recruitment,growth, and survival of Argopecten irradians (L.) and Anomia simplex (D'Orbigny) within eelgrass meadows

Larvae of the bay scallop Argopecten irradians (L.) and the common jingle Anomia simplex (D'Orbigny) use byssal threads to settle onto blades of eelgrass Zostera marina (L.) and other elevated substrata. A 2-yr study of the role of hydrodynamics in recruitment of these bivalves into eelgrass meadows, and in their subsequent growth and survival as juveniles, was conducted in Northwest Harbor, New York. Sea-water flux through the meadow varied significantly among sites and depended on the numerical density of eelgrass shoots. Recruitment of both bivalves varied significantly among sites in a similar manner, indicating that hydrodynamics in eelgrass meadows strongly affects bivalve recruitment. Results from experimental deployments of two types of substrata indicate that hydrodynamics in eelgrass meadows exerts a stronger influence on bivalve recruitment than does either interblade abrasion or possible differences among sites in the nature of the blade substratum. In one of the two study years, hydrodynamics in eelgrass meadows influenced growth and survival of Argopecten following settlement. Abundances and growth of Anomia following settlement appeared unaffected by hydrodynamics. Several lines of evidence suggest that predation is less important than hydrodynamics in determining abundances of recruits and early juvenile stages of these bivalves.     

Spatio-temporal patterns in the mortality of bay scallop recruits in North Carolina: investigation of a life history anomaly

By timing reproduction to occur when predatory mortality on progeny is minimal, organisms may maximize recruitment to adult populations. Accordingly, an hypothesis to explain the greater importance of fall than spring spawning to North Carolina populations of bay scallops (Argopecten irradians) is that predatory mortality of bay scallop recruits is lower in fall and winter than spring and summer. To test this hypothesis, we measured predatory mortality of scallop recruits monthly. To infer the identities of predators that are most important in determining patterns of mortality of bay scallop recruits in spring and in fall, predatory mortality of bay scallop recruits was compared between the edge and interior of sheltered and exposed seagrass patches during the day and at night in May and November. Consideration of predatory mortality throughout the year indicated that mortality of scallop recruits over late spring and summer approaches 100% but is negligible over late fall and winter. In May, predatory mortality of scallop recruits was similar during day and night but greater at exposed than sheltered sites. In November, predatory mortality was greater during night than day and slightly greater at sheltered than exposed sites. In neither month did position within patch influence mortality, and at all times and places, missing and crushed scallops contributed a higher proportion than drilled scallops to the total dead. These spatio-temporal patterns of mortality of scallop recruits suggest that mud crabs, Dyspanopeus sayi, which are more abundant in exposed than sheltered seagrass beds during spring and can feed by day and night, are a likely major contributor to spatio-temporal pattern in mortality of scallop recruits in North Carolina. Blue crabs, Callinectes sapidus, which are many times more abundant in summer than winter, may also contribute to observed seasonal patterns in mortality. The dramatically lower rates of predation on bay scallops over the winter months appear to provide fall settlers with a temporal window of opportunity to recruit to the adult population. Although spring spawning contributes little to adult populations in most years because of high rates of predatory mortality during summer, we hypothesize that spring spawning persists because infrequent devastating perturbations, such as hurricanes and red tides, can result in complete failure of fall recruitment.     

Development and Linkage Analysis of 104 New Microsatellite Markers for Bay Scallop (Argopecten irradians)

For genetic analysis and linkage mapping of bay scallop (Argopecten irradians), a set of 120 novel simple sequence repeat markers were developed from microsatellite-enriched libraries and expressed sequence tags. An inter-subspecies hybrid bay scallop family (CC5) of 46 progeny was analyzed as the reference population to confirm polymorphism and test the segregation patterns of these loci. A total of 104 microsatellite markers were polymorphic in the reference family, among which 36 in female, 28 in male, and 40 in both parents, respectively. Linkage analysis allowed mapping these markers to 15 linkage groups, which is close to the haploid chromosome number of bay scallop (n = 16). Analysis of the 40 markers segregating in both parents showed a higher recombination rate in the female parent, with the average of female-to-male recombination ratio of 1.09:1 between linked pairs of markers. When null alleles were considered, there were 17 loci showing segregation distortion at the 5% significance level using the chi-square test. The microsatellite markers developed in this study provide a useful resource for future linkage mapping and quantitative loci analysis in A. irradians.     

A Consensus Microsatellite-Based Linkage Map for the Hermaphroditic Bay Scallop (Argopecten irradians) and Its Application in Size-Related QTL Analysis

Bay scallop (Argopecten irradians) is one of the most economically important aquaculture species in China. In this study, we constructed a consensus microsatellite-based genetic linkage map with a mapping panel containing two hybrid backcross-like families involving two subspecies of bay scallop, A. i. irradians and A. i. concentricus. One hundred sixty-one microsatellite and one phenotypic (shell color) markers were mapped to 16 linkage groups (LGs), which corresponds to the haploid chromosome number of bay scallop. The sex-specific map was 779.2 cM and 781.6 cM long in female and male, respectively, whereas the sex-averaged map spanned 849.3 cM. The average resolution of integrated map was 5.9 cM/locus and the estimated coverage was 81.3%. The proportion of distorted markers occurred more in the hybrid parents, suggesting that the segregation distortion was possibly resulted from heterospecific interaction between genomes of two subspecies of bay scallop. The overall female-to-male recombination rate was 1.13∶1 across all linked markers in common to both parents, and considerable differences in recombination also existed among different parents in both families. Four size-related traits, including shell length (SL), shell height (SH), shell width (SW) and total weight (TW) were measured for quantitative trait loci (QTL) analysis. Three significant and six suggestive QTL were detected on five LGs. Among the three significant QTL, two (qSW-10 and qTW-10, controlling SW and TW, respectively) were mapped on the same region near marker AiAD121 on LG10 and explained 20.5% and 27.7% of the phenotypic variance, while the third (qSH-7, controlling SH) was located on LG7 and accounted for 15.8% of the phenotypic variance. Six suggestive QTL were detected on four different LGs. The linkage map and size-related QTL obtained in this study may facilitate marker-assisted selection (MAS) in bay scallop.     

Development of Expressed Sequence Tags from the Bay Scallop, Argopecten irradians irradians

The bay scallop, Argopecten irradians irradians, introduced from North America, has become one of the most important aquaculture species in China. Inan effort to identify scallop genes involved in host defense, a high-quality cDNA library was constructed from whole body tissues of the bay scallop. A total of 5828 successful sequencing reactions yielded 4995 expressed sequence tags (ESTs) longer than 100 bp. Cluster and assembly analyses of the ESTs identified 637 contigs (consisting of 2853 sequences) and 2142 singletons, totaling 2779 unique sequences. Basic Local Alignment Search Tool (BLAST) analysis showed that the majority (73%) of the unique sequences had no significant homology (E-value ≤ 0.005) to sequences in GenBank. Among the 748 sequences with significant GenBank matches, 160 (21.4%) were for genes related to metabolism, 131 (17.5%) for cell/organism defense, 124 (16.6%) for gene/protein expression, 83 (11.1%) for cell structure/motility, 70 (9.4%) for cell signaling/communication, 17 (2.3%) for cell division, and 163 (21.8%) matched to genes of unknown functions. The list of host-defense genes included many genes with known and important roles in innate defense such as lectins, defensins, proteases, protease inhibitors, heat shock proteins, antioxidants, and Toll-like receptors. The study provides a significant number of ESTs for gene discovery and candidate genes for studying host defense in scallops and other molluscs.     

Predator–prey interactions in a restored eelgrass ecosystem: strategies for maximizing success of reintroduced bay scallops (Argopecten irradians)

Predation is a key determinant of community structure and function, and thus should play a central role in successful ecological restoration strategies. The bay scallop, Argopecten irradians, was once abundant in the coastal bays of Virginia, U.S.A., until the complete loss of their eelgrass habitat, Zostera marina, in the 1930s. With the successful restoration of Z. marina in these coastal bays, attention has turned to reintroducing A. irradians with the intent of producing a self‐sustaining population. The success of this effort requires an understanding of the sources and degree of natural mortality that A. irradians experiences throughout their ontogeny. The objectives of this study were to: (1) quantify predatory mortality during two successive life history stages of A. irradians, in both spring and fall spawns and (2) identify possible predators of A. irradians in the Virginia coastal bays. We conducted tethering experiments to quantify the proportional losses due to predation, and used otter trawls and suction samples to characterize the predator community over two consecutive years. Losses due to predation ranged from 4 to 80% per day, with smaller juveniles (<15 mm shell height) experiencing greater mortality in 2013, and larger juveniles (>20 mm shell height) in 2014, which we infer is driven by the absence and presence of adult blue crabs in 2013 and 2014, respectively. We propose that managers should look toward relatively inexpensive predator surveys to best judge both when and at what size restored species should be introduced into the wild.     

紫扇贝与海湾扇贝通用SSR的检测及其在杂交子代鉴定中的应用

以紫扇贝DNA为模板,用已开发的147个海湾扇贝微卫星标记引物扩增,结果表明100个微卫星标记能成功扩增,其中有47个表现出多态性,等位基因数目从2到9不等.观测杂合度范围为0.128 0～1.000 0(平均0.660 4),期望杂合度范围为0.503 1～0.862 1(平均0.671 9),有6个位点偏离Hardy-Weinberg平衡(P<0.05).用7对引物分别对紫扇贝、海湾扇贝及其种间正反杂交F1各30个个体进行PCR扩增,发现它们可明确区分紫扇贝与海湾扇贝,且所检测60个杂交子代均同时含海湾扇贝与紫扇贝的相应种特异性条带,证明全部为种间杂交子代.将该7对引物的扩增产物克隆测序,发现这些位点在两种扇贝中的序列同源率为40.22%～91.95%,其中3个位点在紫扇贝中的扩增产物仍然含有微卫星.     

Characterization of 35 microsatellite loci in the Pacific lion‐paw scallop (Nodipecten subnodosus) and their cross‐species amplification in four other scallops of the Pectinidae family

Four microsatellite‐enriched DNA libraries yielded 35 microsatellite loci from 100 primer pairs designed for Pacific lion‐paw scallop, Nodipecten subnodosus. The number of alleles ranged from four to 28. Three of the 35 loci were not in Hardy–Weinberg equilibrium and linkage disequilibrium was found for one pair of loci. These microsatellites will be used to analyse the population structure of the species in Mexico's Baja Peninsula to propose management strategies for scallop aquaculture development. Twenty‐six primer pairs cross‐amplified in Nodipecten nodosus, whereas none (Argopecten ventricosus) or few cross‐amplified in the Argopecten species.     

Warming and hypoxia reduce the performance and survival of northern bay scallops (Argopecten irradians irradians) amid a fishery collapse

Warming temperatures and diminishing dissolved oxygen (DO) concentrations are among the most pervasive drivers of global coastal change. While regions of the Northwest Atlantic Ocean are experiencing greater than average warming, the combined effects of thermal and hypoxic stress on marine life in this region are poorly understood. Populations of the northern bay scallop, Argopecten irradians irradians across the northeast United States have experienced severe declines in recent decades. This study used a combination of high-resolution (~1 km) satellite-based temperature records, long-term temperature and DO records, field and laboratory experiments, and high-frequency measures of scallop cardiac activity in an ecosystem setting to quantify decadal summer warming and assess the vulnerability of northern bay scallops to thermal and hypoxic stress across their geographic distribution. From 2003 to 2020, significant summer warming (up to ~0.2°C year⁻¹) occurred across most of the bay scallop range. At a New York field site in 2020, all individuals perished during an 8-day estuarine heatwave that coincided with severe diel-cycling hypoxia. Yet at a Massachusetts site with comparable DO levels but lower daily mean temperatures, mortality was not observed. A 96-h laboratory experiment recreating observed daily temperatures of 25 or 29°C, and normoxia or hypoxia (22.2% air saturation), revealed a 120-fold increased likelihood of mortality in the 29°C-hypoxic treatment compared with control conditions, with scallop clearance rates also reduced by 97%. Cardiac activity measurements during a field deployment indicated that low DO and elevated daily temperatures modulate oxygen consumption rates and likely impact aerobic scope. Collectively, these findings suggest that concomitant thermal and hypoxic stress can have detrimental effects on scallop physiology and survival and potentially disrupt entire fisheries. Recovery of hypoxic systems may benefit vulnerable fisheries under continued warming.     

Egg size and larval survival of Mercenaria mercenaria (L.) and Argopecten irradians (Lamarck)

Eggs of the hard clam, Mercenaria mercenaria (L.), and the bay scallop, Argopecten irradians (Lamarck), were graded to three size categories, split into replicates and these were replicate-sampled. A chi-square analysis compared the proportion of larvae (48-h survival) to the proportions of eggs in each initial replicate experiment. Statistically significant differences attributed to egg size were found for both species. Large eggs survived better than small eggs, while those of intermediate size showed no difference between expected and observed survival.