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A mass mortality of clam, Meretrix meretrix, occurred in Jiangsu Province of China in the late September of 2007. Of the isolates obtained from the diseased clams, MM21 had the strongest virulence to the clam in the virulence test, with a LD50 value of approximately 6 x 10(6)CFU ml(-1). MM21 was identified as Vibrio parahaemolyticus by the VITEK 2 Compact system and 16S rDNA sequencing. Detection of virulence-associated genes by PCR indicated that MM21 was positive for toxR and tlh, and negative for tdh. Compared with control group, histiocytes from MM21-infected clams displayed a variety of cytopathological changes by transmission electron microscopy examination, which included increased lipid droplets in hepatocytes, deposition of granules in the mantle, excessive secretion in the gill. The results of our study suggested that MM21 may have been an etiological element in the mass mortalities of hard clam (M. meretrix) in Jiangsu Province of China in 2007.

During the summer and fall of 1995, in clam aquaculture leases at two locations on the coast of Massachusetts, significant mortalities were observed to occur primarily in 1frac12;- to 2-year old hard clams (Mercenaria mercenaria, quahog) planted in the leases. Examination of hard clams from those leases suggested a parasite similar to QPX (Quahog Parasite Unknown), as described by Whyte, S. K., Cawthorn, R. J., and McGladdery, S. E. (1994, Can. Dis. Ag. Org. 19, 129-136), was responsible for the poor condition of affected clams and the resulting high mortality. In clam tissues, the QPX-like parasite formed thalli surrounded by a mucofilamentous net. Larger thalli underwent endosporulation producing sporangia that contained approximately 40 endospores. Rupture of the sporangium released the endospores into the surrounding tissue. Inflammatory response by the clam to infection was chronic, active, and granulomatous. The occurrence both of phagocytic multinucleated giant inflammatory cells and inflammatory encapsulation of parasites were commonly identified as part of the response. Moderately to severely infected clams showed significantly reduced growth and a poorer condition index as compared to uninfected or mildly infected clams. Copyright 1998 Academic Press. Copyright 1998 Academic Press

Hemocytes of the hard clam Mercenaria mercenaria migrate toward secreted bacterial products in vitro by chemotaxis (i.e., by detection of an increasing chemical gradient of attractant). The attractants produced by Escherichia coli are peptides or small proteins. Clam hemocytes also migrate toward formyl-methionyl-leucyl-phenylalanine (fMLF), a mammalian neutrophil chemoattractant produced by bacteria, but not toward the related compound formyl-methionyl-valine. Migration of hemocytes to fMLF was blocked with the neutrophil fMLF receptor antagonist, t-Boc-MLF, suggesting that the hemocytes possess this receptor and that the response is receptor-mediated. However, fMLF is not the major bacterial chemoattractant for clam hemocytes, as t-Boc-MLF did not block migration of these cells to secreted bacterial chemoattractants.

QPX (Quahog Parasite Unknown) is a protistan parasite affecting hard clams (Mercenaria mercenaria) along the Northeast coast of the United States. The fact that QPX disease epizootics are usually observed in field sites with high salinities led to the general assumption that salinity represents an important factor for disease distribution. This study was designed to investigate the effect of salinity on QPX disease development as well as constitutive and QPX-induced defense factors in M. mercenaria. Naive and QPX-infected (both experimentally and naturally) clams were submitted to 17 and 30 psu for 4 months. Standard and QPX-specific cellular and humoral defense parameters were assessed after 2 and 4 months. These included total and differential hemocyte counts, reactive oxygen species production, phagocytic activity of hemocytes, lysozyme concentration in plasma, anti-QPX activity in plasma and resistance of hemocytes to cytotoxic QPX extracellular products. Results demonstrated higher QPX-associated mortality in naturally infected clams maintained at high salinity compared to those held at 17 psu. Our findings also showed an increase in mortality following experimental challenge with QPX in clams submitted to 30 psu but not in those held at 17 psu. Constitutive clam defense factors and the response to QPX challenge were also affected by salinity. QPX challenge caused significant but transitory changes in hemolymph parameters that were obvious at 2 months but disappeared at 4 months. Overall, our results show that salinity modulates clam immunity and the progress of QPX disease although its impact appears secondary as compared to findings we reported earlier for temperature.

Quahog Parasite Unknown (QPX) causes disease and mortality in hard clams, Mercenaria mercenaria. Seasonality of QPX disease prevalence in the field and changes in QPX growth and survival in vitro suggest a role of temperature in the hard clam-QPX interaction and disease development. This study specifically examined the effect of temperature on QPX disease development and dynamics. Naturally and experimentally infected clams were separately maintained in the laboratory at 13°C, 21°C, or 27°C for 4 months. Following this initial treatment, temperature was adjusted to 21°C for 5 additional months to simulate seasonal changes of temperature in the field and to investigate the effect of temperature variations on QPX disease dynamics. Mortality was continuously monitored during the experiment and clams were sampled at 2, 4 and 9 months for the assessment of QPX disease prevalence and intensity using our standard histological and quantitative PCR techniques. Results demonstrated significantly higher QPX disease prevalence and intensity, as well as higher mortality, in naturally-infected clams maintained at 13°C as compared to those held at 21°C or 27°C. Similarly, disease development was significantly higher in experimentally infected clams maintained at the colder temperature (70% prevalence after 4 months) as compared to those maintained under warmer conditions (

Rickettsiae are found in the gill epithelium of the hard clam, Mercenaria mercenaria. The procaryotes occur free in the cytoplasm of the epithelial cells at the tip of the filament and in the more proximal cells that support the lateral J cilia. The fine structure of the organisms, showing rippled cell walls, is typical of the rickettsiae. The increasing size of the inclusion representing late phase growth often culminates in lysis of the host cell. Masses (Gram-negative, Feulgen-positive) in ova, similar to those observed in the gill epithelium, suggest that transovarian transmission may occur.

Large granular hemocytes of Mercenaria mercenaria avidly phagocytose a variety of biological particles (red blood cells of six species, yeast, and gram-positive and gramnegative bacteria) as well as polystyrene spheres. Clam hemolymph is not necessary for phagocytosis but may have some opsonic effect in certain circumstances (e.g., low temperature and low particle density). Formaldehyde treatment of red blood cells enhances susceptibility to phagocytosis. Phagocytosis by Mercenaria hemocytes in vitro appears to be a nonspecific process.

The hemolymph of the hard clam, Mercenaria mercenaria, was found to agglutinate nonspecifically 4 of the 30 bacteria tested and a marine alga. The agglutinin is a protein (or a conjugated protein) because it is: (1) precipitated by trichloroacetic acid and ammonium sulfate; (2) inactivated by extraction with chloroform, but not with toluene or xylene; and (3) inactivated by chymotrypsin and protease, but not by deoxyribonuclease. Electrophoretic analysis shows that the agglutinin is composed of subunits each with a molecular weight of approximately 21,000. Calcium ions are required for the activity of the agglutinin and contribute to the heat stability of the molecule. Several saccharides, which may constitute a portion of the bacterial agglutinin receptors, were capable of partially inhibiting agglutination. In vitro studies using clam hemocytes showed that the phagocytosis of a marine bacterium, designated as RS-005, was enhanced by the presence of hemolymph. Adsorption of hemolymph samples with RS-005 bacteria removed the agglutinin activity for all types of cells tested and also abolished the opsonic effect.

Hemocytes of the hard clam, Mercenaria mercenaria , were observed to phagocytose Isochrysis galbana and several other species of unicellular algae, as well as Congo red-stained yeast cells. The “blunt” cytoplasmic granules received degraded materials from the phagosomes containing the algae but not those enclosing a yeast cell. Transfer of the degradation product(s) was traced by observing the fluorescence emission and by microspectrofluorimetric analysis of the phagocytosed material. The blunt granules also participated in the hemocyte's intracellular processing of vital dyes and endotoxin. It is suggested that the blunt granules are involved in the containment and/or further degradation of foreign materials taken up by the hemocytes.

The hard clam, Mercenaria mercenaria , shows a general response to environmental variation and infection. The molar ratio of free taurine to glycine in gill and mantle tissues climbs above 3, while α-amino acids and carbohydrates decrease. Subtle adjustments in the total pattern of free amino acids and fatty acids also occur, but these can be readily seen by changes in biochemical diversity and equitability. In an estuary long polluted with hydrocarbons, the calm population has a patchy distribution and a short life-span. These variations are attributed to a culmination of abnormal complications superimposed on natural responses to a seasonally changing environment. The process apparently starts after a tarlike irritant collects in epithelial tissue and eventually plugs the renal sac. This leads, indirectly, to invasion of shells by Polydora sp., a polychaete which is rarely found in hard clams. A syndrome with many facets soon becomes clear, but the situation can be identified and its results predicted by simply observing the responses of taurine and glycine in stressed and normal populations. When the molar ratio of taurine to glycine is less than 3, the population is normal; if it lies between 3 and 5, a chronic stress is indicated; and at values greater than 5 the situation is acute.