Showing papers on "Oyster published in 2003"

Estimated enhancement of fish production resulting from restoring oyster reef habitat: quantitative valuation

Abstract: We reviewed studies providing quantitative measurements of abundance of fishes and large mobile crustaceans on oyster reefs and on nearby sedimentary habitat in the southeast United States. For each species, we compared density by size (age) class on oyster reefs and sedimentary bottom as a means of estimating the degree to which restoration of oyster reef on sedimentary bottom could augment abundances. By applying published information on growth rates of each species and a combination of empirical data and published information on age-specific survivorship, we calcu- lated the per-unit-area enhancement of production of fishes and large mobile crustaceans expected from the addition of oyster reef habitat. For this calculation, we gave the reef habitat full credit for the expected lifetime production of species whose recruitment was judged to be limited by the area of oyster reefs based on nearly exclusive association of recruits to reefs. For species that were only mod- estly enhanced in abundance by oyster reefs, we gave the reef credit for the fraction of production that is derived from consumption of reef-associated prey, using a combination of gut content data and natural history information. This combination of analyses and calculations revealed that 10 m 2 of restored oyster reef in the southeast United States is expected to yield an additional 2.6 kg yr -1 of pro- duction of fish and large mobile crustaceans for the functional lifetime of the reef. Because the reef is biogenic and self-sustaining, the lifetime of a reef protected from bottom-disturbing fishing gear is limited by intense storms or sedimentation. A reef lasting 20 to 30 yr would be expected to augment fish and large mobile crustacean production by a cumulative amount of 38 to 50 kg 10 m -2 , discounted to present-day value. This set of calculations assumes that oyster reef habitat now limits production of reef-associated fish and crustaceans in the southeast United States. This assumption seems reasonable based on the tight associations of so many fishes with reef-dependent prey, and the depletion of reef habitat over the past century.

Effect of a mono-specific algal diet on immune functions in two bivalve species - Crassostrea gigas and Ruditapes philippinarum

Abstract: The impact of diets upon the fatty acid composition of haemocyte polar lipids and consequently upon immune parameters has been tested in the oyster Crassostrea gigas and the clam Ruditapes philippinarum. Oysters and clams were fed each of three cultured algae: Chaetoceros calcitrans, which is rich in 20:5(n-3) and 20:4(n-6) and poor in 22:6(n-3) fatty acids; T-Iso (Isochrysis sp.), which is rich in 22:6(n-3) and deficient in 20:5(n-3) and 20:4(n-6); and Tetraselmis suecica, which is deficient in 22:6(n-3) and contains only small amounts of 20:5(n-3) and 20:4(n-6). Fatty acid composition of haemocyte polar lipids was greatly affected by the diet. Oysters and clams fed C. calcitrans maintained a higher proportion of 20:5(n-3) and 20:4(n-6) in their haemocyte polar lipids, while these polyunsaturated fatty acids decreased drastically for animals fed T-Iso. However, the T-Iso diet maintained 22:6(n-3) in haemocyte polar lipids of both species. Higher 20:5(n-3) and 20:4(n-6) contents in diets appeared to have a positive effect upon total haemocyte count, granulocyte percentage, phagocytic rate and oxidative activity of clam haemocytes. Similarly, a positive effect of 20:5(n-3) on oxidative activity of oyster haemocytes was observed but to a lesser extent than in clams. Interestingly, when oyster haemocytes are submitted to a stressful condition, a positive effect of a higher dietary 22:6(n-3) content on the phagocytic rate was noticed.

Detection, quantitation, and phylogenetic analysis of noroviruses in Japanese oysters.

Abstract: Noroviruses (NVs) cause many cases of oyster- or clam-associated gastroenteritis in various countries. We collected 191 samples from Japanese oysters intended for raw consumption that had been harvested from the sea in two different areas between December 2001 and February 2002. To detect, quantitate, and phylogenetically analyze the NV genome in purified concentrates from the stomachs and digestive diverticula of these oysters, we amplified the NV capsid gene by reverse transcription-PCR. Phylogenetic analysis was performed by using the neighbor-joining method. We detected the NV genome in 17 of 191 oysters (9%). Phylogenetic analysis indicated genogroup I (Norwalk virus type) in 3 of the 17 oysters and genogroup II (Snow Mountain virus type) in the other 14. Both genogroups showed wide genetic diversity. To quantify the NV capsid gene in these oysters, we performed real-time PCR using genogroup-specific probes. More than 10(2) copies of the NV genome were detected in 11 of 17 oysters. The results suggested that about 10% of Japanese oysters intended for raw consumption harbored NVs, and more than 50% of those oysters in which NVs were detected had a large amount.

Characterization of 79 microsatellite DNA markers in the Pacific oyster Crassostrea gigas.

Abstract: We characterized 79 microsatellite DNA markers, which were obtained from genomic libraries enriched for CA, GA, ATG and TAGA motif repeats, in the Pacific oyster Crassostrea gigas. For eight F1 grandparents or great-grandparents of mapping families, the average heterozygosity, 0.705, and average number of alleles per locus, 5.7, did not vary among motif-repeat or motif-complexity categories. Non-amplifying polymerase chain reaction null alleles, which were confirmed by segregation in the mapping families, were detected at 41 (51.9%) of the 79 loci. Cross-species amplifications from C. angulata, C. sikamea, C. ariakensis and C. virginica showed a precipitous decline with distance from the focal species C. gigas.

Environmental significance of freshets in reducing Perkinsus marinus infection in eastern oysters Crassostrea virginica: potential management applications

Abstract: The effects of extreme freshwater events on Perkinsus marinus-Crassostrea virginica in- teractions remain unexplored. The effects of freshwater events on P. marinus infection in C. virginica and oyster survival were therefore examined in controlled laboratory experiments and a field study. For the laboratory experiments, oysters were collected in spring, summer and winter from an area in Louisiana where P. marinus is endemic. Oysters were placed in 2 recirculating water systems at a salinity and tem- perature similar to their collection site. They were subjected to 2 salinity treatments (freshet and control). Freshet events were simulated by reducing the water to salinities of 0 to 1 ppt over a 48 h period, and maintained for a 21 d period. Control oysters were maintained at the initial salinity. Thirty oysters were sampled prior to the freshet event, and 30 oysters per treatment group (freshet, control) were sampled on Days 7, 14 and 21 after initiation of the freshet event. Oyster mortality, P. marinus infection intensi- ties, oyster condition index and oyster plasma osmolality were measured in weekly samples. All 3 simulated freshet events (i.e. spring, summer, winter) resulted in a significant reduction in P. marinus infection intensity, but failed to eliminate infection. The failure of the oyster plasma to reach very low osmolality (<50 mOsm kg -1 ) provides a likely explanation for the lack of complete P. marinus elimina- tion. The field study involved sampling oysters monthly in the Caloosahatchee estuary, Florida, from September 2000 to February 2002, and determining P. marinus weighted prevalence and condition index of wild oysters, and growth and survival of caged juvenile oysters. The data strongly support the contention that the numerous freshwater releases to the Caloosahatchee River kept P. marinus infection intensities in oysters at low levels, resulting in an overall low weighted prevalence, low oyster mortality and good growth. Data from our field study appear to support the hypothesis that repetitive and well-timed freshet events can prevent infection of oysters with P. marinus or at least maintain P. marinus infections at non-lethal intensities (e.g. <10 6 parasites g -1 wet tissue) in oyster populations. The use of an adaptive management approach involving control of freshwater inflows could be invaluable to the oyster industry in areas close to freshwater diversion projects.

A novel ferritin subunit involved in shell formation from the pearl oyster (Pinctada fucata).

Abstract: Iron is one of the most important minor elements in the shell of bivalves. This study was designed to investigate the involvement of ferritin, the principal protein for iron storage, in shell formation. A novel ferritin cDNA from the pearl oyster (Pinctada fucata) was isolated and characterized. The ferritin cDNA encodes a 206 amino acid polypeptide, which shares high similarity with snail soma ferritin and the H-chains of mammalian ferritins. Oyster ferritin mRNA shows the highest level of expression in the mantle, the organ for shell formation. In situ hybridization analysis revealed that oyster ferritin mRNA is expressed at the highest level at the mantle fold, a region essential for metal accumulation and contributes to metal incorporation into the shell. Taken together, these results suggest that ferritin is involved in shell formation by iron storage. The identification and characterization of oyster ferritin also helps to further understand the structural and functional properties of molluscan ferritins.

Apoptosis in the pathogenesis of infectious diseases of the eastern oyster Crassostrea virginica.

Abstract: Apoptosis, or programmed cell death, has been reported as being pivotal in infectious diseases of different organisms. The effects of apoptosis on the progression and transmission of the protistan parasites Perkinsus marinus and Haplosporidium nelsoni in the eastern oyster Crassostrea virginica were studied. Oysters were diagnosed for their respective infections by standard methods, and apoptosis was detected using in situ hybridization to detect DNA fragments by end labeling on paraffin sections. A digoxigenin nucleotide probe was used to label the 200 bp fragment produced by apoptosis and detected immunohistochemically using an antidigoxigenin peroxidase conjugate. The probe/DNA fragment complex was stained with a peroxidase substrate and tissues were counterstained with methyl green. Uninfected oysters had large numbers of apoptotic hemocytes present in the connective tissue underlying the stomach, gill, and mantle epithelia, whereas oysters infected with P. marinus had a reduced number of apoptotic hemocytes. The parasite may prevent hemocyte apoptosis in order to yield a greater number of hemocytes in which to house itself. Large numbers of P. marinus cells in some infected oysters were eliminated via apoptosis in the stomach epithelia, disabling the spread of infectious particles through seawater. The oysters infected with H. nelsoni also had reduced numbers of apoptotic hemocytes, while part of the vesicular connective tissue cells were apoptotic. H. nelsoni plasmodia were eliminated via apoptosis in some oysters. Apoptosis may enhance progression and prevent transmission of infectious oyster diseases.

Virus-Contaminated Oysters: a Three-Month Monitoring of Oysters Imported to Switzerland

Abstract: Molluscan shellfish are known to be carriers of viral and bacterial pathogens. The consumption of raw oysters has been repeatedly linked to outbreaks of viral gastroenteritis and hepatitis A. Switzerland imports over 300 tons of oysters per year, 95% of which originate in France. To assess the level of viral contamination, a 3-month monitoring study was conducted. Therefore, the sensitivities of several previously described methods for virus concentration were compared, and one protocol was finally chosen by using dissected digestive tissues. Eighty-seven samples consisting of five oysters each were analyzed for Norwalk-like viruses (NLVs), enteroviruses, and hepatitis A viruses from November 2001 to February 2002. The oysters were exported by 31 French, three Dutch, and two Irish suppliers. Eight oyster samples from six French suppliers were positive for NLVs, and four samples from four French suppliers were positive for enteroviruses; two of the latter samples were positive for both viral agents. No hepatitis A viruses were detected. The sequences of NLV and enterovirus amplicons showed a great variety of strains, especially for the NLVs (strains similar to Bristol, Hawaii, Mexico, and Melksham agent). The data obtained indicated that imported oysters might be a source of NLV infection in Switzerland. However, further studies are needed to determine the quantitative significance of the risk factor within the overall epidemiology of NLVs.

Seasonal variation in the reproductive activity and biochemical composition of the Pacific oyster (Crassostrea gigas) from the Marlborough Sounds, New Zealand

Abstract: Reproductive cycles of the Pacific oyster Crassostrea gigas (Thunberg) from the Marlborough Sounds, New Zealand, were followed between June 1998 and January 2000. Histological examination of the gonad confirms an annual cycle with a winter inactive period followed by rapid gonad development and a single short spawning period. The population gonad index correlated with seawater temperature and changes in tissue dry weight, condition index, and biochemical components. In winter, few individuals with early gametogenic stages were present and rapid development of primary oocytes (diam. 11 μm) occurred during spring (September‐November). The developmental rate and the diameter of mature oocytes (37 μm) was similar for the 1998 and 1999 seasons. For a standard 110‐mm‐length oyster, maximal tissue body weight and condition index were recorded in December. Rapid weight loss in January was length dependent and was attributed to spawning. Temperature was the environmental variable which best correlated wit...

Comparative patterns of occupancy by decapod crustaceans in seagrass, oyster, and marsh-edge habitats in a Northeast Gulf of Mexico estuary

Abstract: Decapod crustaceans occupying seagrass, salt marsh edge, and oyster habitats within the St. Martins Aquatic Preserve along the central Gulf coast of Florida were quantitatively sampled using a 1-m2 throw trap during July–August 1999 and March–April 2000. Relative abundance and biomass were used as the primary measures to compare patterns of occupancy among the three habitat types. Representative assemblages of abundant and common species from each habitat were compared using Schoener's Percent Similarity Index (PSI). In all, 17,985 decapods were sampled, representing 14 families and 28 species. In the summer sampling period, mean decapod density did not differ between oyster and seagrass habitats, which both held greater densities of decapods than marsh-edge. In the spring sampling period oyster reef habitat supported greater mean decapod density than both seagrass and marsh-edge, which had similar densities of decapods. Habitat-specific comparisons of decapod density between the two sampling periods indicated no clear seasonal effect. In summer 1999, when seagrasses were well established, decapod biomass among the three habitats was not significantly different. During spring 2000, decapod biomass in oyster (41.40 gm−2) was greater than in marshedge (4.20 gm−2), but did not differ from that of seagrass (9.73 g m−2). There was no significant difference in decapod biomas between seagrass and marsh-edge habitats during the spring 2000 sampling period. The assemblage analysis using Schoener's PSI indicated that decapod assemblages associated with oyster were distinct from seagrass and marshedge habitats (which were similar). The results of this study suggest that in comparison to seagrass and marsh-edge habitats, oyster reef habitats and the distinct assemblage of decapod crustaceans that they support represent an ecologically important component of this estuarine system.

Fouling and predation; how do they affect growth and survival of the blacklip pearl oyster, Pinctada margaritifera, during nursery culture?

Abstract: This paper reports on 5 experiments conducted to assess the effect of cleaning regime and predation on growth and survival of blacklip pearl oyster (Pinctada margaritifera) juveniles in north Queensland, Australia. P. margaritifera juveniles with a mean (±SE) dorso-ventral shell height (DVH) of 4.5 ± 0.1 mm were placed into plastic mesh trays and cleaned either every 4 or 8 weeks or left uncleaned for 16 weeks. Cleaning regime had a significant effect on growth and survival (P < 0.005). Lowest DVH (16.2 ± 1.0) was shown by oysters in uncleaned trays during 16 weeks compared to oysters in cleaned trays; however, there was no significant difference in DVH between oysters held in trays cleaned every 4 (19.4 ± 1.2) or 8 weeks (21.2 ± 0.8). In contrast lowest survival was shown by oysters held in trays that were cleaned every 4 weeks (30 ± 5%), but no differences were noted between oysters cleaned every 8 weeks (63 ± 4%) and oysters that were left uncleaned for 16 weeks (75 ± 8%). Predators of P. margaritifera in northern Australia included crabs, stomatopods, flatworms, gastropods and fish. The stomatopod, Gonodactylus falcatus, was the most destructive predator with individuals consuming in excess of 20 juvenile pearl oysters per week. The leather jacket, Paramonocanthus japonicus, did not kill pearl oysters, but trimmed the margin of oysters shells significantly reducing DVH when compared to control groups cultured without fish. Removing predators monthly had a significant effect on growth of pearl oysters compared to oysters in non-inspected trays; however monthly inspection of culture trays did not significantly improve oyster survival.

Molecular Surveillance of Enterovirus and Norwalk-Like Virus in Oysters Relocated to a Municipal-Sewage-Impacted Gulf Estuary

Abstract: An 18-month survey was conducted to examine the prevalence of enteric viruses and their relationship to indicators in environmentally polluted shellfish. Groups of oysters, one group per 4 weeks, were relocated to a coastal water area in the Gulf of Mexico that is impacted by municipal sewage and were analyzed for enteroviruses, Norwalk-like viruses (NLV), and indicator microorganisms (fecal coliform, Escherichia coli, and male-specific coliphages). The levels of indicator microorganisms were consistent with the expected continuous pollution of the area. Fourteen of the 18 oyster samples were found by reverse transcription (RT)-PCR to harbor NLV and/or enterovirus sequences. Of the four virus-negative oysters, three had exposure to water temperatures of >29°C. Concomitant with these findings, two of these four oysters also accumulated the lowest levels of coliphages. PCR primers targeting pan-enteroviruses and the NLV 95/96-US common subset were utilized; NLV sequences were detected more frequently than those of enteroviruses. Within the 12-month sampling period, NLV and enterovirus sequences were detected in 58 and 42%, respectively, of the oysters (67% of the oysters tested were positive for at least one virus) from a prohibited shellfish-growing area approximately 30 m away from a sewage discharge site. Eight (4.6%) of the 175 NLV capsid nucleotide sequences were heterogeneous among the clones derived from naturally polluted oysters. Overall, enteric viral sequences were found in the contaminated oysters throughout all seasons except hot summer, with a higher prevalence of NLV than enterovirus. Although a high percentage of the oysters harbored enteric viruses, the virus levels were usually less than or equal to 2 logs of RT-PCR-detectable units per gram of oyster meat.

Heavy metals in oysters, mussels and clams collected from coastal sites along the Pearl River Delta, South China.

Abstract: Concentrations of 8 heavy metals: cadmium (Cd), copper (Cu), zinc (Zn), lead (Pb), nickel (Ni), chromium (Cr), antimony (Sb) and tin (Sn) were examined in 3 species of bivalves ( Perna viridis, Crassostrea rivularis and Ruditapes philippinarum) collected from 25 sites along the Pearl River Delta coastal waters in the South China Sea from July to August 1996. In general, Cd, Cu, Zn and Sn concentrations in the three bivalve species collected from the Estuarine Zone were significantly higher than those collected from the Western and Eastern Zones of the Pearl River Delta, which are related to the existence of various anthropogenic activities in the catchment of the Pearl River Delta. The Western Estuarine Zone is mainly impacted hy Cr, Ni and Cu contamination. In Victoria Harbor, heavy metal contamination is mainly due to Cu and Pb, Cd, Cu and Zn concentrations in oysters were significantly higher than those in mussels and clams. This could be explained by the fact that oysters live mainly in the Estuarine Zone of the Pearl River Delta which receives most of the polluting discharges from the catchment of the Delta. During turbid condition, heavy metals( soluble or adsorbed on suspended particulates) discharged from the Delta are filtered from the water column and subsequently accumulated into the soft body tissues of oysters. Heavy metal concentrations in the three bivalve species were compared with the maximum permissible levels of heavy metals in seafood regulated by the Public Health and Municipal Services Ordinance, Laws of Hong Kong, and it was revealed that Cd and Cr concentrations in the three bivalve species exceeded the upper limits. At certain hotspots in the Delta, the maximum acceptable daily load for Cd was also exceeded.

Biomonitoring of trace metal contamination in mangrove-lined Brazilian coastal systems using the oyster Crassostrea rhizophorae : comparative study of regions affected by oil, salt pond and shrimp farming activities

Abstract: The oyster Crassostrea rhizophorae has been used as a biomonitor of trace metal contamination in two Brazilian coastal systems. C. rhizophorae were collected in January 1998 from 15 stations (from 4 coastal inlets (including 1 estuary) and 1 coastal beach) near Macau, Rio Grande do Norte (RN), Brazil, a region affected by the activities of the oil industry and salt manufacture in coastal salt ponds; oysters were also collected in September 1999 from 8 stations in the Curimatau estuary (RN), an estuary becoming increasingly affected by shrimp farming activities. C. rhizophorae is a net accumulator of trace metals and can be used as a biomonitor, the accumulated soft tissue concentrations representing integrated records of bioavailable metal over the life of the oyster. At Macau, significant differences in oyster accumulated concentrations (and hence bioavailabilities to the oyster) of Fe, Zn, Cu, and Mn were found between stations; raised zinc availabilities at the coastal site are in close proximity to oil industry activities but the very high availabilities of Fe, Cu and Mn in the Rio dos Cavalos estuary originate from an unknown source. In the Curimatau estuary, bioavailabilities of Mn, Pb and Cd, but particularly of Cu and Zn, to the oysters are raised at the two most downstream sites, the only sites below the effluent of a large shrimp farming enterprise. The oysters also act as a local food source, and concentrations of Zn, Cu and Pb of some of the oysters are above typical public health recommended limits.

Evaluation of damage in Pacific oyster (Crassostrea gigas) spermatozoa before and after cryopreservation using comet assay.

Abstract: We examined the applicability of the comet assay (single cell gel electrophoresis assay) to estimate the quality of frozen-thawed Pacific oyster (Crassostrea gigas) spermatozoa. Comet assay was performed on semen before and after cryopreservation followed by fluorescent staining with propidium iodide to assess DNA integrity. After cryopreservation, the percentage of spermatozoa with damaged DNA significantly increased, while only about half of the cells displayed intact DNA, even when protected with 10 percent DMSO. All the considered parameters (head length, head area, head intensity, total length, total area, total intensity, tail length percent, tail area percent, and tail intensity percent) were higher than the oyster sperm protected with 10 percent DMSO-artificial sea water after freezing and thawing. Only tail length percent, tail area percent, and tail intensity percent were increased significantly after cryopreservation. The tail length percent was found to be the most sensitive indicator of the cryopreservation-induced DNA damage. Our freeze-thawing procedure significantly affected oyster sperm DNA, as indicated by the reduced fertilization rate when frozen-thawed oyster sperm are used. Irreversible alteration of the genome may prevent fertilization or alter normal embryonic development. This study is the first to demonstrate that the comet assay is an inexpensive, rapid and sensitive method for determining DNA damage in Pacific oyster sperm quality assessments.

The American slipper limpet Crepidula fornicata (L.) in the northern Wadden Sea 70 years after its introduction

Abstract: In 1934 the American slipper limpet Crepidula fornicata (L.) was first recorded in the northern Wadden Sea in the Sylt-Romo basin, presumably imported with Dutch oysters in the preceding years. The present account is the first investigation of the Crepidula population since its early spread on the former oyster beds was studied in 1948. A field survey in 2000 revealed the greatest abundance of Crepidula in the intertidal/subtidal transition zone on mussel (Mytilus edulis) beds. Here, average abundance and biomass was 141 m–2 and 30 g organic dry weight per square metre, respectively. On tidal flats with regular and extended periods of emersion as well as in the subtidal with swift currents in the gullies, Crepidula abundance was low. The main substrate of attachment was live mussels. Compared with the years following their initial introduction, Crepidula is more abundant today and has shifted from the now extinct oyster beds to the epifaunal community of the mussel beds. Their present abundance is considerably lower than at more southern European coasts where the species may dominate the epifauna. Low winter temperatures are suggested to have limited the population expansion in the northern Wadden Sea until now.

Low Dose Gamma Irradiation to Reduce Pathogenic Vibrios in Live Oysters (Crassostrea virginica)

Abstract: Pathogenic strains of Vibrio (Vibrio vulnificus and V. parahaemolyticus), natural inhabitants of estuarine and ocean environments, can cause serious illness and death in susceptible persons from consumption of raw half-shell oysters. Objectives of this study were (1) to establish the irradiation dose needed to reduce pathogenic vibrios to nondetectable levels and (2) to determine consumer's ability to differentiate between irradiated and control oysters. Live oysters, Crassostrea virginica, with naturally incurred and artificially inoculated pathogenic vibrios, were exposed to 0-3 kGy dose Cobalt-60 gamma radiation. Vibrio vulnificus (MO-624) was reduced from 106 cfu/g oyster meat to nondetectable levels (< 3 mpn/g oyster meat) with 0.75-1.0 kGy irradiation exposure. Vibrio parahaemolyticus, 03:K6 (TX-2103), required 1.0-1.5 kGy for reduction to nondetectable levels. Using triangle difference testing, sensory panelists were asked to identify differences between treated (1 kGy) and untreated oyste...

Systematic evaluation of factors controlling Perkinsus marinus transmission dynamics in lower Chesapeake Bay.

Abstract: The transmission of Perkinsus marinus in eastern oysters Crassostrea virginica in rela- tion to water temperature, host oyster mortality, and water-column abundance of anti-P. marinus antibody-labeled cells was systematically examined for 20 mo at a site in the lower York River, Vir- ginia, USA. Uninfected sentinel oysters were naturally exposed to the parasite at 2 wk intervals throughout the course of the study to determine the periodicity and rates of parasite transmission. The timing and magnitude of disease-associated oyster mortalities in a local P. marinus-infected oys- ter population were estimated by monitoring a captive subset of the local oyster population. Flow cytometric immunodetection methods were employed to estimate the abundance of P. marinus cells in water samples collected 3 times each week. The acquisition of P. marinus infections by naive sen- tinel oysters occurred sporadically at all times of the year; however, the highest incidence of infection occurred during the months of August and September. This window of maximum parasite transmis- sion coincided with the death of infected hosts within the captive local oyster population. Counts of antibody-labeled cells ranged from 10 to 11 900 cells l -1 , with the highest abundances in July and August coincident with maximum summer temperatures. A statistically significant relationship between water-column parasite abundance and infection-acquisition rate was not observed; how- ever, highest parasite-transmission rates in both years occurred during periods of elevated water- column abundance of parasite cells. These results support the prevailing model of P. marinus trans- mission dynamics by which maximum transmission rates are observed during periods of maximum P. marinus-associated host mortality. However, our results also indicate that transmission can occur when host mortality is low or absent, so alternative mortality-independent dissemination mechanisms are likely. The results also suggest that atypically early-summer oyster mortality from Haplosporid- ium nelsoni infection, at a time when infections of P. marinus are light, has a significant indirect influ- ence on P. marinus transmission dynamics. Elimination of these hosts prior to late-summer P. marinus infection-intensification effectively reduces the overall number of P. marinus cells disseminated.