Experimental evaluation of the pathogenicity of Perkinsusolseni in juvenile Manila clams Ruditapes philippinarum

We evaluated the pathogenicity of Perkinsus olseni towards the Manila clam, Ruditapes philippinarum, by an experimental challenge. For production of prezoosporangia of P. olseni, we injected uninfected Manila clams with cells of a pure strain of P. olseni and reared them for 7 d. Prezoosporangia were isolated from the soft tissue of the injected clams after culturing in Ray’s fluid thioglycollate medium. Hatchery-reared, uninfected juvenile clams (3-10 mm shell length) were challenged by immersion in one of two concentrations of a prezoosporangial suspension of P. olseni for 6 d. The challenged clams had significantly higher mortality at both the concentrations than the unchallenged clams. The mortality due to infection dose-dependently began approximately 4 weeks and 7 weeks after challenge in the higher and lower concentrations, respectively. This is the first experimental evidence that P. olseni causes direct mortality in Manila clams. The lethal level of infection was estimated at approximately 10⁷ pathogen cells/g soft tissue weight.     

Isolation and identification of Perkinsus olseni from feces and marine sediment using immunological and molecular techniques

Molecular and immunological probes were used to identify various life stages of Perkinsus olseni, a protozoan parasite of the Manila clam Ruditapes philippinarum, from a marine environment and decomposing clam tissue. Western blotting revealed that the antigenic determinants of the rabbit anti-P. olseni antibody developed in this study were peptides with molecular masses of 55.9, 24.0, and 19.2 kDa. Immunofluorescent assay indicated that the rabbit anti-P. olseni IgG was specific to all life stages, including the prezoosporangium, trophozoite, and zoospore. Perkinsus olseni prezoosporangium-like cells were successfully isolated from marine sediment collected from Hwangdo on the west coast of Korea, where P. olseni-associated clam mortality has recurred for the past decade. Purified cells were positively stained with the rabbit anti-P. olseni antibody in an immunofluorescence assay, confirming for the first time the presence of P. olseni in marine sediment. Actively replicating zoospores inside the prezoosporangia were observed in the decomposing clam tissue collected from Hwangdo. P. olseni was also isolated from the feces and pseudofeces of infected clams and confirmed by PCR. The clams released 1-2 prezoosporangia per day through feces. The data suggested that the fecal discharge and decomposition of the infected clam tissue could be the two major P. olseni transmission routes.     

Pathology survey of the short-neck clam Ruditapes philippinarum occurring on sandy tidal flats along the coast of Ariake Bay, Kyushu, Japan

The pathological condition of the short-neck clam Ruditapes philippinarum was surveyed along the coast of Kumamoto, Japan, in June 2004. DNA sequences of the non-transcribed spacer region and internal transcribed spacer region flanking 5.8S rRNA identified Perkinsus olseni among the clams. Ray’s fluid thioglycollate medium assay indicated that 96.7% of the clams surveyed from the Kiguchi River tidal flat (native clams, Stn KR-N) and 96.7% of the clams surveyed from the Midori River tidal flat (Stn MR) were infected with P. olseni with an infection intensity of 464,278 and 199,937 Perkinsus cells/gram tissue wet weight (gWW), respectively. In contrast, 66.7% of the clams imported from China and stored along the Kiguchi River tidal flat (Stn KR-I) and 20.2% of clams from the Arao tidal flat (Stn AT) were infected with P. olseni with an infection intensity of 37,547 and 3382 Perkinsus cells/gWW, respectively. Brown ring disease was detected in the clam population from Stn KR-I at a prevalence of 90.0%. Polymerase chain reaction and the 16S rRNA sequence suggested that the agents of brown ring disease observed at Stn KR-I were Vibrio tapetis-like bacteria. Sporocysts and metacercariae of unidentified trematodes were also observed in the gonads and mantle of the clams from Stn KR-I, Stn MR, and Stn AT, at prevalences of 7.1-42.9%. Metacestodes (larval tapeworms) were found in the foot and digestive gland at a prevalence of 52.5%, 30.0%, and 14.3% in clams from Stns MR, AT, and KR-N, respectively. Histology also showed massive hemocyte infiltration and inflammation among clams heavily infected with P. olseni. Castration of the follicle was typical among clams infected with the trematode. The data indicate that most of the clams along the coast of Kumamoto are infected with various pathogens at various rates of infection, and these pathogens could have negative effects on the clam population in the long term.     

Molecular and histological identification of Marteilioides infection in Suminoe Oyster Crassostrea ariakensis, Manila Clam Ruditapes philippinarum and Pacific Oyster Crassostrea gigas on the south coast of Korea

The oyster ovarian parasite Marteilioides chungmuensis has been reported from Korea and Japan, damaging the oyster industries. Recently, Marteilioides-like organisms have been identified in other commercially important marine bivalves. In this study, we surveyed Marteilioides infection in the Manila clam Ruditapes philippinarum, Suminoe oyster Crassostrea ariakensis, and Pacific oyster Crassostrea gigas, using histology and Marteilioides-specific small subunit (SSU) rDNA PCR. The SSU rDNA sequence of M. chungmuensis (1716 bp) isolated from C. gigas in Tongyoung bay was 99.9% similar to that of M. chungmuensis reported in Japan. Inclusions of multi-nucleated bodies in the oocytes, typical of Marteilioides infection, were identified for the first time in Suminoe oysters. The SSU rDNA sequence of a Marteilioides-like organism isolated from Suminoe oysters was 99.9% similar to that of M. chungmuensis. Marteilioides sp. was also observed from 7 Manila clams of 1840 individuals examined, and the DNA sequences of which were 98.2% similar to the known sequence of M. chungmuensis. Unlike Marteilioides infection of Pacific oysters, no remarkable pathological symptoms, such as large multiple lumps on the mantle, were observed in infected Suminoe oysters or Manila clams. Distribution of the infected Manila clams, Suminoe oysters and Pacific oysters was limited to small bays on the south coast, suggesting that the southern coast is the enzootic area of Marteilioides infection.     

First record of Perkinsus olseni, a protozoan parasite infecting the commercial clam Ruditapes decussatus in Spanish Mediterranean waters

We present the first record in Spanish Mediterranean waters of the protozoan parasite Perkinsus olseni infecting the clam Ruditapes decussatus. Perkinsus infection was detected all year around albeit at a low level of infection intensity. Histological analysis, induction of zoospores and in situ hybridisation assay confirmed the presence of Perkinsus sp. The identity of the parasite was determined by species-specific PCR assay in DNA samples obtained from infected clams. Sequencing of amplified fragments showed 100% identity to the ITS region of P. olseni. We confirmed for the first time the presence of P. olseni in Spanish Mediterranean waters.     

Resistance of the Manila clam (Venerupis philippinarum) to infection with Mikrocytos mackini

Manila clams (Venerupis philippinarum) challenged in laboratory trials via bath exposure proved to be resistant to infections with Mikrocytos mackini (protistan parasite of unknown taxonomic affiliation), while Pacific oysters (Crassostrea gigas) challenged simultaneously using identical conditions developed infections. Although M. mackini was detected by a nucleic acid pathogen specific (PCR) assay in 10-30% of the challenged V. philippinarum that were sampled soon after exposure (0-48 h, n = 40), all of the subsequent V. philippinarum (n = 62) sampled 9-17 weeks post-exposure tested negative for M. mackini by PCR assay. Prevalence of infection for the exposed C. gigas (n = 100) during this same period ranged from 50% to 100% by PCR assay. Infection was confirmed in the oysters (58%, n = 60) by a digoxigenin-labelled DNA probe designed to detect M. mackini by in situ hybridization, but M. mackini was not found in any of the exposed Manila clams (n = 63) using this technique.     

Pathology and immune response of the blue mussel (Mytilus edulis L.) after an exposure to the harmful dinoflagellate Prorocentrum minimum

The harmful dinoflagellate Prorocentrum minimum has different effects upon various species of grazing bivalves, and these effects also vary with life-history stage. Possible effects of this dinoflagellate upon mussels have not been reported; therefore, experiments exposing adult blue mussels, Mytilus edulis, to P. minimum were conducted. Mussels were exposed to cultures of toxic P. minimum or benign Rhodomonas sp. in glass aquaria. After a short period of acclimation, samples were collected on day 0 (before the exposure) and after 3, 6, and 9 days of continuous-exposure experiment. Hemolymph was extracted for flow-cytometric analyses of hemocyte, immune-response functions, and soft tissues were excised for histopathology. Mussels responded to P. minimum exposure with diapedesis of hemocytes into the intestine, presumably to isolate P. minimum cells within the gut, thereby minimizing damage to other tissues. This immune response appeared to have been sustained throughout the 9-day exposure period, as circulating hemocytes retained hematological and functional properties. Bacteria proliferated in the intestines of the P. minimum-exposed mussels. Hemocytes within the intestine appeared to be either overwhelmed by the large number of bacteria or fully occupied in the encapsulating response to P. minimum cells; when hemocytes reached the intestine lumina, they underwent apoptosis and bacterial degradation. This experiment demonstrated that M. edulis is affected by ingestion of toxic P. minimum; however, the specific responses observed in the blue mussel differed from those reported for other bivalve species. This finding highlights the need to study effects of HABs on different bivalve species, rather than inferring that results from one species reflect the exposure responses of all bivalves.     

Development of rRNA and rDNA-targeted probes for fluorescence in situ hybridization to detect Heterosigma akashiwo (Raphidophyceae)

Heterosigma akashiwo (Hada) is a fragile, fish-killing alga. Efforts to understand and prevent blooms due to this harmful species to mitigate the impact on aquaculture require the development of methods for rapid and precise identification and quantification, so that adequate warning of a harmful algal bloom may be given. Here, we report the development and application of rRNA and rDNA-targeted oligonucleotide probes for fluorescence in situ hybridization (FISH) to aid in the detection and enumeration of H. akashiwo. The designed probes were species specific, showing no cross-reactivity with four common HAB causative species: Prorocentrum micans Ehrenberg, P. minimum (Pavillard) Schiller, Alexandrium tarmarense (Lebour) Balech, and Skeletonema costatum (Greville) Cleve, or with four other microalgae, including Gymnodinium sp. Stein, Platy-monas cordiformis (Karter) Korsch, Skeletonema sp.1 Greville and Skeletonema sp.2. The rRNA-targeted probe hybridized to cytoplasmic rRNA, showing strong green fluorescence throughout the whole cell, while cells labeled by rDNA-targeted probe exhibited exclusively fluorescent nucleus. The detection protocols were optimized and could be completed within an hour. For rRNA and rDNA probes, about a corresponding 80% and 70% of targeted cells could be identified and quantified during the whole growth circle, despite the inapparent variability in the average probe reactivity. The established FISH was proved promising for specific, rapid, precise, and quantitative detection of H. akashiwo.     

Maternal and neonate diatom diets impair development and sex differentiation in the copepod Temora stylifera

Development and sex differentiation in the copepod Temora stylifera was studied in the presence of maternal and larval diets of the diatoms Thalassiosira rotula and Skeletonema marinoi, either provided alone or supplemented with the control dinoflagellate Prorocentrum minimum. Both diatoms had deleterious effects on growth compared to the control when used as pure diets, inducing very low or even zero survival from hatching to adulthood. This effect was deleted when the diet was supplemented with a good food (P. minimum) only in the case of T. rotula. By contrast, with a maternal or larval diet of S. marinoi, nauplii did not pass metamorphosis even when this alga was mixed with P. minimum. Arrested development was not due to lack of feeding since early and late nauplii (NII and NV) ingested all three algae at similar rates when used as single diets, and did not show any preference when the algae were offered as mixtures. Since mortality rates with a mixed diet of S. marinoi + P. minimum were similar or even higher than those obtained with a single diet of S. marinoi, we suggest that this diatom is more toxic than nutritionally deficient for T. stlyfera development. No males were produced in cohorts reared on pure diatom diets or with a mixture of S. marinoi + P. minimum, and intermediate male:female sex ratios were obtained with the mixed T. rotula + P. minimum diet. Possibly some diatoms produce compounds such as oxylipins or new molecules that alter sex differentiation in T. stylifera.     

Effects of the mermithid nematode Ovomermis sinensis on the hemocytes of its host Helicoverpa armigera

Little is known about the mechanism by which mermithid nematodes avoid encapsulation responses of insect hosts. In this study, we investigated the influence of the mermithid nematode Ovomermis sinensis on host Helicoverpa armigera hemocyte number, encapsulation activity, spreading behavior and cytoskeleton. Parasitism by O. sinensis caused a significant increase in the total hemocyte counts (THC) and plasmatocyte numbers of H. armigera. However, in vivo encapsulation assays revealed that hemocyte encapsulation abilities of H. armigera were suppressed by O. sinensis. Moreover, parasitism by O. sinensis changed the spreading behavior and cytoskeletons of the host hemocytes. The results suggested that O. sinensis could actively suppress the hemocyte immune response of its host, possibly by destroying the host hemocyte cytoskeleton. This is the first report of a possible mechanism by which mermithid nematodes suppress encapsulation responses of insect hosts.     

Differential in vivo response of soft-shell clam hemocytes against two strains of Vibrio splendidus: Changes in cell structure, numbers and adherence

Host-pathogen interaction models in aquatic species are useful tools for understanding the pathogenicity of diseases in cultured and wild populations. In this study we report the differential in vivo response of soft-shell clam (Mya arenaria) hemocytes against two strains of Vibrio splendidus. Responses were measured 24 h after injecting into the posterior adductor muscle either an endemic wild-type strain (7SHRW) or a strain associated with oyster mortalities (LGP32-GFP). Changes in hemocyte structure (percentage of rounded cells) were assessed microscopically. Changes in adherence and hemocyte numbers were analyzed by flow-cytometric cell counting. Increased percentages of rounded cells were found in response to both strains. However, values from the group infected with LGP32-GFP were significantly higher (p < 0.01) than with 7SHRW. The cell adherence was markedly diminished (p < 0.001) by LGP32-GFP whereas 7SHRW did not change it significantly. Increased numbers of hemocytes (p < 0.001) were induced by LGP32-GFP, while no significant changes were found after infection with 7SHRW. These results show the regulatory capacity of soft-shell clams hemocytes to perform specific responses against different strains of V. splendidus.     

瓦氏黄颡鱼2种寄生车轮虫形态分类

采用国际上使用的"统一特定描述"和"齿体定位描述"方法对寄生在瓦氏黄颡鱼(Peleteobagrusvachelli)鳃上的适度车轮虫(Trichodina modesta)和马氏车轮虫(T.mastu)2种车轮虫的虫体外形、附着盘、齿体、辐线、中央颗粒、核器、口围绕体等形态特征进行了较为详细的描述,并分别与已报道的其他适度车轮虫、马氏车轮虫种群及相似种的形态特征进行了比较。实验证实了瓦氏黄颡鱼为适度车轮虫的寄主新纪录。     

不同年龄段大连群体菲律宾蛤仔EST-SSR多样性

为查明年龄结构对菲律宾蛤仔同一群体内遗传多样的影响,采用14个SSR分子标记对大连石河不同年龄段的野生蛤仔进行了检测。结果表明:不同年龄段(1龄-Age1、2龄-Age2、3龄-Age3)蛤仔均维持着较高的遗传多样性。根据POPGENE 1.31和SPSS16.0统计分析显示,位点Rp-11、Rp-12、Rp-19对3个年龄段蛤仔的等位基因数差异极显著(P<0.01);位点Rp-20、Rp-24、Rp-27、Rp-30对其差异显著(P<0.05);剩余7个位点表现为差异不显著(P>0.05)。在平均水平上,每位点等位基因数目Na为4.3095,有效等位基因数目Ne为2.3729,多态位点百分数P(%)为14。观察杂合度和期望杂合度都比较高,观察杂合度平均为Ho=0.2335,期望杂合度平均为He=0.5140。而且,Ne和He随年龄的变化表现出Age2>Age3>Age1的趋势。各年龄段蛤仔——Age1、Age2、Age3的平均观察杂合度(Ho)和平均期望杂合度(He)分别为0.2357、0.2546、0.2159和0.4951、0.5286、0.5184。Age2的遗传多样性指数高于Age1及Age3,遗传分化相对较低。其中,Age1与Age3蛤仔遗传距离最小,D为0.0195,即变异很小;而Age1与Age2遗传距离较大,D为0.0437,变化范围不大(0.0195—0.0437)。从遗传一致度的数值上看了3个年龄段蛤仔的遗传相似程度很大,平均为0.9655。Age1与Age3遗传相似程度高达0.9807,而Age1与Age2相似程度较小为0.9572。说明不同年龄段蛤仔相似程度非常高。根据不同年龄段蛤仔的遗传距离,采用UPGMA平均聚类方法对其进行聚类可知,Age3与Age1蛤仔间遗传距离较小,与Age2蛤仔差异较大。通过对等位基因频率进行卡方检验发现,随着年龄结构的变化,部分基因基因频率减小;同时随着年龄的增长,有部分等位基因得到了纯化。大连群体蛤仔总的遗传分化较低,其遗传分化指数Fst为0.0248(Fst<0.05),遗传分化系数为0.02,说明总的遗传变异中有2%来自于不同年龄段的蛤仔之间。遗传距离和遗传一致度均值分别为0.035和0.9655,基因流(Nm=9.8238)相对流畅,进一步表明年龄结构对蛤仔种群内遗传分化的影响较小。     

Gymnophallid digenean Parvatrema duboisi uses Manila clam as the first and second intermediate host

To identify the metacercariae of a gymnophallid trematode in the Manila clam Ruditapes philippinarum from the Ariake Sea, experimental infection and molecular analysis were conducted. Based on the morphology of adult worms obtained from experimentally infected mice, the parasite was identified as Parvatrema duboisi. Comparison of internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) sequences for metacercariae and sporocysts from Manila clams and adult worms collected from wild wigeon Anas penelope showed minor differences ranging from 0 to 0.8%. These data strongly suggest that in the Ariake Sea, the parasite has a lifecycle using the Manila clam as the first and second intermediate hosts and wigeon as the definitive host.     

New evidence for the involvement of Paracartia grani (Copepoda,Calanoida) in the life cycle of Marteilia refringens (Paramyxea)

The dynamics of the protozoan parasite Marteilia refringens was studied in Thau lagoon, an important French shellfish site, for 1 year in three potential hosts: the Mediterranean mussel Mytilus galloprovincialis (Mytiliidae), the grooved carpet shell Ruditapes decussatus (Veneriidae) and the copepod Paracartia grani (Acartiidae). Parasite DNA was detected by PCR in R. decussatus. In situ hybridisation showed necrotic cells of M. refringens in the digestive epithelia of some R. decussatus suggesting the non-involvement of this species in the parasite life cycle. In contrast, the detection of M. refringens in mussels using PCR appeared bimodal with two peaks in spring and autumn. Histological observations of PCR-positive mussels revealed the presence of different parasite stages including mature sporangia in spring and autumn. These results suggest that the parasite has two cycles per year in the Thau lagoon and that mussels release parasites into the water column during these two periods. Moreover, PCR detection of the parasite in the copepodid stages of P. grani between June and November supports the hypothesis of the transmission of the parasite from mussels to copepods and conversely. In situ hybridisation performed on copepodites showed labeling in some sections. Unusual M. refringens cells were observed in the digestive tract and the gonad from the third copepodid stage, suggesting that the parasite could infect a copepod by ingestion and be released through the gonad. This hypothesis is supported by the PCR detection of parasite DNA in copepod eggs from PCR-positive females, which suggests that eggs could contribute to the parasite spreading in the water and could allow overwintering of M. refringens. Finally, in order to understand the interactions between mussels and copepods, mussel retention efficiency (number of copepods retained by a mussel) was measured for all P. grani developmental stages. Results showed that all copepod stages could contribute to the transmission of the parasite, especially eggs and nauplii which were retained by up to 90%.     

Comparative studies on the allelopathic effects of two different strains of Ulva pertusa on Heterosigma akashiwo and Alexandrium tamarense

Allelopathic effects of fresh tissue and dry powder of a nonsexual and a sexual strain of the macroalga Ulva pertusa on the growth of the microalgae Heterosigma akashiwo and Alexandium tamarense were evaluated using long-term coexistence culture systems in which several concentrations of macroalga fresh tissue and dry powder were used. The effects of macroalga culture medium filtrate on the two HAB algae were also investigated. Moreover, isolation co-culture systems were built to confirm the existence of allelochemicals and preclude the growth inhibition by direct contact. Short-term algicidal effect assays of macroalgae on H. akashiwo were carried out to measure the rate of algal cell lysis. The results of the coexistence assays showed that the growth of H. akashiwo and A. tamarense was strongly inhibited by fresh tissue and by dry powder of both strains of U. pertusa. The allelochemicals were lethal to H. akashiwo at relatively higher concentrations. The macroalga culture medium filtrate exhibited no apparent growth inhibitory effect on the two HAB algae under initial or semicontinuous filtrate addition, which suggested that continuous release of small quantities of rapidly degradable allelochemicals from the fresh tissue of both strains of U. pertusa was essential to effectively inhibit the growth of H. akashiwo and A. tamarense.     

东海原甲藻与中肋骨条藻的种间竞争数值模拟

研究探讨了两个零维箱式模型在东海典型赤潮藻东海原甲藻和中肋骨条藻竞争与演替研究中的应用。模型在采用不同接种密度下的单种培养实验数据进行参数校正后,被用来模拟不同N/P条件下单种培养实验以及两藻种共培养竞争实验,并以实验数据对其结果进行了验证。模拟结果表明,在单种培养条件下,模型能够较好地重现两种藻在不同N/P环境中的生长及对营养盐的利用;共培养实验的模拟结果显示,在所有初始细胞密度比例条件下,中肋骨条藻的最终密度均会超过东海原甲藻,且PO4的消耗主要源于中肋骨条藻的利用,与实验结果一致,表明模型能够很好地体现两种藻的竞争结果及对营养盐的竞争关系;由于模型不足以模拟除营养盐竞争以外的藻间相互作用,模拟结果未体现东海原甲藻细胞数迅速衰减这一现象,有待进一步研究。     

Functional genomics of hsp-90 in parasitic and free-living nematodes

Heat shock protein 90 (Hsp-90) is a highly conserved essential protein in eukaryotes. Here we describe the molecular characterisation of hsp-90 from three nematodes, the free-living Caenorhabditis elegans (Ce) and the parasitic worms Brugia pahangi (Bp) and Haemonchus contortus (Hc). These molecules were functionally characterised by rescue of a Ce-daf-21 (hsp-90) null mutant. Our results show a gradient of rescue: the C. elegans endogenous gene provided full rescue of the daf-21 mutant, while Hc-hsp-90 provided partial rescue. In contrast, no rescue could be obtained using a variety of Bp-hsp-90 constructs, despite the fact that Bp-hsp-90 was transcribed and translated in the mutant worms. daf-21 RNA interference (RNAi) experiments were carried out to determine whether knock-down of the endogenous daf-21 mRNA in N2 worms could be complemented by expression of either parasite gene. However neither parasite gene could rescue the daf-21 (RNAi) phenotypes. These results indicate that factors other than the level of sequence identity are important for determining whether parasite genes can functionally complement in C. elegans.     

Propagation of the myxozoan parasite Myxobolus cerebralis by different geographic and genetic populations of Tubifex tubifex: An Oregon perspective

Tubifex tubifex are obligate invertebrate hosts in the life cycle of Myxobolus cerebralis, the myxozoan parasite that causes whirling disease in salmonid fishes. This exotic parasite is established to varying degrees across Oregon’s Columbia River system (Pacific Northwest, USA) and characteristics of local T. tubifex populations likely play a role in the pattern of disease occurrence. To better understand these patterns, we collected T. tubifex from three Oregon river basins (Willamette, Deschutes, and Grande Ronde), determined their genotype (mitochondrial 16S rDNA lineage and RAPD genotype) and exposed 10 different populations to M. cerebralis in the laboratory. Four mt lineages were identified: I, III, V and VI. Lineage III was found in all river basins but dominated both central and eastern sites. The RAPD assay further divided these lineages into geographic sub-populations; no RAPD genotype was common to all basins. There was a significant difference in prevalence of infection and level of parasite production among the populations we exposed to M. cerebralis that was attributed to genotypic composition. Only lineage III worms released actinospores and only populations dominated by this lineage amplified the parasite. These populations had the lowest survival, however, the lineage dominant before exposure remained dominant despite the high prevalence of infection. The distribution and infection dynamics of susceptible T. tubifex throughout Oregon may contribute to the differences in M. cerebralis occurrence; our studies further support the influence of oligochaete genotypes on the manifestation of whirling disease in salmonid populations.     

Effects of natural and field-simulated blooms of the dinoflagellate Prorocentrum minimum upon hemocytes of eastern oysters, Crassostrea virginica, from two different populations

Oysters, Crassostrea virginica, from two populations, one from a coastal pond experiencing repeated dinoflagellate blooms (native), and the other from another site where blooms have not been observed (non-native), were analyzed for cellular immune system profiles before and during natural and simulated (by adding cultured algae to natural plankton) blooms of the dinoflagellate Prorocentrum minimum. Significant differences in hemocytes between the two oyster populations, before and after the blooms, were found with ANOVA, principal components analysis (PCA) and ANOVA applied to PCA components. Stress associated with blooms of P. minimum included an increase in hemocyte number, especially granulocytes and small granulocytes, and an increase in phagocytosis associated with a decrease in aggregation and mortality of the hemocytes, as compared with oysters in pre-bloom analyses. Non-native oysters constitutively had a hemocyte profile more similar to that induced by P. minimum than that of native oysters, but this profile did not impart increased resistance. The effect of P. minimum on respiratory burst was different according to the origin of the oysters, with the dinoflagellate causing a 35% increase in the respiratory burst of the native oysters but having no effect on that of the non-native oysters. Increased respiratory burst in hemocytes of native oysters exposed to P. minimum in both simulated and natural blooms may represent an adaptation to annual blooms whereby surviving native oysters protect themselves against tissue damage from ingested P. minimum.