Chronic infections of Haplosporidium nelsoni (MSX) in the oyster Crassostrea virginica

Oysters inhabiting areas enzootic for the parasite Haplosporidium nelsoni (MSX) are exposed to infective particles each summer, and it is often difficult to distinguish newly acquired lesions from older infections. To study long-term parasitism without the complication of new infections, MSX-infected oysters were moved from Delaware Bay to a disease-free area on the New Jersey coast. Because infections seen after the transfer were acquired in Delaware Bay during a known infective period, it was possible to determine how long oysters remained infected and how they were affected by chronic parasitism. Chronic infections displayed the same seasonal cycle (high levels in winter and late spring, low levels in summer and early spring) that occurs in enzootic areas with annual reinfection. Within individual oysters, chronic MSX became localized, relapsed into general infections, and then became localized again in a sequence that was probably controlled by temperature. Some experimental oysters survived with MSX for at least 4 years. Hemocytosis persisted in these oysters, and their poor condition suggested that chronically infected individuals would have lowered resistance to additional stress.     

Infection and mortality patterns in strains of oysters Crassostrea virginica selected for resistance to the parasite Haplosporidium nelsoni (MSX)

Strains of oysters Crassostrea virginica resistant to mortality caused by the parasite Haplosporidium nelsoni (MSX) were developed and tested through 6 generations. In addition, strains in each generation were followed for up to 6 yr of continuous exposure to the parasite in nature. Selected strains responded to challenge by the parasite with gradually improved survival in successive generations. They were slower to develop patent infections than were unselected groups and were able to delay mortality after infections did develop, but under repeated exposure most oysters eventually died with H. nelsoni parasitism. Many selected strains, however, reached market size before significant mortalities occurred. The data suggest that resistance to H. nelsoni mortality is under the influence of many genes. No clear defense mechanism has been described and we hypothesize that resistance to H. nelsoni may, in part, involve a physiological state in which selected oysters temporarily fail to provide a suitable habitat for the parasite. Temporary insusceptibility would, in this view, be followed by an increased ability to tolerate the parasite when conditions for its development are present. Selection would then favor individuals that are able to prolong periods of insusceptibility and/or to carry out basic life processes while parasitized.     

Use of Competitive PCR to Detect and Quantify Haplosporidium nelsoni Infection (MSX disease) in the Eastern Oyster (Crassostrea virginica)

This study was undertaken to develop a quantitative polymerase chain reaction assay that would improve the utility of PCR for detecting Haplosporidium nelsoni (MSX), a serious parasite of the eastern oyster Crassostrea virginica. A competitive PCR sequence was generated from the H. nelsoni small subunit ribosomal DNA fragment, originally described by Stokes and colleagues, that was amplified by the same PCR primers and had similar amplification performance. Assays performed using competitor dilutions ranging from 0.05 to 500 pg/μl DNA were used to test oyster samples designated using histological techniques as having ``light' or ``heavy' MSX infections. Visual diagnoses were confirmed equally well with three methods: densitometry of ethidium-bromide-stained agarose, densitometry of SYBRGreen-stained polyacrylamide gels, and analysis by GeneScan 3.0 of fluorescent products detected in ultrathin gels. Oysters diagnosed as negative for MSX tested as negative or light by PCR. Oysters with light MSX infections generally had less than 5 pg/μl infectious DNA. Oysters with heavy infections generally corresponded to 5 pg/μl or greater competitor dilutions. Received September 3, 1999; accepted March 3, 2000.     

Evidence for Regular Sporulation by Haplosporidium nelsoni (MSX) (Ascetospora; Haplosporidiidae) in Spat of the American Oyster, Crassostrea virginica

The spore stage of Haplosporidium nelsoni , the ascetosporan parasite causing multinucleated sphere unknown (MSX) disease in oysters, Crassostrea virginica , has been reported so rarely (≥0.01% of infected oysters) that a second host has been postulated. However, recent intensive sampling of young (≥1 year) oysters in Delaware Bay, U.S. suggests that spore formation occurs regularly in this group and that spores are produced in at least 75–85% of all infections reaching the advanced stage. Sporulation was seasonal, occurring over two to three weeks in late June/early July and again in late summer/early fall. Our data indicate that sporulation by H. nelsoni in oysters is more common than previously suspected, occurring in a segment of the host population that may not have been sufficiently sampled in the past, and that a direct life cycle should be reconsidered.     

Evidence for regular sporulation by Haplosporidium nelsoni (MSX) (Ascetospora; Haplosporidiidae) in spat of the American oyster, Crassostrea virginica.

The spore stage of Haplosporidium nelsoni, the ascetosporan parasite causing multinucleated sphere unknown (MSX) disease in oysters, Crassostrea virginica, has been reported so rarely (less than 0.01% of infected oysters) that a second host has been postulated. However, recent intensive sampling of young (less than 1 year) oysters in Delaware Bay, U.S. suggests that spore formation occurs regularly in this group and that spores are produced in at least 75-85% of all infections reaching the advanced stage. Sporulation was seasonal, occurring over two to three weeks in late June/early July and again in late summer/early fall. Our data indicate that sporulation by H. nelsoni in oysters is more common than previously suspected, occurring in a segment of the host population that may not have been sufficiently sampled in the past, and that a direct life cycle should be reconsidered.     

Transmission of the haplosporidian parasite MSX Haplosporidium nelsoni to the eastern oyster Crassostrea virginica in an upweller system

The haplosporidian oyster parasite MSX (Multinucleated Sphere X) Haplosporidium nelsoni was transmitted to eastern oysters Crassostrea virginica. Hatchery-raised, MSX-free juvenile oysters were placed in upweller tanks. Water to the tanks was filtered through a screen with 1 mm2 openings and originated from the water column overlaying naturally infected oysters beds (MSX prevalence 17 to 57%). MSX was diagnosed by histopathological analysis. MSX-disease (57% prevalence) with increased mortality (19%) was observed 11 wk after the beginning of the exposure and mortality of 80% after 16 wk. The study demonstrates transmission of MSX via water-borne infectious agents capable of passing through a 1 mm filter.     

Prevalence of Perkinsus marinus (dermo), Haplosporidium nelsoni (MSX), and QPX in bivalves of Delaware's inland bays and quantitative, high-throughput diagnosis of dermo by QPCR

Restoration of oyster reef habitat in the Inland Bays of Delaware was accompanied by an effort to detect and determine relative abundance of the bivalve pathogens Perkinsus marinus, Haplosporidium nelsoni, and QPX. Both the oyster Crassostrea virginica and the clam Mercenaria mercenaria were sampled from the bays. In addition, oysters were deployed at eight sites around the bays as sentinels for the three parasites. Perkinsus marinus prevalence was measured with a real-time, quantitative polymerase chain reaction (PCR) methodology that enabled high-throughput detection of as few as 31 copies of the ribosomal non-transcribed spacer region in 500 ng oyster DNA. The other pathogens were assayed using PCR with species-specific primers. Perkinsus marinus was identified in Indian River Bay at moderate prevalence ( approximately 40%) in both an artificial reef and a wild oyster population whereas sentinel oysters were PCR-negative after 3-months exposure during summer and early fall. Haplosporidium nelsoni was restricted to one oyster deployed in Little Assawoman Bay. QPX and P. marinus were not detected among wild clams. While oysters in these bays have historically been under the greatest threat by MSX, it is apparent that P. marinus currently poses a greater threat to recovery of oyster aquaculture in Delaware's Inland Bays.     

Spore ornamentation of Haplosporidium nelsoni and Haplosporidium costale (Haplosporidia), and incongruence of molecular phylogeny and spore ornamentation in the Haplosporidia

Spore ornamentation of Haplosporidium nelsoni and Haplosporidium costale was determined by scanning electron microscopy. For H. nelsoni, the spore surface was covered with individual ribbons that were tightly bound together and occurred as a single sheet. In some spores, this layer was overlaid with a network of branching fibers, about 0.05 microm in diameter, which often was dislodged from the spore at the aboral pole. For H. costale, ornamentation consisted of a sparse network of branching fibers on the spore surface. Molecular phylogenetic analysis of the phylum Haplosporidia revealed that Urosporidium, Bonamia, and Minchinia were monophyletic but that Haplosporidium was paraphyletic. All species of Minchinia have ornamentation composed of epispore cytoplasm, supporting the monophyly of this genus. The presence of spores with a hinged operculum and spore wall-derived ornamentation in Bonamia perspora confounds the distinction between Bonamia and Haplosporidium. Species with ornamentation composed of outer spore wall material and attached to the spore wall do not form a monophyletic group in the molecular phylogenetic analysis. These results suggest that the widely accepted practice of assigning all species with spore wall-derived ornamentation to Haplospordium cannot be supported and that additional genera are needed in which to place some species presently assigned to Haplosporidium.     

Analysis of Sarcoptes scabiei finds no evidence of infection with Wolbachia

The endosymbiont Wolbachia has been detected in a range of filarial nematodes and parasitic mites and is known to affect host reproductive compatibility and potentially evolutionary processes. PCR of Wolbachia surface protein (wsp), ftsZ and 16SrRNA genes from individual Sarcoptes scabiei mites obtained from a series of individual hosts, and database searches of an S. scabiei var. hominis EST library failed to detect Wolbachia genes. Therefore, Wolbachia appears not to be involved in the genetic subdivision observed between varieties of host-associated S. scabiei or, involved in the inflammatory disease pathogenesis of scabies unlike its activity in filarial infection.     

Spatial genetic features of eastern oysters (Crassostrea virginica Gmelin) in the Gulf of Mexico: northward movement of a secondary contact zone

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (F_ST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long‐term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23‐year span.     

Haplosporidium sp. (Alveolata: Haplosporidia) associated with mortalities among rock oysters Saccostrea cuccullata in north Western Australia

Haplosporidium sp. is described from rock oysters Saccostrea cuccullata Born, 1778 experiencing epizootics on the northwestern coast of Western Australia. All stages were observed as focal infections in the connective tissue of the gills, or as disseminated infections in the mantle and around digestive diverticulae. Haplosporidium sp. occurred between epithelial cells of the gut, in focal lesions in the gills, but not in the epithelium of the digestive diverticulae, and sporulation was confined to the connective tissue. Plasmodia developed into sporonts and sporocysts in a loose syncytium that gave rise to binucleate and uninucleate sporoblasts from which spores developed. Spores were flask-shaped, 5.6-6.7 x 3.3-4.0 microm, with a characteristic operculum, a few filamentous wrappings and rod-like structures in the posterior sporoplasm. Mature spores had a wall comprising inner (90 nm wide), middle (30 nm wide) and outer (130 nm wide) layers, and a surface coat of microtubules giving them a furry appearance. Oysters with empty gonad follicles were most heavily infected, and oyster condition and mortality appeared to be related to degree of infection.     

Epizootiology of Minchinia nelsoni in susceptible wild oysters in Virginia, 1959 to 1971

Twelve years after the haplosporidian parasite Minchinia nelsoni erupted causing severe epizootics of oysters in lower Chesapeake Bay, regular patterns of mortalities and disease prevalence persisted. Distribution changed with salinity and weather patterns, but in mesohaline areas (about 15–25 ‰) infective pressure remained high and relatively stable despite scarcity of oysters.Susceptible disease-free oytsers from low-salinity areas of the James River, the major seed area in Virginia, were transplanted annually to disease-prevalent areas for monitoring in trays. Mortalities were usually over 50% the first year and almost as high in the second year. Prevalences of the pathogen, called MSX, ranged from 35 to 50% in live oysters. Seasonal patterns of disease activity are depicted from 1960 through 1971, and they exhibit exceptional regularity for open-water conditions. Source and history of oysters, and timing of exposure are important variables that affect disease activity as well as size and age. The disease caused by MSX appears to be not contagious in trays.The patterns of disease and mortality obtained from susceptible wild oysters without previous exposure provided a basis for evaluating other stocks including genetic strains selected for disease resistance.     

Uptake and clearance of Vibrio vulnificus from Gulf coast oysters (Crassostrea virginica)

Oysters collected in late winter, when they were free of Vibrio vulnificus, were exposed in the organism in the laboratory. The oysters effectively concentrated the bacteria from seawater, but when the inoculum was removed, the bacteria were rapidly cleared from the oyster tissues. These results suggest that V. vulnificus may be found in oysters as a result of filtration of the bacteria from seawater rather than active multiplication of the bacteria in the oysters.     

Flyingfish Spawning (Parexocoetus brachypterus) in the Northeastern Gulf of Mexico

A spawning aggregation of Parexocoetus brachypterus was observed in the northeastern Gulf of Mexico over the outer continental shelf, 100km south of Mobile, Alabama. The event was considered to be a spawning aggregation given the remarkably high abundance of flyingfish observed, together with unusually vigorous flying, jumping, and swimming activity near the surface, and observations of both males and females exuding ripe sex products when landed. The spawn occurred during May 2001 just after moonrise 2 days post full moon. More than 1000000 flyingfish were estimated to have participated in the spawn. Male flyingfish were three times more abundant than females, consistent with observations of 3–4 flyingfish grouped together before and after the spawning aggregation, if several males were simultaneously pursuing a single female. Although egg attachment to flotsam is the only reproductive mode previously described for flyingfishes with demersal eggs, the P. brachypterus spawn occurred in the absence of flotsam. Other possible egg development modes include egg suspension upon current, mid-water suspension above pycnocline or upon topographically induced turbulence, or benthic.     

Environmental determinants of Gulf Menhaden (Brevoortia patronus) oil content in the northern Gulf of Mexico

Gulf Menhaden (Brevoortia patronus) are a species of commercial and ecological importance in the northern Gulf of Mexico, provisioning the second largest fishery by weight, in the United States, and providing critical ecosystem services in the coastal region. The recruitment and productivity dynamics of the stock are influenced by a suite of environmental factors but an understanding of the factors that determine individual variation in oil content (an indicator of an individual’s commercial value to the fishery and its dietary value to predators) has not been well described. In this work I describe the temporal dynamics of oil content and determine the demographic characteristics that provide predictive power to describe annual contrasts. I relate the predicted patterns in oil yield to a suite of seasonal environmental data series including: the magnitude of spring Mississippi River discharge, spring wind vectors, and the preceding winter El Nino conditions. Two uncorrelated (r = 0.06, p = 0.81) population-level predictor variables were identified that have explanatory power to describe temporal patterns in oil content (L kg⁻¹); a weight-at-length power function parameter (a) and the von Bertalanffy asymptotic fork length (L_∞, mm FL): L kg⁻¹ = − 0.158 − 0.026*a − 0.00163*L_∞ (p < 0.05, R² = 0.42). Analysis of the impacts of environmental variables on the oil content of Gulf Menhaden was evaluated comprehensively in a Bayesian framework by transforming the observed oil content information from two sources to a common scale. Parameters relating oil content to spring Mississippi River discharge and the preceding winter (December–February) El Nino Southern Oscillation index resulted in sample distributions from the posterior where zero was outside the 95% credible interval. This work contributes to the understanding of Gulf Menhaden as a prey species in the Gulf of Mexico and indicates that the value of the species to both the fishery and predators exhibits relatively large inter-annual variability controlled, in part, by seasonal environmental conditions.     

Genetic by environmental variation but no local adaptation in oysters (Crassostrea virginica)

Functional trait variation within and across populations can strongly influence population, community, and ecosystem processes, but the relative contributions of genetic vs. environmental factors to this variation are often not clear, potentially complicating conservation and restoration efforts. For example, local adaptation, a particular type of genetic by environmental (G*E) interaction in which the fitness of a population in its own habitat is greater than in other habitats, is often invoked in management practices, even in the absence of supporting evidence. Despite increasing attention to the potential for G*E interactions, few studies have tested multiple populations and environments simultaneously, limiting our understanding of the spatial consistency in patterns of adaptive genetic variation. In addition, few studies explicitly differentiate adaptation in response to predation from other biological and environmental factors. We conducted a reciprocal transplant experiment of first‐generation eastern oyster (Crassostrea virginica) juveniles from six populations across three field sites spanning 1000 km in the southeastern Atlantic Bight in both the presence and absence of predation to test for G*E variation in this economically valuable and ecologically important species. We documented significant G*E variation in survival and growth, yet there was no evidence for local adaptation. Condition varied across oyster cohorts: Offspring of northern populations had better condition than offspring from the center of our region. Oyster populations in the southeastern Atlantic Bight differ in juvenile survival, growth, and condition, yet offspring from local broodstock do not have higher survival or growth than those from farther away. In the absence of population‐specific performance information, oyster restoration and aquaculture may benefit from incorporating multiple populations into their practices.     

Echinoids (Echinodermata: Echinoidea) from the Gulf of Mexico

The echinoid fauna of the Gulf of Mexico collected during three research cruises (20-1260 m depth) was surveyed from samples were taken at 43 stations. A total of 190 individuals were identified (eight orders, 11 families, 15 genera and 18 species). Six species are new records for the Gulf of Mexico: Stylocidaris lineata, Phormosoma placenta placenta, Plesiodiadema antillarum, Plethotaenia spatangoides, Brissopsis atlantica and Hypselaster limicolus. This adds to the little information available on the echinoid fauna of Tamaulipas, Veracruz, Tabasco, Campeche and Yucatan states in Mexico.     

Echinoderms (Echinodermata) from the Gulf of California, Mexico

A systematic list of the echinoderms of the Gulf of California, based on museum specimens of the Colección Nacional de Equinodermos, Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autónoma de México and the National Museum of Natural History, Smithsonian Institution, Washington, D.C. is presented. A total of 193 echinoderm species is recorded, distributed in 108 genera, 51 families and 19 orders. 12 new records for the Gulf of California are presented: Asteroidea (four), Ophiuroidea (three) and Holothuroidea (five).