Haplosporidium nelsoni and Perkinsus marinus occurrence in waters of Great Bay Estuary, New Hampshire.

In Great Bay Estuary, New Hampshire, USA, Haplosporidium nelsoni and Perkinsus marinus are 2 active pathogens of the eastern oyster Crassostrea virginica (Gmelin), that cause MSX (multinucleated sphere with unknown affinity 'X') and dermo mortalities, respectively. Whereas studies have quantified infection intensities in oyster populations and determined whether these parasites exist in certain planktonic organisms, no studies thus far have examined both infectious agents simultaneously in water associated with areas that do and do not have oyster populations. As in other estuaries, both organisms are present in estuarine waters throughout the Bay, especially during June through November, when oysters are most active. Waters associated with oyster habitats had higher, more variable DNA concentrations from these pathogenic organisms than waters at a non-oyster site. This finding allows for enhanced understanding of disease-causing organisms in New England estuaries, where oyster restoration is a priority.

What do the terms resistance, tolerance, and resilience mean in the case of Ostrea edulis infected by the haplosporidian parasite Bonamia ostreae.

The decline of the European flat oyster Ostrea edulis represents a loss to European coastal economies both in terms of food security and by affecting the Good Environmental Status of the marine environment as set out by the European Council's Marine Strategy Framework Directive (2008/56/EC). Restoration of O. edulis habitat is being widely discussed across Europe, addressing key challenges such as the devastating impact of the haplosporidian parasite Bonamia ostreae. The use of resistant, tolerant, or resilient oysters as restoration broodstock has been proposed by restoration practitioners, but the definitions and implications of these superficially familiar terms have yet to be defined and agreed by all stakeholders. This opinion piece considers the challenges of differentiating Bonamia resistance, tolerance, and resilience; challenges which impede the adoption of robust definitions. We argue that, disease-resistance is reduced susceptibility to infection by the parasite, or active suppression of the parasites ability to multiply and proliferate. Disease-tolerance is the retention of fitness and an ability to neutralise the virulence of the parasite. Disease-resilience is the ability to recover from illness and, at population level, tolerance could be interpreted as resilience. We concede that further work is required to resolve practical uncertainty in applying these definitions, and argue for a collaboration of experts to achieve consensus. Failure to act now might result in the future dispersal of this disease into new locations and populations, because robust definitions are important components of regulatory mechanisms that underpin marine management.

Haplosporidian host:parasite interactions.

The host:parasite interactions of the 3 serious haplosporidian pathogens of oysters, on which most information exists, are reviewed. They are Bonamia ostreae in Ostrea spp. and Crassostrea gigas; Bonamia exitiosa in Ostrea spp.; and Haplosporidium nelsoni in Crassostrea spp. Understanding the haemocytic response to pathogens is constrained by lack of information on haematopoiesis, haemocyte identity and development. Basal haplospridians in spot prawns are probably facultative parasites. H. nelsoni and a species infecting Haliotis iris in New Zealand (NZAP), which have large extracellular plasmodia that eject haplosporosomes or their contents, lyse surrounding cells and are essentially extracellular parasites. Bonamia spp. have small plasmodia that are phagocytosed, haplosporosomes are not ejected and they are intracellular obligate parasites. Phagocytosis by haemocytes is followed by formation of a parasitophorous vacuole, blocking of haemocyte lysosomal enzymes and the endolysosomal pathway. Reactive oxygen species (ROS) are blocked by antioxidants, and host cell apoptosis may occur. Unlike susceptible O. edulis, the destruction of B. ostreae by C. gigas may be due to higher haemolymph proteins, higher rates of granulocyte binding and phagocytosis, production of ROS, the presence of plasma ß-glucosidase, antimicrobial peptides and higher levels of haemolymph and haemocyte enzymes. In B.exitiosa infection of Ostrea chilensis, cytoplasmic lipid bodies (LBs) containing lysosomal enzymes accumulate in host granulocytes and in B. exitiosa following phagocytosis. Their genesis and role in innate immunity and inflammation appears to be the same as in vertebrate granulocytes and macrophages, and other invertebrates. If so, they are probably the site of eicosanoid synthesis from arachidonic acid, and elevated numbers of LBs are probably indicative of haemocyte activation. It is probable that the molecular interaction, and role of LBs in the synthesis and storage of eicosanoids from arachidonic acid, is conserved in innate immunity in vertebrates and invertebrates. However, it seems likely that haplosporidians are more diverse than realized, and that there are many variations in host parasite interactions and life cycles.

Haplosporosomes, sporoplasmosomes and their putative taxonomic relationships in rhizarians and myxozoans.

This paper reviews current knowledge of the structure, genesis, cytochemistry and putative functions of the haplosporosomes of haplosporidians (Urosporidium, Haplosporidium, Bonamia, Minchinia) and paramyxids (Paramyxa, Paramyxoides, Marteilia, Marteilioides, Paramarteilia), and the sporoplasmosomes of myxozoans (Myxozoa - Malacosporea, Myxosporea). In all 3 groups, these bodies occur in plasmodial trophic stages, disappear at the onset of sporogony, and reappear in the spore. Some haplosporidian haplosporosomes lack the internal membrane regarded as characteristic of these bodies and that phylum. Haplosporidian haplosporogenesis is through the Golgi (spherulosome in the spore), either to form haplosporosomes at the trans-Golgi network, or for the Golgi to produce formative bodies from which membranous vesicles bud, thus acquiring the external membrane. The former method also forms sporoplasmosomes in malacosporeans, while the latter is the common method of haplosporogenesis in paramyxids. Sporoplasmogenesis in myxosporeans is largely unknown. The haplosporosomes of Haplosporidium nelsoni and sporoplasmosomes of malacosporeans are similar in arraying themselves beneath the plasmodial plasma membrane with their internal membranes pointing to the exterior, possibly to secrete their contents to lyse host cells or repel haemocytes. It is concluded that these bodies are probably multifunctional within and between groups, their internal membranes separating different functional compartments, and their origin may be from common ancestors in the Neoproterozoic.

Temporal dynamics of infection of cockles Cerastoderma edule with the protistan parasite Minchinia tapetis (Rhizaria: Haplosporida) in Galicia (NW Spain).

Uninucleate and binucleate cells and multinucleate plasmodia of a haplosporidan-like protist associated with heavy haemocytic infiltration were observed in histological sections of cockles, Cerastoderma edule, from the Ría de Noia (Galicia, NW Spain) in the course of a cockle health surveillance programme. Molecular assays provided identification of this protist as Minchinia tapetis, which we thus record from a new host. Prevalence of M. tapetis as high as 93% was recorded but infection intensity was low to moderate, never heavy, and abnormally high cockle mortality was not observed in the ria by shellfishers. A significant positive correlation was found between M. tapetis prevalence and sea water temperature. Sea water temperature increase associated with climate change might contribute to increase the prevalence of this infection in cockles and, as a consequence, this parasite may be considered a threat for cockle production.

Biotic and abiotic factors influencing haplosporidian species distribution in the cockle Cerastoderma edule in Ireland.

The Phylum Haplosporidia consists of four genera (Minchinia, Haplosporidium, Urosporidium and Bonamia) that are endoparasitic protists of a wide range of marine invertebrates including commercial bivalve species. Characterization of haplosporidian species remains a challenge due to their patchy spatial and temporal distributions, host-restricted occurrence, and poorly known life cycles. However, they are commonly associated with significant mortality events in bivalves. Due to the recent sporadic mortality events that have occurred in cockles in Europe, the objectives of this study were to determine the diversity, distribution and seasonality of haplosporidian species in Cerastoderma edule populations at several Irish sites. The role of abiotic (temperature, salinity and dissolved oxygen in water) and biotic (cockle size and age) factors as drivers or inhibitors of haplosporidian infection were also assessed. Cockles (n = 998) from the intertidal were sampled from April/July 2018 to April 2019 at three sites with no commercial fishing activity on the south coast (Celtic Sea) and one site on the northeast coast (Irish Sea) with an active commercial fishery. Screening of the cockles by molecular techniques (PCR, Sanger sequencing) and by histopathology was carried out. Two species were identified and confirmed in Irish C. edule for the first time, Minchinia mercenariae -like (14.8%) and Minchinia tapetis (29.6%). Similar to other haplosporidian parasites, the Minchinia spp. detected in our study were present year-round at all sites, except for M. tapetis in Youghal Bay (Celtic Sea). Coinfection of both Minchinia species was only observed in Cork Harbour (Celtic Sea) and Dundalk Bay (Irish Sea), where Minchinia spp. showed a higher presence compared to Youghal Bay and Dungarvan Harbour (Celtic Sea). Moreover, haplosporidians detected with generic primers, were present at all of the sample sites throughout the year but had a higher occurrence during the winter months and were positively correlated with dissolved oxygen. Likewise, smaller and older C.edule seemed to be more vulnerable to the haplosporidian infection. Furthermore, haplosporidian distribution displayed spatial variability between and within sample sites, with the highest presence being observed in cockles at one of the commercially fished Dundalk beds, while the lowest presence was observed in cockles at the second Dundalk bed that was more influenced by freshwater runoff when the tide was out. Findings from this study provide additional information on the distribution and seasonal presence of novel haplosporidian species and their potential abiotic and biotic drivers/inhibitors of infection.

Is pallial mucus involved in Ostrea edulis defenses against the parasite Bonamia ostreae?

Bonamia ostreae is an intrahemocytic parasite that has been responsible for severe mortalities in the flat oyster Ostrea edulis since the 1970́s. The Pacific oyster Crassostrea gigas is considered to be resistant to the disease and appears to have mechanisms to avoid infection. Most studies carried out on the invertebrate immune system focus on the role of hemolymph, although mucus, which covers the body surface of molluscs, could also act as a barrier against pathogens. In this study, the in vitro effect of mucus from the oyster species Ostrea edulis and C. gigas on B. ostreae was investigated using flow cytometry. Results showed an increase in esterase activities and mortality rate of parasites exposed to mucus from both oyster species. In order to better understand the potential role of mucus in the defense of the oyster against parasites such as B. ostreae, liquid chromatography and tandem mass spectrometry were used to describe and compare mucus protein composition from both species. In all oyster species, pallial mucus contains a high level of proteins; however, O. edulis mucus produced a variety of proteins that could be involved in the immune response against the parasite, including Cu/Zn extracellular superoxide dismutase, thioxiredoxin, peroxiredon VI, heat shock protein 90 as well as several hydrolases. Conversely, a different set of antioxidant proteins, hydrolases and stress related proteins were identified in mucus from C. gigas. Our results suggest an innate immunity adaptation of oysters to develop a specific response against their respective pathogens. The mucosal protein composition also provides new insights for further investigations into the immune response in oysters.

Molecular characterization of Urosporidium tapetis sp. nov., a haplosporidian hyperparasite infecting metacercariae of Parvatrema duboisi (Dollfus 1923), a trematode parasite of Manila clam Ruditapes philippinarum on the west coast of Korea.

Recently, a putative new hyperparasitic haplosporidian in the genus Urosporidium was identified from metacercariae of the trematode Parvatrema duboisi infecting Manila clam Ruditapes philippinarum on the west coast of Korea. In this study, we applied small subunit ribosomal DNA (SSU rDNA) sequences as a marker to substantiate the phylogenetic relationship of the unidentified Urosporidium within the Order Haplosporida. In our phylogenetic analysis, the 1890 bp of SSU rDNA sequences obtained were closely related to a haplosporidian parasite forming a sister clade to Urosporidium group, although the gene sequences were only 89.22-89.70% similar to Urosporidium spp. Such molecular phylogenetic distance within the genus suggested that the unidentified Urosporidium is a new member of the genus. Accordingly, we report the unidentified haplosporidian hyperparasite as Urosporidium tapetis sp. nov.

Infection dynamics of Bonamia exitiosa on intertidal Ostrea angasi farms.

Bonamia spp. cause epizootics in oysters worldwide. In southern Australia, Bonamia exitiosa Hine, Cochennac and Berthe, 2001 threatens aquaculture of Ostrea angasi Sowerby, 1871. Bonamia spp. infections can display strong seasonality, but seasonal dynamics of B. exitiosa-O. angasi are unknown. Ostrea angasi naïve to B. exitiosa infection were stocked onto farms in three growing regions, and B. exitiosa was monitored seasonally for one year. Environmental parameters we measured did not correlate with B. exitiosa prevalence or infection intensities. Extreme temperatures suggest O. angasi culture systems need development. Bonamia exitiosa prevalence increased over time. After three months, O. angasi had B. exitiosa prevalence of 0.08-0.4, and after one year, the prevalence was 0.57-0.88. At some sites, O. angasi had >0.5 B. exitiosa prevalence in >6 months, but at other sites, >9 months passed before prevalence was >0.5. Bonamia exitiosa infection intensities were low with no seasonal pattern but were affected by the interaction of site, season and oyster meat:shell ratio. Understanding infection and initiating a breeding programme for resistance would provide benefits for O. angasi industry expansion.

Rapid transmission of Bonamia exitiosa by cohabitation causes mortality in Ostrea angasi.

The haplosporidian Bonamia was first detected in Australian shellfish in 1991. Australian isolates in Ostrea angasi Sowerby, 1871 were identified as Bonamia exitiosa Hine, Cochennac and Berthe, 2001, which threatens development of an O. angasi aquaculture industry. European field data suggest that Bonamia ostreae Pichot, Comps, Tigé, Grizel and Rabouin, 1980 infections in Ostrea edulis Linnaeus, 1758 build slowly, but infection dynamics of B. exitiosa in O. angasi are unknown. We investigated B. exitiosa infection in O. angasi by cohabiting uninfected juvenile O. angasi with adults infected with B. exitiosa. Oysters were sampled at 10, 21 and 40 days after cohabitation, and B. exitiosa prevalence and intensity were assessed. Bonamia exitiosa rapidly infected and caused disease in O. angasi. Mortalities began at 12 days, with ˜50% mortality by day 21 and >85% mortality by day 40. Mortalities displayed pathology consistent with clinical B. exitiosa infection. Time to first infection is likely influenced by a combination of parasite infectivity, host exposure and host immune capacity. Host death is not required for transmission, but probably facilitates release of parasites from decaying tissue. Understanding B. exitiosa transmission informs design and interpretation of field studies and aids development of management strategies for oyster aquaculture.

Involvement of apoptosis in the dialogue between the parasite Bonamia ostreae and the flat oyster Ostrea edulis.

The protozoan parasite Bonamia ostreae has been associated with the decline of flat oyster Ostrea edulis populations in some European countries. Control of shellfish diseases mostly relies on prevention measures including transfer restrictions and stock management measures such as breeding programmes. These prevention and mitigation measures require a better understanding of interactions between host and pathogens. Previous in vitro studies allowed identifying apoptosis as a mechanism activated by the flat oyster in response to B. ostreae. However, these experiments also suggested that the parasite is able to regulate apoptosis in order to survive and multiply within hemocytes. By simplifying the conditions of infection, in vitro studies allow identifying most distinct features of the response of the host. In order to appreciate the relative importance of apoptosis in this response at the oyster scale, in vivo trials were carried out by injecting with parasites oysters from two French locations, Quiberon Bay (Brittany) and Diana Lagoon (Corsica). Apoptosis was investigated on pools of hemolymph from oysters collected at early and later times after injection using previously developed tools. Apoptotic cellular activities including intracytoplasmic calcium concentration, mitochondrial membrane potential and phosphatidyl serine externalization were analysed using flow cytometry. Moreover, the expression of flat oyster genes involved in both extrinsic and intrinsic pathways was measured using real time quantitative PCR.

Proteomic profile of Ostrea edulis haemolymph in response to bonamiosis and identification of candidate proteins as resistance markers.

European flat oyster Ostrea edulis populations have suffered extensive mortalities caused by bonamiosis. The protozoan parasite Bonamia ostreae is largely responsible for this disease in Europe, while its congener B. exitiosa has been detected more recently in various European countries. Both of these intracellular parasites are able to survive and proliferate within haemocytes, the main cellular effectors of the immune system in molluscs. Two-dimensional electrophoresis was used to compare the haemolymph protein profile between Bonamia spp.-infected and non-infected oysters within 3 different stocks, a Galician stock of oysters selected for resistance against bonamiosis, a non-selected Galician stock and a selected Irish stock. Thirty-four proteins with a presumably relevant role in the oyster-Bonamia spp. interaction were identified; they were involved in major metabolic pathways, such as energy production, respiratory chain, oxidative stress, signal transduction, transcription, translation, protein degradation and cell defence. Furthermore, the haemolymph proteomic profiles of the non-infected oysters of the 2 Galician stocks were compared. As a result, 7 proteins representative of the non-infected Galician oysters selected for resistance against bonamiosis were identified; these 7 proteins could be considered as candidate markers of resistance to bonamiosis, which should be further assessed.

Flat oyster follows the apoptosis pathway to defend against the protozoan parasite Bonamia ostreae.

The in vitro model Ostrea edulis hemocyte - Bonamia ostreae is interesting to investigate host-parasite interactions at the cellular level. Indeed, this unicellular parasite infects the flat oyster Ostrea edulis and multiplies within hemocytes, the central effectors of oyster defenses. Apoptosis is a mechanism used by many organisms to eliminate infected cells. In order to study the potential involvement of this mechanism in the oyster response to B. ostreae, in vitro experiments were carried out by exposing hemocytes from the naturally susceptible oyster O. edulis and a resistant oyster species Crassostrea gigas to live and heat-inactivated parasites. Hemocyte apoptotic response was measured using a combination of flow cytometry and microscopy analyses. Whatever the host species was, the parasite was engulfed in hemocytes and induced an increase of apoptotic parameters including intracytoplasmic calcium concentration, mitochondrial membrane potential or phosphatidyl-serine externalization as well as ultrastructural modifications. However, the parasite appears more able to infect flat oyster than cupped oyster hemocytes and the apoptotic response was more important against live than dead parasites in the natural host than in C. gigas. Our results suggest that O. edulis specifically responds to B. ostreae by inducing apoptosis of hemocytes.

Bonamia ostreae in the New Zealand oyster Ostrea chilensis: a new host and geographic record for this haplosporidian parasite.

Previous reports of the haplosporidian parasite Bonamia ostreae have been restricted to the Northern Hemisphere, including Europe, and both eastern and western North America. This species is reported for the first time in New Zealand infecting the flat oyster Ostrea chilensis. Histological examination of 149 adult oysters identified 119 (79.9%) infected with Bonamia microcells. Bonamia generic PCR of several oysters followed by DNA sequencing of a 300 bp portion of the 18S rDNA gene produced a 100% match with that of B. ostreae. All DNA-sequenced products also produced a B. ostreae PCR-restriction fragment length polymorphism (PCR-RFLP) profile. Bonamia species-specific PCRs further detected single infections of B. exitiosa (2.7%), B. ostreae (40.3%), and concurrent infections (53.7%) with these 2 Bonamia species identifying overall a Bonamia prevalence of 96.6%. Detailed histological inspection revealed 2 microcell types. An infection identified by PCR as B. ostreae histologically presented small microcells (mean ± SE diameter = 1.28 ± 0.16 µm, range = 0.9-2 µm, n = 60) commonly with eccentric nuclei. A B. exitiosa infection exhibited larger microcells (mean ± SE diameter = 2.12 ± 0.27 µm, range = 1.5-4 µm, n = 60) with more concentric nuclei. Concurrent infections of both Bonamia species, as identified by PCR, exhibited both types of microcells. DNA barcoding of the B. ostreae-infected oyster host confirmed the identification as O. chilensis. A suite of other parasites that accompany O. chilensis are reported here for the first time in mixed infection with B. ostreae including apicomplexan X (76.5%), Microsporidium rapuae (0.7%) and Bucephalus longicornutus (30.2%).

Oral immunostimulation of the oyster Ostrea edulis: Impacts on the parasite Bonamia ostreae.

Bioactive compounds were orally administered to the native European oyster Ostrea edulis to evaluate the immune response and the progression of infection of the protozoan parasite Bonamia ostreae. The immunostimulants lipopolysaccharide and zymosan directly administrated to the water column induced an increase in lysozyme activity and the percentage of granulocytes in naïve oysters over a period of 7 days. In another set of experiments, zymosan and curdlan were microencapsulated in alginate and also administered to the water column to naïve and B. ostreae infected O. edulis. Oyster mortality, prevalence and intensity of infection and several immune parameters were evaluated up to 28 days post-administration. Lysozyme activity, nitric oxide production and the expression of galectin, lysozyme and superoxide dismutase increased after 24 h in both infected and uninfected oysters. Zymosan immunostimulated oysters displayed a decrease in the prevalence of B. ostreae infection not attributed to mortalities but which could be associated to the enhanced ability of immunostimulants to evoke an enhanced immune response in the oysters and reduce infection.

Microcell parasites of molluscs: introduction to DAO Special 7.

First discovered decades ago, microcell protistan parasites of the genera Bonamia and Mikrocytos remain relevant today for their economic impacts on growing molluscan aquaculture industries and fisheries. Bonamia parasites have received more attention over the years in part because they are more widespread and thus of wider concern, but there has been renewed interest in Mikrocytos recently with the generation of important new findings. Among these has been the surprising observation that Mikrocytos has phylogenetic affinities to the Rhizaria, which includes the haplosporidian protists and the genus Bonamia. This Diseases of Aquatic Organisms Special, emerging from the 5th Meeting of the Microcell Working Group held at the Central Veterinary Institute, Lelystad, the Netherlands, in February 2012, presents new insights into Mikrocytos and Bonamia diversity, distributions, diagnostics, ultrastructure, and infection dynamics, and captures major developments in the field since the last review of these genera in 2004.

Ultrastructural comparison of Bonamia spp. (Haplosporidia) infecting ostreid oysters.

The ultrastructure of Bonamia from Ostrea angasi from Australia, Crassostrea ariakensis from the USA, O. puelchana from Argentina and O. edulis from Spain was compared with described Bonamia spp. All appear conspecific with B. exitiosa. The Bonamia sp. from Chile had similarities to the type B. exitiosa from New Zealand (NZ), but less so than the other forms recognized as B. exitiosa. Two groups of ultrastructural features were identified; those associated with metabolism (mitochondrial profiles, lipid droplets and endoplasmic reticulum), and those associated with haplosporogenesis (Golgi, indentations in the nuclear surface, the putative trans-Golgi network, perinuclear granular material and haplosporosome-like bodies). Metabolic features were regarded as having little taxonomic value, and as the process of haplosporogenesis is not understood, only haplosporosome shape and size may be of taxonomic value. However, the uni-nucleate stages of spore-forming haplosporidians are poorly known and may be confused with Bonamia spp. uni-nucleate stages. The many forms of NZ B. exitiosa have not been observed in other hosts, which may indicate that it has a plastic life cycle. Although there are similarities between NZ B. exitiosa and Chilean Bonamia in the development of a larger uni-nucleate stage and the occurrence of cylindrical confronting cisternae, the clarification of the identity of Chilean Bonamia must await molecular studies.

Epidemiology of Bonamia in the UK, 1982 to 2012.

Bonamiasis, caused by Bonamia ostreae, was confirmed in native flat oysters Ostrea edulis L. in England in 1982. Hudson & Hill (1991; Aquaculture 93:279-285) documented investigations into the initial spread of the disease in wild and cultivated stocks of native oysters in the UK. They also described the controls that were initially applied to prevent the further spread of the pathogen. This paper reports on subsequent controls and associated monitoring applied in the UK and reports on the epidemiology of the disease in the 30 yr from 1982 to 2012. Bonamiasis remained confined to the zones in England as documented by Hudson & Hill (1991) until 2005, when it was confirmed in Lough Foyle, Northern Ireland. In 2006 it was found in 2 new areas, one in Wales and one in Scotland. Subsequent further spread to additional areas in all parts of the UK has resulted in 9 zones being currently designated as infected with the disease. In addition, a single oyster from one area has tested positive for the closely related B. exitiosa. In general, analysis of the results of the monitoring programme in England and Wales shows no clear trend in infection levels over time, although there has been an apparent decrease in the level of infection in some fishery areas. In an autumn sampling programme the highest levels of infection were detected in October.

Bonamiosis status in natural Ostrea puelchana beds in San Matías Gulf (Patagonia, Argentina), 14 years after an epizootic.

Between 1995 and 1996, Bonamia exitiosa caused an epizootic in San Matías Gulf, Argentina, that spread from a commercial culture site of Ostrea puelchana to natural beds located at the northeastern coast of the gulf. A mortality rate of 95% was registered in cultured oysters, and oysters from natural beds were also affected. The aims of this study were to assess the parasite prevalence in oyster beds and the demographic structure 14 yr after the epizootic. Two different oyster beds were studied during 2009 and 2010. Parasite prevalence was studied related to oyster aggregation, density, sex, and oyster size. Prevalence reached 35.3% at Las Grutas and 18.9% at Banco Reparo and was proportionally associated with density. Prevalence was also associated with the type of aggregation in Banco Reparo, where carrier oysters were more infected. Infection was independent of sex category, and infected oysters were larger than the non-infected ones. Oyster density decreased markedly compared to previous studies in both beds and mean sizes were lower, while prevalence doubled. Because of the persistence of the beds in this period, disease seems to control the population structure.

Pathology of Haplosporidium patagon affecting siphonariid gastropods in Patagonia.

Haplosporidium patagon was found parasitizing Siphonaria lessonii and S. lateralis, 2 siphonariid gastropods co-occurring on the littoral rocky shore at Puerto Deseado, Santa Cruz, Argentina. Gastropods from 2 habitats representing 2 different levels of environmental harshness were studied. In both cases, S. lessonii showed a higher prevalence of infection (3.78%) over the entire 14 mo study period than S. lateralis (0.13%). Very different values of prevalence of infection were observed at the different sampling sites: Site 1, the more restrictive habitat (exposed for long periods to desiccation during low tides, higher ultraviolet exposure, and high ranges of temperature variation) showed a higher prevalence value (5.99%) than Site 2 (1.46%). Statistical differences in prevalence were also found between values corresponding to the austral spring (3.35% at Site 1 and 0.74% at Site 2) and winter (13.79% at Site 1 and 2.13% at Site 2). The presence/absence of H. patagon did not vary significantly with gastropod shell length. Infection affected the digestive gland, whose normal histology was greatly modified. The hermaphroditic gonads were also affected; the female germinal cells disappeared or only a few primary or previtellogenic oocytes were present, and vitellogenesis was inhibited. The function of the male germinal epithelium, as well as spermatogenesis and spermiogenesis processes and associated organs (seminal vesicles and seminal receptacles), were not affected. However, the glandular pallial complex of the reproductive systemwas affected, and we observed a significant reduction in development in parasitized gastropods. H. patagon sporocysts also invaded the supporting connective tissues of both the kidney and pseudobranch.