Vesicular Connective Tissue Research Articles

Pathology, epidemiology, and phylogeny of mussel egg disease due to the microsporidian Steinhausia mytilovum (Field, 1924) in the Mediterranean mussel (Mytilus galloprovincialis)

Microsporidia are well known fungal pathogens of aquatic animals. However, the taxonomy of microsporidia is generally poorly resolved, which has consequently constrained our understanding of their pathology and epidemiology in marine animals. To date, microsporidia have been reported in both bivalves and gastropods, and microsporidia from mollusks have been classified in different genera. Despite ongoing work to better describe these genera, including detailed microscopic and ultrastructural images, so far we lack information on microsporidian phylogeny and pathogenicity of species within these genera. Here we investigate the microsporidian parasite Steinhausia mytilovum associated with the mussel, Mytilus galloprovincialis, in natural beds and farms along coast of southern Italy. A survey of M. galloprovincialis was conducted in 13 mussel farms and one natural bed between 2009 and 2020. We found the presence of S. mytilovum in 10 of the investigated farms, with a prevalence ranging between 14 and 100% of female mussels, depending on the population and season in which they were sampled. The parasite developed in the oocytes within a sporophorous vesicle (SV) where it produced 1–3 spores per cell, both in the cytoplasm and in the nucleus. Stenhausia mytilovum elicited an infiltrative (24.8%) or a strong capsular inflammatory response (43.4%) at gonadal follicles and surrounding vesicular connective tissue, in some cases accompanied by gonadal atresia (24.8%), leading to loss of gonadal architecture. In 7% of cases no reaction was observed. Ultrastructural observations revealed a mitochondrial re-organization to interact with all the phases of parasite development; the mitochondria were arranged outside the parasitophorous vesicle (PV) or directly interacting with the spore inside vesicle. There are five taxonomic clades of microsporidians as identified by SSU ribosomal gene sequence data. Maximum likelihood analysis assigned S. mytilovum within the Clade IV, defined as the Class Terresporidia, with closest genetic relationship (83.6% identity) to an undetermined invertebrate ovarian microsporidian. The constant presence, prevalence, and severity of S. mytilovum in coastline populations of M. galloprovincialis populations in southern Italy may indirectly reflect immunocompetence at both individual and population levels.

Microscopic anatomy of gonadal area in the deep-sea clam Calyptogena pacifica (Bivalvia: Vesicomyidae) with emphasis on somatic cells

Somatic cells in the gonadal area of male and female deep-sea clams, Calyptogena pacifica, were examined using light and transmission electron microscopy. Acini both at the pre-spawning stage and at the stage of active spermatogenesis were observed to be simultaneously present in sections through a male gonad. Oocytes of various degrees of maturity were simultaneously present in female acini. No storage tissue and cells similar to adipogranular cells or vesicular connective tissue of other mollusks were found in the gonadal area of C. pacifica. Instead, in both males and females, numerous hemocytes surround the acini. Among hemocytes, two types of granulocytes and erythrocytes were identified. Bundles of muscle cells were also found in the interacinar space. Male intraacinar accessory cells were rather large, glycogen-rich, with lipid inclusions, and phagosomes contained spermatogenic cells. Female accessory cells had well-developed endoplasmic reticulum, but they did not form any follicles around oocytes, being in their basal part, closer to basal lamina. Such a specific pattern of gonad organization can presumably be explained by both symbiosis with sulfide-oxidizing bacteria and phylogenetic aspects that should be further studied. Some evidence for continuous gametogenesis are discussed.

Effects of temperature and salinity on the sub-Antarctic snail Xymenopsis muriciformis (King and Broderip, 1832): Medium-term exposure to different climate change scenarios

As a consequence of climate change, benthic marine species that inhabit the sub-Antarctic oceans could be impacted as a result of changes in temperature and salinity that are affecting areas of high latitudes, where stenoic marine species are common. Therefore, the physiological performance of the species is essential to understand the ability of species to persist within their current range and to predict their rates of redistribution in response to the impacts of climate change impact. Xymenopsis muriciformis, an endemic sub-Antarctic benthic snail of the Strait of Magellan (53° 47′S; 70° 58′W), was used as a model species to understand the physiological response of a species that inhabits the Southern Oceans, where environmental conditions are very stable. We tested a medium-term exposure to the effects of climate change, including changes in temperature and salinity. The organisms were acclimatized for three months in the laboratory, at a temperature of 9 °C and salinity of 30 psu (control). The experiment was carried out for 60 days, where the ingestion rate and oxygen uptake were measured every 10 days. At the end of the experiments, the physiological condition of the organisms was determined by applying the metabolic indicators: carbohydrate indices (CHI), total lipids (TLI) and lipofuscins (LI), which were obtained by histological-histochemical analysis. The results indicated that temperature had significant effects on the ingestion rate and oxygen consumption, while salinity and the interaction between salinity and temperature had no significant effects. Organisms exposed to low temperatures (1 °C) and (5 °C), expected for the southern hemisphere summer, exhibited cellular degradation, which would limit the access and colonization of Antarctic environments. The organisms exposed to the 15 °C treatment (expected for the Magellan region), presented a significantly higher amount of lipofuscins in the vesicular connective tissue cells with morphological characteristics in the autophagosomes. We concluded that the snail X. muriciformis is a characteristic stenoic species, highly sensitive to temperature variations, as those projected in climate change scenarios for the Antarctic and sub-Antarctic regions.

Implementation of various approaches to study the prevalence, incidence and progression of disseminated neoplasia in mussel stocks

Marine mussel production is of substantial economic interest in numerous coastal areas worldwide, making crucial the study of pathologies that affect them. Disseminated neoplasia (DN) has recently been suggested to be linked to blue mussel, Mytilus edulis, mortality outbreaks observed in France since 2014, although the evidence remains indirect. In order to improve DN detection and monitoring, we compared the sensitivity of four diagnostic tools, namely haemocytology, histology, flow cytometry, and genetics. Haemocytological examination gave the best results in sensitivity and had the advantage of being non-invasive, allowing disease progression to be followed in affected mussels. Using this approach, we showed that DN progression is usually slow, and we provide evidence of remission events. We observed a high diversity of forms and mitotic features of neoplastic cells located in the vesicular connective tissue but rarely in the haemolymph. Circulating cells occur as four main types but are homogenous in morphology and DNA content within a single individual. Polyploidy proved very high, from 8 N to 18 N. Genetic analysis of haemolymph DNA showed that a Mytilus trossulus genetic signal was associated with almost all the DN cases here diagnosed by haemocytological examination, regardless of the DN type. This result corroborates DN is a transmissible cancer that first originated in a M. trossulus host and subsequently crossed into M. edulis. No pre-neoplastic conditions were detectable. The prevalence of the disease was quite low, which, together with the low morbidity observed in the lab, suggest DN is unlikely to be the direct cause of mortality outbreaks in France.

Autophagy in nutrient storage cells of the Pacific oyster, Crassostrea gigas

In oysters, nutrients are stored in a special type of cells referred to as vesicular-connective tissue cells (VCT-cells). These cells accumulate and provide nutrient to satisfy various needs of the organism, including gametogenesis. During the annual reproductive cycle, VCT-cells pass through a series of changes in their morphology associated with nutrients mobilization for developing germ cells. The results presented here show an approximately 33–35% increase in the number of autophagic vesicles in cytoplasm of VCT-cells in the gonadal area of C. gigas during the stage of active gametogenesis as compared to the resting stage of reproductive cycle. No destruction of VCT-cells due to autophagy or any other factors was observed, both in males and females. Our results indicate that autophagy does increase in VCT-cells of C. gigas and plays a certain role in nutrient mobilization from these cells.

The Mussel Mytilus galloprovincialis in the Bay of Naples: New Insights on Oogenic Cycle and Its Hormonal Control.

The aim of the present article was to investigate the oogenic cycle of Mytilus galloprovincialis sampled in the Bay of Naples, and to immunolocalize 3β-hydroxysteroid dehydrogenase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), and P450 aromatase, enzymes involved in the synthesis of two sex hormones: testosterone and 17β-estradiol. We demonstrate that the oogenic cycle starts in late summer-early fall and continues in early winter when the first event of spawning occurs; other spawning events take place until June, when the ovary is spent and contains a few empty ovarian follicles and numerous somatic cells, that is, adipogranular cells and vesicular connective tissue cells. During the oogenic cycle, apoptotic events occur at the level of oogonia, previtellogenic oocytes, as well as follicle cells; by contrast, necrosis events probably take place in vitellogenic oocytes, which, once degenerated, transfer their content to healthy oocytes. Finally, the present data demonstrate that 3β-HSD, 17β-HSD, and P450 aromatase are present in the ovary both during the reproductive and nonreproductive phases. The possible role of these enzymes during the Mytilus galloprovincialis reproductive cycle is discussed. Anat Rec, 302:1039-1049, 2019. © 2019 Wiley Periodicals, Inc.

Morphology of nutrient storage cells in the gonadal area of the Pacific oyster, Crassostrea gigas (Thunberg, 1793).

Successful gametogenesis in invertebrates is tightly associated with functioning of specific nutrient-storing cells. In oysters, cells of vesicular connective tissue (VCT-cells), also referred to as storage cells, which form a meshwork around gonadal acini, are the major population of cells that accumulate and provide nutrients for developing gametes. During the annual reproductive cycle, populations of developing germ cells and VCT-cells demonstrate the inversely proportional size dynamics: the larger the acini, the smaller the VCT-cells. In the present study, the morphology of VCT-cells in the Pacific oyster, Crassostrea gigas, at the active gametogenesis stage of reproductive cycle has been studied using light and transmission electron microscopy. At this stage, VCT-cells are big, irregularly shaped cells containing large nucleus with a single large nucleolus. The cytoplasm contains weakly developed endoplasmic reticulum, mitochondria in the perinuclear area and at periphery of the cell, numerous lipid droplets, and glycogen particles. Ultrastructure of VCT-cells is similar to the organization of brown adipocytes in mammals. The surface of cells has numerous cytoplasmic processes that are presumably associated with the transport function and provide close interaction with adjacent cells. The spatial relationship between VCT-cells and myoepithelial elements of the gonad area is demonstrated and discussed.

Histological and statistical studies on the gonadal cycle of the mussel Lithophaga truncata (Gray, 1843) along immature and mature phases (Pelecypoda - Mytilidae)

The main purpose of this study was to investigate the gonadal cycle of the musselLithophaga truncata (Gray, 1843). Specimens were sampled regularly twice every season at depth 1- 2 meters in the year (2015 - 2016) along the northern estuarine harbor of the Arabian Gulf - Saudi Arabia. Parts of gonads (ovaries and testes) were prepared for histological and statistical studies. The histological observations of the gonad indicated that there was only one gonadal cycle in the year. The ovaries produced oocytes which undergone vitellogenesis to change these oocytes into comparatively ripe ones condensed with yolk. With slide micrometer a fixed length and width of the ovary was determined in serial histological sections (30000 µm). The different stages of oocyte development were counted and statistically analyzed. During November - February the gonadal follicles appeared small in size and in a quiescent state. Follicles were few in number and scattered throughout the vesicular connective tissue. In males the follicles were small and scattered throughout the connective tissue in the same way as females. Follicles contained undifferentiated residual cells, spermatogonia and negligible number of primary spermatocytes. The oogonia were the most dominant type. Few number of previtellogenic oocytes were observed. The ooplasm of these stages, oogonia and previtellogenic oocytes, was homogeneous and with comparatively large nucleus. In previtellogenic oocytes small granules of the same nature were scattered randomly in the ooplasm. These granules consisted of proteins (peptide linkage). Carbohydrates and lipids were entirely absent.During April - July previtellogenic oocytes began to undergo growth phase and extending towards the center of the follicles. In males spermatogenesis became more apparent and proceeded at a faster rate. In some mussels spermiogenesis was formed in great number. The gonadal follicles ramified rapidly, and the development of gametes of both sexes was very fast. In females the vitellogenic oocytes showed prominent development. Atretic oocytes and their debris were negligible. Very few number of oogonia were observed. Yolk bodies (glycoproteins) increased rapidly and replaced the vacuoles in the ooplasm. Other large granules gained a dark blue color when Aqueuos bromophenol blue was applied. These granules are basic proteins. Other large-sized granules began to appear at the periphery of the ooplasm. These granules gained positive reaction with Alcian blue, PAS and Alcian blue- PAS reactions. The vitelline envelope became conspicuous under the follicle cells. Mucosubstances were positively intense at the periphery of the oocyte while proteins surrounded the nucleus. Gradually fat deposition increased. The size range of these oocytes was 130 - 180µm with the ratio of the nucleus to the ooplasm 0.5:2. During August - October both sexes had ripe gametes. In males, the central portion of the follicles was empty because of the recent discharge of large quantities of ripe gametes. At the end of this step resorption of the gonads proceeded very rapidly and emptied follicles began to acquire an elongated form. At this step the oocyte attains its maximal size 180 - 200µm. The mean of the total number of oocyte developmental stages in all seasons showed that the ovary was active in all seasons of the year but different labors were present. It concluded that the oogonia , the postvitellogenic oocyte development and atretic oocytes were conspicuous during the first step of oogenesis ; the vitellogenic oocytes were dominant during the second step of oogenesis whereas the postvitellogenic and atretic oocytes were dominant during the third step of oogenesis. All data obtained were subjected to analysis of variance (ANOVA) means significant difference P>0.05 or P 0.05.

Ultrastructure of ovary and oogenesis in Chamelea gallina (Linné, 1758) (Bivalvia, Veneridae)

This is an ultrastructural study of the development of oocytes in the female venerid clam Chamalea gallina that is an economically valuable species. Specimens were collected from the Western Black Sea and the ultrastructural changes from oogonia to mature oocytes are described. The interfollicular tissue between the acini of the ovary of this species is composed of vesicular connective tissue, and a small number of follicle cells surround previtellogenic oocytes completely and move towards the stalk area at the late vitellogenic stage. Vitellogenesis may occur autosynthetically within the collaboration of Golgi complex, mitochondria and rough endoplasmic reticulum from the previtellogenic stage to the formation of mature egg. Besides autosythetic activity, intense endocytotic activity contributes to vitellogenesis heterosynthetically from the late vitellogenic stage to the end of yolk deposition. Despite high nuclear blebbing activity that is assumed to be performed to provide thenecessary membrane for vesicule formation, oogenesis in C. gallina is similar to that in many bivalve species studied so far.

Ultrastructure of oocytes during oogenesis and oocyte degeneration associated with follicle cells in female sinonovacula constricta (BIVALVIA: PHARIDAE) in western Korea

The ultrastructure of oocytes during oogenesis and oocyte degeneration associated with follicle cells in female Sinonovacula constricta (Lamarck, 1818) were investigated by electron microscope observations. Ovarian follicles are surrounded by a matrix of vesicular connective tissue cells (VCT cells). VCT cells contain large quantities of glycogen particles and several lipid droplets in their cytoplasm. It is suggested that VCT cells act as a source of nutrients for vitellogenesis during oogenesis. In early vitellogenic oocytes, several coated vesicles, which appear at the basal region of the oocyte, lead to the formation of membrane‐bound vesicles via endocytosis. The uptake of nutritive materials in coated vesicles formed by endocytosis appears through the formation of coated pits on the oolemma during vitellogenesis. During the late stage of oogenesis, yolk precursors (yolk granules), mitochondria and lipid droplets are present in the cytoplasm of late vitellogenic oocytes. In particular, proteinaceous yolk granules containing several different components are intermingles and form immature yolk granules. In the mature oocyte, small immature yolk granules are intermingled and form large mature yolk granules. Vitellogenesis occurs through a process of autosynthesis, involving combined activity of the Golgi complex, mitochondria and rough endoplasmic reticulum in the cytoplasm of vitellogenic oocytes. The process of heterosynthesis is where extraovarian precursors are incorporated into oocytes by endocytosis at the basal region of early vitellogenic oocytes before the formation of the vitelline coat. Follicle cells appear to play an important role in vitellogenesis and oocyte degeneration. The functions of attached follicle cells to the oocyte during oocyte degeneration are phagocytosis and digestion of phagosomes originating from oocyte degeneration. After digestion of phagosomes, it is assumed that the function of follicle cells can permit a transfer of yolk precursors necessary for vitellogenesis and allows for the accumulation of glycogen and lipid during oocyte degeneration, which can be employed by vitellogenic oocytes. Follicle cells of S. constricta may possess a lysosomal system for induction of oocyte breakdown and might resorb phagosomes in the cytoplasm for nutrient accumulation during oocyte degeneration.

Histochemical and ultrastructural characterisation of mantle storage cells in the hydrothermal-vent bivalve Bathymodiolus azoricus

This study reports on two types of storage cells that are present in the mantle connective tissue of the deep-sea mussel Bathymodiolus azoricus. One type corresponds to the adipogranular cells, a kind of storage cell previously described in other bivalves. In these cells extensive regions of the cytoplasm are filled with glycogen deposits and these zones became strongly stained after histochemical (PAS) or ultrastructural detection of polysaccharides. Several lipid droplets and membrane bound granules containing homogeneous electron-dense material are also present in adipogranular cells. A second type of cell contains large lysosomes in addition to numerous lipid droplets, but lacking cytoplasmic glycogen deposits. Due to these characteristics we named them adipolysosomal cells. They can be identified in semi-thin sections stained with PAS reaction because the lysosomes are the only positively stained structures. In the connective tissue of the mantle, some cells containing many lysosomes and a few lipid droplets were also observed. These cells differ from the adipolysosomal cells mainly because they have a reduced amount of lipid reserves, and could be an initial stage in the development of adipolysosomal cells. The vesicular connective tissue cells that in other Mytilidae are specialised in glycogen storage were not detected in B. azoricus. The reserves accumulated in the two types of storage cells described in B. azoricus may be important for the survival of these hydrothermal-vent bivalves if their nutrition is affected by a temporary loss or reduction of endosymbiotic bacteria due to sulphide and/or methane shortage caused by oscillations in vent activity.

Sensitivity of a digoxigenin-labelled DNA probe in detecting Mikrocytos mackini, causative agent of Denman Island disease (mikrocytosis), in oysters

The protistan parasite Mikrocytos mackini, causative agent of Denman Island disease (mikrocytosis), induces mortality and reduces marketability in the Pacific oyster, Crassostrea gigas, in British Columbia, Canada. This parasite is a pathogen of international concern because it infects a range of oyster species, and because its life cycle and mode of transmission are unknown. A digoxigenin-labelled DNA probe in situ hybridisation technique (DIG–ISH) was developed, and its detection sensitivity was compared to standard histological sections stained with haematoxylin and eosin stain (H&E-histo). In H&E-histo preparations, the detection of M. mackini was certain only when the parasite occurred within the vesicular connective tissue of adult oysters. However, the DIG–ISH technique clearly demonstrated the presence of infection in all other host tissues as well as in juvenile oysters with poorly developed vesicular connective tissue. The probe hybridised strongly to M. mackini, did not hybridise to oyster tissues or with the other shellfish parasites tested, and was more sensitive for detecting infections when compared to H&E-histo.

Susceptibility of juvenile Crassostrea gigas and resistance of Panope abrupta to Mikrocytos mackini

Samples from the field and laboratory exposure to Mikrocytos mackini (a tiny protistan parasite of unknown taxonomic affiliation) confirmed that juvenile Pacific oysters ( Crassostrea gigas) are susceptible to infection and the resulting disease. In the laboratory bath exposure experiment, a prevalence of infection approaching 100% and mortalities were observed in the small oysters (about 18 mm in shell length). However, in the same laboratory exposure experiment, similar aged geoduck clams ( Panope abrupta, about 8 mm in shell length) were resistant to infection. The main route of infection in the oysters appeared to be via the digestive tract and possibly the gills where the parasite multiplied within host cells. Other tissues such as the adductor muscle and vesicular connective tissue were subsequently colonized. Although the infection resulted in the mortality of some oysters, others appeared to overcome the disease.

Ultrastructure of Mikrocytos mackini, the cause of Denman Island disease in oysters Crassostrea spp. and Ostrea spp. in British Columbia, Canada.

An ultrastructural study was carried out on Mikrocytos mackini, the cause of Denman Island disease in Pacific oysters Crassostrea gigas in western Canada. Three forms were identified, quiescent cells (QC), vesicular cells (VC) and endosomal cells (EC). QC occurred in the vesicular connective tissue (VCT), haemocytes (hyalinocytes), adductor and heart myocytes, and extracellularly. They had a central round to ovoid nucleus, < 7 cisternae of inactive nuclear membrane-bound Golgi, few vesicles and lysosome-like bodies. VC were rarely extracellular and usually occurred in adductor and heart myocytes, in close association with host cell mitochondria. The contents of the host cell mitochondria appeared to pass through a tubular extension into the cytoplasm of the parasite. Cytoplasmic vesicles resembled the tubular structure in appearance and size. EC occurred in the VCT, in haemocytes and extracellularly. They had a dilated nuclear membrane, sometimes containing a looped membranous structure that appeared to derive from the nucleus, and pass into the cytoplasm. A well-developed anastomosing endoplasmic reticulum connected the nuclear and plasma membranes, and endosomes were present in the cytoplasm. QC and EC cells were frequently observed tightly against, or between, the nuclear membranes of the host cell. Few organelles occurred in all forms of M. mackini, especially QC. The lack of organelles found in most eukaryotic cells, including mitochondria or their equivalents, may be due to obligate parasitism and the utilization of host cell organelles reducing the need for parasite organelles. Alternatively, perhaps M. mackini is a primitive eukaryote. Although phylogenetic affinities could not be determined, it is not a haplosporidian. A developmental cycle is proposed from these findings.

Schistosoma mansoni Infection Inhibits Maturation of Ovotestis Allografts in Biomphalaria glabrata (Mollusca: Pulmonata)

The posterior tip of juvenile snails, consisting of immature ovotestis, vesicular connective tissue, and digestive gland acini, all surrounded by body wall, was implanted into the hemocoel of allogeneic adult Biomphalaria glabrata. Recipient snails were either uninfected or had been infected 15 days previously with Schistosoma mansoni. Graft recipients were dissected at 30 or 60 days postimplantation (DPI), and the implants, along with preimplantation controls, were examined histologically. The gonads in preimplantation controls consisted of a partially coiled collecting canal partitioned dorsally by connective tissue septa and contained mainly spermatogonia-like cells and small oocytes. In uninfected recipients, implanted gonads underwent rapid development, i.e., had formed mature acini and were carrying out vigorous gametogenesis, both spermatogenesis and oogenesis, by 30 DPI. At 60 DPI, numbers of spermatozoa had increased substantially in most implants. In infected recipients, relatively little development or gametogenesis occurred at either 30 or 60 DPI, and oogenesis was more strongly inhibited than spermatogenesis. Except for 1 implanted gonad (out of 25) that had undergone necrosis at 60 DPI and a second otherwise healthy 60-day implant partially encapsulated at its cut surface (both in uninfected recipients), no significant hemocytic response occurred against the allografts.

Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. II. Testis and spermatogenesis

The ultrastructural features of the testis and spermatogenesis have been described from the eastern oyster,Crassostrea virginica (Gmelin, 1791). The testis is a diffuse organ consisting of branching acini containing differentiating sperm in a variety of stages. Spermatogonia are located nearest the outer wall of the acinus, while spermatocytes and spermatids are positioned nearer the lumen. Mature spermatozoa are all confined to the central region of the acinus. The acinus is surrounded by an intermittent layer of myoepithelial cells and is bathed by a $uid-filled hemocoel. Vesicular connective tissue (VCT cells) fills the region between adjacent acini, and the cells contain glycogen granules and lipid droplets. Each acinus is divided radially into subcompartments that are partially separated by pleomorphic accessory cells which remain in close contact with sperm until late stages of development. Sperm are similar to those described in other oysters, except that five midpiece mitochondria were observed in some sperm rather than the usual four, and the acrosomal vesicle lacked the “whorled” substructure described in some other oyster sperm. We suggest that the neutral term “accessory cells” be applied to bivalve testicular somatic cells until more detailed studies are available to justify the use of “Sertoli cell” and other descriptive terms which have previously been adopted from other taxa only distantly related to bivalves.

Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. I. Ovary and oogenesis

The ultrastructural features of the ovary and oogenesis are described in the eastern oyster,Crassostrea virginica (Gmelin, 1791). The ovary is a diffuse organ consisting of highly branching acini in which oocytes develop. The acini are surrounded by a matrix of vesicular connective tissue (VCT tells) which serves a nutrient storage function. Each acinus is bathed by fluid within a surrounding connective tissue compartment, the hemocoel, which likely serves as a means of transporting nutrients to the oocytes. Oocytes begin growth while positioned near to the inner acinus wall. As differentiation proceeds and they enter the late stages of vitellogenesis, they become stalle-shaped and project into the acinus lumen. Follicle tells are closely associated with oocytes during the early and middle stages of vitellogenesis but they are largely confined to the basal, stalked region of late-stage oocytes. Vitellogenesis occurs through a process of autosynthesis, involving the combined activity of the Golgi complex and rough endoplasmic reticulum, and heterosynthesis in which extraovarian precursors are incorporated into oocytes via receptor-mediated endocytosis involving the basal surface of the oocytes. It is suggested that the follicle tells play some important role during oogenesis but probably are not the major source of yolk precursors. The VCT celas are probably the main source of nutrients for vitellogenesis.

The effect of geographical location on the cellular composition of the mantle tissue of the mussel, Mytilus edulis

The cellular composition of the mantle tissue of Mytilus edulis L. was determined, using quantitative microscopy (stereology) techniques, in three populations of mussels from differing environments, and the data compared. The results indicated that mussels from an estuarine or sheltered coastal environment had a greater proportion of adipogranular tissue than animals from an exposed, open coastal environment. The difference in cellular composition was attributed to differences in nutrient availability in the water column between the various sites. A 15-month transplantation of mussels from an open coast to estuarine conditions resulted in a decrease in the proportion of vesicular connective tissue in the mantle and an increase in gamete production. This suggests that Mytilus edulis can exhibit a high degree of phenotypic plasticity by using not only the more conservative strategy of nutrient storage during periods of high food availability, but also the more opportunistic approach of maximizing gamete production when conditions are favourable.

Storage tissue metabolism and reproduction in marine bivalves—a brief review

Summary The diversity of storage tissue cell types in bivalve molluscs is evident. In some species there are no specific storage cells, and the adductor muscle plays the role of a reserve tissue (Pectinidae). In others both muscular cells and specific storage cells are involved in storage and release of nutrients for gametogenesis (Veneridae). A few species have a specific storage tissue such as the mantle in Mytilus containing two complementary types of cells. This specialisation can be correlated with the fertility of the species and with the variation of available food organisms. Vesicular connective tissue is found in species with a high level of fecundity which are subject to marked fluctuations in food availability (Mytilidae, Ostreidae). This can be considered as an adaptive behaviour to profit from uncertain supply of food supplies in order to produce the maximum number (and quality) of gametes in a precise period of the year.

Occurrence of the Pathogen Haplosporidium nelsoni in Oysters, Crassostrea virginica, in South Carolina, U.S.A.

The presence of the pathogenic protozoan Haplosporidium nelsoni is documented for the first time in the American oyster, Crassostrea virginica, from South Carolina, U.S.A. Initially incriminated in 1957 as the causative agent of mass mortalities in the American oyster, Crassostrea virginica, in Delaware Bay, Haplosporidium nelsoni was given the acronym MSX (multinucleated sphere X) because its systematic relationship to other organisms was unknown. Subsequently, it was designated as Minchinia nelsoni by Haskin et al. (1966). Still later, Sprague (1979) transferred the species to the genus Haplosporidium (Balanosporida: Haplosporidiidae). The devastating effects of H. nelsoni on C. virginica are well known (see reviews by Lauckner, 1983; Sindermann, 1990; Sparks, 1985). It, along with Perkinsus marinus, another protistan parasite, are undoubtedly the major causes of the decline of the oyster industry in both the Delaware and Chesapeake Bays. Although H. nelsoni has been known by a few investigators to parasitize C. virginica in South Carolina, its occurrence in this state has never been recorded (Burrell, 1986). MATERIALS AND METHODS During November and December 1991 and January 1992, we examined 35 young oysters, 5-6 cm long, collected intertidally in Charleston Harbor (salinity 15-17%oo at Fort Johnson. The soft tissues of these bivalves appeared watery, with recessed mantles, and in each one of the six clusters studied, at least one or two were dead. These were native, wild oysters, the ancestors of which had inhabited Charleston Harbor for at least 100 years. No drought had occurred for at least three years prior to the time of collection. Moreover, no mass mortalities of oysters had been reported at the time. 1 This research was supported by a grant (NA16FL0408-01) from the National Marine Fisheries Service, U.S. Department of Commerce. TRANS. AM. MICROSC. Soc., 112(1): 75-77. 1993. ? Copyright, 1993, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.224 on Wed, 14 Dec 2016 05:02:50 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. All of the living oysters were removed from their shells and processed for histopathologic examination. All were fixed in 10% seawater formalin, embedded in paraffin, sectioned at 7 ,im, and stained with hematoxylin and eosin for light microscopy. RESULTS AND DISCUSSION Intercellular plasmodia of Haplosporidium nelsoni were found in each oyster. All of the observed plasmodia were in relatively early stages of karyokinesis in that none was observed to include more than 10 nuclei. A few dividing nuclei, with conspicuous spindle fibers, also were observed (Farley, 1967). The plasmodia observed measured 6-10 ,um in greatest diameter. Although the majority of the plasmodia occurred between Leydig cells (i.e., vesicular connective tissue cells) situated between acini of the digestive diverticula and in the matrices of gill filaments, some also were found in various hemolymph sinuses and vessels as well as suprabranchial chambers, thus indicating systematic infections (Kanaley & Ford, 1990). In addition to plasmodia, spores, each measuring 6-8 Aim in length, were observed. These were located between Leydig cells in the hosts' digestive diverticula. H. nelsoni spores are seen rarely and to date have been reported only by Couch et al. (1966) and Barrow & Taylor (1966). The fact that we observed spores in the oysters examined during November through January in South Carolina is of interest because in the Chesapeake and Delaware Bays, sporulation of H. nelsoni usually occurs in June and July (Couch et al., 1966). Our finding of spores during the stated collection period may reflect the warmer temperatures that prevail during autumn and winter in South Carolina coastal waters. Haplosporidium nelsoni has been reported along the Atlantic coast of the United States from Massachusetts, Connecticut, New York, New Jersey, Delaware, Maryland, Virginia, North Carolina, and the Atlantic Coast of Florida (Haskin & Andrews, 1988). Recently, H. nelsoni was reported in C. virginica from coastal Georgia (Lewis et al., 1992). Our observations document the first record of the occurrence of H. nelsoni in South Carolina.