Occurrence of the pathogen Haplosporidium nelsoni in oysters, Crassostrea virginica, in South Carolina, USA.

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.