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Larvae of hatchery-reared Pacific oysters Crassostrea gigas Thunberg ceased feedlng 3 to 4 d after spawning, and 60 to l00 O/ O mortalities occurred after 7 to l l d. Up to 25 '!?o of moribund larvae had putative phagocyte precursors and fibroblasts with enlarged nuclei of abnormal shape and chromatin pattern, which contained ovoid to hexagonal, non-enveloped, 97 + 4 nm diameter capsids and nucleocapsids of a herpes-like virus. Nucleocapsids became enveloped and de-enveloped as they passed through the nuclear membranes. Some became enveloped in intranuclear sacs containing tubular elements Naked cytoplasmic nucleocapsids became enveloped by budding into vesicles or Golgi cisternae, and probably acquired a tegument from large dense granular masses near Golgi complexes. Extracellular enveloped virions usually had a tegument, and were 131 + 9 nm In diameter Extracellular naked nucleocapsids were thought to denve from lysed cells. Non-enveloped ovoid to hexagonal 86 t 12 nm virions, thought to be degraded herpesvirions, were abundant in inter-digitating processes of putative phagocytes in > 5 0 % of moribund larvae. Comparison w~t h groups within the Herpesviridae was made, and similarities to cytomegaloviruses noted. Elevated temperature and crowding may increase susceptibility of oyster larvae to herpes-like virus infection.

Bonamia sp. is a pathogenic parasite that occurs in the haemocytes of dredge oysters Ostrea chilensis Philippi in New Zealand. Ultrastructurally it resembles other haplosporidians in the possession of haplosporosomes, haplosporogenesis, persistence of mitotic microtubules during interphase and of the nuclear envelope during mitosis, and occurrence of a diplokaryotic or multi-nucleate plasmodial stage. Another stage containing a large vacuole derived from enlargement of 1 or more mitochondria has not previously been described from other haplosporidians. It most closely resembles B. ostreae Pichot et al., 1979, which parasitises and is pathogenic in haemocytes of European flat oysters, O. edulis. However, B. ostreae is smaller and denser, and has fewer lipoid bodies and haplosporosomes. We have nearly completely sequenced the small ribosomal gene of the organism from O. chilensis. Initial comparisons of these sequences with those of other protozoans showed similarities to B. ostreae. Polymorphism within Bonamia sp. was confirmed by restriction fragment length polymorphism analysis. On the basis of ultrastructural and molecular considerations it is proposed that this organism be named Bonamia exitiosus sp. nov. KEY WORDS: Bonamia exitiosus · Ostrea chilensis · Ultrastructure · 18S rDNA · New Zealand Resale or republication not permitted without written consent of the publisherDis Aquat Org 47: [63][64][65][66][67][68][69][70][71][72] 2001 MATERIALS AND METHODS Ultrastructural studies. Infected Ostrea chilensis (n = 237) from Foveaux Strait were opened one at a time, the heart was removed and cut in two, imprints were made with one half, and the other half was fixed for ultrastructural studies. Imprints were stained with Hemacolor™ (Merck) and microscopically examined, and heavily infected oysters were selected for further study. Hearts were fixed in 2.5% glutaraldehyde in 0.22 µm filtered seawater for 1 to 2 h, washed twice in filtered seawater, post-fixed in 1% OsO 4 for 1 h, stained en bloc with 5% uranyl acetate in 0.1 M sodium acetate buffer for 45 min, dehydrated in ascending (50 to 100%) ethyl alcohol, embedded in Araldite, sectioned, stained with 5% uranyl acetate for 10 min and in 5% lead citrate for 5 to 6 min, and examined on a Philips 420ST TEM.To compare Bonamia sp. with B. ostreae, quantitative data were taken from published studies on the ultrastructure of B. ostreae (Pichot et al. 1979, Brehélin et al. 1982, Balouet et al. 1983, Grizel 1985, Hervio et al. 1991 and from archived material held at the IFREMER laboratory at Ronce-les-Bains, La Tremblade, France. Samples were compared directly under the TEM from blocks of B. ostreae infected Ostrea edulis kindly lent by Steve Feist of the MAFF Fish Diseases Laboratory, Weymouth, England.Base sequence studies. DNA extraction: Genomic DNA was extracted from Bonamia sp. infected Ostrea chilensis using the following procedure. Ethanol fixed tissues were suspended in 1 ml of extraction buffer (NaCl 100 mM, EDTA 25 mM, pH 8, SDS 0.5%) with proteina...

Examination of the oyster Ostreola equestris as a potential reservoir host for a species of Bonamia discovered in Crassostrea ariakensis in North Carolina (NC), USA, revealed a second novel Bonamia sp. Histopathology, electron microscopy, and molecular phylogenetic analysis support the designation of a new parasite species, Bonamia perspora n. sp., which is the first Bonamia species shown to produce a typical haplosporidian spore with an orifice and hinged operculum. Spores were confirmed to be from B. perspora by fluorescent in situ hybridization. Bonamia perspora was found at Morehead City and Wilmington, NC, with an overall prevalence of 1.4% (31/2,144). Uninucleate, plasmodial, and sporogonic stages occurred almost exclusively in connective tissues; uninucleate stages (2-6 microm) were rarely observed in hemocytes. Spores were 4.3-6.4 microm in length. Ultrastructurally, uninucleate, diplokaryotic, and plasmodial stages resembled those of other spore-forming haplosporidians, but few haplosporosomes were present, and plasmodia were small. Spore ornamentation consisted of spore wall-derived, thin, flat ribbons that emerged haphazardly around the spore, and which terminated in what appeared to be four-pronged caps. Number of ribbons per spore ranged from 15 to 30, and their length ranged from 1.0 to 3.4 microm. Parsimony analysis identified B. perspora as a sister species to Bonamia ostreae.

The distribution, specificity and pathogenicity of the acanthocephalan Pomphorhynchus laevis (Muller) were studiedin the River Avon, Hampshire. The parasite occurredin every fish species, but three categories of hosts could be distinguished. Using growth and maturation as criteria of specificity, only chub Leuciscus cephalus (L.) and barbel Eurbus barbus (L.) were recognized as its preferred hosts. Parasites occasionally matured in trout Sulmo truttu (L.) and dace Leuciscus Ceuciscus (L.), but none grew or matured in other host species.Changes in the abundance of P. laevis along the river were related to changes in the abundance of both the intermediate host, Gammarus pulex, and the preferred hosts, and its occurrence in fish to the importance of G. pulex in their diet. In the upper reaches of the River Avon and in other localities populations could be maintained at a low level by parasites maturing in trout, and presence and abundance at any site depended upon the presence and abundance of both intermediate and definitive hosts especially and upon the dietary preferences of the latter. The absence of P. laevis from many parts of Britain is nevertheless still inexplicable. The parasite caused local damage to the intestinal wall of fish, the extent of which varied from species to species, but did not affect host growth rate or cause direct mortality and P. Iuevis cannot be regarded as an important pathogen in the River Avon or any other British River.