M. F. Mulcahy

Gametogenesis of the oyster Crassostrea gigas in southern Ireland

The Pacific oyster Crassostrea gigas (Mollusca: Bivalvia) was introduced to Ireland in 1965 and is farmed at many sites around the coast. The reproductive biology of 1377 oysters from two sites on the south coast of Ireland was examined from April 1996 until December 1997 for variations in maturation rate and condition indices. Qualitative data were compiled by staging gonadal development using histological sections. Environmental parameters of temperature, dissolved oxygen and chlorophyll -a levels, as well as parasites and pathology were monitored. Unusually high sea temperatures led to oysters in Dungarvan (site 1) spawning in both years of the study. Although sea temperatures were significantly higher, oysters in Cork Harbour (site 2) did not spawn but instead reached ripeness and then started a process of gametic degeneration called resorption. Lack of spawning was not attributed to environmental conditions monitored but was tentatively attributed to levels of pollutants in the water. Oyster condition in Cork Harbour was significantly affected by the presence of blistering due to tributyltin levels in the water and also by Polydora sp. (Polychaete) in the shell. Oyster condition in Dungarvan was not affected by the presence of the exotic species Mytilicola orientalis (Copepoda: Cyclopoida).

Season-, age-, and sex-related variation in the prevalence of bonamiasis in flat oysters (Ostrea edulis L.) on the south coast of Ireland

Abstract Monitoring of two age groups of native flat oysters Ostrea edulis L. was undertaken in Cork harbour on the south coast of Ireland over 24 months to determine prevalence and intensity of infection of Bonamia ostreae (Pichot et al., 1980) and levels of mortalities occurring on the beds. Seasonal occurrence, as well as age- and sex-related variation were investigated. Two years appears to be a critical age in relation to disease development; bonamiasis was not detected in oysters younger than this, but once the infection appeared in this group, the disease quickly spread, and prevalence and intensity of infection and mortalities rose rapidly in the following months. Gonad smears indicated that males and females in both age groups were equally affected by the parasite.

An evaluation of anaesthetics for Ostrea edulis (L.)

Abstract An effective anaesthetic for oysters was sought, in order to allow for experimental manipulation of live oysters with minimal stress. A range of anaesthetics commonly used with gastropods and cephalopods, as well as a fish anaesthetic, were tested. Most of the chemicals tested were found to be unsuitable for oysters. Magnesium chloride was the most successful agent, inducing anaesthesia quickly, allowing rapid recovery with minimal stress and mortality.

Cell-free transmission of lymphosarcoma in the northern pikeEsox lucius L. (Pisces; Esocidae)

Beim HechtEsox lucius L. liess sich ein Lymphosarkom durch zellfreie Filtrate übertragen und damit die Virus-Aetiologie nachweisen.

An investigation into the relative resistance of Irish flat oysters Ostrea edulis L. to the parasite Bonamia ostreae

Abstract The parasite Bonamia ostreae has caused significant mortalities in the flat oyster Ostrea edulis. To date, methods of control and eradication have proved largely unsuccessful. Research is now concentrating on development of a population of oysters that has increased resistance to bonamiasis. This study evaluated the relative resistance of three Irish strains of flat oysters to this disease: two naive strains that had not previously been exposed to the parasite, and one that has been exposed to B. ostreae since the 1980s and has been selectively bred from survivors. In both field and laboratory trials, oysters from the selected strain showed lower prevalence of infection, intensity of infection and mortalities compared to the two naive strains. Development of a strain showing some resistance to B. ostreae would allow oysters to be grown to market size before significant mortalities occurred, and would also allow restocking of areas that have been decimated by the disease.

THE SUSCEPTIBILITY OF YOUNG PRESPAWNING OYSTERS, OSTREA EDULIS, TO BONAMIA OSTREAE

Abstract Young prespawning oysters, Ostrea edulis, were held over 6 mo at two different Bonamia ostreae-endemic sites in Ireland, to determine to what extent they could become infected with this protozoan parasite. Prevalence and intensity of infection were monitored, using the traditional method of ventricular heart smears and polymerase chain reaction (PCR). Results showed that 0+ and 1+ oysters were susceptible to infection. Infection was observed in the naïve and previously exposed oysters 2 months post relaying. Of ventricular heart smears and PCR, PCR was the more sensitive diagnostic technique in detecting B. ostreae in most of the oysters. Current methods recommended by the Office International Epizooties (OIE) and the European Union (EU), histology and screening of heart smears for B. ostreae, may be inadequate because certain low levels of infection may go undetected.

The interrelation of growth and disease resistance of different populations of juvenile Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract Growth of juvenile Atlantic halibut from three areas of the North Atlantic (Canada, Iceland and Norway) was studied in an experiment using individual tagged fish reared at 15°C for 85 days. Fish from each population were subsequently split into two groups and acclimatised to either 12°C or 18°C. The fish were then injected intra-peritoneally with a Vibrio anguillarum bacteria suspension and mortality monitored for 4 weeks. Growth rates of the Canadian population ranked lowest, whereas the Norwegian population had the highest mean growth rates (SGR=1.70% day −1 , 1.62% day −1 and 1.53% day −1 for the Norwegian, Icelandic and Canadian populations, respectively). The halibut from Norway had the best survival following bacterial challenge (80%, 50% and 55% survival for the Norwegian, Icelandic and Canadian populations, respectively). Mortality was higher at 18°C than at 12°C in the Icelandic (62% at 12°C and 27% at 18°C) and Canadian (56% at 12°C and 32% at 18°C) fish, whereas a smaller difference between temperatures was observed in the Norwegian fish (25% at 12°C and 13% at 18°C). Fish that survived the challenge test were those that had grown fastest in the growth trial. Low, but significant, correlations between survival and size and growth were seen, but these correlations varied between populations. In the Canadian population, no correlation between size and growth and survival were seen; only size was correlated ( r =0.27) with survival in the Icelandic population, whereas both size ( r =0.18) and growth ( r =0.17) were correlated with survival in the Norwegian population.

The detection of Bonamia ostreae (Pichot et al. 1980) in frozen oysters (Ostrea edulis L.) and the effect of the parasite on condition

Two year classes of frozen flat oysters, Ostrea edulis L., from four different beds in Cork Harbour (southern Ireland) were analysed. Bonamia ostreae, a parasitic protozoan, was detected from frozen heart tissue. Freezing did not appear to affect the ability to diagnose the infection. The detection of Bonamia from oysters frozen in 1986 pre-dates by nine months the initial diagnosis of the disease from fresh oysters in Ireland. The effect of the endoparasite on condition was also examined. Overall, oysters with heavy Bonamia infections were in poorer condition than uninfected oysters.

Living with bonamiasis: Irish research since 1987

The main oyster species produced in Ireland up to the 1980s was the European flat oyster Ostrea edulis. However, since then, production of this species has been severely affected by the presence of the protistan Bonamia ostreae, which was diagnosed in a population of flat oysters on the south coast following heavy mortalities. Research has been ongoing since the first diagnosis in Ireland and has concentrated on aspects of the biology of both the host and the parasite. In recent years research has concentrated on screening populations of oysters to identify any with reduced susceptibility to the parasite.

COMPARISON OF TECHNIQUES FOR DIAGNOSIS OF BROWN RING DISEASE AND DETECTION OF VIBRIO TAPETIS IN THE MANILA CLAM, VENERUPIS (RUDITAPES) PHILIPPINARUM

Abstract The bacterium, Vibrio tapetis, is the aetiological agent of Brown Ring Disease (BRD), which affects the Manila clam, Venerupis (Ruditapes) philippinarum. Two PCR assays for detection of V. tapetis were applied to a sample of 52 Manila clams, and evaluated in comparison with the more traditional V. tapetis detection method of microbiological isolation and characterization, as well as the traditional BRD diagnostic technique of shell valve analysis. The pathogen was detected in 15.4% of the sample using the PCR assay of Rodríguez et al. (2003, 2006) in 50% of the sample using the PCR assay of Paillard et al. (2006) and in 36.5% of the sample by microbiological methods. Whereas shell valve analysis was the least sensitive technique, detecting BRD in 7.7% of the sample, it was an essential diagnostic tool because it was the only technique that identified the disease, rather than the aetiological agent. None of the four techniques was sufficient on its own for effective BRD diagnosis; rather various combinations of two techniques were the minimum required. The combination of shell valve analysis with the assay of Paillard et al. (2006) proved to be the most sensitive and rapid of those tested. Shell valve analysis was the most time-efficient and cost-effective technique, whereas microbiological characterization was the most time-consuming, and PCR detection, using either set of primers, the most expensive. These results may need to be considered in light of diagnosis and management of BRD.