The aetiology of environmental stress responses and disease in bivalve molluscs

Abstract

The aim of this study was to provide a comprehensive set of quantitative and qualitative baseline responses at physiological, metabolical and immunological levels, in the Pacific oyster Crassostrea gigas (Thunberg), the European flat oyster Ostrea edulis (L.), and the Manila clam Tapes philippinarum (Adams and Reeve). The energetics of these species were compared across a matrix of temperature and salinity conditions. Field trids examined the effect of exposure of three O. edulis populations to infection by the protozoan parasite Bonamia ostreae. and enzyme electrophoresis investigated the genetic basis for any differences. Changes in immunocompetence were monitored from field samples and with controlled Vibrio anguillarum bacterial challenges. Haemolymph and haemocytic responses were recorded. Filtration rate had the most significant effect on scope for growth (SFG) indices measured in all species. C. gieas showed a much wider range of filtration rates than O. edulis. and consequently had much higher SFG. Optimum environmental conditions for C. pi gas occurred at 20-25 °C and 19-25 %o, compared with 20°C and 33%o for O. edulis. and 15-20°C at 33%o in T. philippinarum. Separate winter and summer physiological behaviour was detected in C. gigas and O. edulis. with the change occurring at 15°C and 10-12°C respectively. Body condition indices were inversely proportional to SFG and were probably related to the reproductive cycle. Temperature was shown to have the most significant influence on energetic factors, with salinity having little effect. Field trials investigating Bonamia effects in three O. edulis populations found a significant, inverse size relationship with most of the physiological measurements. The largest animals, from Scotland, showed the lowest mortality, corresponding with increased energy input and decreased energy expenditure. The apparent resistance of this population probably arose from the fate rather than amount of assimilated energy. The Conwy population of O. edulis showed similar energy partitioning to the C. gigas and T. philippinarum animals, which also came from Conwy. Enzyme electrophoresis showed the three populations to be genetically very similar, with little genetic variation. Thus the differences in physiology were probably a result of phenotypic plasticity. A phagocytic index on V. anguillarum uptake by O. edulis large granulocytes showed seasonal changes with higher levels in winter than summer. This was related to temperature stress inducing the physiological stress of reproduction. Hydrogen peroxide production varied similarly, but lysozyme activity was very variable. Bacterial challenge under seasonal conditions showed peroxide activity to increase significantly after 48 hours. Pathogen challenge in C. gigas under controlled environmental conditions also showed highest peroxide concentration at low temperature, with variable lysozyme activity. Hydrogen peroxide was considered the primary haemolymph defence mechanism.

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ORCID for Lawrence Hawkins: orcid.org/0000-0002-9236-2396

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