The effects of environmental stressors on immunological activity in Pinctada imbricata

Pearl oyster farming is a significant component of the Australian mariculture industry. However, disease often threatens its productivity. Previous work has identified a strong relationship between environmental stress and altered immunological activity in many molluscs, often resulting in the spread of opportunistic pathogens. My studies investigate this relationship, focusing on the effect of stress induced neuroendocrine secretions. -- The First Chapter provides a general introduction to the history and existing knowledge of Akoya pearl oyster aquaculture. This Chapter examines subjects that are to be further investigated by this thesis, namely; the relationship between farmed marine invertebrates and the influence of husbandry and handling on their immune and endocrine systems. -- The Second Chapter provides a detailed introduction of the morphology and cytochemistry of P. imbricata haemocytes; granulocytes, hyalinocytes, and serous cells. Granulocytes represent the most abundant cells type (62%), and are shown to have the highest capacity to phagocytose Congo red-stained yeast cells. Granules contained within granulocytes were also found to stain positively for a number of immuno-reactive enzymes or products including; phenoloxidase (PO), peroxidase, superoxide, and melanin. Hyalinocytes were the second most abundant cell type (36%), and also engulfed Congo red-stained yeast cells. Serous cells represented the smallest component of the haemocyte population (2%) and were not involved in clearance activities. -- Chapter Three investigates the effects of common environmental stressors (low salinity, mechanical agitation (MA), and exposure to air) on the immunological activity in P. imbricata haemocytes. Both phagocytosis and PO activity decreased significantly when oysters were exposed to all three stressors. The responses to other parameters tested (granulocyte to hyalinocyte ratio, total haemocyte counts, acid phosphatase activity, and total protein content) were found to vary, and the different immunological parameters tested were influenced uniquely according to the type of stressor. -- Chapter Four addresses the relationship between stress and hormone-induced apoptosis in defensive haemocytes from P. imbricata. Haemocytes were exposed to 0.0, 2.5, 5.0 and 10.0 ng noradrenaline (NA) per μg of protein. Both DNA fragmentation and Annexin V-FITC staining was greatest in haemocytes exposed to 10 ng NA/ μg protein. The ability of haemocytes to adhere to glass slides was also negatively affected by NA exposure. Cytoskeletal re-arrangement was evident in NA- treated haemocytes stained with the F-actin selective stain, Phalloidin Alexa Fluor 488. Morphological and ultrastructural changes in NA-exposed haemocytes identified by transmission and scanning electron microscopy included; chromatin and cytoplasmic condensation, the formation of apoptotic bodies, vacuolization and blebbing.