The Resilience of Leaf Oysters (Isognomon ephippium) to Variable Water Quality in Northern New South Wales Estuaries

Chamara Benthotage

doi:10.25918/thesis.302

Bivalves play important ecological roles in marine and freshwater ecosystems. When bivalves aggregate in dense populations, they form shellfish reefs made of living assemblages and dead shells. Isognomon ephippium commonly known as the leaf oyster is one of the largest intertidal bivalves known to form shellfish reefs. However, very little is known about the biology and ecology of the species. This thesis sought to advance our knowledge of leaf oyster population ecology, environmental tolerance, the immune system and shell microstructure. To investigate the environmental tolerance of leaf oysters, population assessments were undertaken across four mid to north coast estuaries in New South Wales. Leaf oyster densities ranged from scattered individuals (≪1 per m2) to dense beds with over 250 leaf oysters per m2 extending over areas of 500 m2. Leaf oyster reefs were mostly found on muddy and rocky substrates in the intertidal and shallow subtidal zone, with low flow rates and often coated by a layer of fine silt. Seventeen water quality parameters were assessed at each site and distance-based linear modelling (DBLM) revealed that a combination of water quality parameters explained a high proportion of the variation (> 85%) in leaf oyster density and condition index among sites. Poor body condition typically occurred at sites with low salinity, low dissolved oxygen and high inorganic nitrogen levels. To gain further insight into the health of leaf oysters in relation to water quality, the hemocytes of I. ephippium were characterised, along with a preliminary proteomics study from the hemolymph. The proteome did not differ between leaf oyster populations, however, there were significant differences in the total haemocyte and bacterial counts. DBLM revealed that bacterial counts in the water, along with dissolved and particulate nitrogen explained most of the variation in the total hemocyte count of leaf oysters. Inductively coupled plasma mass spectrometry (ICP-MS) was used to investigate the elemental profile of leaf oyster flesh, shells and the sediment from different estuaries. Trace metals/metalloids including Cd, Zn, Hg, Ag and As were found to bioaccumulate in leaf oyster flesh. However, elements like Pb, Cr, Mn, and Ni were underrepresented in leaf oysters compared to the sediment. High levels of As and Cd in the flesh were correlated with the lower body condition of leaf oysters, whereas elements like Cr, Cu, Se and Ni increased in the flesh with increased shell size. Scanning electron microscopy was used to investigate the microstructure and composition of leaf oyster shells. Leaf oyster shell characteristics were consistent with those of pearl oyster species (Pterridae), and showed consistent surface ultrastructure between two sites, one of which experiences periodic acid sulphate soil run-off. Overall, this study found that leaf oysters are able to tolerate a range of estuarine conditions. However, many of their biological parameters correlated with variation in estuarine water quality. This study provides baseline data for future monitoring of leaf oyster populations and facilitates their inclusion in shellfish reef restoration programs.

The Resilience of Leaf Oysters (Isognomon ephippium) to Variable Water Quality in Northern New South Wales Estuaries

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The Resilience of Leaf Oysters (Isognomon ephippium) to Variable Water Quality in Northern New South Wales Estuaries

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