The pulmonate mud snail, Amphibola crenata as a potential bioindicator of estuarine trace metal and nutrient pollution in New Zealand.

The present study was the first to examine the toxicological, biochemical, physiological, and population level responses of an estuarine pulmonate snail, Amphibola crenata, to trace metal and nutrient pollution using both laboratory and field-based studies. The main objectives of this study were to (a) determine toxic mechanisms of A. crenata to both acute and chronic cadmium (Cd) exposure, and to investigate bioaccumulation of Cd in different tissues as a function of exposure concentrations and time; (b) assess multiple biomarker responses of field-collected snails in relation to trace metal and nutrient pollution levels present; and (c) evaluate the potential utility of A. crenata as a bioindicator for estuarine contamination in NZ. The laboratory-based acute toxicity test showed that Amphibola crenata is highly tolerant to waterborne Cd, at least over acute (48 h) exposures. This high tolerance to Cd could be associated with large internal energy reservoirs (e.g. glycogen reserves), efficient antioxidant defences (e.g. catalase activity) and mechanisms that possibly restrict Cd exposure (e.g. mucus production and/or opercular closure). Exposure to chronic (21 days) Cd has shown more detrimental effects on mud snail health. Long-term exposure to Cd significantly induced oxidative stress in A. crenata, represented by tissue catalase activity and lipid peroxidation. Under both acute and chronic exposures, Cd induced a cascade of toxicological, physiological, and biochemical effects in A. crenata. During both exposures, individuals of A. crenata showed a significant accumulation of Cd in their tissues relation to availability of Cd in the seawater, demonstrating that this mud snail is a net accumulator of metal contaminants from their surroundings. The acute toxicity responses, however, indicated that short-term exposure to high levels of Cd may result in saturation of Cd in certain tissues including viscera and remaining tissues (i.e., kidney, mantle, remaining digestive tissues and heart) with increasing Cd concentration. During both acute and chronic exposure to Cd, oxygen consumption of mud snails was significantly reduced, with a concomitant increase in excretion rate. In addition, significant declines in O:N ratio and tissue glycogen levels were observed in Cd-exposed mud snails. The reduced O:N ratio suggests that protein is being mobilized for use as an energy source in this species, while depletion of tissue glycogen could be attributed to the utilisation of glycogen reserves by the organism through glycogenesis, to meet energy demand during stress. Catalase activity in certain tissues (e.g. viscera), haemolymph glucose, and protein levels of A. crenata also increased under both acute and chronic Cd exposure. Overall, the laboratory-based studies clearly demonstrated that the toxicological, biochemical, and physiological responses of A. crenata significantly in relation to the Cd exposure concentration, tissue type (e.g. toxicological and biochemical responses) and the duration of exposure. This confirms a potential use of this species as an indicator of trace metal pollution in the environment. Concentrations of different trace metals (As, Cd, Cu, Ni, Pb, and Zn) in field-collected snails were measured to assess the use of A. crenata as a bioindicator for trace metal pollution in New Zealand estuaries. Out of 6 trace metals, there were significant correlations between metal concentrations in the sediment and snail soft tissues for Cu (R² = 0.43, p < 0.01), As (R² = 0.36, p < 0.05) and Pb (R² = 0.3, p < 0.05). The sites were distinguished according to the metal content of the snail tissues, metal contamination in sediment, and biomarker responses. Of the biomarkers examined in field collected snails, those with the greatest promise for detecting environmental impact, related to metal contamination, included the condition index, glutathione-S-transferase, catalase activity, lipid peroxidation, haemolymph glucose, and protein. In addition, this thesis for the first-time evaluated A. crenata population attributes (e.g. length and density) as indicators of estuarine pollution using both contrasting areas within and between estuaries. The results indicated significant site-specific and regional differences in snail population structure, and significant positive correlations between sediment total recoverable phosphorous (TRP), to snail density (R = 0.9) and shell length characteristics including minimum, mean, and median length to certain sediment trace metals (e.g. Zn and Cd). The mudsnail, A. crenata, can be an effective biomonitoring tool for New Zealand estuaries. As mud snail populations can be used to identify areas that are potentially affected by trace metals and other contaminants. Biomarkers can be used to follow the effects of habitat disturbances, or the recovery of particular areas following habitat remediation, and will provide useful information for environmental managers