Thesis (MSc (Botany and Zoology))--University of Stellenbosch, 2007. / The ability of various molluscan species to accumulate toxicants such as cadmium from natural waters in quantities that are many orders of magnitude higher than background levels are well-known. This phenomenon of bioaccumulation might cause certain stress responses in these organisms at the cellular level, which can be measured using biomarkers. A biomarker response test known as the neutral red retention assay was employed in this study to measure responses in four intertidal species. Specimens of Cymbula oculus (Born), Scutellastra longicosta (Lamarck), Cymbula granatina (Linnaeus) and Scutellastra granularis (Linnaeus) were collected at two localities on the coast of False Bay, South Africa. Laboratory exposures in static flow tanks at three different concentrations i.e. 0.8, 1 and 1.2 mg/L of CdCl2 were done respectively for each species over a three day exposure period i.e. each exposure concentration had an exposure period of 24, 48 and 72 hours. After every 24 hour exposure period the lysosomal membrane integrity was determined using the neutral red retention method to establish which species is the most sensitive to Cd. Both control and exposure groups for all species showed a decrease in retention times with an increase in Cd concentration over the exposure period. This decrease was particularly prominent at the highest exposure concentration after 72 hours. At 0.8 and 1.2 mg/L CdCl2 exposures an indirectly proportional relationship between neutral red retention time and heavy metal concentration was prominent in C. oculus, indicating a dose related response. In all species there was a moderate increase in heavy metal concentration over the 72 hour exposure period. EC50 values indicated that S. granularis and C. granatina had a “high” sensitivity to Cd contamination, while C. oculus had “medium” sensitivity and S. longicosta “low” sensitivity to Cd contamination. The sensitivity data obtained from the analysis of the experimental species in this study may contribute to the eventual establishment of a species sensitivity distribution model (SSD).