The worlds’ oceans are increasingly exposed to a diversity of contaminants stemmed from anthropogenic activities, many considered to be ‘contaminants of emerging concern’ (CECs). For several CECs, their influence on marine ecosystems is unknown. Historically, biomonitors have been used as an effective time-integrated tool to monitor contaminant loads in ecosystems. The upside-down jellyfish (Cassiopea) has shown promise as a biomonitor to a range of marine contaminants. Using manipulative laboratory experiments, Cassiopea were exposed to a range of CECs to assess both the host jellyfish and endosymbiont response to chronic exposure. Changes in jellyfish size, photosynthetic efficiency, and zooxanthellae density were measured as sub-lethal responses to contaminant exposure. The results from this study will be summarised and the relative toxicities of the respective contaminants discussed. Findings from this research will determine the effectiveness of Cassiopea as a biomonitor to a range of CECs in coastal marine systems. This will aid in developing early detection and quantification tools to help assess risks associated with these contaminants, thereby supporting the maintenance of healthy coastal marine systems.