Echinoderm embryos are commonly used for toxicity assays because of their extreme sensitivity to pollutants in general, and metals in particular (Kobayashi 1980, Phillips et al. 2003, Bielmyer et al. 2005, Grosell et al. 2007). However, our results show that H. iris is even more sensitive to copper and zinc than is E. chloroticus, and highlights the importance of obtaining metal toxicity data for species of special importance or concern instead of relying exclusively on indicator species.
Many factors affect the toxicity of metal in aquatic organisms in addition to variation between species; for example, exposure time, abiotic factors such as salinity, pH,temperature,and organic matter (Eisler 2007, Green et al. 2007, Rozman et al. 2010).Ecotoxicity studies use test duration ranging from 24 h to several days. This is because assays typically end when test animals reach the pluteus stage for echinoderms (ASTM 2012) or the veliger stage for molluscs (Hunt & Anderson 1993), and development time is regulated by temperature (Strathmann 1987).Tropical species typically reach these stages after 24 h (Kobayashi 1980, Rumbold & Snedaker 1997, Bielmyer et al. 2005), warm temperate species in 48 h (Manzo 2004,Gorski & Nugegoda 2006), and colder temperate species in 72 to 96 h (King & Riddle 2001, Phillips et al. 2003). A study even used a 20-day test for an extremely slow developing Antarctic sea urchin (King & Riddle 2001). Longer assays may allow more time for metal absorption and result in generally lower EC50 values than shorter assays. Indeed, tropical species are often less sensitive than their temperate counterparts (Table 2.6 and Table 2.7); similarly, the only toxicity value reported for an Antarctic sea urchin was the lowest reported so far (King & Riddle 2001).Therefore, the relatively low toxicity values reported in this study may in part be due to longer test duration because of slower development time than most but not all other species.