While early life stages are usually the most sensitive to pollutants and environmental disturbances, a specific developmental stage may be particularly sensitive -sometimes called the ‘critical life stage’ (Kobayashi 1980, Hunt & Anderson 1993,Conroy et al. 1996, Jezierska et al. 2009). As a general rule (although not always the case), younger organisms tend to be more vulnerable than older ones (King & Riddle 2001, Jezierska et al. 2009, Aronzon et al. 2011). In echinoids, the early pluteus stage(occurring between fertilisation and the two-armed pluteus stage, which takes one to few days post-fertilisation depending on the species) has been shown to be the most sensitive in many species (Kobayashi 1980, King & Riddle 2001, Bielmyer et al.2005). As a result, most studies on pollutants have focused their observations during this critical life stage and little is known about the sensitivity of later pluteus stages(but see Kobayashi 1980, Phillips & Shima 2006) and young juveniles. Understanding the effect of pollutants outside of the critical life stage period is important for evaluating the impact of pollutants in the field and for natural populations.
The aim of this study was to investigate the ecologically relevant impact (i.e. shortterm,low concentrations) of copper pollution on marine invertebrate larvae and young juveniles, with a focus on latent and carry-over effects, using the sea urchin Evechinus chloroticus as the focal species. Stage-dependent toxicity was evaluated by exposing larvae either early (4 – 6 d post-fertilisation) or late (11 – 13 d postfertilisation)during development, and as young juveniles (25 – 27 d post-settlement).I then tested resistance of the surviving juveniles to a further exposure to the same stressor. All copper concentrations used were derived from field measures in low to moderately polluted areas of New Zealand, and were expected to produce no major direct effects on larval survival or development when administered in a very short pulse and outside of the critical life stage of early pluteus development. Young larvae were expected to be more vulnerable than older larvae and juveniles.