Effects of metal toxicity on the early life stages of the sea urchin Evechinus chloroticus
Authors:   , Rouchon Agnes 
Published: November 2021

<p>Metals are a common source of pollution in coastal waters, and have long been recognised as a major concern for many marine species, especially their early life stages. Although effects have been examined using standard toxicity assays, the impact of metals in more complex and realistic exposure regimes is still poorly known, in particular with regards to latent effects across multiple life stages and the interaction of multiple stressors. In this thesis, the effects of metals were investigated for multiple life stages of the endemic New Zealand sea urchin Evechinus chloroticus.  Standard short-term bioassays were performed on the early life stage of E. chloroticus and also the endemic abalone Haliotis iris, for comparison. These assays evaluated the toxicity of three major pollutants (copper, lead and zinc) alone and in combination, on these species. Embryos of both species were highly vulnerable to copper (EC50s: 5.4 and 3.4 µg/L respectively for E. chloroticus and H. iris) and zinc (27.7 and 13.1 µg/L) but relatively tolerant to lead (52.2 and 775 µg/L), and there was no evidence of synergistic effects of metal mixtures.  The latent effects of copper across two life stages in E. chloroticus, larval and juvenile, were investigated with laboratory experiments using realistic scenarios of low copper concentration, short pulses of exposure and examining exposure through dietary intake as well as waterborne exposure. Strong latent and carry-over effects were observed even at low concentration and short exposure time. For example, individuals exposed as larvae to 10.4 µg/L Cu for two days developed normally during the larval stage but had strongly impaired subsequent growth, with average body size decreasing by 24% in the 25 d following settlement. Moreover, juveniles previously exposed to copper as larvae were less resistant to a subsequent exposure, with up to four times higher mortality. Latent effects were especially important when copper was present in the diet rather than dissolved in water. For example, E. chloroticus larvae exposed to 2.3 µg/L Cu in water and fed with an algal diet cultured in the same concentration had a settlement success three times lower than those exposed only to waterborne copper. Furthermore, a short pulse exposure (4 days) to copper in the algal diet was generally more toxic than chronic exposure, showing that a short-lived bloom of contaminated phytoplankton may have a more severe impact on zooplankton than chronic pollution.  Because metal discharge in coastal water is generally associated with freshwater (e.g. storm water or river plumes), the toxicity of copper was evaluated in both normal and low salinity seawater. Low salinity (24 ppt) increased copper toxicity in E. chloroticus larvae under chronic exposure to high levels (15 µg/L; 43% and 80% lower survival and normal development rate, respectively) but not under a single pulse exposure (4 days) to low concentration (5 µg/L). This highlights the importance of using realistic exposure in laboratory assays.  Finally, the effect of copper on adult E. chloroticus and in particular on their fertilisation success was evaluated. Strong sublethal effects were observed after exposure to 50 µg/L Cu for two weeks including spawning impairment (especially in females) and elevated copper burden in gonads (25-times higher than control animals). However, the fertilisation success of successfully spawning males was not affected. The prevalence of local metal contamination was also measured at the mouth of local river plumes and in E. chloroticus gonads at sites expected to vary in likely exposure to pollution. Copper levels exceeding water quality criteria were found in two instances in coastal agricultural runoff (Makara stream). Other metals were within water quality cirteria in all samplings. Adult E. chloroticus had an elevated copper burden in gonads in an urban site compared to a control site (0.77 µg/g vs. 0.27 µg/g).  In total, this research demonstrates the need for considering toxic effects across multiple life stages and using realistic exposure regimes (e.g. timing, concentration, multiple stressors) to better understand the likely impact of metal pollution on marine populations. It also provides the first measure of metal toxicity on early life stages of an endemic species of cultural and commercial importance in New Zealand.</p>