Abstract:
In many places, the natural concentrations of heavy metals in seawater are low, but with the activity of humans, such as the depositing of industrial wastewater and solid waste into the environment, increasingly soil, water, air, plants and animals are being contaminated with heavy metals. The toxicity of heavy metals commonly impacts the survival of crustacean and bivalve larvae in hatchery culture, and this has led to the widespread use of the chelating agent ethylenediaminetetraacetic acid (EDTA) to decrease this toxicity.
The research presented in this thesis improves our understanding about the known benefit of applying chelating agents to shellfish hatchery seawater, particularly at the earliest stages of development. Significant improvements in survival, which were associated with lower concentrations of several toxic heavy metals, were observed with EDTA treatment over the first 2 days of P. canaliculus larval development. The concentration and spatial arrangement of heavy metals in larvae was consistent with reduced bioavailability of several metals, especially copper and zinc.
Following the first study, P. canaliculus larvae were experimentally raised to 22 days post-fertilisation in seawater with and without 12 μM EDTA. The survival, shell length, growth, algal ingestion rate, swimming activity and potential toxic metal accumulation by the larvae were compared over this period. There were minimal benefits from continuing addition of EDTA beyond the first 2 days. However, significant changes in metal concentrations within the larvae were observed. Zinc, cadmium and mercury were detected at significantly lower concentrations in 22-day-old larvae reared with EDTA versus those reared without EDTA.
Since EDTA has a very poor biodegradability leading to potential persistent environmental effects, alternative methods to prevent heavy metal toxicity to shellfish larvae are needed. Ethylenediaminedisuccinic acid (EDDS) is a biodegradable potential alternative to EDTA for this application and was tested as a treatment of the seawater used for rearing aquaculture Greenshell™ mussel (Perna canaliculus) larval embryos in this study. Mussel embryos reared with EDTA or EDDS had significantly better survival than without. Similar improvements to those with EDTA were observed in the survival of P. canaliculus embryos over the first 2 days of development when reared with EDDS added to the seawater, resulting in significantly lower concentrations of zinc and higher concentrations of calcium observed in the embryos when compared to the control treatment which had no chelating agent added.