Development of an embryo-larval chronic toxicity test using the sea urchins Tripneustes gratilla and Echinometra mathaei
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Date
2019
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University of Zululand
Abstract
Toxicity tests use organisms to assess the effects of the bioavailable fraction of toxicants in effluents and environmental media, such as water and sediment. Test organisms must be highly sensitive and ecologically relevant. Gametes of the sea urchins Echinometra mathaei and Tripneustes gratilla are used for acute toxicity testing purposes in South Africa through the fertilisation test (e.g. for testing effluents and seawater samples). However, this test has raised questions on the longer-term, sub-lethal effects of toxicants in effluents and other environmental media tested. The aim of this study was thus to develop a chronic toxicity test using the larvae of these sea urchins, to allow for more comprehensive toxicity testing. To achieve this aim, the influence of temperature on larval development to the 4-arm pluteus stage was first evaluated, to determine the duration of the toxicity test. Embryos of the sea urchins were exposed to three temperatures, namely 20, 23 and 26 °C, and development was documented at 24 h intervals until the 4-arm pluteus larva was attained. The optimum temperature for normal larval development for both species was 23 °C, with the 4-arm pluteus stage attained at 72 h for E. mathaei and 96 h for Tripneustes gratilla. The sensitivity of the sea urchin larvae to potential reference toxicants copper and zinc was evaluated. Reference toxicants are critical for quality assurance in toxicity testing. The EC50s were used to construct control charts for each species. The control charts and coeffiecient of variation identifies the preferred reference toxicant, for E. mathaei as zinc and for T. gratilla as copper. The larval development test was then used to test the toxicity of seawater desalination brine, effluent from two wastewater treatment works and water collected in a marine environment that receives effluent from a pulp mill effluent concurrently with the fertilisation test. This was to compare the sensitivity of each test in detecting toxicity. The minimum acceptable toxicant dilution (MATD) was calculated from dose-response curves using the linear interpolation model and the index of difference was used a measure of sensitivity. The larval development test was generally less sensitive in detecting toxicity of seawater desalination brine and effluent compared to the fertilisation test, but the difference was negligible. In contrast, the larval development test was more sensitive in detecting toxicity of seawater samples collected from the receiving water of pulp mill effluent, but the results were highly variable. The fertilisation test would be the preferred choice for toxicity testing of seawater desalination brine, effluent and receiving water samples of pulp mill effluent as it is generally more sensitive, rapid and cost effective for sample analysis. However, this is not a definitive
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decision because the larval development test may be more sensitive in other applications using different types of effluent or water samples from receiving environments.
Description
A dissertation submitted in fulfillment of the academic requirements for the degree of Master of Science in the Department of Agriculture in the Faculty of Science, Agriculture and Engineering, University of Zululand, 2020.
Keywords
Sea urchin, Echinometra mathaei, Tripneustes gratilla, acute toxicity