In this paper we report on our investigations of the toxic effects caused by the increased levels of lead in the sea water of the Persian Gulf on the embryonic and early larval growth stages of the Echinometra mathae sea urchin. Our toxicity tests were performed after the spawn induction and fertilization in 6 levels of lead concentration (250, 500, 1000, 2000, 4000 and 8000 mg/L). The resulting toxic effects of this metal were recorded on the development of the embryos until they reached the 4-arms pluteus larvae. To determine the effects of lead toxicity on embryonic and larval stages, the samples were studied using a microscope and, in addition the morphometric data of larval growth which were based on 3 parameters: overall length (OL), post-oral arm length (PL), and body length (BL), was collected. Our results confirmed that increasing concentrations of lead arrested development in early embryonic stages and led to morphologic abnormalities in the larval growth. In addition we demonstrated that morphometric studies showed a graduate decrease in larvae size by increasing the concentration of lead. Furthermore, median effective concentration (EC50) value calculated for lead was 2178.94 mg/L.

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