This proposal focuses on differences in the susceptibility to genotoxicants of early and late stage embryos with respect to embryological development. Grass shrimp will be used as a model organism to characterize the difference in susceptibility of early and late stage embryos to contaminant induced DNA damage. Grass shrimp embryos have been used for toxicity testing. Adult grass shrimp will be exposed to sediments and food containing chromium, mercury, or polycyclic aromatic hydrocarbons (PAH?s). Embryo stages produced from control and genotoxicant exposed adults will be followed through the various stages of development. DNA strand breakage and repair will be measured using the comet assay, including modifications for quantification of specific lesions and crosslinks. A DNA repair enzyme will be amplified, cloned and sequenced. Based on this sequence, primers will be prepared and tested to begin studies on the expression of a DNA repair gene in early and late stage embryos. The significance of this work is that it provides a model for the study of genotoxicity. By working with grass shrimp embryos, which have well studied developmental processes, a quick reproductive cycle which is easily manipulated in the laboratory, and are routinely exposed to genotoxicants in contaminated environments, we can understand the processes which make early embryonic stages more sensitive to contaminants.