Polyspermic eggs usually die after a short period of abnormal embryogenesis, but eggs of many animals and at least two plants are protected from polyspermy by "fast" and "slow" blocks. The studies proposed here will extend out general understanding of the fast block (a transient, ion-mediated blockage) using fucoid algae as a model system. Physiological and electrophysiological techniques will be used to describe the ionic dependence of the fast block and the fertilization potential. The effect of the sperm/egg ratio on polyspermy will be evaluated in several fucoid species, and data on fertilization and sperm/egg rations in the field will be collected. Natural levels of polyspermy will be determined by placing tiles within appropriate portions of the intertidal zone and observing the development of colonizing zygotes. Such studies will examine the possibility that recruitment of zygotes is affected by failure of the fast block during periods of natural osmotic stress. Little information is available on polyspermic blocks in plants. The proposed work will make a significant contribution in this area, both through the proposed studies of fucoid algae and by examining several other marine and freshwater algae for the presence of an ionic block to polyspermy. %%% Eggs that are fertilized by more than one sperm usually die after a short period of abnormal embryogenesis. In many organisms polyspermy is prevented by two successive mechanisms- a fast acting block and a slower one. The fast block is mediated by transient ionic fluxes that begin immediately after the first fertilization. In spite of its widespread occurrence, there is little ecological data available to evaluate the importance of this fast block under natural circumstances as opposed to in the laboratory where ionic conditions are manipulated or sperm/egg ratios are high. Dr. Brawley proposes to investigate the natural situation using fucoid algae in tide pools where ionic conditions can vary dramatically. She will thus obtain ecological data that are currently lacking on natural sperm/egg concentrations, and an evaluation of how common polyspermy is in nature.