Absence of a complete block to polyspermy after fertilization of Mytilus galloprovincialis (Mollusca,Pelecypoda) oocytes

Mytilus galloprovincialis oocytes undergo monospermic fertilizations (1 sperm nucleus/oocyte) over a wide range of sperm-oocyte ratios beyond which the number of penetrating sperm increases either linearly or exponentially over 10 min. Artificial activation of oocytes by KCl or the ionophore A 23187, up to the polar body extrusion stage, allows successful fertilizations upon a subsequent insemination. No organized and complete detachment of supernumerary oocyte-bound sperm is detected after fertilization. Reducing the external Na⁺ concentration promotes a higher rate of fertilizations. These results suggest that no complete block to polyspermy is established in this species but that a partial block, Na⁺ dependent, might be sufficient to ensure monospermic fertilizations under natural conditions.