. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 2345678 Time of Re-insemination (min) 10 Figure 7. Sperm binding to fertilized oocytes. Fertilized oocytes were heavily re-inseminated at appropriate times and fixed at 30 s after re- insemmation. Time zero represents heavily inseminated sperm binding to unfertilized oocytes (control). Values are represented as a ratio of the control value (mean ± SEM of six experiments). 10' and 2 X 10:, suggesting that no complete block to polyspermy is established in this species. In Mytihts ednlis, we found here that the oocyte exhibits
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 2345678 Time of Re-insemination (min) 10 Figure 7. Sperm binding to fertilized oocytes. Fertilized oocytes were heavily re-inseminated at appropriate times and fixed at 30 s after re- insemmation. Time zero represents heavily inseminated sperm binding to unfertilized oocytes (control). Values are represented as a ratio of the control value (mean ± SEM of six experiments). 10' and 2 X 10:, suggesting that no complete block to polyspermy is established in this species. In Mytihts ednlis, we found here that the oocyte exhibits a complete block to polyspermy after fertilization (Fig. 4), but that the block lasts only 30 min after spawning. Because the incidence of polyspermy increases with time after oocytes are spawned (Fig. 3), use of old oocytes for insemination would lead to the same conclusion for At. edulis as Dufresne-Dube et al. (1983) reached for M. gallo- provincialis—, that the species lacks a mechanism for complete polyspermy block. In the brown alga Fucus (Brawley, 1991) and in marine invertebrates such as the sea urchin, the starfish, the echiuroid Urechis. and the nemertean Cerebratnlus (Jaffe, 1976, 1980; Miyazaki and Hirai, 1979; Gould-Somero et al., 1979; Kline?/ 1985), the membrane of the oocyte depolarizes at fertilization. In crustaceans, hyperpolariza- tion was observed at fertilization (Goudeau and Goudeau, 1989a, b). All the above reports, except those for crusta- ceans, also showed that polyspermic fertilization occurs in low-sodium ASW by suppression of the depolarization, and that fertilization is inhibited when the membrane po- tential is clamped at a positive value. Depolarization of the oocyte plasma membrane also occurred at fertilization in M. edulis. and less depolarization and higher poly- spermic fertilization were found in low-sodium ASW (Figs. 5 and 6, Table I), suggesting that an electrical event at the plasma membrane acts as the fast polyspermy block
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