Comparative study on the Structure of Zona Radiata in Pre- and Post-fertilized Mature Eggs of the Viviparous Molly (Poecilia sphenops) and the Oviparous Bighead Carp (Hypophthalmichthys nobilis)

In the present research, modification of the zona radiata (ZR) around the molly (Poecilia sphenops, Valencinnes 1846) and bighead carp (Hypophthalmichthys nobilis, Richardson 1845) oocytes and in the zygotes were studied for pre- and post-fertilization stages and its relationship with the reproductive strategy. Light and scanning electron microscopic studies on the histological sections of ovary in viviparous molly and oocytes of the oviparous bighead carp showed that in post-vitellogenic (mature) oocytes, the traces of pore-canal system (organization) of the ZR were seen and after fertilization, the zona radiata of both species gained further simplicity and reduced in thickness, that could show a change in its role from nutritional type to protectoral one particularly in bighead carp. Obvious difference in thickness, architecture and number of pore-canals was seen among the ZR of the pre- and post-fertilized eggs of two species. It appeared that in both pre- and post-fertilization stages, the ZR of the molly eggs was simpler and thinner when compared to other fish eggs. The result was logical, because of reproductive manner of these fishes. Bighead carp spawns in fast waters and ZR protects its eggs from large mechanical pressure, but molly is viviparous and its eggs do not undergo large mechanical pressure.