Reproductive Systems Gametogenesis and Early Development
In most marine vertebrates (fish), physical copulation does not occur; instead, eggs and sperm are simply released into the water, where they can meet and fertilization may occur. Humans have various blocks to polyspermy, and fishes have similar mechanisms to prevent fertilization of an egg by more than one sperm. In addition, their method of fertilization probably requires:
|A||The release of far greater amounts of eggs by females.|
|B||Mechanisms to prevent the fertilization of more than one egg by a single sperm.|
|C||Mechanisms to avoid potential cross-species fertilization, which would be couterproductive since members of different species are genetically incomplete.|
|D||Both A and C are correct.|
Since the eggs and sperm released into the water by fish must find each other virtually “by chance”, eggs (and sperm) must be released in very large quantities to ensure that some fertilization occurs (choice A is correct). In addition, since so many species of fish release their eggs and sperm in this fashion (not to mention hundreds of marine invertebrates), species-specific identification methods must exist that allow only sperm and eggs from the same species to attempt to join. While it is theoretically possible for the sperm of one species to fertilize the egg of another, this would be counterproductive so mechanisms have evolved to prevent it (Choice C is correct). Choice B is incorrect as it is impossible for a sperm to fertilize more than one egg in any environment.