External fertilization occurs mostly in wet environments and requires both the male and the female to release their gametes into their surroundings (usually water). An advantage of external fertilization is that it results in the production of a large number of offspring. One disadvantage is that environmental hazards such as predators greatly reduce the chance of surviving into adulthood.
The entire process of development of new individuals is called procreation, the act of species reproduction. Consideration as to whether an animal (more specifically a vertebrate) uses internal or external fertilisation is often dependent on the method of birth. Fertilization outside of the animal's body occurs in aquatic animals such as sea urchins, fish, and frogs.
In sea urchins, several billion sperm are released into the water and swim towards eggs
released in the same area, in most of fish and in many amphibians, the eggs and sperms are released in the water around the animals and fertilization takes place there. When the fish are about to reproduce, the males and females swim close together.
Oviparous animals laying eggs with thick calcium shells, such as chickens, or thick leathery shells generally reproduce via internal fertilisation so that the sperm fertilise the egg without having to pass through the thick, protective, tertiary layer of the egg. Ovoviviparous and euviviparous animals also use internal fertilisation. It is important to note that although some organisms reproduce via amplexus, they may still use internal fertilisation, as with some salamanders. Advantages to internal fertilisation include: minimal waste of gametes;
greater chance of individual egg fertilisation, relatively "longer" time period of egg protection, and selective fertilisation; many females have the ability to store sperm for extended periods of time and can fertilize their eggs at their own desire, minimal contact and transmission of bodily fluids; decreasing the risk of disease transmission, and greater genetic variation (especially during broadcast spawning external fertilisation methods).
82 External Fertilization
3.1.1.Aquatic Animals with External fertilization 1 The sea urchin
To illustrate the gametes and fertilization process in animals, we will begin with the sea urchin, an aquatic invertebrate.
Sea urchins are echinoderms that live in marine environments throughout the world. We are using the sea urchin Arbacia punctulata from the Florida coast.
Like most aquatic animals, the sea urchin sheds its gametes into the water and fertilization is external.
Gametes are produced by meiosis as described in Animal Development I. The production of sperm by the male (spermatogenesis) entails the usual meiotic divisions to produce haploid daughter cells. However, these cells are not sperm until they undergo morphological changes. Review the structure of sperm before proceeding. The production of eggs by the female (oogenesis) is also by meiosis, but there are several deviations from the "classic"
meiotic process. A major feature of oogenesis is the arrest of meiosis at the first meiotic prophase during which large quantities of yolk and other substances are stored within the cell cytoplasm. When these large cells divide at telophase I and telophase II, they divide unequally so that all stored materials are retained within one cell. Thus oogenesis of a mother cell produces one large egg and 3 tiny (non-functional) cells called polar bodies.
Also, eggs are surrounded by protective coverings; a vitelline envelope and jelly coat in the case of sea urchins. Review oogenesis and egg structure before proceeding to the fertilization experiment.
We can perform fertilization in the laboratory by inducing fertile sea urchins to release their gametes. Play the following video to see how this is done, then observe the eggs and sperm as they appear using the light microscope.
Sperm are obtained using the same procedure, except that the milky sperm suspension is collected from the urchin's body surface and diluted in a small amount of sea water. The resulting sperm suspension can be viewed in the video below.
Unfertilized sea urchin egg Suspension of sea urchin sperm Note the difference in size of egg vs. sperm (micrographs are at similar magnification)
To view fertilization, a drop of sperm is added to the slide containing eggs, and the eggs are observed under the microscope as shown in the following video. Note that shortly after a sperm has contacted the egg surface, the surrounding vitelline envelope rises creating what is called the "fertilization membrane". Prior to fertilization, the vitelline membrane adheres to the egg surface and is not visible as a separate structure. The fertilization membrane soon hardens to provide a protective covering for the developing embryo. The rising fertilization membrane also pushes other sperm away from the egg and forms a permanent block to polyspermy (i.e. no additional sperm can enter the egg).
You have learned from the topic Animal Development I that the sperm of one species canno usually t fertilize the eggs of a another species. We will now put this to the test. The following video shows a slide containing eggs from two species of sea urchin. The eggs of Lytechinus pictus are large and those of Strongylocentrotus purpuratus are small and brown. Sperm from Lytechinus pictus are added to the mixture at the beginning of the video.
To complete the fertilization process, the nucleus of sperm and egg must fuse. The compacted DNA within the sperm nucleus expands and can be seen within the egg. The fusing nuclei are called pronuclei and the resulting diploid cell is called a zygote. Observe pronuclear fusion in the following video.