Fertilization

External and Internal Fertilization

External and internal fertilization are forms of reproduction that vary in method and embryo development.

Learning Objectives

Compare and contrast external and internal methods of fertilization

Key Takeaways

Key Points

  • External fertilization is characterized by the release of both sperm and eggs into an external environment; sperm will fertilize the egg outside of the organism, as seen in spawning.
  • Internal fertilization is characterized by sperm fertilizing the egg within the female; the three methods include: oviparity (egg laid outside female body), ovoviparity (egg held within female), and viviparity (development within female followed by live birth).
  • Internal fertilization protects the fertilized egg or embryo from predation and harsh environments, which results in higher survival rates than can occur with external fertilization.
  • Ovoviparity is characterized by an organism retaining a fertilized egg inside the body where development occurs and nourishment is received from the yolk.
  • Viparity is characterized by an organism which has its young develop within the female and nourishment is received directly from the mother via a placenta.

Key Terms

  • oviparous: egg-laying; depositing eggs that develop and hatch outside the body as a reproductive strategy
  • viviparous: being born alive, as are most mammals, some reptiles, and a few fish (as opposed to being laid as an egg)
  • ovoviparity: eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk

External and Internal Fertilization

External Fertilization

External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization can then take place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The release of the reproductive material may be triggered by water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a small area, enhancing the possibility of fertilization.

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Anemone fish protecting its spawn: An image of an Anemone fish protecting its spawn. The anemone fish utilizes a form of external fertilization.

External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment. For sessile aquatic organisms such as sponges, broadcast spawning is the only mechanism for fertilization and colonization of new environments. The presence of the fertilized eggs and developing young in the water provides opportunities for predation, resulting in a loss of offspring. Therefore, millions of eggs must be produced by individuals. The offspring produced through this method must mature rapidly. The survival rate of eggs produced through broadcast spawning is low.

Internal Fertilization

Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization: oviparity, ovoviparity, and viviparity.

In oviparity, fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. These animals are classified as oviparous.

In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk; the young are fully developed when they are hatched. This occurs in some bony fish (such as the guppy, Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake, Thamnophis sirtalis), some vipers, and some invertebrate animals (such as the Madagascar hissing cockroach, Gromphadorhina portentosa).

In viviparity, the young develop within the female, receiving nourishment from the mother’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles, making these animals viviparous.

Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization also enhances the fertilization of eggs by a specific male. Even though fewer offspring are produced through this method, their survival rate is higher than that for external fertilization.

The Evolution of Reproduction

Sexually-reproducing organisms have evolved specialized gonads, along with a variety of ways to transfer sperm during reproduction.

Learning Objectives

Differentiate among the types of reproductive systems that have evolved

Key Takeaways

Key Points

  • Annelids undergo sexual reproduction by producing sperm or eggs within the coelom and storing them within the cavity until they are ready to be released through an excretory opening.
  • Insects have developed complete reproductive systems for the separate sexes and will often have a specialized sac for sperm called the spermatheca.
  • Non-mammals will utilize a common body opening called the cloaca to transfer sperm between animals.
  • The means by which sperm is transferred varies and can include releasing sperm into the environment as well as direct delivery to the vagina.

Key Terms

  • coelom: a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
  • gonad: a sex organ that produces gametes; specifically, a testicle or ovary
  • cloaca: the common duct in fish, reptiles, birds, and some primitive mammals that serves as the anus as well as the genital opening
  • spermatheca: a small sac within the reproductive tract of some female invertebrates, such as insects, which stores sperm until it is used to fertilize the ova

The Evolution of Reproduction

Several competing scientific hypotheses have been proposed to explain the evolution of sexual reproduction. All sexually-reproducing eukaryotic organisms derive from a common ancestor that was a single-celled eukaryotic species. Many protists reproduce sexually, as do multicellular plants, animals, and fungi. However, there are a few species which have secondarily lost this feature. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction.

Once multicellular organisms evolved and developed specialized cells, some also developed tissues and organs with specialized functions. The evolution of reproductive organs arrived with the development of gonads that produced sperm and eggs. These cells develop through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell by half, while increasing the number of cells through cell division. The development of specialized gonads to produce sperm and egg was a major step in the evolutionary process.

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Meiosis: The development of sperm and eggs occurs via the process of meiosis which reduces the number of chromosomes in each cell by half.

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Sperm fertilizing an egg: The evolution of sex-specific gonads led to the development of sperm (male) and eggs (female). When fertilized, if the conditions are favorable, the egg may develop into a new organism.

An early development in reproduction occurred in the Annelids. These organisms produce sperm and eggs from undifferentiated cells in their coelom, storing them in that cavity. When the coelom becomes filled, the cells are released through an excretory opening or by the body splitting open. Further evolution of reproductive systems resulted in the development of reproductive systems that are sex specific. In these more advanced systems, sperm is made in the testes and then travels through coiled tubes to the epididymis for storage. Additionally, in these more advanced systems, eggs are matured in the ovary; when released, they travel to the uterine tubes for fertilization. These types of reproductive systems developed in insects (compared to annelids which have a coelom for storage). Specifically, in the insect reproductive system, a specialized sac developed, called a spermatheca, which is used to store sperm for later use, sometimes up to a year. This was a key development since fertilization in insects can be timed with environmental or food conditions that are optimal for offspring survival.

Vertebrates have similar structures (i.e., gonads that specialize in sex cell production) with a few differences in their reproductive systems. Non-mammals, such as birds and reptiles, have a common body opening, called a cloaca, for the digestive, excretory, and reproductive systems. Coupling between birds usually involves positioning the cloaca openings opposite each other for transfer of sperm. In mammals, there are separate openings for the systems in the female and a uterus for support of developing offspring. Depending on the type of species, there are differences in the uterus. In species that produce large numbers of offspring, the uterus has two chambers. In other species that produce one offspring, such as in primates, there is a single uterus.

Another development in the evolution of reproduction is the means by which sperm is transferred. During reproduction, sperm transfer from the male to the female ranges from releasing the sperm into the watery environment for external fertilization, to the joining of cloaca in birds, to the development of a penis for direct delivery into the female’s vagina in mammals. All of these methods of sperm transfer represent the varying ways reproduction has evolved and become specialized to specific organisms.