OVIPAROUS: CHARACTERISTICS, REPRODUCTION, EXAMPLES, EMBRYONIC DEVELOPMENT - BIOLOGY - 2024

Oviparous are those animals that are born from eggs, that is, those whose embryonic development occurs in structures external to the parents and that may or may not be cared for by them during the first stages of the development process.

The term literally means "egg" and "birth", and is used to denote those animals whose sexual reproduction gives rise to an egg that is covered by a kind of protective "shell" that is usually formed after the fertilization of the egg cell.

Example of an oviparous animal, a bird (Source: Bill Byrne / FWS via Wikimedia Commons)

Oviparous animals differ from viviparous animals, for example, in that the latter develop within or in some part of the mother's body and usually depend on her for food and growth during the early stages of development.

Oviparity is recognized as the “ancestral condition” in many animal lineages and some authors consider that, although it may not be as efficient a reproductive mechanism as that of viviparous ones, it allows the animals that present it to produce much more offspring in shorter periods of time.

General characteristics

Oviparous animals reproduce sexually and once the ovum is fertilized, the developing embryo is protected within the oocell thanks to the formation of a resistant outer shell or membrane.

Oviparous animals can be terrestrial or aquatic, and their oviposition patterns vary considerably.

Some terrestrial species take care of their eggs until they hatch and even help them to come out of the shell, while others bury them and abandon them, so the young are independent from the moment of their birth.

Depending on the species and its reproductive strategy, as well as these oviposition patterns, oviparous animals can lay one or multiple eggs, which is directly related to the survival rates of the offspring.

Usually, these eggs contain enough space and nutritious reserve substances for the development of the embryos; which ensures that the young will be able to develop all the organs and most of the body systems necessary to survive before hatching.

Eggs represent a "controlled" environment that isolates the embryo to some degree from the environment that surrounds it, making it capable of withstanding some of the environmental shocks it may be subjected to after oviposition.

Reproduction

When oviparous animals reproduce sexually, the process of fertilization (fusion of gametes) can be internal or external.

Internal fertilization implies that, in one of the parents, usually the female (which contains the egg cells) receives the gametes from the other, which implies physical contact between both cells and the fusion of their nuclei inside the reproductive system of the female.

The zygote thus produced is protected inside the egg, a structure that is formed by the cells of the mother that surround the ovum and that may or may not mineralize and harden.

The composition of the "shells" of animal eggs varies greatly depending on the species. Thus, some eggs have more or less flexible layers or membranes, composed of fibrous proteins, and others are covered by membranes on which resistant materials such as calcium carbonate, for example, are deposited.

External fertilization, on the other hand, occurs when both parents release their sex cells into the environment around them and these cells randomly fuse outside the body of reproducing animals.

Despite the above, Lodé, in 2012, proposed that oviparity is characteristic only of animal species in which fertilization is internal and the embryos are arranged in the genital tract of females.

This author also establishes that oviparous animals are characterized by a lecithotrophic reproduction, that is, a reproduction where the embryos feed on an abundant yolk (nutritive cytosol of the ovum) contained inside the egg.

It is important to note that many species of oviparous animals have a "cloacal" reproduction, that is, fertilization occurs after the animals "join" their cloaca and the male deposits the semen with the sperm in the female.

Embryonic development

The embryonic development of all animal species (oviparous or not) begins with the formation of a zygote, which is the cell resulting from the fusion of gametes (ovum and sperm) from animals of the opposite sex that mate or reproduce sexually.

- The ovum, the oocell or the egg

The female gametes, that is, the ovules or oocells, vary considerably in size. However, they are usually large cells that accumulate a substance known as vitellogenin, which is transformed into the "yolk" or yolk of the egg and serves as a nutrient storage substance to support the embryo that is formed inside.

Depending on the amount of vitellogenin that accumulates, eggs can be classified as microlecytes, mesolecytes or macrolecytes, this if they have too little, a moderate amount or too much reserve substance, respectively.

In addition, eggs can also be classified according to the way in which the reserve material is distributed, so there are isolecyte eggs (with the yolk equally distributed) or telolecyte eggs (with the yolk concentrated in a single place in the egg).

Each egg cell is surrounded by three membranes or "shells." The first separates the plasma membrane of the egg from the other cells of the ovary where it occurs and is often known as the yolk membrane.

The second layer or envelope is composed of the cells of the ovary that surround the egg and contribute in the transport or transfer of nutrients to it, meanwhile the third layer is formed in the oviducts and is one that in many species is a hard and resistant.

In many oviparous animals, this layer is formed after fertilization and helps protect the zygote during development, since fibrous proteins and other resistant or leathery substances are usually deposited in it.

Divisions or cleavage of the zygote

The zygote undergoes multiple mitotic cell divisions during the early stages of development, divisions that give rise to structures known as morula, blastula and gastrula, in which the definition of the embryo itself and the tissues that surround and nourish it begins (extraembryonic tissues).

As the process continues, the embryo that came from the zygote undergoes a process of organogenesis (formation of organs) from germ layers that have been previously defined through successive cell divisions and the establishment of specific “functions”.

The germ layers are known as ectoderm, mesoderm and endoderm, which normally form the epidermis and organs in contact with the environment, part of the digestive tract and lungs, and the musculature, skeleton, gonads and the excretory system., respectively.

Post embryonic development

The embryonic development of oviparous animals occurs inside the eggs, outside the body of the females.

In birds, for example, the temperature is carefully controlled by the females or males that "hatch" or "nest" on their eggs, while poikilothermic animals such as reptiles depend on environmental conditions for the maintenance of their eggs..

Once the embryos have consumed all the reserve substances of the yolk, they hatch and leave the egg.

Depending on the amount of nutritional reserves that the egg has, development can be direct or indirect.

In other words, animals such as birds and reptiles hatch from their eggs only to grow and mature reproductively, as their eggs contain enough food; meanwhile other oviparous with micro or mesolecyte eggs hatch as larvae and must undergo different metamorphosis processes (indirect development) until they acquire the adult form.

Examples

There are numerous examples of oviparous animals in nature, beyond birds, which are one of the first groups of animals that can be brought to mind when thinking of animals that hatch from eggs.

Thus, in the natural world, in addition to birds, insects, reptiles, fish, mammals and amphibians are obtained whose origin begins with an egg-like structure.

- Oviparous mammals

Although it is not very common among this group of animals, monotremes ("primitive" mammals) such as the platypus are the classic example of oviparous mammals, since they are the only ones within this group that share oviposition characteristics with the group of the reptiles.

This animal, with a truly unique appearance, is a semi-aquatic mammal endemic to the Australian continent, of which there are around 6 species. It has only one reproductive period per year during which it lays two to 3 eggs that are fertilized in the oviduct, where the leathery shell is formed.

Ornithorhynchus anatinus (Source: Dr. Philip Bethge via Wikimedia Commons)

Unlike other mammals, platypuses have a cloaca, that is, feces, urine and eggs are expelled through the same hole, as is the case with birds and reptiles.

The eggs that these animals oviposit are quite large and oviposition usually occurs in nests that are excavated by the same animal. Since they are mammals, after the hatching of the eggs, the young are fed with the milk that is produced by the mother.

- Insects

Although there are many viviparous and ovoviviparous arthropods, there are some oviparous species where the females lay eggs that develop outside the body. These animals are generally internally fertilized and may incubate their eggs or have some form of parental care during early development.

Bees and their eggs (Source: Image by Christa Mahler on pixabay.com)

Dragonflies, beetles, grasshoppers, bees, and butterflies are good examples of oviparous insects. However, since their development is indirect, the hatching of the eggs gives rise to larvae, which are worm-like structures that have to undergo successive metamorphic changes to reach adulthood.

- Fish

Fish have great diversity in terms of their sexual reproduction, but practically all species are oviparous. In these, the developing embryos grow at the expense of the nutritional content found inside or the "yolk" of the egg, although the nutritional content of the eggs varies with the species.

However, there is a great difference with other animal groups: the fertilization of the ovules by the sperm is often external, that is, it occurs outside the parents (as well as the development of the eggs).

Photograph of a salmonid during oviposition (Source: Image by ArtTower on pixabay.com)

Simply put, females and males release their gametes into large aquatic spaces. The females release the eggs that are fertilized by the sperm produced by the males and, after fertilization, the eggs usually swell with water and harden.

The fish spawn in fairly defined conditions, as females and males make sure that the temperature is adequate, since otherwise the survival of the young would be considerably low.

The characteristics of the eggs also depend on the species considered, with small, translucent and floating eggs, large, non-floating and adhesive eggs or non-floating eggs, for example.

- Amphibians

Most amphibians are oviparous and, as in many fish, their fertilization is external and their development indirect, as they hatch from the eggs as larvae. The eggs are deposited in bodies of water, where the larvae (tadpoles) can develop as they have tails and gills to breathe.

A frog and its eggs in the background (Source: Image by NiklasPntk on pixabay.com)

The tadpoles of frogs and toads, to name a few representative amphibians, eventually lose their tails and acquire their locomotive limbs.

- Birds

Absolutely all birds are oviparous. A good example of this group are chickens, animals domesticated thousands of years ago that, like other birds, nest and provide parental care to their chicks before and after hatching from the eggs.

The birds are all oviparous (Source: Fischchen via Wikimedia Commons)

Many species of birds make sure to have their young in safe places and when they are able to provide the conditions and resources necessary for the survival of their offspring. Some species exhibit complex courtship, territorial defense, and nesting behaviors during the reproductive season.

- Reptiles

Reptiles are an extremely diverse group of animals. The vast majority of these are oviparous; All turtles, for example, hatch from a few to hundreds of eggs that are buried under the ground by the mothers, but these eggs are not cared for by the mothers once they are laid.

Young crocodile after hatching from the egg (Source: Image by skeeze on pixabay.com)

Lizards and lizards are also generally oviparous, although there are ovoviviparous and viviparous. Oviparous are snakes, although there are some cases of snakes that "give birth" to live juveniles, instead of laying eggs.

Crocodiles and alligators are oviparous, but they differ from turtles, for example, in that they jealously take care of their eggs and the young that hatch from them, which is why it is said that they have a "nesting" behavior and a certain "care parental ”.

References

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