External fertilization

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External fertilization is a mode of reproduction in which a male organism's sperm fertilizes a female organism's egg outside of the female's body. [1] It is contrasted with internal fertilization, in which sperm are introduced via insemination and then combine with an egg inside the body of a female organism. [2] External fertilization typically occurs in water or a moist area to facilitate the movement of sperm to the egg. [3] The release of eggs and sperm into the water is known as spawning. [4] In motile species, spawning females often travel to a suitable location to release their eggs.

Contents

However, sessile species are less able to move to spawning locations and must release gametes locally. [4] Among vertebrates, external fertilization is most common in amphibians and fish. [5] Invertebrates utilizing external fertilization are mostly benthic, sessile, or both, including animals such as coral, sea anemones, and tube-dwelling polychaetes. [3] Benthic marine plants also use external fertilization to reproduce. [3] Environmental factors and timing are key challenges to the success of external fertilization. While in the water, the male and female must both release gametes at similar times in order to fertilize the egg. [3] Gametes spawned into the water may also be washed away, eaten, or damaged by external factors.

Sexual selection

Sexual selection may not seem to occur during external fertilization, but there are ways it actually can. The two types of external fertilizers are nest builders and broadcast spawners. For female nest builders, the main choice is the location of where to lay her eggs. A female can choose a nest close to the male she wants to fertilize her eggs, but there is no guarantee that the preferred male will fertilize any of the eggs. Broadcast spawners have a very weak selection, due to the randomness of releasing gametes. [4] To look into the effect of female choice on external fertilization, an in vitro sperm competition experiment was performed. The results concluded that there was a decreased importance of sperm number, but increased the importance of the sperm velocity, thus changing the outcome of sperm competition. The ovarian fluid also increased the paternity for the preferred male because they release fewer and faster sperm. [1] The success of a male fertilizing an egg relies on the ability of a male's sperm to outcompete other sperm that is looking to fertilize the same egg. Sperm chemotaxis is the use of chemical signals to give sperm the ability to navigate an egg and is a huge contributor to reproductive success. [6]

Invertebrates

Benthic sessile animals that make up the majority of invertebrates using external fertilization rely on ambient water motion to bring the sperm and eggs together. Other invertebrates that externally fertilize are organisms like the sea urchin, are confined to shallow burrows on exposed shores. Turbulent flows in the surf zone also create a transport of gametes. [3] Hydrodynamic conditions and gamete properties control the efficiency of fertilization because they influence the rate at which the water mixes. [7] The only dilemma with turbulence is the possibility of dilution of sperm and egg because of over mixing. Rapid mixing can cause a lower probability to fertilize. [3] Sessile adult staged animals commonly produce gametes at the same times, also known as a synchronized release of gametes, for external fertilization in the water column. This is helpful because of the lack of mobility these organisms share. [8] They also can rely on turbulent mixing and sperm mobility to enhance the chances of fertilization. [3]

The presence of food, resources, favorable environmental conditions, and the lack of predators are thought of when thinking of survival of the next generation. [9] When the female is producing eggs, they perform predator satiation, which is when multiple females will release gametes in a mass reproductive event. [10] The Great Barrier Reef is known for having a “mass spawn.” This occurs the week after the full moon in October. [11] This mass reproductive event is given by reef forming corals, which perform a synchronized release of gametes in one evening from dusk to midnight. Up to 130 species release gametes during this time. [12] In some cases, fertilization can take place on a spawning animal's surface and when the animals are in the turbulent wake. [7] Although fertilization is usually thought of as a short-term process, there is the possibility of gametes being retained on the surface of an animal for an extended period of time. [13] In order to release an egg or sperm over time, clumps are formed that float in the water column. [14] This allows for a variation in locations and time differences of fertilization taking place by the same invertebrate. [7]

Vertebrates

Amphibians

The earliest amphibians were all internal fertilizers. It was not until 300 million years ago that the Anura (early internal fertilizer) and Caudata (early external fertilizer) orders had begun. Most anurans now externally fertilize. [15] Anurans are the amphibians lacking a tail such as frogs and toads. [16] Anurans are commonly used as a model organism for amphibians, because of the large, easy to manipulate eggs, fast developmental rate, high fecundity rate, no parental involvement, and external fertilization. Males will congregate near a lake or pond and establish calling stations. Females approach the area and listen to all of the different male calls, and then continue to move towards the mate she chooses. This is the anuran's sexual selection. It has been concluded that females prefer a male with a more attractive call, which is also the larger male. [17] Copulation occurs when a male anuran hops onto the back of a female. They then move to a spot near water to simultaneously release their sperm and eggs. Other males in the area can also release sperm onto the eggs to also attempt to fertilize the eggs. If the female does not want to reproduce with the male that jumps onto her back, she will wait until the male leaves or move to a new location. [18] Sperm released into the water must be in close proximity, get to the egg first, and then enter the gel layer of the egg to have the best chance of fertilizing. When the anurans are not close to eggs, they sometimes release their sperm into oocyte containing foam nests, or terrestrial breeders go right to the gel coat of the oocyte to release their sperm. [15] Over the course of a breeding season, males can copulate numerous times by releasing sperm anywhere he finds unfertilized eggs or encounters a female who is/wants to spawn. Females, however, can only release eggs once per breeding season. [18] Releasing sperm directly into the water increases sperm competition through agonistic behavior and spawning in groups. This has been tested, with good evidence associated with a larger sperm number and testes size. [15] Smaller testes size and a slower sperm velocity were seen with the anurans that released the sperm into the foam nests instead of into open water. [19] To further increase sperm competition, there is a larger thickness to an anuran oocyte gel, specifically the green tree frog. Anuran sperm also have high longevity and osmotic tolerance compared to fresh water fish. [15]

The Caudata order contains all of the salamanders and newts, amphibians that have tails. [15] Within this, the only subgroups that externally fertilize are Cryptobranchidae (giant salamanders) Sirenidae, and Hynobiidae. [20] The females release egg sacs onto stones or branches and the male later hovers over the eggs to release the sperm to them. Males are seen to be very protective over the eggs and may continue to hover over the eggs after sperm release to decrease sperm competition. In some cases, males may even latch onto the females while they lay their eggs to ensure that they fertilize them first. Other times there may be numerous males surrounding a single sac of eggs, creating scramble competition. [20] Cryptobranchid sperm is seen to have higher longevity. This is about 600 times greater than in freshwater fish, but not even close to as high as anurans. [15]

Fish

Salmon, cod, trout, and char are all examples of the fish that externally fertilize. The females release roe (an egg mass) and the males release milt (seminal fluid containing sperm) into the water, where they diffuse together and fertilize. [21] On top of the sperm locating the oocyte and penetrating the gel layer, it must also infiltrate the mycropyle. [15] If there is turbulent water or even in open, calm water, the closer fish that releases sperm has the higher chance of fertilizing the eggs. If sperm is released too early, it can become too dilute or die before it ever reaches the eggs. If sperm is released too late, there is a higher chance that a different fish's sperm has already reached the eggs. Also, the faster and more numerous the sperm, the better. [21] There are instances where males will create habitats in an attempt to monopolize females and increase their chance of fertilizing eggs. [15]

Fishes can be iteroparous, and spawn more than once, but there are some who only spawn once before death, known as semelparous. Within iteroparous fish, they usually give no parental care with external fertilization. [22] The sperm present in male fish are immotile while in testes and in seminal fluid, and the fertilization environment determines when the sperm become motile. In salmon, a decrease of potassium in fresh water will initiate the motility of the sperm. A decrease in osmolality after spawning in fresh water makes a cyprinid fish's sperm motile. [23]

See also

Related Research Articles

is the process of male gamete formation in animals This process also involves

<span class="mw-page-title-main">Gametophyte</span> Haploid stage in the life cycle of plants and algae

A gametophyte is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte. The sporophyte can produce haploid spores by meiosis that on germination produce a new generation of gametophytes.

<span class="mw-page-title-main">Sex</span> Trait that determines an organisms sexually reproductive function

Sex is the trait that determines whether a sexually reproducing organism produces male or female gametes. A male organism produces small mobile gametes, while a female organism produces larger, non-mobile gametes. An organism that produces both types of gamete is called a hermaphrodite. During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent.

<span class="mw-page-title-main">Fertilisation</span> Union of gametes of opposite sexes during the process of sexual reproduction to form a zygote

Fertilisation or fertilization, also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. While processes such as insemination or pollination, which happen before the fusion of gametes, are also sometimes informally referred to as fertilisation, these are technically separate processes. The cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms, the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation.

<span class="mw-page-title-main">Egg cell</span> Female reproductive cell in most anisogamous organisms

The egg cell, or ovum, is the female reproductive cell, or gamete, in most anisogamous organisms. The term is used when the female gamete is not capable of movement (non-motile). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified as oogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere. When fertilized, the oosphere becomes the oospore.

<span class="mw-page-title-main">Amplexus</span> Type of mating behavior exhibited by some externally fertilizing species

Amplexus is a type of mating behavior exhibited by some externally fertilizing species in which a male grasps a female with his front legs as part of the mating process, and at the same time or with some time delay, he fertilizes the eggs, as they are released from the female's body. In amphibians, females may be grasped by the head, waist, or armpits, and the type of amplexus is characteristic of some taxonomic groups.

<span class="mw-page-title-main">Gobiidae</span> Family of fishes

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<span class="mw-page-title-main">Egg</span> Organic vessel in which an embryo first begins to develop

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<span class="mw-page-title-main">Anisogamy</span> Sexual reproduction involving a large, female gamete and a small, male gamete

Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a sperm cell, whereas the larger gamete is female, typically an egg cell. Anisogamy is predominant among multicellular organisms. In both plants and animals gamete size difference is the fundamental difference between females and males.

Reproductive biology includes both sexual and asexual reproduction.

<span class="mw-page-title-main">Internal fertilization</span> Union of an egg and sperm to form a zygote within the female body

Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For internal fertilization to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract.

<span class="mw-page-title-main">Fish reproduction</span> Reproductive physiology of fishes

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<span class="mw-page-title-main">Human fertilization</span> Union of a human egg and sperm

Human fertilization is the union of an egg and sperm, occurring primarily in the ampulla of the fallopian tube. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scientists discovered the dynamics of human fertilization in the 19th century.

In biology, polyspermy describes the fertilization of an egg by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome—one from the egg and one each from multiple sperm. Usually, the result is an unviable zygote. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. Such a situation is often deleterious to the female: in other words, the male–male competition among sperm spills over to create sexual conflict.

<span class="mw-page-title-main">Hermaphrodite</span> Sexually reproducing organism that produces both male and female gametes

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References

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