{{Short description|Form of reproduction where fertilization occurs outside of the female's body}} '''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.<ref name="Alonzo et al 2016">{{cite journal |doi=10.1038/ncomms12452 |pmid=27529581 |pmc=4990696 |title=Ovarian fluid allows directional cryptic female choice despite external fertilization |journal=Nature Communications |volume=7 |article-number=12452 |year=2016 |last1=Alonzo |first1=Suzanne H. |last2=Stiver |first2=Kelly A. |last3=Marsh-Rollo |first3=Susan E. |bibcode=2016NatCo...712452A }}</ref> 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.<ref name=pmid26642825>{{cite journal |doi=10.1016/j.ympev.2015.11.011 |pmid=26642825 |title=Molecular phylogeny and evolution of internal fertilization in South American seasonal cynopoeciline killifishes |journal=Molecular Phylogenetics and Evolution |volume=95 |pages=94–9 |year=2016 |last1=Costa |first1=Wilson J.E.M. |last2=Amorim |first2=Pedro F. |last3=Mattos |first3=José Leonardo O. |bibcode=2016MolPE..95...94C }}</ref>
In animals, external fertilization typically occurs in water or a moist area to facilitate the movement of sperm to the egg.<ref name="Denny & Shibata 1989">{{cite journal |doi=10.1086/285018 |jstor=2462013 |title=Consequences of Surf-Zone Turbulence for Settlement and External Fertilization |journal=The American Naturalist |volume=134 |issue=6 |pages=859–89 |year=1989 |last1=Denny |first1=Mark W. |last2=Shibata |first2=Mark F. |bibcode=1989ANat..134..859D |s2cid=84201209 }}</ref> The release of eggs and sperm into the water is known as spawning.<ref name=pmid22408733>{{cite journal |doi=10.1002/ece3.77 |pmid=22408733 |pmc=3297185 |title=Northern refugia and recent expansion in the North Sea: The case of the wrasse ''Symphodus melops'' (Linnaeus, 1758) |journal=Ecology and Evolution |volume=2 |issue=1 |pages=153–64 |year=2012 |last1=Robalo |first1=Joana I. |last2=Castilho |first2=Rita |last3=Francisco |first3=Sara M. |last4=Almada |first4=Frederico |last5=Knutsen |first5=Halvor |last6=Jorde |first6=Per E. |last7=Pereira |first7=Ana M. |last8=Almada |first8=Vitor C. |bibcode=2012EcoEv...2..153R }}</ref> In motile species, spawning females often travel to a suitable location to release their eggs. However, sessile species are less able to move to spawning locations and must release gametes locally.<ref name=pmid22408733/> Among vertebrates, external fertilization is most common in amphibians and fish.<ref name="Kondo & Kashiwagi 2004">{{cite journal |doi=10.1670/160-02A |jstor=1565777 |title=Experimentally Induced Autotetraploidy and Allotetraploidy in Two Japanese Pond Frogs |journal=Journal of Herpetology |volume=38 |issue=3 |pages=381–92 |year=2004 |last1=Kondo |first1=Yasuyuki |last2=Kashiwagi |first2=Akihiko |s2cid=86149061 }}</ref> Invertebrates utilizing external fertilization are mostly benthic, sessile, or both, including animals such as coral, sea anemones, and tube-dwelling polychaetes.<ref name="Denny & Shibata 1989"/> Benthic marine plants also reproduce through external fertilization.<ref name="Denny & Shibata 1989"/> 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.<ref name="Denny & Shibata 1989"/> 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.<ref name=pmid22408733/> 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.<ref name="Alonzo et al 2016"/> 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.<ref name=pmid26994183>{{cite journal |doi=10.1242/jeb.134924 |pmid=26994183 |title=Sperm chemotaxis promotes individual fertilization success in sea urchins |journal=The Journal of Experimental Biology |volume=219 |issue=10 |pages=1458–66 |year=2016 |last1=Hussain |first1=Yasmeen H. |last2=Guasto |first2=Jeffrey S. |last3=Zimmer |first3=Richard K. |last4=Stocker |first4=Roman |last5=Riffell |first5=Jeffrey A. |doi-broken-date=15 October 2025 |doi-access=free }}</ref>
==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.<ref name="Denny & Shibata 1989"/> Hydrodynamic conditions and gamete properties control the efficiency of fertilization because they influence the rate at which the water mixes.<ref name="Thomas et al 2013">{{cite journal |doi=10.3354/meps10601 |title=Fertilization in a sea urchin is not only a water column process: Effects of water flow on fertilization near a spawing female |journal=Marine Ecology Progress Series |volume=494 |pages=231–40 |year=2013 |last1=Thomas |first1=FIM |last2=Kregting |first2=LT |last3=Badgley |first3=BD |last4=Donahue |first4=MJ |last5=Yund |first5=PO |bibcode=2013MEPS..494..231T |doi-access=free }}</ref> 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.<ref name="Denny & Shibata 1989"/> 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.<ref name="Mercier & Hamel 2010">{{cite journal |doi=10.1007/s00265-010-0987-z |jstor=40962419 |title=Synchronized breeding events in sympatric marine invertebrates: Role of behavior and fine temporal windows in maintaining reproductive isolation |journal=Behavioral Ecology and Sociobiology |volume=64 |issue=11 |pages=1749–65 |year=2010 |last1=Mercier |first1=Annie |last2=Hamel |first2=Jean-François |bibcode=2010BEcoS..64.1749M |s2cid=11539543 }}</ref> They also can rely on turbulent mixing and sperm mobility to enhance the chances of fertilization.<ref name="Denny & Shibata 1989"/>
The presence of food, resources, favorable environmental conditions, and the lack of predators are thought of when thinking of survival of the next generation.<ref name=pmid20819806>{{cite journal |doi=10.1098/rstb.2010.0145 |pmid=20819806 |pmc=2981948 |title=Toward a synthetic understanding of the role of phenology in ecology and evolution |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=365 |issue=1555 |pages=3101–12 |year=2010 |last1=Forrest |first1=J. |last2=Miller-Rushing |first2=A. J. }}</ref> When the female is producing eggs, they perform predator satiation, which is when multiple females will release gametes in a mass reproductive event.<ref name="Kelly & Sork 2002">{{cite journal |doi=10.1146/annurev.ecolsys.33.020602.095433 |title=Mast Seeding in Perennial Plants: Why, How, Where? |journal=Annual Review of Ecology and Systematics |volume=33 |pages=427–47 |year=2002 |last1=Kelly |first1=Dave |last2=Sork |first2=Victoria L.|issue=1 |bibcode=2002AnRES..33..427K |author-link2=Victoria Sork }}</ref> The Great Barrier Reef is known for having a "mass spawn." This occurs the week after the full moon in October.<ref name="Babcock et al 1985">{{cite conference |last1=Willis |first1=B.L. |last2=Babcock |first2=R.C. |last3=Harrison |first3=P.L. |last4=Oliver |last5=J.K |last6=Wallace |first6=C.C. |year=1985 |chapter=Patterns in the mass spawning of corals on the Great Barrier Reef from 1981 to 1984 |chapter-url=http://researchonline.jcu.edu.au/24208/ |title=Proceedings of the Fifth International Coral Reef Congress |pages=343–8 |conference=Fifth International Coral Reef Congress, 27 May – 1 June 1985 |location=Tahiti, French Polynesia }}</ref> 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.<ref name=pmid17742935>{{cite journal |doi=10.1126/science.223.4641.1186 |pmid=17742935 |title=Mass Spawning in Tropical Reef Corals |journal=Science |volume=223 |issue=4641 |pages=1186–9 |year=1984 |last1=Harrison |first1=P. L. |last2=Babcock |first2=R. C. |last3=Bull |first3=G. D. |last4=Oliver |first4=J. K. |last5=Wallace |first5=C. C. |last6=Willis |first6=B. L. |bibcode=1984Sci...223.1186H |s2cid=31244527 }}</ref> In some cases, fertilization can take place on a spawning animal's surface and when the animals are in the turbulent wake.<ref name="Thomas et al 2013"/> 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.<ref name="Marshall 2002">{{cite journal |doi=10.3354/meps236113 |title=In situ measures of spawning synchrony and fertilization success in an intertidal, free-spawning invertebrate |journal=Marine Ecology Progress Series |volume=236 |pages=113–9 |year=2002 |last1=Marshall |first1=DJ |bibcode=2002MEPS..236..113M |doi-access=free }}</ref> In order to release an egg or sperm over time, clumps are formed that float in the water column.<ref name=pmid9317771>{{cite journal |pmid=9317771 |url=http://jeb.biologists.org/cgi/pmidlookup?view=long&pmid=9317771 |year=1994 |last1=Thomas |first1=F |title=Physical Properties of Gametes in Three Sea Urchin Species |journal=The Journal of Experimental Biology |volume=194 |issue=1 |pages=263–84 |doi = 10.1242/jeb.194.1.263| bibcode=1994JExpB.194..263T |doi-access=free }}</ref> This allows for a variation in locations and time differences of fertilization taking place by the same invertebrate.<ref name="Thomas et al 2013"/>
==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.<ref name=pmid25442393>{{cite journal |doi=10.1016/j.theriogenology.2014.09.018 |pmid=25442393 |title=Sperm motility of externally fertilizing fish and amphibians |journal=Theriogenology |volume=83 |issue=1 |pages=1–13 |year=2015 |last1=Browne |first1=R.K. |last2=Kaurova |first2=S.A. |last3=Uteshev |first3=V.K. |last4=Shishova |first4=N.V. |last5=McGinnity |first5=D. |last6=Figiel |first6=C.R. |last7=Mansour |first7=N. |last8=Agnew |first8=D. |last9=Wu |first9=M. |last10=Gakhova |first10=E.N. |last11=Dzyuba |first11=B. |last12=Cosson |first12=J. }}</ref> Anurans are the amphibians lacking a tail such as frogs and toads.<ref>{{cite book |last1=Arak |first1=Anthony |chapter=Male–male competition and mate choice in anuran amphibians |chapter-url={{Google books|HY-onXFuTcoC|page=181|plainurl=yes}} |pages=181–210 |editor1-first=Patrick |editor1-last=Bateson |year=1983 |title=Mate Choice |publisher=Cambridge University Press |isbn=978-0-521-27207-0 }}</ref> 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.<ref name=pmid20053661>{{cite journal |doi=10.1098/rsbl.2009.0938 |pmid=20053661 |pmc=2880058 |jstor=2407594 |title=Turgid female toads give males the slip: A new mechanism of female mate choice in the Anura |journal=Biology Letters |volume=6 |issue=3 |pages=322–4 |year=2010 |last1=Bruning |first1=B. |last2=Phillips |first2=B. L. |last3=Shine |first3=R. }}</ref> 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.<ref name="Zhao et al 2016">{{cite journal |doi=10.1016/j.anbehav.2015.10.007 |title=Male pursuit of higher reproductive success drives female polyandry in the Omei treefrog |journal=Animal Behaviour |volume=111 |pages=101–10 |year=2016 |last1=Zhao |first1=Mian |last2=Li |first2=Chenliang |last3=Zhang |first3=Wei |last4=Wang |first4=Hui |last5=Luo |first5=Zhenhua |last6=Gu |first6=Qi |last7=Gu |first7=Zhirong |last8=Liao |first8=Chunlin |last9=Wu |first9=Hua |s2cid=53189745 }}</ref> 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.<ref name=pmid25442393/> Over the course of a breeding season, a male can copulate numerous times by releasing sperm anywhere he finds unfertilized eggs or encounters a female who wants to spawn. Females, however, can only release eggs once per breeding season.<ref name="Zhao et al 2016"/> 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.<ref name=pmid25442393/> 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.<ref name="Byrne et al 2002">{{cite journal |doi=10.1046/j.1420-9101.2002.00409.x |title=Sperm competition selects for increased testes mass in Australian frogs |journal=Journal of Evolutionary Biology |volume=15 |issue=3 |pages=347–55 |year=2002 |last1=Byrne |first1=P. G. |last2=Roberts |first2=J. D. |last3=Simmons |first3=L. W. |doi-access=free }}</ref> 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.<ref name=pmid25442393/> The Caudata order contains all of the salamanders and newts, amphibians that have tails.<ref name=pmid25442393/> Within this, the only subgroups that externally fertilize are Cryptobranchidae (giant salamanders), Sirenidae, and Hynobiidae.<ref name="Houck & Arnold 2003">{{cite book |first1=Lynne D. |last1=Houck |first2=Stevan J. |last2=Arnold |year=2003 |chapter=Courtship and Mating Behavior |chapter-url=http://eherp.com/pdf/88410.pdf |pages=383–424 |editor1-first=David M. |editor1-last=Sever |title=Reproductive Biology and Phylogeny of Urodela |publisher=Taylor & Francis |isbn=978-1-57808-285-8 }}</ref> 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.<ref name="Houck & Arnold 2003"/> 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.<ref name=pmid25442393/>
===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.<ref name=pmid17040384>{{cite journal |doi=10.1111/j.1420-9101.2006.01165.x |pmid=17040384 |title=Sperm competition in a fish with external fertilization: The contribution of sperm number, speed and length |journal=Journal of Evolutionary Biology |volume=19 |issue=6 |pages=1873–81 |year=2006 |last1=Stoltz |first1=J. A. |last2=Neff |first2=B. D. |s2cid=29115283 }}</ref> On top of the sperm locating the oocyte and penetrating the gel layer, it must also infiltrate the mycropyle.<ref name=pmid25442393/> 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.<ref name=pmid17040384/> There are instances where males will create habitats in an attempt to monopolize females and increase their chance of fertilizing eggs.<ref name=pmid25442393/>
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.<ref>{{cite journal |last1=Murua |first1=Hilario |date=March 2014 |title=Fish reproduction assortment: a wonderful diversity |journal=Environmental Biology of Fishes |volume=97 |issue=3 |pages=329–33 |doi=10.1007/s10641-013-0154-2 |bibcode=2014EnvBF..97..329M |s2cid=18570324 }}</ref> 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.<ref name=pmid25152513>{{cite journal |doi=10.1016/j.repbio.2013.12.005 |pmid=25152513 |title=Motility of fish spermatozoa: From external signaling to flagella response |journal=Reproductive Biology |volume=14 |issue=3 |pages=165–75 |year=2014 |last1=Dzyuba |first1=Viktoriya |last2=Cosson |first2=Jacky }}</ref>
==See also== *Fertilization
==References== {{Reflist|30em}}
{{Animalbirth}} {{Sex (biology)}}
{{DEFAULTSORT:External Fertilization}} Category:Sexual reproduction Category:Reproduction in animals