Editing Seahorse (section)

==Reproduction==
{{see also|Animal sexual behavior#Seahorse}}
[[File:Seahorse lifecycle.svg|thumbnail|Seahorse life-cycle]]
The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side of the tail. When mating, the female seahorse deposits up to 1,500 eggs in the male's pouch. The male carries the [[egg (biology)#Fish and amphibian eggs|egg]]s for 9 to 45 days until the seahorses emerge fully developed, but very small. The young are then released into the water, and the male often mates again within hours or days during the breeding season.<ref>{{cite journal |author1=Foster S.J |author2=Vincent C.J. |title=Life history and ecology of seahorses: implications for conservation and management |journal=Journal of Fish Biology |volume=65 |pages=1–61 |year=2004 |issue=1 |doi=10.1111/j.0022-1112.2004.00429.x|doi-access=free |bibcode=2004JFBio..65....1F }}</ref>

===Courtship===
Before breeding, seahorses may court for several days. Scientists believe the [[Courtship display|courtship]] behavior synchronizes the animals' movements and reproductive states, so that the male can receive the eggs when the female is ready to deposit them. During this time, they may change color, swim side by side holding tails or grip the same strand of sea grass with their tails, and wheel around in unison in what is known as a "predawn dance". They eventually engage in a "true courtship dance" lasting about 8 hours, during which the male pumps water through the egg pouch on his trunk which expands and opens to display its emptiness. When the female's eggs reach maturity, she and her mate let go of any anchors and drift upward snout-to-snout, out of the sea grass, often spiraling as they rise. They interact for about 6 minutes, reminiscent of courtship.<ref name="autogenerated2" /> The female inserts her [[ovipositor]] into the male's brood pouch and deposits dozens to thousands of eggs. As the female releases her eggs, her body slims while his swells. Both animals then sink back into the sea grass and she swims away.<ref name="autogenerated2" />

==== Phases of courtship ====
Seahorses exhibit four phases of courtship that are indicated by clear behavioral changes and changes in the intensity of the courtship act. Phase 1, the initial courtship phase, typically takes place in the early morning one or two days before physical [[copulation (zoology)|copulation]]. During this phase the potential mates brighten in colour, quiver, and display rapid side-to-side body vibrations. These displays are performed alternately by both the male and the female seahorse. The following phases, 2 through 4, happen sequentially on the day of copulation. Phase 2 is marked by the female pointing, a behaviour in which the female will raise her head to form an oblique angle with her body. In phase 3 males will also begin the same pointing behaviour in response to the female. Finally, the male and female will repeatedly rise upward together in a [[water column]] and end in mid-water copulation, in which the female will transfer her eggs directly into the male's brood pouch.<ref name="Masonjones-1996">{{Cite journal |last1=Masonjones |first1=Heather D. |last2=Lewis |first2=Sara M. |date=1996 |title=Courtship Behavior in the Dwarf Seahorse, ''Hippocampus zosterae'' |journal=Copeia |volume=1996 |issue=3 |pages=634–640 |doi=10.2307/1447527 |jstor=1447527}}</ref>

===== Phase 1: Initial courtship =====
This initial courtship behaviour takes place about 30 minutes after dawn on each courtship day, until the day of copulation. During this phase the males and females will remain apart during the night, but after dawn they will come together in a side-by-side position, brighten, and engage in courtship behaviour for about 2 to 38 minutes. There is repeated reciprocal quivering. This starts when the male approaches the female, brightens and begins to quiver. The female will follow the male with her own display, in which she will also brighten and quiver about 5 seconds later. As the male quivers, he will rotate his body towards the female who will then rotate her body away. During phase 1 the tails of both seahorses are positioned within 1&nbsp;cm of each other on the same [[Holdfast (biology)|hold-fast]] and both of their bodies are angled slightly outward from the point of attachment. However, the female will shift her tail attachment site, causing the pair to circle their common hold-fast.<ref name="Masonjones-1996" />

===== Phase 2: Pointing and pumping =====
This phase begins with the female beginning her pointing posture, by leaning her body towards the male, who will simultaneously lean away and quiver. This phase can last up to 54 minutes. Following phase 2 is a latency period (typically between 30 minutes and four hours), during which the seahorses display no courtship behaviour and females are not bright; males will usually display a pumping motion with their body.<ref name="Masonjones-1996" />

===== Phase 3: Pointing – pointing =====
[[File:Seahorse mating dance.JPG|thumb|Seahorses in Phase 2 of courtship]]
The third phase begins with the females brightening and assuming the pointing position. The males respond with their own brightening and pointing display. This phase ends with the male departing. It usually lasts nine minutes and can occur one to six times during courtship.<ref name="Masonjones-1996" />

===== Phase 4: Rising and copulation =====
The final courtship phase includes 5–8 bouts of courtship. Each bout of courtship begins with both the male and female anchored to the same plant about 3&nbsp;cm apart; usually they are facing each other and are still bright in colour from the previous phase. During the first bout, following the facing behaviour, the seahorses will rise upward together anywhere from 2 to 13&nbsp;cm in a water column. During the final rise, the female will insert her [[ovipositor]] and transfer her eggs through an opening into the male's brood pouch.<ref name="Masonjones-1996" />

=== Fertilization ===
During fertilization in ''Hippocampus kuda'', the brood pouch was found to be open for only six seconds while egg deposition occurred. During this time seawater entered the pouch where the spermatozoa and eggs meet in a seawater milieu. This hyperosmotic environment facilitates sperm activation and motility. The fertilization is therefore regarded as being physiologically 'external' within a physically 'internal' environment after the closure of the pouch.<ref>{{Cite journal |title=Dimorphic sperm and the unlikely route to fertilisation in the yellow seahorse |journal=Journal of Experimental Biology |date=2007-02-01 |issn=0022-0949 |pmid=17234612 |pages=432–437 |volume=210 |issue=3 |doi=10.1242/jeb.02673 |first1=Katrien J. W. Van |last1=Look |first2=Borys |last2=Dzyuba |first3=Alex |last3=Cliffe |first4=Heather J. |last4=Koldewey |first5=William V. |last5=Holt |doi-access=free}}</ref> It is believed that this protected form of fertilization reduces [[sperm competition]] among males. Within the Syngnathidae (pipefishes and seahorses) protected fertilization has not been documented in the pipefishes but the lack of any distinct differences in the relation of testes size to body size suggests that pipefishes may also have evolved mechanisms for more efficient fertilization with reduced sperm competition.<ref>{{cite journal |journal=Biological Journal of the Linnean Society |year=2004 |volume=83 |issue=3 |pages=369–376 |title=Testes investment and spawning mode in pipefishes and seahorses (Syngnathidae) |author1=Kvarnemo, Charlotta |author2=Simmons, Leigh W. |doi=10.1111/j.1095-8312.2004.00395.x |doi-access=free}}</ref>

===Gestation===
[[File:Hippocampus haema mating.jpg|thumb|Seahorses in Phase 4 of courtship]]
The fertilized eggs are then embedded in the pouch wall and become surrounded by a spongy tissue.<ref name="Project Seahorse">{{cite web |title=The biology of seahorses: Reproduction |publisher=The Seahorse Project |url=http://seahorse.fisheries.ubc.ca/biology5.html |access-date=8 May 2007 |archive-url=https://web.archive.org/web/20090303051206/http://seahorse.fisheries.ubc.ca/biology5.html |archive-date=3 March 2009}}</ref> The pouch provides oxygen,<ref>{{cite journal |last1=Dudley |first1=Jessica |title=Structural changes to the brood pouch of male pregnant seahorses (Hippocampus abdominalis) facilitate exchange between father and embryos |journal=Placenta |date=October 2021 |volume=114 |pages=115–123 |doi=10.1016/j.placenta.2021.09.002 |pmid=34517263 |s2cid=237505281 |hdl=2123/31726 |hdl-access=free }}</ref> as well as a controlled environment incubator. Though the egg yolk contributes nourishment to the developing embryo, the male sea horses contribute additional nutrients such as energy-rich lipids and also calcium to allow them to build their skeletal system, by secreting them into the brood pouch that are absorbed by the embryos. Further they also offer immunological protection, osmoregulation, gas exchange and waste transport.<ref>{{Cite journal |title=Seahorse Brood Pouch Transcriptome Reveals Common Genes Associated with Vertebrate Pregnancy |journal=Molecular Biology and Evolution |date=2015-09-01 |issn=0737-4038 |pmid=26330546 |pages=3114–31 |doi=10.1093/molbev/msv177 |first1=Camilla M. |last1=Whittington |first2=Oliver W. |last2=Griffith |first3=Weihong |last3=Qi |first4=Michael B. |last4=Thompson |first5=Anthony B. |last5=Wilson |volume=32 |issue=12 |doi-access=free|hdl=20.500.11850/110832 |hdl-access=free }}</ref>

The eggs then hatch in the pouch, where the salinity of the water is regulated; this prepares the newborns for life in the sea.<ref name="autogenerated2">{{Cite journal |doi=10.2307/4012130 |url=http://ase.tufts.edu/biology/labs/lewis/news/articles/2000ScienceNews.pdf |archive-url=https://web.archive.org/web/20110818082935/http://ase.tufts.edu/biology/labs/lewis/news/articles/2000ScienceNews.pdf |archive-date=2011-08-18 |url-status=deviated |jstor=4012130 |title=Pregnant: And Still Macho |journal=Science News |volume=157 |issue=11 |pages=168–170 |year=2000 |last1=Milius |first1=S.}}</ref><ref>{{Cite journal |pmid=10640362 |year=2000 |last1=Masonjones |first1=H. D. |last2=Lewis |first2=S. M. |title=Differences in potential reproductive rates of male and female seahorses related to courtship roles |journal=Animal Behaviour |volume=59 |issue=1 |pages=11–20 |doi=10.1006/anbe.1999.1269 |s2cid=5999610}}</ref><ref name="autogenerated1">{{cite web |author=Danielson, Stentor |work=National Geographic News |date=14 June 2002 |title=Seahorse Fathers Take Reins in Childbirth |url=http://news.nationalgeographic.com/news/2002/06/0614_seahorse_recov.html|archive-url=https://web.archive.org/web/20020624035320/http://news.nationalgeographic.com/news/2002/06/0614_seahorse_recov.html|url-status=dead|archive-date=24 June 2002}}</ref> 

===Birth===
The number of young released by the male seahorse averages 100–1000 for most species, but may be as low as 5 for the smaller species, or as high as 2,500.<ref name="Project Seahorse"/> When the [[fry (biology)|fry]] are ready to be born, the male expels them with muscular contractions. He typically gives birth at night and is ready for the next batch of eggs by morning when his mate returns. Like almost all other fish species, seahorses do not nurture their young after birth. Infants are susceptible to predators or ocean currents which wash them away from feeding grounds or into temperatures too extreme for their delicate bodies. Less than 0.5% of infants survive to adulthood, explaining why litters are so large. These survival rates are actually fairly high compared to other fish, because of their protected gestation, making the process worth the great cost to the father. The eggs of most other fish are abandoned immediately after fertilization.<ref name="autogenerated1" />

===Reproductive roles===
[[File:Diagram of a pregnant male seahorse.png|thumb|left| Diagram of a pregnant male seahorse (''[[Hippocampus comes]]'')<ref name=nature20595/>]]
[[File:Tehotny morsky konik.jpg|thumb|right|Pregnant male seahorse at the [[New York Aquarium]]]]

Reproduction is energetically costly to the male. This brings into question why the sexual role reversal even takes place. In an environment where one partner incurs more energy costs than the other, [[Bateman's principle]] suggests that the lesser contributor takes the role of the aggressor. Male seahorses are more aggressive and sometimes fight for female attention. According to [[Amanda Vincent]] of [[Project Seahorse]], only males tail-wrestle and snap their heads at each other. This discovery prompted further study of energy costs. To estimate the female's direct contribution, researchers chemically analyzed the energy stored in each egg. To measure the burden on the males, oxygen consumption was used. By the end of incubation, the male consumed almost 33% more oxygen than before mating. The study concluded that the female's energy expenditure while generating eggs is twice that of males during incubation, confirming the standard hypothesis.<ref name="autogenerated2" />

Why the male seahorse (and other members of the Syngnathidae) carries the offspring through gestation is unknown, though some researchers believe it allows for shorter birthing intervals, in turn resulting in more offspring.<ref>{{cite journal |title=Operational Sex Ratios in Seahorses |author=Vincent, Amanda C. J. |journal=Behaviour |volume=128 |issue=1/2 |year=1994 |pages=153–167 |jstor=4535169 |doi=10.1163/156853994X00091}}</ref> Given an unlimited number of ready and willing partners, males have the potential to produce 17% more offspring than females in a breeding season. Also, females have "time-outs" from the reproductive cycle 1.2 times longer than those of males. This seems to be based on mate choice, rather than physiology. When the female's eggs are ready, she must lay them in a few hours or eject them into the water column. Making eggs is a huge cost to her physically, since they amount to about a third of her body weight. To protect against losing a clutch, the female demands a long courtship. The daily greetings help to cement the bond between the pair.<ref>{{cite web |url=http://petseahorse.com/male-seahorses-pregnant/ |archive-url=https://archive.today/20130717075359/http://petseahorse.com/male-seahorses-pregnant/ |url-status=usurped |archive-date=17 July 2013 |title=Why Do Male Seahorses Get Pregnant? |publisher=Petseahorse.com }}</ref>

===Monogamy===
Though seahorses are not known to mate for life, many species form [[pair bond]]s that last through at least the breeding season. Some species show a higher level of mate fidelity than others.<ref>{{cite journal |author1=Kvarnemo C |author2=Moore G.I |author3=Jones A.G |author4=Nelson W.S |author5=Avise J.C. |title=Monogamous pair bonds and mate switching in the Western Australian seahorse ''Hippocampus subelongatus'' |journal=J. Evol. Biol. |volume=13 |issue=6 |pages=882–8 |year=2000 |doi=10.1046/j.1420-9101.2000.00228.x|s2cid=40777563 |url=https://escholarship.org/uc/item/2zm2d5gx }}</ref><ref>{{cite journal |doi=10.1016/0003-3472(95)80011-5 |author1=Vincent C.J. |author2=Sadler L.M. |title=Faithful pair bonds in wild seahorses, ''Hippocampus whitei'' |journal=Anim. Behav. |volume=50 |pages=1557–1569 |year=1995 |url=http://courses.umass.edu/wfcon470/Vincent%20and%20Sadler%2095.pdf |archive-url=https://web.archive.org/web/20110723140601/http://courses.umass.edu/wfcon470/Vincent%20and%20Sadler%2095.pdf |archive-date=23 July 2011 |issue=6 |s2cid=53192875 }}</ref> However, many species readily switch mates when the opportunity arises. ''H.&nbsp;abdominalis'' and ''H.&nbsp;breviceps'' have been shown to breed in groups, showing no continuous mate preference. Many more species' mating habits have not been studied, so it is unknown how many species are actually monogamous, or how long those bonds actually last.<ref>{{cite web |url=http://www.fusedjaw.com/biology/seahorse-monogamy/ |title=What's Love Got to Do With It? The Truth About Seahorse Monogamy |publisher=fusedjaw.com |author=Weiss, Tami |date=10 April 2010}}</ref>

Although [[Monogamy in animals|monogamy]] within fish is not common, it does appear to exist for some. In this case, the [[mate guarding|mate-guarding]] hypothesis may be an explanation. This hypothesis states, "males remain with a single female because of ecological factors that make male parental care and protection of offspring especially advantageous."<ref>{{cite book |author=Alcock, John |author-link=John Alcock (behavioral ecologist) |title=Animal Behavior |publisher=Sinauer |location=Massachusetts |year=2005 |isbn=978-0878930050 |pages=370–1 |edition=8th }}</ref> Because the rates of survival for newborn seahorses are so low, incubation is essential. Though not proven, males could have taken on this role because of the lengthy period the females require to produce their eggs. If males incubate while females prepare the next clutch (amounting to a third of body weight), they can reduce the interval between clutches.{{citation needed|date=July 2015}}