Editing Echinoderm (section)

==Reproduction==

===Sexual reproduction===

Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. Almost all species have separate [[gonochoric|male and female sexes]], though some are [[hermaphrodite|hermaphroditic]]. The eggs and sperm cells are typically released into open water, where fertilisation takes place. The release of sperm and eggs is synchronised in some species, usually with regard to the lunar cycle. In other species, individuals may aggregate during the reproductive season, increasing the likelihood of successful fertilisation. Internal fertilisation has been observed in three species of sea star, three brittle stars and a deep-water sea cucumber. Even at [[Abyssal zone|abyssal depths]], where no light penetrates, echinoderms often synchronise their reproductive activity.<ref>{{harvnb|Young|Eckelbarger|1994|pp=179–194}}</ref>

Some echinoderms [[Egg incubation|brood their eggs]]. This is especially common in cold water species where planktonic larvae might not be able to find sufficient food. These retained eggs are usually few in number and are supplied with large yolks to nourish the developing embryos. In starfish, the female may carry the eggs in special pouches, under her arms, under her arched body, or even in her cardiac stomach.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|pp=887–888}}</ref> Many brittle stars are hermaphrodites; they often brood their eggs, usually in special chambers on their oral surfaces, but sometimes in the ovary or coelom.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=895}}</ref> In these starfish and brittle stars, development is usually direct to the adult form, without passing through a bilateral larval stage.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=888}}</ref> A few sea urchins and one species of sand dollar carry their eggs in cavities, or near their anus, holding them in place with their spines.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=908}}</ref> Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back, where they are retained. In a very small number of species, the eggs are retained in the coelom where they develop [[Viviparity|viviparously]], later emerging through ruptures in the body wall.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=916}}</ref> In some crinoids, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happens to find them.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=922}}</ref>

===Asexual reproduction===
{{See also|#Regeneration}}
[[File:Comet form of Linckia.png|thumb|upright|'Comet' form of ''[[Linckia]]'']]

One species of [[seastar]], ''[[Ophidiaster granifer]]'', reproduces asexually by [[parthenogenesis]].<ref name="Yamaguchi1984">{{harvnb|Yamaguchi|Lucas|1984}}</ref> In certain other [[asterozoa]]ns, adults reproduce asexually until they mature, then reproduce sexually. In most of these species, asexual reproduction is by [[transverse fission]] with the disc splitting in two. Both the lost disc area and the missing arms regrow, so an individual may have arms of varying lengths.<ref name=Fisher1925/><ref>{{harvnb|McGovern|2002}}</ref> During the period of regrowth, they have a few tiny arms and one large arm, and are thus often known as "comets".<ref name=Hotchkiss2000/><ref name="Monks1904">{{harvnb|Monks|1904}}</ref>

Adult sea cucumbers reproduce asexually by transverse fission. ''[[Holothuria parvula]]'' uses this method frequently, splitting into two a little in front of the midpoint. The two halves each regenerate their missing organs over a period of several months, but the missing genital organs are often very slow to develop.<ref name="Kille1942">{{harvnb|Kille|1942}}</ref>

The larvae of some echinoderms are capable of asexual reproduction. This has long been known to occur among starfish and brittle stars, but has more recently been observed in a sea cucumber, a sand dollar and a sea urchin.<ref name="Eaves2003">{{harvnb|Eaves|Palmer|2003}}</ref> This may be by [[Fragmentation (reproduction)|autotomising]] parts that develop into secondary larvae, by [[budding]], or by [[paratomy|splitting transversely]]. Autotomised parts or buds may develop directly into fully formed larvae, or may pass through a [[gastrula]] or even a [[blastula]] stage. New larvae can develop from the preoral hood (a mound like structure above the mouth), the side body wall, the postero-lateral arms, or their rear ends.<ref name=Eaves2003/><ref name="Jaeckle1994">{{harvnb|Jaeckle|1994}}</ref><ref name="Vaughn2009">{{harvnb|Vaughn|2009}}</ref>

Cloning is costly to the larva both in resources and in development time. Larvae undergo this process when food is plentiful<ref name="McDonald2010">{{harvnb|McDonald|Vaughn|2010}}</ref> or temperature conditions are optimal.<ref name=Vaughn2009/> Cloning may occur to make use of the tissues that are normally lost during metamorphosis.<ref name="Vaughn2008">{{harvnb|Vaughn|Strathmann|2008}}</ref> The larvae of some sand dollars clone themselves when they detect dissolved fish mucus, indicating the presence of predators.<ref name=Vaughn2009/><ref name=Vaughn2008/> Asexual reproduction produces many smaller larvae that escape better from planktivorous fish, implying that the mechanism may be an anti-predator adaptation.<ref name="Vaughn2010">{{harvnb|Vaughn|2010}}</ref>

===Larval development===
[[File:Pluteus001.jpg|thumb|A [[bilaterally symmetric]] echino[[pluteus larva]] with larval arms]]

Development begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. [[Gastrulation]] marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host the skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are often [[plankton]]ic, but in some species the eggs are retained inside the female, while in some the female broods the larvae.<ref>{{harvnb|Dorit|Walker|Barnes|1991|p=778}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|pp=997–998}}</ref>

The larvae pass through several stages, which have specific names derived from the taxonomic names of the adults or from their appearance. For example, a sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. A starfish has a '[[bipinnaria]]' larva, which develops into a multi-armed '[[brachiolaria]]' larva. A sea cucumber's larva is an 'auricularia' while a crinoid's is a 'vitellaria'. All these larvae are [[bilaterally symmetrical]] and have bands of cilia with which they swim; some, usually known as 'pluteus' larvae, have arms. When fully developed, they settle on the seabed to undergo metamorphosis, and the larval arms and gut degenerate. The left-hand side of the larva develops into the oral surface of the juvenile, while the right side becomes the aboral surface. At this stage, the pentaradial symmetry develops.<ref>{{harvnb|Dorit|Walker|Barnes|1991|p=778}}</ref><ref>{{harvnb|van Egmond|2000}}</ref>

A [[planktotrophic|plankton-eating]] larva, living and feeding in the water column, is considered to be the ancestral larval type for echinoderms, but in extant echinoderms, some 68% of species develop using a [[lecithotrophic|yolk-feeding]] larva.<ref name=Uthicke/> The provision of a yolk-sac means that smaller numbers of eggs are produced, the larvae have a shorter development period and a smaller dispersal potential, but a greater chance of survival.<ref name=Uthicke/>