Editing Sea cucumber (section)

== Life history and behaviour ==

=== Habitat ===
{{multiple image
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 | image1    = Pelagothuria natatrix (cropped).png
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 | caption1  = The mysterious ''[[Pelagothuria natatrix]]'' is the only truly [[pelagic]] echinoderm known to date.
 | image2    = Enypniastes sp.jpg
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 | caption2  = [[Elasipodida|Benthopelagic sea cucumber]]s, such as this ''[[Enypniastes]]'', are often confused with jellyfish, have webbed swimming structures enabling them to swim up off the surface of the seafloor and journey as much as {{convert|1000|m}} up the water column
 | image3    = Expl0790 - Flickr - NOAA Photo Library.jpg
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 | caption3  = Spanish dancer (''Benthodytes'' sp.), another swimming sea cucumber, hovering at {{convert|2789|m}} by the [[Davidson Seamount]]
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Sea cucumbers can be found in great numbers on the deep seafloor, where they often make up the majority of the animal biomass.<ref>{{cite web | url =http://jrscience.wcp.muohio.edu/fieldcourses05/PapersMarineEcologyArticles/SeaCucumbers.html| title = Sea Cucumbers | last = Miller | first = Nat | access-date =2007-10-03 }}</ref> At depths deeper than {{convert|5.5|mi|m|abbr=on|order=flip}}, sea cucumbers comprise 90% of the total mass of the macrofauna.<ref>{{cite web|url=http://www.answers.com/topic/sea-cucumberSci-Tech|title=Answers - The Most Trusted Place for Answering Life's Questions|work=Answers.com|access-date=12 June 2015}}</ref> Sea cucumbers form large herds that move across the bathygraphic features of the ocean, hunting food. The body of some deep water holothuroids, such as ''Enypniastes eximia'', ''Peniagone leander'' and ''Paelopatides confundens'',<ref>{{cite web|url=http://www7a.biglobe.ne.jp/~grafish/fa_abyssopelagic03.html|title=深海底帯底生魚|access-date=12 June 2015}}</ref> is made of a tough gelatinous tissue with unique properties that makes the animals able to control their own buoyancy, making it possible for them to either live on the ocean floor or to actively swim <ref>''Blue Planet'' Discovery Channel</ref> or float over it in order to move to new locations,<ref>{{cite web | url =http://www.oceanexplorer.noaa.gov/explorations/07mexico/logs/june18/june18.html| title = The Kingdom of the Echinoderm | date = 2007-06-18 | first =Bob | last = Carney | access-date = 2007-10-03 }}</ref> in a manner similar to how the group [[Torquaratoridae]] floats through water.

Holothuroids appear to be the echinoderms best adapted to extreme depths, and are still very diversified beyond {{convert|5,000|m}} deep: several species from the family [[Elpidiidae]] ("sea pigs") can be found deeper than {{convert|9,500|m}}, and the record seems to be some species of the genus ''[[Myriotrochus]]'' (in particular ''[[Myriotrochus bruuni]]''), identified down to {{convert|10,687|m}} deep.<ref name="MahDeepest">{{cite web|url=http://echinoblog.blogspot.fr/2014/04/what-are-deepest-known-echinoderms.html|title=What are the Deepest known echinoderms ? |last1=Mah|first1=Christopher L. |date=8 April 2014 |website=The Echinoblog }}</ref>
In more shallow waters, sea cucumbers can form dense populations. The strawberry sea cucumber (''Squamocnus brevidentis'') of [[New Zealand]] lives on rocky walls around the southern coast of the South Island where populations sometimes reach densities of {{convert|1,000|/m2|/sqft|abbr=on|disp=preunit|animals}}. For this reason, one such area in [[Fiordland]] is called the strawberry fields.<ref>{{cite journal | url =http://www.niwa.co.nz/news-and-publications/publications/all/abb/2003-03/cucumber| journal = Aquatic Biodiversity & Biosecurity Update|title= Sheddingnew light on the humble sea cucumber | access-date = 2007-10-03 | last =Alcock | first = Niki | year = 2003 | issue=3 }}</ref>

=== Locomotion ===
Some abyssal species in the abyssal order [[Elasipodida]] have evolved to a "benthopelagic" behaviour: their body is nearly the same density as the water around them, so they can make long jumps (up to {{convert|1000|m|disp=or}} high), before falling slowly back to the ocean floor. Most of them have specific swimming appendages, such as some kind of umbrella (like ''[[Enypniastes]]''), or a long lobe on top of the body (''[[Psychropotes]]''). Only one species is known as a true completely [[pelagic]] species, that never comes close to the bottom: ''[[Pelagothuria natatrix]]''.<ref>{{cite web|url=http://echinoblog.blogspot.com/2012/09/deep-sea-swimming-sea-cucumbers-and.html|title=Deep-Sea Swimming Sea Cucumbers and the "most bizarre holothurian species in existence" ! |last1=Mah |first1=Christopher L.|date=2012-09-18 |website=The Echinoblog }}</ref>

=== Diet ===
Holothuroidea are generally [[scavenger]]s, feeding on debris in the [[benthic zone]] of the ocean. Exceptions include some [[Pelagic zone|pelagic]] cucumbers and the species ''Rynkatorpa pawsoni'', which has a [[commensalism|commensal]] relationship with deep-sea [[anglerfish]].<ref>{{cite book|last1=Brusca|first1= R.C.|last2=Brusca|first2= G.J.| title=Invertebrates|url=https://archive.org/details/invertebrates0000brus|url-access=registration|publisher= SinauerAssociates|location= Massachusetts|year=1990|isbn=978-0-87893-097-5}}</ref> The [[diet (nutrition)|diet]] of most cucumbers consists of [[plankton]] and decaying organic matter found in the sea. Some sea cucumbers position themselves in [[Ocean current|currents]] and catch food that flows by with their open tentacles. They also sift through the bottom [[sediment]]s using their tentacles. Other species can dig into bottom silt or sand until they are completely buried. They then extrude their feeding tentacles, ready to withdraw at any hint of danger.

In the South Pacific, sea cucumbers may be found in densities of {{convert|40|/m2|/sqft|abbr=on|disp=preunit|individuals}}. These populations can process {{convert|19|kg/m2|lb/sqft}} of sediment per year.<ref>{{cite book |title=Coral Reefs: Cities Under TheSeas|last=Murphy |first=Richard C.|year=2002 |page=36|isbn=978-0-87850-138-0 |publisher=The Darwin Press, Inc.}}</ref>

The shape of the tentacles is generally adapted to the diet, and to the size of the particles to be ingested: the filter-feeding species mostly have complex arborescent tentacles, intended to maximize the surface area available for filtering, while the species feeding on the substratum will more often need digitate tentacles to sort out the nutritional material; the detritivore species living on fine sand or mud more often need shorter "peltate" tentacles, shaped like shovels. A single specimen can swallow more than {{convert|45|kg}} of sediment a year, and their excellent digestive capacities allow them to reject a finer, purer and homogeneous sediment. Therefore, sea cucumbers play a major role in the biological processing of the sea bed (bioturbation, purge, homogenization of the substratum etc.).

<gallery style="text-align:center;" mode="packed">
Image:Euapta godeffroyi, détail.jpg|The mouth of ''[[Euapta godeffroyi]]'', showing pinnate tentacles.
Image:Apostichopus californicus.004 - Aquarium Finisterrae.jpg|Mouth of ''[[Holothuria]] sp.'', showing peltate tentacles.
Image:Cucumaria main.jpg|Mouth of ''[[Cucumaria miniata]]'', with dendritic tentacles, for filtering the water.
Image:Holothurie sp..jpg|Faeces of a holothuroid. This participates in sediment homogenization and purification.
</gallery>

=== Communication and sociability ===
{{Unreferenced section|date=January 2018}}

=== Reproduction ===
[[File:Haeckel Amphoridea-12.jpg|thumb|upright=0.8|"Auricularia" larva (by [[Ernst Haeckel]])]]
Most sea cucumbers reproduce by releasing [[sperm]] and [[ovum|ova]] into the ocean water. Depending on conditions, one organism can produce thousands of [[gamete]]s. Sea cucumbers are typically [[dioecious]], with separate male and female individuals, but some species are [[Sequential hermaphroditism|protandric]]. The reproductive system consists of a single [[gonad]], consisting of a cluster of tubules emptying into a single duct that opens on the upper surface of the animal, close to the tentacles.<ref name=IZ/>

At least 30 species, including the red-chested sea cucumber (''[[Pseudocnella insolens]]''), fertilize their eggs internally and then pick up the fertilized zygote with one of their feeding tentacles. The egg is then inserted into a pouch on the adult's body, where it develops and eventually hatches from the pouch as a juvenile sea cucumber.<ref>Branch GM, Griffiths CL, Branch ML and Beckley LE(2005) ''Two Oceans'' {{ISBN|0-86486-672-0}}</ref> A few species are known to brood their young inside the body cavity, giving birth through a small rupture in the body wall close to the anus.<ref name=IZ/>

=== Development ===
In all other species, the egg develops into a free-swimming [[larva]], typically after around three days of development. The first stage of larval development is known as an '''auricularia''', and is only around {{convert|1|mm|mil|lk=out|abbr=on}} in length. This larva swims by means of a long band of [[cilia]] wrapped around its body, and somewhat resembles the [[bipinnaria]] larva of starfish. As the larva grows it transforms into the '''doliolaria''', with a barrel-shaped body and three to five separate rings of cilia. The '''pentacularia''' is the third larval stage of sea cucumber, where the tentacles appear. The tentacles are usually the first adult features to appear, before the regular tube feet.<ref name=IZ/>

=== Symbiosis and commensalism ===
[[File:Periclimenes imperator (Emperor shrimp) on Bohadschia argus (Sea cucumber).jpg|thumb|right|Emperor shrimp ''[[Periclimenes imperator]]'' on a ''[[Bohadschia ocellata]]'' sea cucumber]]
Numerous small animals can live in [[symbiosis]] or [[commensalism]] with sea cucumbers, as well as some parasites.

Some cleaner shrimps can live on the tegument of holothuroids, in particular several species of the genus ''[[Periclimenes]]'' (genus which is specialized in echinoderms), in particular ''[[Periclimenes imperator]]''.<ref name="Mah Crabs & Shrimps">{{cite web|url=http://echinoblog.blogspot.fr/2013/10/crabs-that-live-in-sea-cucumber-anuses.html|title=Crabs & Shrimps that Live in Sea Cucumber Anuses |last1=Mah|first1=Christopher L. |date=1 October 2013|website=The Echinoblog }}.</ref>
A variety of fish, most commonly [[pearl fish]], have evolved a [[commensalism|commensalistic]] symbiotic relationship with sea cucumbers in which the pearl fish will live in sea cucumber's cloaca using it for protection from predation, a source of food (the nutrients passing in and out of the anus from the water), and to develop into their adult stage of life. Many [[polychaete]] worms (family [[Polynoidae]]<ref name="Mah Worms & Snails">{{cite web|url=http://echinoblog.blogspot.fr/2014/01/worms-snails-that-live-onin-sea.html|title=Worms & Snails that Live On/In Sea Cucumbers!  |last1=Mah|first1=Christopher L. |date=22 January 2014|website=The Echinoblog }}</ref>) and [[crab]]s (like ''[[Lissocarcinus orbicularis]]'') have also specialized to use the mouth or the cloacal respiratory trees for protection by living inside the sea cucumber.<ref>{{cite journal| journal=Advanced Aquarist's Online Magazine|volume=2|issue=3| url =http://www.advancedaquarist.com/issues/mar2003/invert.htm | title =Aquarium Invertebrates | first = Rob, Ph.D.| last = Toonen | access-date =2007-10-03 |date=March 2003}}</ref> Nevertheless, holothuroids species of the genus ''[[Actinopyga]]'' have anal teeth that prevent visitors from penetrating their anus.<ref name="Mah Pearlfish-Sea Cucumber">{{cite web|url=http://echinoblog.blogspot.fr/2010/05/when-fish-live-in-your-cloaca-how-anal.html|title=When Fish Live in your Cloaca & How Anal Teeth are Important: The Pearlfish-Sea Cucumber Relationship |last1=Mah|first1=Christopher |date=11 May 2010 |website=The Echinoblog }}</ref>

Sea cucumbers can also shelter bivalves as endocommensals, such as ''[[Entovalva]] sp''.<ref name="Mah CLAMS that live in their throat">{{cite web|url=http://echinoblog.blogspot.fr/2010/08/sea-cucumbers-got-fish-that-live-in.html|title=Sea cucumbers got fish that live in their anus and CLAMS that live in their throat! (but not at the same time) |last1=Mah |first1=Christopher L. |date=31 January 2010|website=The Echinoblog }}</ref>

<gallery style="text-align:center;" mode="packed">
Image:Swimming Crab - Lissocarcinus Orbicularis on Sea Cucumber.jpg|''[[Lissocarcinus orbicularis]]'', a symbiotic crab.
Image:Shrimp on Sea Cucumber.jpg|''[[Periclimenes imperator]]'', a symbiotic shrimp.
Image:Polychètes sur T. anax.JPG|[[Polynoidae|Polynoid]] worms on a king sea cucumber.
</gallery>

=== Predators and defensive systems ===
[[Image:Tonna Perdix en train de se nourrir.JPG|thumb|''[[Tonna perdix]]'', a selective predator of tropical sea cucumbers]]
[[File:Sea cucumber defence.JPG|thumb|left|A sea cucumber in [[Mahé, Seychelles]] ejects sticky filaments from the anus in self-defense.]]
Sea cucumbers are often ignored by most of the marine predators because of the toxins they contain (in particular, [[holothurin]]) and because of their often spectacular defensive systems. However, they remain a prey for some highly specialized predators which are not affected by their toxins, such as the big mollusks ''[[Tonna galea]]'' and ''[[Tonna perdix]]'', which paralyzes them using powerful poison before swallowing them completely.<ref>{{cite journal |author1=Philippe Bourjon |author2=Sébastien Vasquez |date=2016 |title=Predation by the gastropod ''Tonna perdix'' (Gastropoda: Tonnoidea) on the holothurian ''Actinopyga echinites'' (Echinodermata: Holothuroidea) on a reef of Réunion |url=http://www.spc.int/DigitalLibrary/Doc/FAME/InfoBull/BDM/36/BDM36_45_Bourjon.pdf |journal=SPC Beche-de-mer Information Bulletin |volume=36 }}</ref>
Some other less specialized and opportunist predators can also prey on sea cucumbers sometimes when they cannot find any better food, such as certain species of fish ([[triggerfish]], [[pufferfish]]) and crustaceans (crabs, lobsters, [[hermit crab]]s).

Some species of coral-reef sea cucumbers within the order [[Aspidochirotida]] can defend themselves by expelling their sticky [[cuvierian tubules]] (enlargements of the respiratory tree that float freely in the [[Body cavity|coelom]]) to entangle potential predators. When startled, these cucumbers may expel some of them through a tear in the wall of the [[cloaca]] in an [[Autotomy|autotomic]] process known as [[Evisceration (autotomy)|evisceration]]. Replacement tubules grow back in one and a half to five weeks, depending on the species.<ref name=PLoSone/><ref>{{cite journal | title = Biomechanics of adhesion in sea cucumber Cuvierian tubules (Echinodermata, Holothuroidea) | journal = Integrative and Comparative Biology | date =2002-12-01 | last = Flammang | first = Patrick |author2=Ribesse, Jerome |author3=Jangoux, Michel | doi=10.1093/icb/42.6.1107|volume=42|issue=6|pages=1107–1115 | pmid = 21680394 |doi-access=free }}</ref> The release of these tubules can also be accompanied by the discharge of a toxic chemical known as [[holothurin]], which has similar properties to soap. This chemical can kill animals in the vicinity and is one more method by which these sedentary animals can defend themselves.<ref name=piper/>

=== Estivation ===
If the water temperature becomes too high, some species of sea cucumber from temperate seas can [[aestivate]]. While they are in this state of dormancy, they stop feeding, their gut atrophies, their metabolism slows down and they lose weight. The body returns to its normal state when conditions improve.<ref name=PLoSone/>