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{{short description|Class of echinoderms}}
{{pp|small=yes}}
{{Automatic taxobox
| fossil_range = {{fossil range|Darriwilian|present|Middle Ordovician-present}}
| image = Actinopyga echinites1.jpg
| image_caption = A sea cucumber (''[[Actinopyga echinites]]''), displaying its feeding tentacles and [[tube feet]]
| taxon = Holothuroidea
| authority = [[Henri Marie Ducrotay de Blainville|Blainville]], 1834
| subdivision_ranks = Orders
| subdivision = 
* [[Apodida]] <small>Brandt, 1835</small>
* †[[Arthrochirotida]] <small>Seilacher, 1961</small>
* [[Dendrochirotida]] <small>Grube, 1840</small>
* [[Elasipodida]] <small>Théel, 1882</small>
* [[Holothuriida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small>
* [[Molpadida]] <small>Haeckel, 1896</small>
* [[Persiculida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small>
* [[Synallactida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small>
}}
[[File:Thelenota ananas.jpg|thumb|''[[Thelenota ananas]]'', a giant sea cucumber from the [[Indo-Pacific]] tropics]]

'''Sea cucumbers''' are [[echinoderm]]s from the [[class (biology)|class]] '''Holothuroidea''' ({{IPAc-en|ˌ|h|ɒ|l|ə|ˌ|θj|ʊəˈr|ɔɪ|d|i|ə|,_|ˌ|h|oʊ|l|ə|-}} {{respell|HOL|ə|thyuu|ROY|dee|ə|,_|HOH|lə|-}}). They are [[benthic]] [[marine animal]]s found on the [[sea floor]] worldwide, and the number of known '''holothuroid''' species worldwide is about 1,786,<ref>{{cite web |url=http://www.marinespecies.org/aphia.php?p=browser&id=148744&expand=true#ct |title=Holothuroidea |last=Paulay |first=G. |year=2014 |work=[[World Register of Marine Species]] |access-date=2 March 2014}}</ref> with the greatest number being in the [[Asia–Pacific]] region.<ref name=PLoSone>{{cite journal |author1=Du, H. |author2=Bao, Z. |author3=Hou, R. |author4=Wang, S. |author5=Su, H. |year=2012 |title=Transcriptome Sequencing and Characterization for the Sea Cucumber ''Apostichopus japonicus'' (Selenka, 1867) |journal=[[PLOS One]] |volume=7 |issue=3 |pages=e33311 |doi=10.1371/journal.pone.0033311 |pmid= 22428017 |pmc=3299772 |display-authors=etal |bibcode=2012PLoSO...733311D |doi-access=free}}</ref> Sea cucumbers serve a useful role in the [[marine ecosystem]] as [[detritivore]]s who help recycle nutrients, breaking down [[detritus]] and other [[organic matter]], after which microbes can continue the [[decomposition]] process.<ref name=PLoSone/>

Sea cucumbers have a leathery [[skin]] and an elongated body containing a single, branched [[gonad]], are named for their overall resemblance to the fruit of the [[cucumber]] plant. Like all [[echinoderm]]s, sea cucumbers have a calcified [[dermal]] [[endoskeleton]], which is usually reduced to isolated microscopic [[ossicle (echinoderm)|ossicles]] (or sclerietes) joined by [[connective tissue]]. In some species these can sometimes be enlarged to flattened plates, forming an armoured [[cuticle]]. In some abyssal or [[pelagic]] species such as ''[[Pelagothuria natatrix]]'' (order [[Elasipodida]], family [[Pelagothuriidae]]), the skeleton is absent and there is no [[calcareous]] ring.<ref>{{cite journal |first=Mike |last=Reich |url=http://www.geobiologie.uni-goettingen.de/people/mreich/pdf/PDFs/POST_Dijon_Seegurken1.pdf |archive-url=https://web.archive.org/web/20090225190556/http://www.geobiologie.uni-goettingen.de/people/mreich/pdf/PDFs/POST_Dijon_Seegurken1.pdf |url-status=dead |archive-date=February 25, 2009 |title=Cambrian holothurians? – The early fossil record and evolution of Holothuroidea |editor=Lefebvre, B. |editor2=David, B. |editor3=Nardin, E. |editor4=Poty, E. |journal= Journées Georges Ubaghs |date=30–31 January 2006 |pages=36–37}}</ref>

Many species of sea cucumbers are [[sea cucumbers as food|foraged as food]] by [[human]]s, and some species are cultivated in [[aquaculture]] systems. They are considered a [[delicacy]] [[seafood]], especially in [[Asian cuisines]], and the harvested product is variously referred to as ''[[trepanging|trepang]]'', ''namako'', ''[[wikt:bêche-de-mer|bêche-de-mer]]'', or ''balate''.

== Overview ==
[[File:Sjogurka stor ugglan.gif|thumb|upright=1.1|Sea cucumber : a - Tentacles, b - Cloaca, c - Ambulacral feet on the ventral side, d - Papillae on the back]]

Most sea cucumbers have a soft and cylindrical body, rounded off and occasionally fat in the extremities, and generally without solid appendages. Their shape ranges from almost spherical for "sea apples" (genus ''[[Pseudocolochirus]]'') to serpent-like for [[Apodida]] or the classic sausage-shape, while others resemble caterpillars. The mouth is surrounded by tentacles, which can be pulled back inside the animal.<ref>{{Cite book|title=Encyclopedia of Animals|last1=Walters|first1=Martin|last2=Johnson|first2=Jinny|publisher=Marks and Spencer p.l.c|year=2003|isbn=978-1-84273-964-8|pages=68}}</ref> Holothuroids measure generally between {{convert|10|and|30|cm}} long, with extremes of some millimetres for ''[[Rhabdomolgus ruber]]'' and up to more than {{convert|3|m}} for ''[[Synapta maculata]]''. The largest [[Americas|American]] species, ''[[Holothuria floridana]]'', which abounds just below low-water mark on the [[Florida]] reefs, has a volume of well over {{convert|500|cm3|sp=us}},<ref>For an example, in Charles Edward Lincoln, "Variation development and growth in ''Holothuria floridana''" ''Biometrika'' '''6'''.2–3 July 1908:279.</ref> and {{convert|10|–|12|in|cm|abbr=on|order=flip}} long. Most possess five rows of tube feet (called "[[tube feet|podia]]"), but [[Apodida]] lacks these and moves by crawling; the podia can be of smooth aspect or provided with fleshy appendages (like ''[[Thelenota ananas]]''). The podia on the dorsal surface generally have no locomotive role, and are transformed into papillae. At one of the extremities opens a rounded mouth, generally surrounded with a crown of tentacles which can be very complex in some species (they are in fact modified podia); the anus is postero-dorsal.

Holothuroids do not look like other echinoderms at first glance, because of their tubular body, without visible skeleton nor hard appendixes. Furthermore, the fivefold symmetry, classical for echinoderms, although preserved structurally, is doubled here by a bilateral symmetry which makes them look like [[chordate]]s. However, a central symmetry is still visible in some species through five 'radii', which extend from the mouth to the anus (just like for sea urchins), on which the tube feet are attached. There is thus no "oral" or "aboral" face as for sea stars and other echinoderms, but the animal stands on one of its sides, and this face is called ''trivium'' (with three rows of tube feet), while the dorsal face is named ''bivium''.
A remarkable feature of these animals is the "catch" [[collagen]] that forms their body wall.<ref group=Notes>"Catch" collagen has two states, soft and stiff, that are under neurological control.[http://www.biolbull.org/cgi/reprint/190/2/243.pdf Jose del Castillo and David S. Smith. (1996) "We Still Invoke Friction and Occam's Razor to Explain Catch in the Spines of ''Eucidaris Tribuloides''." ''Biological Bulletin'' 190:243-244]</ref> This can be loosened and tightened at will, and if the animal wants to squeeze through a small gap, it can essentially liquefy its body and pour into the space. To keep itself safe in these crevices and cracks, the sea cucumber will hook up all its collagen fibers to make its body firm again.<ref name=piper>{{cite book|first=Ross|last=Piper|year=2007|title=Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals|url=https://archive.org/details/extraordinaryani0000pipe|url-access=registration|publisher=Greenwood Press|isbn=978-0-313-33922-6}}</ref>

The most common way to separate the subclasses is by looking at their oral tentacles. Order [[Apodida]] have a slender and elongate body lacking tube feet, with up to 25 simple or [[pinnate]] oral tentacles. [[Aspidochirotida]] are the most common sea cucumbers encountered, with a strong body and 10 to 30 leaflike or shield-like oral tentacles. [[Dendrochirotida]] are filter-feeders, with plump bodies and eight to 30 branched oral tentacles (which can be extremely long and complex).

<gallery style="text-align:center;" mode="packed">
Image:Apostichopus californicus.004 - Aquarium Finisterrae.jpg|Details of the mouth with its tentacles.
Image:Synaptula lamberti.JPG|''[[Synaptula lamperti]]'' lives on sponges (here in [[Indonesia]]).
Image:Reef1118 - Flickr - NOAA Photo Library.jpg|''[[Synapta maculata]]'', the longest known sea cucumber ([[Apodida]]).
Image:Thelenota anax géante.JPG|The king sea cucumber (''[[Thelenota anax]]'', family [[Stichopodidae]]) is one of the heaviest known holothuroids.
Image:Chiridota heheva.jpg|''[[Chiridota hydrothermica]]'', abyssal species.
Image:Cucumaria big.jpg|''[[Cucumaria miniata]]'', a filter-feeding sea cucumber.
Image:Pseudocolochirus axiologus.jpg|''[[Pseudocolochirus]]'' ("sea apple").
Image:Holothuria leucospilota.jpg|''[[Holothuria leucospilota]]''
Image:Isostichopus badionotus.jpg|''[[Isostichopus badionotus]]''
Image:Treasure chest candycane sea cucumber 2.jpg|''[[Thelenota rubralineata]]''
Image:Holothuria fuscopunctata.jpg|''[[Holothuria fuscopunctata]]''
Image:Bohadschia argus.jpg|''[[Bohadschia argus]]''
</gallery>

==Anatomy==
Sea cucumbers are typically {{cvt|10|to|30|cm|in|0}} in length, although the smallest known species are just {{cvt|3|mm|in}} long, and [[Largest organisms#Echinoderms (Echinodermata)|the largest]] can reach {{convert|3|m|ft|sp=us|0}}. The body ranges from almost spherical to worm-like, and lacks the arms found in many other echinoderms, such as [[starfish]]. The anterior end of the animal, containing the mouth, corresponds to the oral pole of other echinoderms (which, in most cases, is the underside), while the posterior end, containing the anus, corresponds to the aboral pole. Thus, compared with other echinoderms, sea cucumbers can be said to be lying on their side.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher=Holt-Saunders International |location= Philadelphia, PA|pages=981–997|isbn= 978-0-03-056747-6}}</ref>
[[File:Conspicuous Sea Cucumber, Coconut Island, Hawaii.JPG|thumb|Conspicuous Sea Cucumber, [[Coconut Island (Hawaii Island)|Coconut Island]], Hawaii]]

===Body plan===
The body of a holothuroid is roughly cylindrical. It is radially symmetrical along its longitudinal axis, and has weak bilateral symmetry transversely with a dorsal and a ventral surface. As in other [[echinozoa]]ns, there are five [[Ambulacral|ambulacra]] separated by five ambulacral grooves, the interambulacra. The ambulacral grooves bear four rows of tube feet but these are diminished in size or absent in some holothuroids, especially on the dorsal surface. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium.<ref name=Lander>[http://webs.lander.edu/rsfox/invertebrates/cucumaria.html ''Cucumaria frondosa''] {{webarchive|url=https://web.archive.org/web/20110903211033/http://webs.lander.edu/rsfox/invertebrates/cucumaria.html |date=2011-09-03 }} Invertebrate anatomy online. ed2012-02-12.</ref>

At the anterior end, the mouth is surrounded by a ring of tentacles which are usually retractable into the mouth. These are called the primary tentacles and were present in the common ancestor of echinoderms, but have been lost in all the other classes of the phylum,<ref>{{cite journal | url=https://link.springer.com/article/10.1134/S0031030122080020 | doi=10.1134/S0031030122080020 | title=Origin of Echinodermata | date=2022 | last1=Ezhova | first1=O. V. | last2=Malakhov | first2=V. V. | journal=Paleontological Journal | volume=56 | issue=8 | pages=938–973 | bibcode=2022PalJ...56..938E | url-access=subscription }}</ref> and may be simple, branched or arborescent. They are known as the introvert and posterior to them there is an internal ring of large calcareous ossicles. Attached to this are five bands of muscle running internally longitudinally along the ambulacra. There are also circular muscles, contraction of which cause the animal to elongate and the introvert to extend. Anterior to the ossicles lie further muscles, contraction of which cause the introvert to retract.<ref name=Lander/>

The body wall consists of an epidermis and a dermis and contains smaller calcareous ossicles, the types of which are characteristics which help to identify different species. Inside the body wall is the coelom which is divided by three longitudinal [[Mesentery (zoology)|mesenteries]] which surround and support the internal organs.<ref name=Lander/>

===Digestive system===
[[File:Holothuroidea (Sea cucumber feeding).jpg|thumb|right|A sea cucumber atop gravel, feeding]]
A [[pharynx]] lies behind the mouth and is surrounded by a ring of ten [[calcareous]] plates. In most sea cucumbers, this is the only substantial part of the skeleton, and it forms the point of attachment for muscles that can retract the tentacles into the body for safety as for the main muscles of the body wall. Many species possess an [[oesophagus]] and [[stomach]], but in some the pharynx opens directly into the [[intestine]]. The intestine is typically long and coiled, and loops through the body three times before terminating in a [[cloaca]]l chamber, or directly as the [[anus]].<ref name=IZ/>

===Nervous system===
Sea cucumbers have no true [[brain]]. A ring of neural tissue surrounds the oral cavity, and sends nerves to the tentacles and the [[pharynx]]. The animal is, however, quite capable of functioning and moving about if the nerve ring is surgically removed, demonstrating that it does not have a central role in nervous coordination. In addition, five major nerves run from the nerve ring down the length of the body beneath each of the ambulacral areas.<ref name=IZ/>

Most sea cucumbers have no distinct sensory organs, although there are various nerve endings scattered through the skin, giving the animal a sense of touch and a sensitivity to the presence of light. There are, however, a few exceptions: members of the Apodida order are known to possess [[statocyst]]s, while some species possess small eye-spots near the bases of their tentacles.<ref name=IZ/>

===Respiratory system===
Sea cucumbers use [[cloacal respiration]] via a pair of "respiratory trees" that branch in the [[cloaca]] just inside the [[anus]], so that they "breathe" by drawing water in through the anus, extracting [[dissolved oxygen]] from water, and then expelling it.<ref>{{cite web | url = http://www.fegi.ru/prim/sea/golot.htm | title = Holothurians or sea cucumbers | access-date = 2007-10-03 | archive-date = 2007-09-30 | archive-url = https://web.archive.org/web/20070930154932/http://www.fegi.ru/prim/sea/golot.htm | url-status = dead }}</ref><ref>{{cite web | url = http://www.comoxvalleynaturalist.bc.ca/knowing_nature/2006/sea_cucumbers.html | title = Knowing Nature... Cool as a Sea Cucumber | access-date = 2007-10-03 | last = Ingram | first = Jocie | date = 2006-06-16 | url-status = dead | archive-url = https://web.archive.org/web/20071021183031/http://www.comoxvalleynaturalist.bc.ca/knowing_nature/2006/sea_cucumbers.html | archive-date = 2007-10-21 }}</ref> The "trees" consist of a series of narrow tubules branching from a common duct, and lie on either side of the [[digestive tract]]. [[Gas exchange]] occurs across the thin walls of the tubules, to and from the fluid of the main body cavity.

Together with the intestine, the "respiratory trees" also act as [[excretory]] organs, with nitrogenous waste diffusing across the tubule walls in the form of [[ammonia]] and [[phagocytosis|phagocytic]] [[coelomocyte]]s depositing particulate waste.<ref name=IZ/>

===Circulatory systems===
Like all echinoderms, sea cucumbers possess both a [[water vascular system]] that provides hydraulic pressure to the tentacles and tube feet, allowing them to move, and a ''haemal system''. The latter is more complex than that in other echinoderms, and consists of well-developed vessels as well as open [[Hemocoel|sinuses]].<ref name=IZ/>

A central haemal ring surrounds the pharynx next to the ring canal of the water vascular system, and sends off additional vessels along the radial canals beneath the ambulacral areas. In the larger species, additional vessels run above and below the intestine and are connected by over a hundred small muscular ampullae, acting as miniature hearts to pump blood around the haemal system. Additional vessels surround the respiratory trees, although they contact them only indirectly, via the [[Coelom#Coelomic fluid|coelomic fluid]].<ref name=IZ/>

Indeed, the blood itself is essentially identical with the coelomic fluid that bathes the organs directly, and also fills the water vascular system. Phagocytic coelomocytes, somewhat similar in function to the [[white blood cell]]s of [[vertebrates]], are formed within the haemal vessels, and travel throughout the body cavity as well as both circulatory systems. An additional form of coelomocyte, not found in other echinoderms, has a flattened discoid shape, and contains [[hemoglobin]]. As a result, in many (though not all) species, both the blood and the coelomic fluid are red in colour.<ref name=IZ/>

{{multiple image
 | align = right
 | direction = vertical
 | width     = 220
 | image1    = Pearsonothuria graeffei bouche.JPG
 | alt1      = 
 | caption1  = ''[[Pearsonothuria graeffei]]'' showing its three rows of podia on its ''trivium''
 | image2    = Zeekomkommer-insluitselen.jpg
 | alt2      = 
 | caption2  = Sea cucumber ossicles (here "wheels" and "anchors")
}}

[[Vanadium]] has been reported in high concentrations in holothuroid blood,<ref>{{cite book | last = Phillips | first = Alexander |title = A Possible Source of Vanadium in Sedimentary Rocks | publisher =Princeton University | year = 1918 | url={{google books |plainurl=y |id=O6KxwrrC_LMC|page=473}}|page=473}}</ref> however researchers have been unable to reproduce these results.<ref>{{cite journal | last1 = Ciereszko | first1 = L. |title = On the Occurrence of Vanadium in Holothurians | year = 1962 |doi=10.1016/0010-406X(62)90034-8  |last2 = Ciereszko | first2 = E | last3 = Harris | first3 = E | last4 =Lane | first4 = C | journal = Comparative Biochemistry and Physiology |volume = 7 | issue = 1–2 | pages = 127–9 | pmid = 14021342}}</ref>

=== Locomotive organs ===
Like all echinoderms, sea cucumbers possess [[pentaradial symmetry]], with their bodies divided into five nearly identical parts around a central axis. However, because of their posture, they have secondarily evolved a degree of bilateral symmetry. For example, because one side of the body is typically pressed against the substratum, and the other is not, there is usually some difference between the two surfaces (except for [[Apodida]]). Like [[sea urchin]]s, most sea cucumbers have five strip-like ambulacral areas running along the length of the body from the mouth to the anus. The three on the lower surface have numerous [[tube feet]], often with suckers, that allow the animal to crawl along; they are called ''trivium''. The two on the upper surface have under-developed or vestigial tube feet, and some species lack tube feet altogether; this face is called ''bivium''.<ref name=IZ/>

In some species, the ambulacral areas can no longer be distinguished, with tube feet spread over a much wider area of the body. Those of the order [[Apodida]] have no tube feet or ambulacral areas at all, and burrow through sediment with muscular contractions of their body similar to that of worms, however five radial lines are generally still obvious along their body.<ref name=IZ/>

Even in those sea cucumbers that lack regular tube feet, those that are immediately around the mouth are always present. These are highly modified into retractile [[tentacle]]s, much larger than the locomotive tube feet.  Depending on the species, sea cucumbers have between 10 and 30 such tentacles and these can have a wide variety of shapes depending on the diet of the animal and other conditions.<ref name=IZ/>

Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.<ref>{{cite web|url=http://species-identification.org/species.php?species_group=nasc&selected=definitie&menuentry=woordenlijst&record=PapillaMarine|title=Marine Species Identification Portal : North Australian Sea Cucumbers : Glossary : PapillaMarine|access-date=12 June 2015}}</ref> These can even evolve into long antennae-like structures, especially on the abyssal genus ''[[Scotoplanes]]''.

=== Endoskeleton ===
Echinoderms typically possess an internal skeleton composed of plates of [[calcium carbonate]] within the [[dermis]]. In most sea cucumbers, however, these have become reduced to microscopic ossicles embedded beneath the skin. A few genera, such as ''[[Sphaerothuria]]'', retain relatively large plates, giving them a scaly armour.<ref name=IZ/>

== Life history and behaviour ==

=== Habitat ===
{{multiple image
 | align = right
 | direction = vertical
 | width     = 220
 | image1    = Pelagothuria natatrix (cropped).png
 | alt1      = 
 | caption1  = The mysterious ''[[Pelagothuria natatrix]]'' is the only truly [[pelagic]] echinoderm known to date.
 | image2    = Enypniastes sp.jpg
 | alt2      = 
 | 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
 | alt3      = 
 | caption3  = Spanish dancer (''Benthodytes'' sp.), another swimming sea cucumber, hovering at {{convert|2789|m}} by the [[Davidson Seamount]]
}}

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/>

==Phylogeny and classification==
{{see also|List of prehistoric sea cucumbers|List of echinodermata orders}}
[[Image:Euapta godeffroyi1.jpg|thumb|[[Apodida]] like this ''[[Euapta godeffroyi]]'' are snake-shaped, without podia, and have pinnate tentacles. ]]
[[Image:Holothuria cinerascens Réunion.jpg|thumb|[[Holothuriida]] like this ''[[Holothuria cinerascens]]'' are sausage-shaped, with peltate tentacles. ]]
[[Image:Cercodemas anceps Red box sea cucumber PC260152.JPG|thumb|[[Dendrochirotida]] like this ''[[Cercodemas anceps]]'' are curled-bodied and have arborescent tentacles. ]]
[[Image:Scotoplanes globosa and crab.jpg|thumb|[[Elasipodida]] like this "sea pig" ''[[Scotoplanes]]'' have a translucent body with specific appendages; they live in the abyss. ]]
[[Image:Stichopus herrmanni (Mayotte).jpg|thumb|upright=1|[[Synallactida]] like this ''[[Stichopus herrmanni]]'' still lack a definition.]]

Holothuroidea (sea cucumbers) are one of five [[extant taxon|extant]] classes that make up the phylum Echinodermata. This is one of the most distinctive and diverse phyla, ranging from starfish to urchins to sea cucumbers and many other organisms. The echinoderms are mainly distinguished from other phyla by their body plan and organization. The earliest sea cucumbers are known from the middle [[Ordovician]], over 450 million years ago.<ref>{{Cite journal|last1=Miller|first1=Allison K.|last2=Kerr|first2=Alexander M.|last3=Paulay|first3=Gustav|last4=Reich|first4=Mike|last5=Wilson|first5=Nerida G.|last6=Carvajal|first6=Jose I.|last7=Rouse|first7=Greg W.|date=June 2017|title=Molecular phylogeny of extant Holothuroidea (Echinodermata)|journal=Molecular Phylogenetics and Evolution|volume=111|pages=110–131|doi=10.1016/j.ympev.2017.02.014|pmid=28263876|issn=1055-7903|doi-access=free|bibcode=2017MolPE.111..110M }}</ref> The apodida is the sister group to the other orders of sea cucumbers.<ref>{{cite journal | doi=10.1038/s42003-022-03176-4 | title=The genome of an apodid holothuroid (Chiridota heheva) provides insights into its adaptation to a deep-sea reducing environment | year=2022 | last1=Zhang | first1=Long | last2=He | first2=Jian | last3=Tan | first3=Peipei | last4=Gong | first4=Zhen | last5=Qian | first5=Shiyu | last6=Miao | first6=Yuanyuan | last7=Zhang | first7=Han-Yu | last8=Tu | first8=Guangxian | last9=Chen | first9=Qi | last10=Zhong | first10=Qiqi | last11=Han | first11=Guanzhu | last12=He | first12=Jianguo | last13=Wang | first13=Muhua | journal=Communications Biology | volume=5 | issue=1 | page=224 | pmid=35273345 | pmc=8913654 }}</ref>

All echinoderms share three main characteristics. When mature, echinoderms have a pentamerous radial symmetry. While this can easily be seen in a sea star or brittle star, in the sea cucumber it is less distinct and seen in their five primary tentacles. The pentamerous radial symmetry can also be seen in their five ambulacral canals.<ref>{{Cite journal|last=Smirnov|first=A. V.|date=2014-12-01|title=Sea cucumbers symmetry (Echinodermata: Holothuroidea)|journal=Paleontological Journal|language=en|volume=48|issue=12|pages=1215–1236|doi=10.1134/S0031030114120107|bibcode=2014PalJ...48.1215S |s2cid=84952228|issn=0031-0301}}</ref> The ambulacral canals are used in their water vascular system which is another characteristic that binds this phylum together.

The water vascular system develops from their middle coelom or hydrocoel. Echinoderms use this system for many things including movement by pushing water in and out of their podia or "tube feet". Echinoderms tube feet (including sea cucumbers) can be seen aligned along the side of their axes.

While echinoderms are invertebrates, meaning they do not have a spine, they do all have an endoskeleton that is secreted by the [[mesenchyme]]. This endoskeleton is composed of plates called ossicles. They are always internal but may only be covered by a thin epidermal layer like in sea urchin's spines. In the sea cucumber, the ossicles are only found in the dermis, making them a very supple organism. For most echinoderms, their ossicles are found in units making up a three dimensional structure. However, in sea cucumbers, the ossicles are found in a two-dimensional network.<ref>{{Cite web|url=https://www.elsevier.com/books/chemical-zoology-v3/florkin/978-0-12-395536-4|title=Chemical Zoology V3 - 1st Edition|last=Elsevier|website=www.elsevier.com|language=en|access-date=2017-12-05|archive-url=https://web.archive.org/web/20171206140016/https://www.elsevier.com/books/chemical-zoology-v3/florkin/978-0-12-395536-4|archive-date=2017-12-06|url-status=dead}}</ref>

All echinoderms also possess anatomical feature(s) called [[mutable collagenous tissues]], or MCTs.<ref name=":0">{{Cite journal|last=Wilkie|first=I. C.|date=2005|title=Mutable collagenous tissue: overview and biotechnological perspective|journal=Progress in Molecular and Subcellular Biology|volume=39|pages=221–250|issn=0079-6484|pmid=17152700|doi=10.1007/3-540-27683-1_10|isbn=978-3-540-24402-8}}</ref> Such tissues can rapidly change their passive mechanical properties from soft to stiff under the control of the nervous system and coordinated with muscle activity. Different echinoderm classes use MCTs in different ways. The asteroids, sea stars, can detach limbs for self-defense and then regenerate them. The Crinoidea, sea fans, can go from stiff to limp depending on the current for optimal filter feeding. The Echinoidea, sand dollars, use MCTs to grow and replace their rows of teeth when they need new ones. The Holothuroidea, sea cucumbers, use MCTs to eviscerate their gut as a self-defense response. MCTs can be used in many ways but are all similar at the cellular level and in mechanics of function. A common trend in the uses of MCTs is that they are generally used for self-defense mechanisms and in regeneration.<ref name=":0" />

Holothuroid classification is complex and their paleontological phylogeny relies on a limited number of well-preserved specimens. The modern taxonomy is based first of all on the presence or the shape of certain soft parts (podia, lungs, tentacles, peripharingal crown) to determine the main orders, and secondarily on the microscopic examination of ossicles to determine the genus and the species. Contemporary genetic methods have been helpful in clarifying their classification.

Taxonomic classification according to [[World Register of Marine Species]]:
* subclass Actinopoda <small>Ludwig, 1891</small>
** order [[Dendrochirotida]] <small>Grube, 1840</small>
*** family [[Cucumariidae]] <small>Ludwig, 1894</small>
*** family [[Cucumellidae]] <small>Thandar & Arumugam, 2011</small>
*** family [[Heterothyonidae]] <small>Pawson, 1970</small>
*** family †[[Monilipsolidae]] <small>Smith & Gallemí, 1991</small>
*** family [[Paracucumidae]] <small>Pawson & Fell, 1965</small>
*** family [[Phyllophoridae]] <small>Östergren, 1907</small>
*** family [[Placothuriidae]] <small>Pawson & Fell, 1965</small>
*** family [[Psolidae]] <small>Burmeister, 1837</small>
*** family [[Rhopalodinidae]] <small>Théel, 1886</small>
*** family [[Sclerodactylidae]] <small>Panning, 1949</small>
*** family [[Vaneyellidae]] <small>Pawson & Fell, 1965</small>
*** family [[Ypsilothuriidae]] <small>Heding, 1942</small>
** order [[Elasipodida]] <small>Théel, 1882</small>
*** family [[Elpidiidae]] <small>Théel, 1882</small>
*** family [[Laetmogonidae]] <small>Ekman, 1926</small>
*** family †[[Palaeolaetmogonidae]] <small>Reich, 2012</small>
*** family [[Pelagothuriidae]] <small>Ludwig, 1893</small>
*** family [[Psychropotidae]] <small>Théel, 1882</small>
** order [[Holothuriida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017 </small>
*** family [[Holothuriidae]] <small>Burmeister, 1837</small>
*** family [[Mesothuriidae]] <small>Smirnov, 2012</small>
** order [[Molpadida]] <small>Haeckel, 1896</small>
*** family [[Caudinidae]] <small>Heding, 1931</small>
*** family [[Eupyrgidae]] <small>Semper, 1867</small>
*** family [[Gephyrothuriidae]] <small>Koehler & Vaney, 1905</small>
*** family [[Molpadiidae]] <small>Müller, 1850</small>
** order [[Persiculida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017 </small>
*** family [[Gephyrothuriidae]] <small>Koehler & Vaney, 1905</small>
*** family [[Molpadiodemidae]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small>
*** family [[Pseudostichopodidae]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small>
** order [[Synallactida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017 </small>
*** family [[Deimatidae]] <small>Théel, 1882</small>
*** family [[Stichopodidae]] <small>Haeckel, 1896</small>
*** family [[Synallactidae]] <small>Ludwig, 1894</small>
* subclass †Arthrochirotacea<small>Smirnov, 2012</small>
** order †[[Arthrochirotida]] <small>Brandt, 1835</small>
*** family †[[Palaeocucumariidae]] <small>Frizzell & Exline, 1966</small>
* subclass [[Paractinopoda]] <small>Ludwig, 1891</small>
** order [[Apodida]] <small>Brandt, 1835</small>
*** family [[Chiridotidae]] <small>Östergren, 1898</small>
*** family [[Myriotrochidae]] <small>Théel, 1877</small>
*** family [[Synaptidae]] <small>Burmeister, 1837</small>

== Scientific history and naming ==

The word Holothurion (ὁloqoύrion or "holothoύrion") was first coined by the Greek philosopher [[Aristotle]] in his ''Historia animalium'' (Book I, Part 1), although it remains unsure whether he is referring to the same animal as us. 

The earliest known mention of the Greek term ὁλοθούριον (''holotoúrion'') <ref>{{cite web |access-date=23 October 2023 |author1=Anatole Bailly |author2=Hugo Chávez |author3=Gérard Gréco |author4=André Charbonnet |author5=Mark De Wilde |author6=Bernard Maréchal |date=2020 |title=Le Bailly |url=https://bailly.app/holothourion}}<!-- auto-translated from French by Module:CS1 translator -->.</ref>{{,}}<ref>{{cite web|access-date=23 October 2023 |author=Maria Pantelia |date=February 2011 |title=The Online Liddell-Scott-Jones Greek-English Lexicon |url=https://stephanus.tlg.uci.edu/lsj/#eid=75554}}<!-- auto-translated from French by Module:CS1 translator -->.</ref> is found in the poet [[Epicharmus]] around 450 BC.<ref name="A Philology of Òλοθóυριου"/> [[Plato]], [[Aristotle]] and [[Theophrastus]] all used the generic name πλεύμον θαλάσσιος (''pleúmon thalássios'') to refer to a soft, flabby marine zoophyte – often translated as "sea lung"; the term may designate sea cucumbers, but possibly also [[ascidiacea|ascidians]] or even [[jellyfish]].<ref>[[Plato]], ''[[Philebus (Plato)|Philebus]]'' (21b); [[Aristotle]], ''[[History of Animals (Aristotle)|History of Animals]]'' (Book V, 15); [[Theophrastus]], ''On Weather Signs'' (Frag. 40).</ref>

One of the oldest scientific texts concerning sea cucumbers dates back to [[Aristotle]], in his ''[[Parts of Animals]]''<ref name="Aristote">{{cite book |last=Aristotle |title=Parts of Animals |volume=IV |orig-date=−343 |url=http://remacle.org/bloodwolf/philosophes/Aristote/partieslivre4.htm |access-date=6 February 2025}}</ref> (around 343 BC): he names an animal "holothurion" without describing it, but classifies it among the animals lacking sensation (along with [[Porifera|sponges]] and "sea lungs", apparently corresponding to [[tunicates]]) ; this name would later be retained and used to refer to sea cucumbers, though there is no definitive proof that this was the animal the Philosopher meant.<ref name="A Philology of Òλοθóυριου">{{cite journal | last = Kerr | first = Alexander M. | title = A Philology of Òλοθóυριου: From Ancient Times to Linnaeus, Including Middle and Far Eastern Sources | journal = University of Guam Marine Laboratory Technical Report | issue = 151 | year = 2013 | url = http://www.guammarinelab.com/publications/uogmltechrep151.pdf | access-date = 6 February 2025 | language = en}}</ref>

Today, the word "Holothurian" is often used, although it is considered wrong as it would refer mostly to the genus ''[[Holothuria]]'' rather than to the whole class of Holothurioidea, which should rather be called in English "Holothuroids".<ref>Annie Mercier, Andrey Gebruk, Antonina Kremenetskaia, and Jean-François Hamel, "An overview of taxonomic and morphological diversity in sea cucumbers (Holothuroidea: Echinodermata)", ''The World of Sea Cucumbers - Challenges, Advances, and Innovations'', 2024, Pages 3–15.</ref> 

In the East, medical or zoological treatises mention sea cucumbers as early as the 8th centuryth century, notably the ''[[Kojiki]]'' in China (712), then the ''[[Wamyō ruijushō]]'' in Japan (934), initiating a long tradition of excellent representations of these animals in Chinese and Japanese treatises.<ref name="A Philology of Òλοθóυριου"/>

Western scientists began to take renewed interest in echinoderms during the Renaissance, and [[Pierre Belon]] in 1553 was the first to propose a link between them and starfish and sea urchins.<ref name="Smirnov 2012">{{cite journal | last = Smirnov | first = A.V. | title = System of the Class Holothuroidea | journal = Paleontological Journal | volume = 46 | issue = 8 | year = 2012 | pages = 793–832 | doi = 10.1134/s0031030112080126 | bibcode = 2012PalJ...46..793S | url = https://doi.org/10.1134/s0031030112080126 | access-date = 6 February 2025 | language = en| url-access = subscription }}</ref> The first unambiguous use of this term to name a sea cucumber, accompanied by an illustration, is found in the ''Libri de Piscibus Marinis'' by [[Guillaume Rondelet]], published in 1554 (although he describes two species, the second being an [[Ascidiacea|ascidian]], and he wrongly separates the "''vit de mer''"<ref name="Rondelet">[[Guillaume Rondelet]], [https://gallica.bnf.fr/ark:/12148/bpt6k97271k ''De piscibus marinis, libri XVIII, in quibus veræ piscium effigies expressæ sunt''], Lyon, apud Matthiam Bonhomme, 1554, p.86</ref>).<ref name="A Philology of Òλοθóυριου"/> He noted that these beings "are of a middle nature between plants and animals".<ref name="Rondelet"/> 

Real progress came during the [[Age of Enlightenment]]: in 1751, an article titled "Holothurie" was written for the [[Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers|Encyclopédie]], based on the commentaries of Aristotle and Rondelet, but their taxonomic position (and even their description) remained unclear:  
{{citation bloc|HOLOTHURIE, s.f. ''holothurium'', (''Nat. Hist. Zool.'') marine animal. [[Carl von Linné|Mr. Linnæus]] placed it among the zoophytes, which are naked and have limbs. [[Guillaume Rondelet|Rondelet]] mentioned two species of holothuries which he illustrated. The first species has a hard shell, it is oblong; one end is blunt and finished with a shell pierced with several holes. The second species has a body covered with spines; it ends at one end with a sort of round head pierced with a round, wrinkled hole that opens and closes, and which is the animal’s mouth; the other end is narrow and elongated like a tail. On each side is an extension which is a leg, or rather a fin, since the animal uses it to move. One of the extensions is narrower than the other, scalloped all around, and pointed.<ref>L’''[[Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers|Encyclopédie]]'', 1st&nbsp;édition, 1751 (Volume 8, {{p.|247}}). [[:s:L’Encyclopédie/1re édition/HOLOTHURIE|read online]].</ref>}}

In 1758, sea cucumbers appeared in the ''[[Systema Naturae]]'' of [[Carl von Linné]], but the term still did not refer specifically to echinoderms,<ref name="Rowe 1969">{{cite book | last = Rowe | first = Francis W.E. | title = A Review of the Family Holothuriidae (Holothuroidea: Aspidochirotida) | volume = 18 | chapter = 4 | publisher = Bulletin of the British Museum (Natural History) | year = 1969 | pages = 54 | url = https://www.vliz.be/imisdocs/publications/276172.pdf | access-date = 6 February 2025| language = en}}.</ref> and included diverse creatures such as the [[physalia]].<ref>Gustav Paulay, ''[http://www.marinespecies.org/aphia.php?p=taxdetails&id=123456 Holothuria]'' on the ''[[World Register of Marine Species]]''.</ref> It was only in 1767 that Linnaeus revised the ''Holothuria'' entry.<ref name="Rowe 1969"/>

[[Nathanael Gottfried Leske]] created the phylum [[echinoderm]]s in 1778 (systematized by [[Jean-Guillaume Bruguière]] in 1791<ref name="Rowe 1969"/>), thereby formally incorporating this clade into scientific classifications. [[Henri-Marie Ducrotay de Blainville]] provided the scientific description of the subclass Holothuroidea in 1834, identifying it specifically with sea cucumbers (then still grouped under a single genus).

During the 19th centuryth century, many species were discovered (notably by [[Edmond Perrier]]), and were rapidly divided into orders and families, particularly by Grube, Théel, and Haeckel.

In the 20th centuryth century, increasingly intensive commercial fishing driven by Asian markets led to the rapid collapse of numerous stocks.<ref>Hampus Eriksson, Purcell, S., Conand, C., Muthiga, N., & Lovatelli, A. (2013), ''Report on the FAO Workshop on Sea Cucumber Fisheries: An Ecosystem Approach to Management in the Indian Ocean (SCEAM Indian Ocean), Mazizini, Zanzibar, the United Republic of Tanzania'', 12–16 November 2012. FAO Fisheries and Aquaculture Report, 1038, 92.</ref> This situation began to alarm the industry and governments from the 1970s onward, prompting scientific studies on population status, which helped revive interest in sea cucumber research. In 1990, the ''SPC Beche-de-mer Information Bulletin'' was launched, the first scientific journal devoted exclusively to holothuroids.<ref name="Ziegler">{{cite journal | last1 = Ziegler | first1 = Alexander | last2 = Jangoux | first2 = Michel | last3 = Mirantsev | first3 = Georgy | last4 = Kroh | first4 = Andreas | title = Historical Aspects of Meetings, Publication Series, and Digital Resources Dedicated to Echinoderms | journal = Zoosystematics and Evolution | volume = 90 | issue = 1 | year = 2014 | pages = 45–56 | doi = 10.3897/zse.90.7201 | doi-access = free | url = https://zse.pensoft.net/articles.php?id=1090 | access-date = 6 February 2025 | language = en}}</ref> Today, sea cucumbers are studied by numerous specialists from around the world, including [[Chantal Conand]], Gustav Paulay, Sven Uthicke, [[Nyawira Muthiga]], Maria Byrne, Steven Purcell, François Michonneau, and Yves Samyn.<ref name="ConandFAO"/>

==Relation to  humans==

=== Food ===
{{Main|Sea cucumber as food}}
[[File:Yokohama Chinese Medicine Sea cucumber 2.jpg|thumb|Dried sea cucumbers in a Japanese pharmacy]]
To supply the markets of [[Southern China]], [[Makassar]] [[trepanger]]s traded with the [[Indigenous Australians]] of [[Arnhem Land]] from at least the 18th century and probably earlier. This is the first recorded example of [[Makassan contact with Australia|trade between the inhabitants of the Australian continent and their Asian neighbours]].<ref>{{cite book|isbn=978-0-522-84088-9|last=MacKnight |first=CC |year=1976|title= The Voyage to Marege: Macassan Trepangers in Northern Australia|publisher= Melbourne University Press}} {{Cite journal|last= Tippett| first= A. R.|title=The Voyage to Marege: Macassan Teripangers in Northern Australia}}</ref>

There are many commercially important species of sea cucumber that are harvested and dried for export for use in [[Chinese cuisine]] as ''hoisam''.<ref name="Purcell2023">{{cite journal|last1=Purcell|first1= Steven W.|last2=Lovatelli|first2=Alessandro|last3=González-Wangüemert|first3=Mercedes|last4=Solís-Marín|first4=Francisco A.|last5=Samyn|first5=Yves|last6=Conand|first6=Chantal|name-list-style=&|title=Commercially important sea cucumbers of the world|year=2023|journal=FAO Species Catalogue for Fishery Purposes|number=6|edition=2nd|location=Rome|publisher=[[Food and Agriculture Organization|Food and Agriculture Organization of the United Nations (FAO)]]|doi=10.4060/cc5230en|doi-access=free|issn=1020-8682|isbn=978-92-5-137793-2|url=https://openknowledge.fao.org/items/83beabbc-bdeb-4ecb-bef6-38716e4bfb88|access-date=18 May 2024}}</ref> Some of the more commonly found species in markets include:<ref name="Purcell2023"/><ref>{{Cite journal|doi=10.1071/MF02145 |last1= Ramofafia |first1= C. |last2= Byrne |first2= M. |last3= Battaglene| first3= S. C. |title=Development of three commercial sea cucumbers, ''Holothuria scabra'', ''H. fuscogilva'' and ''Actinopyga mauritiana'': larval structure and growth|journal=Marine and Freshwater Research |volume=54|pages=657–667 |year=2003 | issn=1323-1650 |issue=5|bibcode= 2003MFRes..54..657R }}</ref>
* ''[[Acaudina molpadioides]]''
* ''[[Actinopyga echinites]]''
* ''[[Actinopyga mauritiana]]''
* ''[[Actinopyga palauensis]]''
* ''[[Apostichopus californicus]]''
* ''[[Apostichopus japonicus]]''
* ''[[Holothuria nobilis]]''
* ''[[Holothuria scabra]]''
* ''[[Holothuria fuscogilva]]''
* ''[[Isostichopus fuscus]]''
* ''[[Thelenota ananas]]''

=== Medicine ===
According to the [[American Cancer Society]], although it has been used in traditional Asian folk medicine for a variety of ailments, "there is little reliable scientific evidence to support claims that sea cucumber is effective in treating cancer, arthritis, and other diseases" but research is examining "whether some compounds made by sea cucumbers may be helpful against cancer".<ref>{{cite web|url=http://www.cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/pharmacologicalandbiologicaltreatment/sea-cucumber |url-status=dead |archive-url=https://web.archive.org/web/20100714040906/http://www.cancer.org/Treatment/TreatmentsandSideEffects/ComplementaryandAlternativeMedicine/PharmacologicalandBiologicalTreatment/sea-cucumber |archive-date=July 14, 2010 |title=Sea Cucumber |publisher=[[American Cancer Society]] |date=November 2008 |access-date=September 12, 2016 }}</ref>

Various pharmaceutical companies emphasize ''[[gamat]]'', the [[Malay language|Malay]] traditional medicinal usage of this animal.<ref>{{Cite journal|last1=Pangestuti|first1=Ratih|last2=Arifin|first2=Zainal|date=2018-07-01|title=Medicinal and health benefit effects of functional sea cucumbers|journal=Journal of Traditional and Complementary Medicine|volume=8|issue=3|pages=341–351|doi=10.1016/j.jtcme.2017.06.007|pmid=29992105|pmc=6035309|issn=2225-4110}}</ref> Extracts are prepared and made into oil, cream or cosmetics. Some products are intended to be taken internally.

A review article found that [[chondroitin sulfate]] and related compounds found in sea cucumbers can help in treating joint-pain, and that dried sea cucumber is "medicinally effective in suppressing [[arthralgia]]".<ref>{{Cite journal|last1=Bordbar|first1=Sara|last2=Anwar|first2=Farooq|last3=Saari|first3=Nazamid|date=10 October 2011|title=High-Value Components and Bioactives from Sea Cucumbers for Functional Foods—A Review|journal=[[Marine Drugs]]|volume=9|issue=10|pages=1761–1805|doi=10.3390/md9101761|pmc=3210605|pmid=22072996|doi-access=free}}</ref>

Another study suggested that sea cucumbers contain all the fatty acids necessary to play a potentially active role in tissue repair.<ref>{{cite journal|title=Fatty acid compositions in local sea cucumber, ''Stichopus chloronotus,'' for wound healing|journal=General Pharmacology|volume=33|issue=4| date=October 1999| pages= 337–340| doi=10.1016/S0306-3623(98)00253-5 |pmid=10523072| first1= B. D. |last1=Fredalina|first2= B. H. |last2= Ridzwan|first3= A. A. |last3= Zainal Abidin|first4= M. A. |last4= Kaswandi|first5= H. |last5= Zaiton|first6= I. |last6= Zali|first7= P. |last7= Kittakoop|first8= A. M. |last8=Mat Jais}}</ref> Sea cucumbers are under investigation for use in treating ailments including [[colorectal cancer]].<ref>{{cite journal|last1= Bordbar|first1=S. |last2= Anwar| first2= F. |last3= Saari| first3= N.|title=High-value components and bioactives from sea cucumbers for functional foods—a review.|journal=Marine Drugs|year=2011|volume=9|issue=10|pages=1761–805|pmid=22072996|doi=10.3390/md9101761|pmc=3210605|doi-access=free }}</ref> Surgical probes made of nanocomposite material based on the sea cucumber have been shown to reduce brain scarring.<ref>{{cite journal|title=Mechanically adaptive intracortical implants improve the proximity of neuronal cell bodies|first1= J. P. |last1= Harris| first2= J. R. |last2= Capadona| first3= R. H. |last3= Miller |first4= B. C. |last4= Healy| first5= K. |last5= Shanmuganathan|author6-link=Stuart Rowan| first6= S. J. |last6= Rowan| first7= C. |last7= Weder| first8= D. J. |last8= Tyler| journal= Journal of Neural Engineering|volume=8|issue=6|pages=066011|date=2011|bibcode=2011JNEng...8f6011H|doi=10.1088/1741-2560/8/6/066011|pmid=22049097|pmc=3386315}}</ref> One study found that a [[lectin]] from ''[[Dendrochirotida|Cucumaria echinata]]'' impaired the development of the [[malaria|malaria parasite]] when produced by transgenic [[Anopheles|mosquitoes]].<ref>{{cite journal |last= Yoshida |first=S.|title=Hemolytic C-Type Lectin CEL-III from Sea Cucumber Expressed inTransgenic Mosquitoes Impairs Malaria Parasite Development |journal=PLOS Pathogens |volume=3 |issue=12 |pages=e192 |year=2007 |pmid=18159942|doi=10.1371/journal.ppat.0030192|pmc=2151087 |name-list-style=vanc|author2=Shimada Y |author3=Kondoh D|display-authors=3 |last4=Kouzuma |first4=Yoshiaki |last5=Ghosh|first5=Anil K. |last6=Jacobs-Lorena |first6=Marcelo |last7=Sinden|first7=Robert E. |doi-access=free }}</ref>

<gallery style="text-align:center;" mode="packed">
File:Dried sea cucumber.jpg|"Teripang" in a market in Asia.
File:Sea cucumber dish 2.jpg|Sea cucumber in sauce in China.
File:Haisom Cah Jamur 1.JPG|''Haisom cah jamur'', [[Chinese Indonesian cuisine|Chinese Indonesian]] sea cucumber with mushroom.
File:Sea cucumber dish 3.jpg|[[Deep fried]] sea cucumbers.
File:Keripik teripang 1.JPG|''[[Kripik]] teripang'', Indonesian sea cucumber cracker.
</gallery>

==Procurement==
Sea cucumbers are harvested from the environment, both legally and illegally, and are increasingly farmed via [[aquaculture]]. The harvested animals are normally dried for resale.<ref name="NYT031913" /> In 2016, prices on [[Alibaba Group|Alibaba]] ranged up to {{convert|1,000|$/kg}}.<ref>{{Cite web|url=http://www.chinadaily.com.cn/world/2016-03/02/content_23706803.htm|title=Sea cucumbers slither into US market|last=代艳|website=www.chinadaily.com.cn|access-date=2018-06-22}}</ref>

===Commercial harvest===
In recent years, the sea cucumber industry in Alaska has increased due to increased demand for the skins and muscles to China.<ref name="NF">{{cite web|url=http://www.nationalfisherman.com/2008.asp?ItemID=1800&pcid=373&cid=375&archive=yes|last=Ess|first=Charlie|title=Wild product's versatility could push price beyond $2 for Alaska dive fleet|publisher=National Fisherman|access-date=2008-08-01|url-status=dead|archive-url=https://web.archive.org/web/20090122074025/http://www.nationalfisherman.com/2008.asp?ItemID=1800&pcid=373&cid=375&archive=yes|archive-date=2009-01-22}}</ref> Wild sea cucumbers are caught by divers. Wild Alaskan sea cucumbers have higher nutritional value and are larger than farmed Chinese sea cucumbers. Larger size and higher nutritional value has allowed the Alaskan fisheries to continue to compete for market share.<ref name="NF" />

One of Australia's oldest fisheries is the collection of sea cucumber, harvested by divers from throughout the [[Coral Sea]] in far [[North Queensland]], [[Torres Straits]] and [[Western Australia]]. In the late 1800s, as many as 400 divers operated from [[Cook Town, Queensland]].

[[Overfishing]] of sea cucumbers in the Great Barrier Reef is threatening their population.<ref>{{cite news |title= Overfishing threatens sea slugs: study| agency= [[Australian Associated Press|AAP]] |url=http://www.theaustralian.com.au/higher-education/overfishing-threatens-sea-slugs-study/story-e6frgcjx-1226846325407 |newspaper= [[The Australian]] |date=March 6, 2014 |access-date= November 17, 2014}}</ref> Their popularity as luxury seafood in [[East Asia]]n countries poses a serious threat.<ref>{{cite journal|title=The cost of being valuable: predictors of extinction risk in marine invertebrates exploited as luxury seafood |journal= Proceedings of the Royal Society B: Biological Sciences|first1=Steven W. |last1=Purcell |first2=Beth A. |last2=Polidoro |first3=Jean-François |last3=Hamel |first4=Ruth U. |last4=Gamboa |first5=Annie |last5=Mercier |doi=10.1098/rspb.2013.3296 |pmid=24598425 |date=5 March 2014 |volume=281 |issue=1781 |pages=20133296 |pmc=3953849 }}</ref>

===Black market===
As of 2013, a thriving [[Wildlife trade#Illegal wildlife trade|black market]] was driven by demand in China where {{convert|1|lb|g|order=flip}} at its peak might have sold for the equivalent of {{USD|300}}<ref name=NYT031913>{{cite news|title=Quest for Illegal Gain at the Sea Bottom Divides Fishing Communities |url=https://www.nytimes.com/2013/03/20/world/americas/quest-for-illegal-gain-at-the-sea-bottom-divides-fishing-communities.html|access-date=March 20, 2013|newspaper=The New York Times|date=March 19, 2013|first=Karla |last=Zabludovsky}}</ref> and a single sea cucumber for about {{USD|160}}.<ref name= crackdown>{{cite news| url= http://www.vocativ.com/money/the1/the-sea-cucumbers-pickle-in-china/| title= China's Sea Cucumber Crackdown: President Xi Jinping puts a stop to elites' dining on rarefied and mystical delicacy| website= Vocativ.com| date= June 25, 2013| access-date= September 12, 2016| archive-url= https://web.archive.org/web/20170116040720/http://www.vocativ.com/money/the1/the-sea-cucumbers-pickle-in-china/| archive-date= January 16, 2017| url-status= dead}}</ref> A crackdown by governments both in and out of China reduced both prices and consumption, particularly among government officials who had been known to eat (and were able to afford purchasing) the most expensive and rare species.<ref name= crackdown /> In the [[Caribbean Sea]] off the shores of the [[Yucatán Peninsula]] near fishing ports such as Dzilam de Bravo, illegal harvesting had devastated the population and resulted in conflict as rival gangs struggled to control the harvest.<ref name=NYT031913 />

===Aquaculture===
{{main|Aquaculture of sea cucumber}}

[[Overexploitation]] of sea cucumber stocks in many parts of the world provided motivation for the development of sea cucumber [[aquaculture]] in the early 1980s. The Chinese and Japanese were the first to develop successful hatchery technology on ''[[Apostichopus japonicus]]'', prized for its high meat content and success in commercial hatcheries.<ref name="Baskar">James, B. D. (2004).[http://www.fao.org/docrep/007/y5501e/y5501e17.htm Captive breeding of the sea cucumber, ''Holothuria scabra'', from India]. In Lovatelli, A. (comp./ed.); Conand, C.; Purcell, S.; Uthicke, S.; Hamel, J.-F.; Mercier, A. (eds.) ''Advances in sea cucumber aquaculture and management''. FAO Fisheries Technical Paper. No. 463. Rome, FAO. 2004.425p.</ref> Using techniques pioneered by the Chinese and Japanese, a second species, ''[[Holothuria scabra]]'', was cultured for the first time in India in 1988.<ref name="james">James, D. B., Gandhi, A. D., Palaniswamy, N., & Rodrigo, J. X. (1994).[http://eprints.cmfri.org.in/3452/Hatchery Techniques and Culture of the Sea-cucumber ''Holothuria scabra'']. CMFRI Special Publication(57),1-40.</ref> In recent years Australia, Indonesia, New Caledonia, Maldives, Solomon Islands and Vietnam have successfully cultured ''H. scabra'' using the same technology, and now culture other species.<ref name="Baskar" />

==Conservation==
In India, the commercial harvest and transportation of sea cucumbers has been strictly banned under Schedule I of the [[Wild Life (Protection) Act, 1972]] (WLPA) since 2001. In 2020, the Indian government created the world's first sea cucumber conservation area, the [[Dr. K.K. Mohammed Koya Sea Cucumber Conservation Reserve]], to protect the sea cucumber species.<ref name="Badri Chatterjee">{{Cite news|url=https://m.hindustantimes.com/mumbai-news/world-s-first-sea-cucumber-conservation-area-in-lakshadweep/story-cdwuvZwVkr1d1CR2cwNUZP.html|title=World's first sea cucumber conservation area in Lakshadweep|author=Badri Chatterjee|newspaper=Hindustan Times|date=29 February 2020|access-date=22 July 2020}}</ref><ref name="Shaji2020">{{cite news|last1=Shaji|first1=K.A.|title=Lakshadweep unveils world's first sea cucumber conservation reserve|date=5 May 2020|website=[[Mongabay|Mongabay-India]]|url=https://india.mongabay.com/2020/05/lakshadweep-unveils-worlds-first-sea-cucumber-conservation-reserve/|access-date=18 May 2024}}</ref>

==In popular culture==
[[File:Haeckel Thuroidea.jpg|thumb|Holothurians plate by [[Ernst Haeckel]] from his ''[[Kunstformen der Natur]]'' (1904)]]
Sea cucumbers have inspired thousands of [[haiku]] in [[Japan]], where they are called ''namako'' (海鼠), written with characters that can be translated as "sea mice" (an example of [[gikun]]). In English translations of these haiku, they are usually called "sea slugs". According to the ''[[Oxford English Dictionary]]'', the English term "sea slug" was originally applied to holothuroids during the 18th century. The term is now applied to several groups of [[sea snail]]s, [[Marine (ocean)|marine]] [[gastropod]] [[mollusk]]s that have no shell or only a very reduced shell, including the [[nudibranch]]s. Almost 1,000 Japanese holothuroid haiku translated into English appear in the book ''Rise, Ye Sea Slugs!'' by [[Robin D. Gill]].<ref>Gill, RobinD.[https://books.google.com/books?id=hFWpECE_RtwC&q=rise+ye+sea+slugs ''Rise, Ye Sea Slugs!'']. Paraverse Press, 2003, {{ISBN|0-9742618-0-7}}</ref>

==See also==
{{portal|Marine life|Oceans}}
* [[Gamat]]
* [[Trepanging]]

==References==
'''Informational notes'''
{{Reflist|group=Notes}}

'''Citations'''
{{Reflist}}

==External links==
{{Commons category|Holothuroidea}}
*{{Wikispecies-inline|Holothuroidea}}

{{commercial fish topics}}
{{Authority control}}
{{Taxonbar|from=Q127470}}

[[Category:Holothuroidea| ]]
[[Category:Extant Ordovician first appearances]]
[[Category:Edible shellfish]]
[[Category:Commercial echinoderms]]