Sea cucumber
(
Extant Ordovician First Appearances
)
- Overview
- Anatomy
- Body plan
- Digestive system
- Nervous system
- Respiratory system
- Circulatory systems
- Locomotive organs
- Endoskeleton
- Life history and behavio..
- Habitat
- Locomotion
- Diet
- Communication and sociab..
- Reproduction
- Development
- Symbiosis and commensali..
- Predators and defensive ..
- Estivation
- Phylogeny and classifica..
- Scientific history and n..
- Relation to humans
- Procurement
- Conservation
- In popular culture
- See also
- References
- External links
Sea cucumbers are from the class Holothuroidea ( ). They are benthic found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number being in the Asia–Pacific region. Sea cucumbers serve a useful role in the marine ecosystem as who help recycle nutrients, breaking down detritus and other organic matter, after which microbes can continue the decomposition process.
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 , sea cucumbers have a calcified dermal endoskeleton, which is usually reduced to isolated microscopic 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.
Many species of sea cucumbers are foraged as food by , 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, namako, , or balate.
Overview
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 . 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."Catch" collagen has two states, soft and stiff, that are under neurological control. 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 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.
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).
Anatomy
Body plan
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, 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.
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 mesenteries which surround and support the internal organs.
Digestive system
Nervous system
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 , while some species possess small eye-spots near the bases of their tentacles.
Respiratory system
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 depositing particulate waste.
Circulatory systems
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 coelomic fluid.
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 cells 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.
Vanadium has been reported in high concentrations in holothuroid blood, however researchers have been unable to reproduce these results.
Locomotive organs
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.
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 , 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.
Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices. These can even evolve into long antennae-like structures, especially on the abyssal genus Scotoplanes.
Endoskeleton
Life history and behaviour
Habitat
Holothuroids appear to be the echinoderms best adapted to extreme depths, and are still very diversified beyond deep: several species from the family Elpidiidae ("sea pigs") can be found deeper than , and the record seems to be some species of the genus Myriotrochus (in particular Myriotrochus bruuni), identified down to deep. 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 . For this reason, one such area in Fiordland is called the strawberry fields.
Locomotion
Diet
In the South Pacific, sea cucumbers may be found in densities of . These populations can process of sediment per year.
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 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.).
Communication and sociability
Reproduction
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.Branch GM, Griffiths CL, Branch ML and Beckley LE(2005) Two Oceans 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.
Development
Symbiosis and commensalism
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.. A variety of fish, most commonly pearl fish, have evolved a commensalism 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) and (like Lissocarcinus orbicularis) have also specialized to use the mouth or the cloacal respiratory trees for protection by living inside the sea cucumber. Nevertheless, holothuroids species of the genus Actinopyga have anal teeth that prevent visitors from penetrating their anus.
Sea cucumbers can also shelter bivalves as endocommensals, such as Entovalva sp.
Predators and defensive systems
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) 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 process known as evisceration. Replacement tubules grow back in one and a half to five weeks, depending on the species. 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.
Estivation
Phylogeny and classification
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. 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.
All echinoderms also possess anatomical feature(s) called mutable collagenous tissues, or MCTs.
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.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 Ludwig, 1891
- order Dendrochirotida Grube, 1840
- family Cucumariidae Ludwig, 1894
- family Cucumellidae Thandar & Arumugam, 2011
- family Heterothyonidae Pawson, 1970
- family †Monilipsolidae Smith & Gallemí, 1991
- family Paracucumidae Pawson & Fell, 1965
- family Phyllophoridae Östergren, 1907
- family Placothuriidae Pawson & Fell, 1965
- family Psolidae Burmeister, 1837
- family Rhopalodinidae Théel, 1886
- family Sclerodactylidae Panning, 1949
- family Vaneyellidae Pawson & Fell, 1965
- family Ypsilothuriidae Heding, 1942
- order Elasipodida Théel, 1882
- family Elpidiidae Théel, 1882
- family Laetmogonidae Ekman, 1926
- family †Palaeolaetmogonidae Reich, 2012
- family Pelagothuriidae Ludwig, 1893
- family Psychropotidae Théel, 1882
- order Holothuriida Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017
- family Holothuriidae Burmeister, 1837
- family Mesothuriidae Smirnov, 2012
- order Molpadida Haeckel, 1896
- family Caudinidae Heding, 1931
- family Eupyrgidae Semper, 1867
- family Gephyrothuriidae Koehler & Vaney, 1905
- family Molpadiidae Müller, 1850
- order Persiculida Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017
- family Gephyrothuriidae Koehler & Vaney, 1905
- family Molpadiodemidae Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017
- family Pseudostichopodidae Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017
- order Synallactida Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017
- family Deimatidae Théel, 1882
- family Stichopodidae Haeckel, 1896
- family Synallactidae Ludwig, 1894
- subclass †ArthrochirotaceaSmirnov, 2012
- order †Arthrochirotida Brandt, 1835
- family †Palaeocucumariidae Frizzell & Exline, 1966
- subclass Paractinopoda Ludwig, 1891
- order Apodida Brandt, 1835
- family Chiridotidae Östergren, 1898
- family Myriotrochidae Théel, 1877
- family Synaptidae Burmeister, 1837
Scientific history and naming
The earliest known mention of the Greek term ὁλοθούριον ( holotoúrion) .. is found in the poet Epicharmus around 450 BC. 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 or even jellyfish.Plato, Philebus (21b); Aristotle, History of Animals (Book V, 15); Theophrastus, On Weather Signs (Frag. 40).
One of the oldest scientific texts concerning sea cucumbers dates back to Aristotle, in his Parts of Animals (around 343 BC): he names an animal "holothurion" without describing it, but classifies it among the animals lacking sensation (along with Porifera 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.
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".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.
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.
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. 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, and he wrongly separates the " vit de mer"Guillaume Rondelet, De piscibus marinis, libri XVIII, in quibus veræ piscium effigies expressæ sunt, Lyon, apud Matthiam Bonhomme, 1554, p.86). He noted that these beings "are of a middle nature between plants and animals".
Real progress came during the Age of Enlightenment: in 1751, an article titled "Holothurie" was written for the Encyclopédie, based on the commentaries of Aristotle and Rondelet, but their taxonomic position (and even their description) remained unclear:
In 1758, sea cucumbers appeared in the Systema Naturae of Carl von Linné, but the term still did not refer specifically to echinoderms,. and included diverse creatures such as the physalia.Gustav Paulay, Holothuria on the World Register of Marine Species. It was only in 1767 that Linnaeus revised the Holothuria entry.
Nathanael Gottfried Leske created the phylum in 1778 (systematized by Jean-Guillaume Bruguière in 1791), 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.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. 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. 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.
Relation to humans
Food
There are many commercially important species of sea cucumber that are harvested and dried for export for use in Chinese cuisine as hoisam.
- Acaudina molpadioides
- Actinopyga echinites
- Actinopyga mauritiana
- Actinopyga palauensis
- Apostichopus californicus
- Apostichopus japonicus
- Holothuria nobilis
- Holothuria scabra
- Holothuria fuscogilva
- Isostichopus fuscus
- Thelenota ananas
Medicine
Various pharmaceutical companies emphasize gamat, the Malay language traditional medicinal usage of this animal. 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".
Another study suggested that sea cucumbers contain all the fatty acids necessary to play a potentially active role in tissue repair. Sea cucumbers are under investigation for use in treating ailments including colorectal cancer. Surgical probes made of nanocomposite material based on the sea cucumber have been shown to reduce brain scarring. One study found that a lectin from Dendrochirotida impaired the development of the malaria when produced by transgenic Anopheles.
Procurement
Commercial harvest
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. Their popularity as luxury seafood in countries poses a serious threat.
Black market
Aquaculture
Conservation
In popular culture
See also
Informational notes
Citations
External links
|
|
