Pseudocolochirus+violaceus+(Théel,+1886)

//Pseudocolochirus violaceus// (Théel, 1886)
 * Sea Apple Cucumber **

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 * "Oh my, look! There's an apple in the water!" **

Figure 1. //Pseudocolochirus violaceus// seen on Beting Bronok, Singapore. (Photo taken by Ria Tan, permission obtained)

1. Apple or cucumber?
Neither an apple, nor a cucumber we find in our supermarket, the sea apple cucumber is in fact an animal that can be found in the tropical waters. The sea apple cucumber belongs to a group of animals called the sea cucumbers and is related to the sea stars and sea urchins which we might be more familiar with. The common name "sea apple" is also used for a plant, //Syzygium grande//. Hence to avoid confusion, //P. violaceus// is referred to as sea apple cucumber in this page.

Sea cucumbers, Holothurians, are sedentary marine invertebrates commonly known as //trepan, bêche-de-mer,// //khiar daravaei// or //gamat// 1]. You might have encounter dried sea cucumbers in the market, or cooked sea cucumbers on your dining table, but not all sea cucumbers can be eaten and they aren’t always drab-looking (Fig. 2). Sea apple cucumber, for example, is one of the most colorful sea cucumbers! Also, the sea apple cucumber is NOT an apple-flavored cucumber by any sense and they CANNOT BE EATEN.

Figure 2. A sea apple cucumber found at Beting Bronok, Singapore (left)in comparison with a dried sea cucumber found in Hong Kong market (right). (Photo on left taken by Ria Tan, permission obtained;Photo on right taken by Poly Ho, permission obtained) The sea apple cucumber is more well-known in the aquarium trade (Fig. 3) but it is also known infamously for its deadly chemical defense that can kill off other inhabitants in an enclosed tank [see section 4.5 for more information on its chemical defenses]. While it can be tempting to keep such a beautiful gem in your home, remember that..."most good things are wild and free!" Plus, it is more thrilling and rewarding to find them in the wild! Use the distribution information in section 2 to find your own wild sea apple cucumber. Read on to find out more about this colorful and fascinating animal!

The table of contents on the side can help you navigate through the page. Some other sea cucumbers that are documented in this species page platform are //Stichopus hermanni, // //Holothuria leucospilota,// and //Holothuria (Metriatyla) scabra //<span style="font-family: Arial,Helvetica,sans-serif;">.



<span style="font-family: Arial,Helvetica,sans-serif; line-height: 0px; overflow: hidden;">Figure 3. Two sea apple cucumber found in an aquarium tank. <span style="color: #616161; font-family: Arial,Helvetica,sans-serif; line-height: 0px; overflow: hidden;">(Photo taken by Stefanie Seskin, Creative Commons)

<span style="color: #e80606; font-family: Arial,Helvetica,sans-serif; font-size: 120%;">2. Where can I find them?
<span style="font-family: Arial,Helvetica,sans-serif;">The sea apple cucumber is widely distributed in the Tropical Indo-Pacific Ocean 20]. The map below (Map 1) marks out the various places where this animal has been recorded at. Locally in Singapore, the sea apple cucumber has also been sighted at Changi, Beting Bronok, Chek Jawa and Pulau Sekudu (Map 1). These local records are represented by the red pins in the map. Blue pins represent non-local distributions and the star-pin represent the location where the **holotype** was collected. 

media type="custom" key="28862342" Map 1. Google map showing locations of sea cucumber record. (Permission obtained under permission guidelines for [|Google Maps and Google Earth]) <span style="font-family: Arial,Helvetica,sans-serif;">They are usually found anchored to the substrate by their tube feet 22], at intertidal to subtidal<span style="font-family: Arial,Helvetica,sans-serif;"> areas (depth of 0-13m) 20, 23]. In the past, dredging efforts have also recorded sea apple cucumber at depths of 20m on very fine gray mud, and 50m on stony corals 6].

<span style="font-family: Arial,Helvetica,sans-serif;">Back to top]

<span style="color: #e80606; font-family: Arial,Helvetica,sans-serif; font-size: 120%;">3. Basic anatomy
<span style="font-family: Arial,Helvetica,sans-serif;">If you are new to sea cucumbers and want to first understand the major anatomical features of a sea cucumber, continue reading section 3.1. If you want to jump straight into sea apple cucumber, head to section 3.2.

**<span style="color: #e86100; font-family: Arial,Helvetica,sans-serif;">3.1. General sea cucumber anatomy **
<span style="font-family: Arial,Helvetica,sans-serif;">Sea cucumbers have an elongated body with 4 main sides: ventral, dorsal surface, and posterior, anterior surface.

<span style="font-family: Arial,Helvetica,sans-serif;">The ventral surface tends to be the side of the sea cucumber that is in contact with the substrate. On the ventral surface, are the locomotory (ventral podia) which **podia** help<span style="font-family: Arial,Helvetica,sans-serif;"> the sea cucumber to move about. The dorsal surface is the side facing up in the water and there are also rows of podia known as the dorsal podia,<span style="font-family: Arial,Helvetica,sans-serif;"> or papillae. The mouth of the sea cucumber is found on the anterior end and it has evident **tentacles** that the animal extends to acquire food. Some sea cucumbers also possess an where the tentacles **introvert** insert<span style="font-family: Arial,Helvetica,sans-serif;"> into at the center<span style="font-family: Arial,Helvetica,sans-serif;"> of the mouth. Both the tentacles and introvert<span style="font-family: Arial,Helvetica,sans-serif;"> can be retracted into the body. The anus is at the posterior end of the animal 9].

Figure 4. Main external anatomical features of a sea cucumber (Image from published paper by FAO, permission obtained)

**<span style="color: #e86100; font-family: Arial,Helvetica,sans-serif;">3.2. Sea apple cucumber anatomy **<span style="font-family: Arial,Helvetica,sans-serif;">
<span style="font-family: Arial,Helvetica,sans-serif;">Sea apple cucumber can generally grow to 10-20cm 22]. As described by Théel, who first gave a name to this animal, they have a body shaped like a cucumber with the anterior and posterior equally rounded 30]. The bulk of the body is colored in red, blue or purple 29]. The body is also decorated with rows of yellow, orange or red bands of podia. They have ten tentacles that are extended into the water during feeding 30]. The tentacles are dendritic, meaning they branch out in a tree-like manner. At the center<span style="font-family: Arial,Helvetica,sans-serif;"> of the mouth where the tentacles insert<span style="font-family: Arial,Helvetica,sans-serif;"> into, is the introvert of the sea apple cucumber. Both the tentacles and introverts are retractable 9].

Figure 5. Lateral view of sea cucumber showing main exterior features (Photo taken by Ria Tan, permission obtained. Edited by Rachel Lee) For the taxonomists interested in comparing //P. violaceus// with other similar-looking //Pseudocolochirus//, note that various synonyms are used for //P. violaceus// but only two species are officially recognized under genus //Pseudocolochirus: P. violaceus and P. unica.// Section 6.4 contains a key to differentiating between these two species and more on the complexity regarding the scientific name of //P. violaceus// is also explained in the section. <span style="font-family: Arial,Helvetica,sans-serif;">[ Back to top<span style="font-family: Arial,Helvetica,sans-serif;">]
 * [[image:Dolmatov photo 2.JPG width="460" height="358"]] || [[image:Screenshot 2016-11-10 14.40.04.png width="472" height="354"]] ||
 * Figure 6. Close up shot showing dendritic tentacles of //P. violaceus//. (Photo taken by Dolmatov, permission obtained)  || Figure 7. Anterior view of sea apple cucumber.  (Still image from video uploaded by Tom Hoffman. Edited by Rachel Lee)  ||

**<span style="color: #e86100; font-family: Arial,Helvetica,sans-serif;">4.1. Feeding behaviors and diet **
<span style="color: #0054f7; font-family: Arial,Helvetica,sans-serif;">{An apple a day keeps the plankton away!}

<span style="font-family: Arial,Helvetica,sans-serif;">Sea apple cucumber feed by extending their tentacles into the water to gather plankton and detritus (Fig. 9) 12, 22]. Being sensitive to the presence of food in the water column, sea apple cucumber can conserve energy by keeping their tentacles tucked in when the amount of food is low (Fig. 8) 33]. The animal will also retract the tentacles into the body at slight disturbance 12].
 * [[image:Ria Tan tentacles partially retracted.jpg width="468" height="344"]] || [[image:Dolmatov photo 3.JPG width="447" height="347"]] ||
 * Figure 8. Introvert and tentacles of sea apple cucumber partially retracted (Photo taken by Ria Tan, permission obtained)  || Figure 9. Introvert and tentacles of sea apple cucumber fully extended during feeding (Photo taken by Dolmatov, permission obtained)  ||

<span style="font-family: Arial,Helvetica,sans-serif;">They are very patient feeders and only one tentacle will be inserted into their mouth at one point of time. Each time they put a tentacle in, they are not only sucking clean the food that’s on their tentacles, they are also coating it with a layer of mucus. The mucus is being secreted from within the mouth and is used to trap food particles on the tentacles 33]. Turn the speed up, and watch up close in the video below how a sea apple cucumber feed.

media type="youtube" key="RL5jBIQ867I" width="560" height="315" align="center" Video 1. Sea apple cucumber feeding (Video uploaded by Tom Hoffman)

**<span style="color: #e86100; font-family: Arial,Helvetica,sans-serif;">4.2. Reproduction **
<span style="color: #0054f7; font-family: Arial,Helvetica,sans-serif;">{You’re the apple of my eye.} Figure 10. Diagram showing a male and female sea cucumber spawning. (Diagram by Tom Carefoot, Creative Commons) <span style="font-family: Arial,Helvetica,sans-serif;">Sea apple cucumbers are **dioecious** and undergo sexual reproduction 8, 17]. Like most of the other sea cucumbers, they are broadcast spawners that release large quantities of sperms and eggs into the water column. Even though no published studies have looked at the natural reproduction of sea apple cucumber, there has been an artificial breeding study carried out in Sri Lanka.

<span style="font-family: Arial,Helvetica,sans-serif;">It has been found that the presence of food, a microalgae //Chaetoceros// spp<span style="font-family: Arial,Helvetica,sans-serif;">. can effectively induce the spawning of P. violaceus 17]. Males were observed to spawn first and females responded about 40-50 minutes after the first male. The presence of sperm in the water can be the stimulation for a reaction in the female 13]. Males released sperms for about 35 minutes while females spawned for only a short 20-30 seconds 17].

**<span style="color: #e86100; font-family: Arial,Helvetica,sans-serif;">4.3. Larvae development **
<span style="font-family: Arial,Helvetica,sans-serif;">In general, fertilized <span style="font-family: Arial,Helvetica,sans-serif;">eggs of sea cucumber develops<span style="font-family: Arial,Helvetica,sans-serif;"> into free-floating larvae found as before settling down on the sea bottom 32]. For the sea apple cucumber, Kumara //et al//. briefly documented the growth of the larvae 17].

<span style="font-family: Arial,Helvetica,sans-serif;">Three stages of the larvae growth have been documented in Figure 11 below. Figure 11. Larval stages of //P. violaceus//: (1) gastrula, (2) pentactula and (3) juveniles. (Image from published paper by Kumara et al.  Use of figure for scientific, educational, and research purposes allowed under newsletter regulations. See [|Page 72 of SPC Beche-de-mer Information Bulletin]) Following the typical stages cell division after fertilization, the **blastula** stage occurs within a few hours and the **gastrula** stage was reached after one day 17]. After 13-14 days, the non-feeding **doliolaria** stage occurred. After which, a gradual metamorphosis takes place to give rise to a young animal that still retains most of the larval features 14]. This is the **pentaculata** stage. This is also the final part of its planktonic existence. Finally, the sea apple cucumber juvenile will settle to the bottom and take on its adult, sedentary lifestyle 14]. Figure 12. Diagram showing the typical larval stages of sea cucumber, time adapted to that of P. violaceus  (Diagram by Tom Carefoot, Creative Commons. Edited by Rachel Lee.)

** 4.4. Autotomy and regenerations **
Sea apple cucumbers, like the other members of the sea cucumber group, are able to literally puke their guts out when faced with adverse conditions and stresses. More specifically, sea cucumbers can expel **cuvierian tubules** 33]. These tubules are effective in deterring and can entangle smaller predators such as crabs 10]. The behavior of losing a body part as a self-defense mechanism is known as autotomy. While many cases of such evisceration occur through the anterior end of the body, the sea apple cucumber has been found to eject its organs out through the anus 8]. A laboratory experiment was conducted to study the abilities of sea apple cucumber to regenerate its lost organs after evisceration 8]. Such regeneration capability is well-known for other kinds of sea cucumber. The study found out that long exposure to unfavorable conditions, such as that during transportation, was able to make the sea apple cucumber eject their intestine, gonadal tubes and respiratory tree (Fig. 13). However, the study did not manage to examine the regeneration ability of sea apple cucumber as those who have undergone evisceration have also lost their lives. The study did however show that //P. violaceus// was able to regenerate its anterior end of the body after a dissection procedure. Signs of regeneration was only observed after 23 days of ablation. Figure 13. Evisceration in //P. violaceus//. Scale: 4cm. (Image from published paper by Dolmatov, permission obtained. Edited by Rachel Lee)

** 4.5. Chemical defense **
One of the more popular researches done on //P. violaceus// is the analysis of a chemical compound, known as the glycosides. Gycosides of //P. violaceus// have been found to have cytotoxic and hemolytic effects 24]. These chemicals are speculated to be used against predators 1]. Aquarists have also described sea apple cucumbers as “Death in a Colorful Package” as its potent chemical defenses can be highly toxic to fish and other coral reef inhabitants in an enclosed habitat 33]. However, toxic chemical defenses are usually used as a last resort, sometimes released in conjunction with the expelling of its Cuvieren tubules 33]. Back to top]

** 5.1. Threats **
With the potent pack of chemicals that the sea apple cucumber has, it manages the escape from the food trade industry. However, there are other anthropogenic threats that can cause a decline in their wild population. The first and most obvious one is that brightly colored sea apple cucumber is popular in the aquarium trade. It has been showcased in aquariums since the early 1970s and its presence in aquariums became common near the end of 1980s 32]. Figure 14. Photo of an aquarium habitat at SEA Aquarium, Singapore, displaying sea apple cucumber alongside other marine creatures (Photo by Rachel Lee) The other major threat that the sea apple cucumber face is habitat degradation and loss 32]. At this age, habitat degradation and loss is a common problem faced by many other wild animals. Habitats such as the coral reefs and coral beds are being degraded due to climate change. The intertidal to subtidal shores where sea apple cucumbers are found in are also damaged due to unsustainable fishing practices, coastal pollution and sedimentation 32].

** 5.2. Conservation status and challenges **
No comprehensive assessment of the sea apple cucumber has been done globally and it has not yet been assessed for the IUCN Red List. It has however attained certain levels of protection at more local scales. In Sri Lanka, //P. violaceus// is listed as a protected species under the Fauna and Flora Protection Ordinance 17]. In Singapore, the sea cucumber was listed as ‘Vulnerable’ on the Red List of Threatened Animals of Singapore 35] and was more recently considered at ‘Endangered’ 22]. The lack of assessment is perhaps one of the main challenges this species will face in its conservation efforts. A further conservation challenge can arise due to the confusion of the animal’s scientific names, as explained further in section 6.3. In a report by the Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES), sea apple cucumber is recognized as both //P. violaceus// and //P. axiologus// when the later scientific name is in actual fact a synonym. CITES seems to distinguish the two via locality, with the former from Indonesia and the later from Australia 32]. Such confusion can lead to difficulties in assessing and devising management plans to conserve the animal as it can lead to serious underestimation or overestimation of populations. It will also present a challenge if trade in the species is to be restricted. Back to top]

6. Taxonomy & systematics
This section is catered more for taxonomic enthusiasts and for those with a budding interest in taxonomy. However, like most cases in taxonomy and systematics, it is rarely straightforward and this can not be more true for the sea apple cucumber.

** 6.1. Classification **
Here is where it still remains relatively simple and straightforward. ** 6.2. Taxonomy ** This is where things get messy. Let's start with the most congruent information found from the World Register of Marine Species (WORMS) 18], Global Biodiversity Information Facility (GBIF) 36] and a scientific paper published by Thanndar & Samyn 29] regarding the genus //Pseudocolochirus//. __Recognized synonyms of //P. violaceus//__ __About the genus //Pseudocolochirus//__ Recognized by WORMS and Thanndar & Samyn, under the genus //Pseudocolochirus// there are only two extant species: //P. violaceus// and //P. unica//. A key to the identification of these two species can be found below (Fig. 15) 29]. To differentiate the two species, one has to consider both the external features and the structure of the internal skeleton (ossicles). //P. unica// appears to be much less well-known and was described by based on the morphology of only two specimens. Figure 15. Key to the species of //Pseudocolochirus// provided in Thandar & Samyn (2004) In contrast, GBIF listed three species under the genus //Pseudocolochirus////: P. arae, P. unica, P. violacues//. However, //P. arae// appers to refer to the synonym //Pentacta arae// instead of being a standalone species. __Unrecognized synonyms__ and [|Atlas of Living Australia] || //Pseudocolochirus arae// Boone (1938) || Appears to be misnamed from //Pentacta arae// since both scientific names are "described" by Boone (1938). It is stated as another species under genus genus //Pseudocolochirus//. || and CITES 2007 Report || //Pseudocolochirus axiologus// (Clark, 1914) || Appears to be misnamed from //Colochirus axiologus// since both scientific names are "described" by Clark (1914) || cucumbers in Inida (Ragathunathan & Venkataraman, 2014) || //Pseudocolochirus echinatus// (von Marenzellar, 1881) || It is stated as a standalone species recorded alongside //P. violaceus// and //P. tricolor//. It was found along the eastern coast of Indian waters || cucumbers in Inida || //Pseudocolochirus tricolor// (Sluiter, 1901) || It is stated as a standalone species recorded alongside //P. violaceus// and //P. echinatus//. It was found along the eastern coast of Indian waters || Some of the synonyms of //P. violaceus// has been used in relation with its locality, which can be inaccurate as well. Dr. Tooren confuses //C. axiologus// as the Indonesian Sea Apple even though //Colochirus axiologus// was first described in Australia. In contrast, //P. violaceus// is the one being described by Dr. Tooren as an Australian Sea Apple 33]. As shown via the distribution map in section 2, //P. violaceus// can actually be found through a wide range including both Northern Australia and Indonesia.
 * Kingdom || Animalia ||
 * Phylum || Echinodermata ||
 * Subphylum || Echinozoa ||
 * Class || Holothuroidea ||
 * Order || Dendrochirotida Grube, 1840 ||
 * Family || Cucumariidae Ludwig, 1894 ||
 * Genus || //Pseudocolochirus// Pearson, 1910 ||
 * Species || //Pseudocolochirus violaceus// (Théel, 1886) ||
 * //Colochirus axiologus// Clark, 1914
 * //Colochirus violaceus// Théel, 1886
 * //Cucumaria tricolor// Sluiter, 1901
 * //Pentacta arae// Boone, 1938
 * //Pseudocolochirus bicolor// Cherbonnier, 1970
 * Platform || Unrecognized synonym || Context ||
 * [|Global Biodiversity Information Facility]
 * [|Marine Species Identification Portal]
 * 2014 Report on status survey of sea
 * 2014 Report on status survey of sea

** 6.3. Type information **

 * Original name: //Colochirus violaceus// Théel, 1886
 * Holotype: Holotype Natural History Museum, London (England) 1886.10.2.79
 * Type locality: Philippines (11°06'N 123°09'E) ; depth, 20 fathoms; mud; Collected on October 31, 1874
 * Original description in: [|Report on the Holothuriodea dredged by H. M. S. . 'Challenger' during the years 1873-76. Chall. Rep. Zool. No. xxxix: 290 pp. 78-79]
 * Original drawing of plate of ossicles 31]

Figure 16. Original descriptions of //P. violaceus// (basiosynonym: //Colocohirus violaceus//) (Pages taken from original report by Théel) Figure 17. (1) Original drawings of ossicles of //P. violaceus// and (2) original label for the ossicles (basiosynonym: //Colocohirus violaceus//) (Image adapted from original report by Thomson et al., 1889) ** 6.4. Phylogeny ** The phylogeny of //P. violaceus// remains unexamined. Other than DNA barcoding, there are also two other taxonomic characters in Holothuroids that help sort out its phylogenetic relationship with other Holothuroids. First is through the comparisons of ossicles. The structure of ossicles has been used as the main taxonomic character in the Holutroids and is well-described for most species 29]. A disadvantage is that it was found to change with increasing age so the age of specimen needs to be considered when using this characteristic. The second method is through chemical fingerprinting. It has been suggested that since Holothuroidea have a vast variety of triterpene glycosides, these compounds can be used as a chemotaxonomic marker to position the species within a phylogenetic tree 1]. Currently, the lowest taxonomic rank containing //P. violaceus// with published phylogenetic tree is at the family level: Cucumariidae (Order: Dendrochirotida) (Fig. 18). Order Dendrochirotida appears to be a monophyletic group but relationship within the family Cucumariidae remains mostly unresolved. 37] Figure 18. Phylogenetic tree based on maximum likelihood analysis of 18s rDNA sequences for class Holothuroidea. Numbers above branches represent the bootstrap proportions based on maximum likelihood (percentage of 100 replicates) and maximum parsimony (percentage of 1,000 replicates). (Image from published paper by Lacey et al., permission pending)

** 6.5. DNA barcode ** There are only 2 sequences in BOLD, with >500bp. It belongs to non-public access. See: [] Back to top]

7. Glossary
Here is a list of technical terms bolded in the main text that has not already been explained. To return to the main text and continue reading from where you left off, simply click the bolded words here in the glossary.


 * Blastula:** an early stage of embryonic development in animals; a hollow sphere of cells surrounding the blastocoele
 * Cuvierian tubules:** clusters of fine threads found near the base of the respiratory tree of some sea cucumbers. Threads become sticky when ejected and are used as a defense mechanism.
 * Dioecious:** having separate sexes in a reproduction system
 * Doliolaria:** A non-feeding larval stage; precedes the pentactula stage
 * Gastrula:** an embryo stage following the blastula stage; usually contains two layers of cell, the ectoderm and entoderm.
 * Holotype:** a specimen (mostly physical) in which a scientific name is attached to and formally described.
 * Introvert:** the anterior area of a sea cucumber consisting the tentacles and the mouth
 * Pentactula:** the later stage of larvae development; In this stage, bilateral symmetry is observed, with five tentacles around mouth and hydrocoel already separated from Coelom
 * Podia:** also known as tube feet, refers to tiny tubes filled with water on the body of echinoderms.
 * Tentacles:** modified buccal podia extended from the mouth for feeding purposes.

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8. Other species page
>  9. References
 * //P. violaceus// on [|World Register of Marine Species]
 * //P. violaceus// on [|Global Biodiversity Information Facility]
 * //P. violaceus// on [|WildSingapore]


 * 1) Bahrami, Y. & Franco, C. M. M. (2016) Acetylated Triterpene Gylcosides and their Biological Activity from Jolothuroidea Reported in the Past Six Decades. Marine Drugs 14, 147.
 * 2) Boone, L. (1938). The marine Algae, Coelenterata, Annelida Polychaeta, Echinodermata, Crustacea and Mollusca of the world cruises of the yachts Ara 1928-1929, and Alva, 1931 Alva Mediterranean cruise, and Alva South American cruise, 1933, William K. Vanderbilt, commanding. Part IV-Echinodermata. Bull. Vanderbilt Mar. Mus. Huntingdon 7: pp. 115-193.
 * 3) Carefoot, T. (N.A.) Spawning and fertilisation. Retrieved on 2016-11-23, from A Snail's Odyssey: []
 * 4) Carefoot, T. (N.A.) Metamorphosis and symmetry. Retrieved on 2016-11-23, from A Snail's Odyssey: []
 * 5) Cherbonnier, G., 1970. //Pseudocolochirus bicolor// n. sp., nouvelle holothurie dendrochirote de Madagascar. Bulletin Muséum National Histoire Naturelle Paris. 2 série. 42(2): 424-427.
 * 6) Cherbonnier, G. (1988) Echinoderms: Holothurides. Faune de Madagascar 70: 1-292.
 * 7) Clark, H. L. (1914). The echinoderms of the Western Australian Museum. Records of the Western Australian Museum. 1(3): 132-173.
 * 8) Dolmatov, I. Y., Khang, N. A., & Kamenev, Y. O. (2012). Asexual reproduction, evisceration, and regeneration in holothurians (Holothuroidea) from Nha Trang Bay of the South China Sea. Russian Journal of Marine Biology, 38(3), 243-252.
 * 9) Food and Agricultural Organization of the United Nations (2012) Authors: <span style="background-color: #ffffff; color: #222222; font-family: Arial,sans-serif;">Purcell, S. W., Samyn, Y., & Conand, C.. Commercially important sea cucumbers of the World. []. Reproduced with permission.
 * 10) Hamel, J. F., & Mercier, A. (2000). Cuvierian tubules in tropical holothurians: usefulness and efficiency as a defence mechanism. Marine & Freshwater Behaviour & Phy, 33(2), 115-139.
 * 11) Hoffman, T. (2011). Sea Apple Feeding. Video retrieved on 2016-11-23, from YouTube: []
 * 12) James, D. B. (1984). Studies on Indian echinoderms-15. On Psolus mannarens1s sp. Nov. and other Dendrochirotids from the Indian Seas. Journal of the Marine Biological Association of India, 26(1&2), 109-122.
 * 13) James D.B., Rajapandian M.E., Gopinathan C.P. and Baskar B.K. 1994. Breakthrough in induced breeding and rearing of the larvae and juveniles of //Holothuria (Metriatyla)// scabra //Jaeger// at Tuticorin. CMFRI Bulletin 46:66–70.
 * 14) Khanna, D. R. (2005). Biology of Echinodermata. Discovery Publishing House.
 * 15) <span style="background-color: #ffffff; color: #222222; font-family: Arial,sans-serif;">Lacey, Katrina MJ, Grace P. McCormack, Brendan F. Keegan, and Richard Powell. "Phylogenetic relationships within the class Holothuroidea, inferred from 18S rRNA gene data." //<span style="color: #222222; font-family: Arial,sans-serif;">Marine Biology //<span style="background-color: #ffffff; color: #222222; font-family: Arial,sans-serif;"> 147, no. 5 (2005): 1149-1154.
 * 16) Resources Research and Development Agency (NARA). Scientifc Sessions 2012.
 * 17) Kumara, P. A., Jayanatha, J., Pushpakumara, J., Bandara, W., & Dissanayake, D. C. T. (2013). Artificial breeding and larval rearing of three tropical sea cucumber species–//Holothuria// scabra, //Pseudocolochirus violaceus// and //Colochirus quadrangularis//–in Sri Lanka. SPC Beche-de-Mer Inf. Bull, 33, 30-37.
 * 18) Paulay, G. (2015). //Pseudocolochirus violaceus// (Théel, 1886). Retrieved on 2016-11-23, on World Register of Marine Species: []
 * 19) Raghunathan, C. and Venkataraman, K., 2014. Status Survey of Holothurians (Sea Cucumber) in the Territorial waters of Andaman and Nicobar Islands : 1–96, (Published by the Director, Zool. Surv. India, Kolkata).
 * 20) Rowe, F. W. E & Gates, J. (1995). Echinodermata. In ‘Zoological Catalogue of Australia’. 33 (Ed A. Wells.) pp xiii + 510.
 * 21) Lane, D. (2008). Echinodermata. In p. 129-143, Davison, G.W.H.; Ng, P.K.L.; Ho, H.C., 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Singapore: The Nature Society, 285pp.
 * Ng, P. K. L., Corlett R. T. & Tan H. T. W. (eds.) (2011) Singapore Biodiversity: An Encyclopedia of the Natural Environment and Sustainable Development. Editions Didier Millet, Singapore, 552 pp.
 * 1) Ong, J. Y., & Wong, H. P. S. (2015). Sea cucumbers (Echinodermata: Holothuroidea) from the Johor Straits, Singapore. The Raffles Bulletin of Zoology, Supplement, 31, 273-291.
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