Sea Apple Cucumber
Pseudocolochirus violaceus (Théel, 1886)

"Oh my, look! There's an apple in the water!"
Apple lookalike image.jpg
























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.

Colorful sea apple.jpgCanadian sea cucumber on sale.jpgFigure 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.

Stefanie Seskin flickr sea apple.jpg

Figure 3. Two sea apple cucumber found in an aquarium tank.


(Photo taken by Stefanie Seskin, Creative Commons)

2. Where can I find them?


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.


Map 1. Google map showing locations of sea cucumber record.(Permission obtained under permission guidelines for Google Maps and Google Earth)
They are usually found anchored to the substrate by their tube feet [22], at intertidal to subtidal 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].

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3. Basic anatomy


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.
3.1. General sea cucumber anatomy

Sea cucumbers have an elongated body with 4 main sides: ventral, dorsal surface, and posterior, anterior surface.

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 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, 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 into at the center of the mouth. Both the tentacles and introvert can be retracted into the body. The anus is at the posterior end of the animal [9].

Main exterior features .png
Figure 4. Main external anatomical features of a sea cucumber(Image from published paper by FAO, permission obtained)
3.2. Sea apple cucumber anatomy

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 of the mouth where the tentacles insert into, is the introvert of the sea apple cucumber. Both the tentacles and introverts are retractable [9].

Sea apple lateral anatomy.jpg
Figure 5. Lateral view of sea cucumber showing main exterior features(Photo taken by Ria Tan, permission obtained. Edited by Rachel Lee)
Dolmatov photo 2.JPG
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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)

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.
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4. Natural History and Biology


4.1. Feeding behaviors and diet

{An apple a day keeps the plankton away!}

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].
Ria Tan tentacles partially retracted.jpg
Dolmatov photo 3.JPG
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)

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.


Video 1. Sea apple cucumber feeding (Video uploaded by Tom Hoffman)
4.2. Reproduction
{You’re the apple of my eye.}
Cameron&Fankboner1986Fig10.gif
Figure 10. Diagram showing a male and female sea cucumber spawning.(Diagram by Tom Carefoot, Creative Commons)
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.

It has been found that the presence of food, a microalgae Chaetoceros spp. 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].

4.3. Larvae development

In general, fertilized eggs of sea cucumber develops 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].

Three stages of the larvae growth have been documented in Figure 11 below.
Sea apple larvae development.png
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].
Cam&FankFig2.gif
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.

Sea apple evisceration image.png
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].
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5. Conservation


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].
SEA Aquarium focus globe with sea apple.JPG
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.
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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.
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)
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
  • Colochirus axiologus Clark, 1914
  • Colochirus violaceus Théel, 1886
  • Cucumaria tricolor Sluiter, 1901
  • Pentacta arae Boone, 1938
  • Pseudocolochirus bicolor Cherbonnier, 1970

About the genus PseudocolochirusRecognized 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.
Key to species of Pseudocolochirus.png
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
Platform
Unrecognized synonym
Context
Global Biodiversity Information Facility
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.
Marine Species Identification Portal
and CITES 2007 Report
Pseudocolochirus axiologus
(Clark, 1914)
Appears to be misnamed from Colochirus axiologus since both scientific
names are "described" by Clark (1914)
2014 Report on status survey of sea
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
2014 Report on status survey of sea
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.
6.3. Type information


Challenger sea apple original (1).pngChallenger sea apple orginal (2).pngFigure 16. Original descriptions of P. violaceus (basiosynonym: Colocohirus violaceus)(Pages taken from original report by Théel)

Original challenger sea apple ossicles.jpg
Original ossicles label.png
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]

Phylogenetic tree order D.jpg
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: http://www.boldsystems.org/index.php/Taxbrowser_Taxonpage?taxid=222829
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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

  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: http://www.asnailsodyssey.com/LEARNABOUT/CUCUMBER/cucuSpaw.php
  4. Carefoot, T. (N.A.) Metamorphosis and symmetry. Retrieved on 2016-11-23, from A Snail's Odyssey: http://www.asnailsodyssey.com/LEARNABOUT/CUCUMBER/cucuMeta.php
  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.
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  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: Purcell, S. W., Samyn, Y., & Conand, C.. Commercially important sea cucumbers of the World. http://www.fao.org/docrep/017/i1918e/i1918e.pdf. 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: https://www.youtube.com/watch?v=RL5jBIQ867I
  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. Lacey, Katrina MJ, Grace P. McCormack, Brendan F. Keegan, and Richard Powell. "Phylogenetic relationships within the class Holothuroidea, inferred from 18S rRNA gene data." Marine Biology 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: http://www.marinespecies.org/aphia.php?p=taxdetails&id=210930
  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.
  22. 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.
  23. 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.
  24. Silchenko, A. S., … (2014) Structures of Violaceusosides C, D, E and G, Sulfated Triterpene Glycosides from the Sea Cucumber Pseudocolochirus violaceus (Cucumariidae, Dendrochirotida). Natural Product Communications Vol. 9(3).
  25. Sluiter, C. P. (1901). Die Holothurien der Siboga-Expedition. Monogr. xliv. Uitkomst. H. M. Siboga...uitgegeven Max Weber. Die Holothurien der Siboga-Expedition. Monogr. xliv. Uitkomst. H. M. Siboga...uitgegeven Max Weber.: 142 pp.
  26. Seskin, S. (2008). Photo of sea apple cucumber found on Flickr.
  27. Tan, R. (N.A.) Images of Pseudocolochirus violaceus found in Wild Singapore's Flickr album. Permission to use images obtained. https://www.flickr.com/search/?text=Pseudocolochirus%20violaceus&sort=date-posted-desc&user_id=54527470%40N00
  28. Thandar, A.S. (1991). The Cucumariid holothurians of southern Africa with the erection of a new genus. S. Afr. J. Zool. 26(3): 115 - 139.
  29. Thandar, A. S., & Samyn, Y. V. E. S. (2004). Taxonomy of the monotypic genus Koehleria Cherbonnier, 1988 (Echinodermata: Holothuroidea: Cucumariidae). Zootaxa, 652, 1-11
  30. Théel, H. (1886). Report on the Holothurioidea dredged by H.M.S. 'Challenger' during the years 1873-76. Chall. Rep. Zool. No. xxxix: 290 pp. 78-79. http://19thcenturyscience.org/HMSC/HMSC-Reports/Zool-39/htm/doc078.html
  31. Thomson, C. W., Murray, J., Nares, G. S., & Thomson, F. T. (1889). Report on the scientific results of the voyage of HMS Challenger during the years 1873-76 under the command of Captain George S. Nares... and the late Captain Frank Tourle Thomson, RN (Vol. 2). HM Stationery Office.
  32. Toral-Granda, V. (2007) Biological and Trade Status of Sea Cucumbers in the Families Holothuriidae and stichopodidae. In Fourteenth meeting of the Conferencee of Parties. CITIES.
  33. Toonen, R. J. (2003). Aquarium Invertebrates: Sea Apples. In Advanced Aquarists Online Magazine. March 2003. Retrieved on 2016-11-23, from http://www.advancedaquarist.com/2003/3/inverts
  34. Zaidnuddin, I. (2002). Sea cucumber species diversity and abundance from the south–western waters of Balik Pulau, Penang. In Workshop on the impact of development on the coastal fisheries off south–west Penang Island, Malaysia. ed. Choo, P.S., Ismail, I., Chee, P. E. and Chuah, T. T. Fisheries Research Institute, Penang, Malaysia. pp. 57–62.
  35. Davison, G.W. H., Ng, P. K. L. and Ho, H. C. (2008). The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore).
  36. GBIF Secretariat: GBIF Backbone Taxonomy. doi:10.15468/39omei. Retrieved on 2016-11-22, from http://www.gbif.org/species/5721357 .
  37. Lacey, K. M. J., McCormack, G. P., Keegan, B. F., Powell, R. (2005). Phylogenetic relationship with the class Holothuroidea, inferred from 18S rRNA gene data.

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Site updated as of 2016-11-23