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Asian Arowana (Curtesy of Frank Zijlmans)

Scleropages formosus (Muller and Schlegel, 1844)

Asian Arowana (Dragonfish)

formosus = esthetic (referring to the majestic appearance of Asian arowana)


Content

  1. Taxonomic Classification
  2. General Introduction
  3. Biology
  4. Conservation Status and Issue Faced by Singapore
  5. Species Description
  6. Type Species Information Retrieval
  7. Evolution and Taxonomy
  8. References
  9. Comment

Taxonomic Classification

Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Osteoglossiformes
Family: Osteoglossidae
Genus: Scleropages
Species: S. formosus

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General Introduction

Asian arowana was first recorded in 1844 (Schlegel & Müller, 1844) and they are often the best coloured arowana in the family. They were originally thought to be confined to Borneo, Bangka and Sumatra. Over the years, it had been documented in different localities in Malaya. However, Asian arowana is not a native species to Singapore but instead, has been introduced into the freshwater ecosystem by fish owners who released the fish into several reservoirs in Singapore (National Park, 2009).
The Asian arowana comprises four different natural occurring colour varieties with each found in specific geographical region. They include the following
1) The “Green” variety: They are most common and are widespread throughout the Southeast Asia
2) The “Super Red”: They are only known to be found at the upper part of the Kapuas River (Danau Sentarum Reserve, West Borneo, Indonesia)
3) The “Cross Back Golden” or “Blue Malayan”: They are known from the Pahang State and Bukit Merah of Perak State, Peninsula Malaysia
4) The “Red Tail Golden”: They are originated from the area of Pekanbaru and Jambi (North Sumatra, Indonesia)
Asian arowana has special cultural significance in locations influenced by Chinese culture. With a presumed resemblance to the “Chinese Ancient Dragon”, many Chinese believe that these fish symbolize lucks and fortune (Pouyaud et at, 2003). As a result, coupled with the colourful appearance, they have acquired a popular status in many Asian countries since 1970 as an expensive aquarium fish, which consequently led to overexploitation on the wild population. It has been estimated that the population size has declined drastically of over 50% and it is currently still decreasing (The IUCN Red List of Threatened Species, 2011). It is now listed as “Endangered” on the IUCN.

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Biology

Habitat and Feeding

The Asian arowana are mainly found in slow-moving streams flowing through forested swamps and wetlands. It has been suggested that a stable water parameter contribute to the bright colouration of these fish (Freshwaterstingray, n.d.). They survive optimally in slightly acidic water with a pH of 5-6 with a water temperature of 26-30°C (Pouyaud et al, 2003). Asian arowanas also require a low conductivity, maxing at around 200us. These knowledge with regards to habitat requirements has allowed fish farms to breed this popular aquarium fish.
The fish generally spend the daylights in the midst of water plants, emerging to feed after dark. They are predatory and they feed on smaller fishes, insects and even birds. Their food also consists of some small terrestrial insects and anarhnids. The following video depicts the aggressiveness of an Asian arowana when they feed.





Reproductive Biology

Asian arowana generally take up to four years to reach reproductive maturity. During courtship, the male and female arowanas will swim side by side in circle. They will rub each other as they swim. The courtship ritual will end in water splashing. During courtship period, both sexes do not eat. If the courtship is successful, the duty of the female arowana is to lay eggs. After which, she would declare her duty done and swim away to look for food, leaving the male arowana which would then fertilize the eggs.
The spawning period of Asian arowana is around August to October each year (Scott and Fuller, 1976). After spawning, it is the male, not the female parent which broods the young. It had been misunderstood due to the fish not having apparent external sexual difference and the gender could only be precisely determined by examination of gonads (Scott and Fuller, 1976). The male carries the brood in its mouth for over six weeks before allowing them to swim free (Kesava, 2008), during which, the male arowana completely stop eating. The young fish found in the mouth of the male parent generally would still have the yolk sac intact with their body.
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Baby Asian Arowana (yolk intact) (Suleiman, 2003)
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Brood inside the mouth of male parent




The mature male fish have only one testis with a maximum size being 0.1% of body weight. The low gonadosomatic ratio indicates that little sperm was produced. The fully ripe males have seminiferous tubules which are packed with sperms. After spawning, the seminiferous tubules would become occluded and at the same time, the testis would become highly vascularized (Scott and Fuller, 1976).
The female fish have one ovary. The size ranges from negligible in immature fish to 11.5% of total body weight in mature fish. The ripe ovary would contain 20-30 ova. Ripe ova are characterized by the centrifugal replacement of the cytoplasm by yolk which will progressively mature until October (Scott and Fuller, 1976).

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Conservation Status and Issue faced by Singapore

Scleropages formosus have been the targets for aquarium fish since 1970 which led to overexploitation. Consequently, they have been listed as one of the most endangered animal species since 1975 by CITES, thereby restricting the trades of these fish. They have also been listed as “Endangered” by IUCN Red List with the most recent evaluation in 1996. Now trade is regulated, but enforcement is not optimal and there is still pressure on some of the wild populations. This species is assessed as Endangered based on a decline in area of occupancy, extent of occurrence and habitat quality, and levels of exploitation . Despite being endangered, there have been no systematic survey works or quantitative assessments on wild population (CITES, 2003).
In the context of Singapore, Asian arowana is not a native species but an introduced species. Being an island, Singapore is particularly prone to invasion because the native species have faced few strong competitors and predators (Stachowicz and Tilman, 2005). This raises controversy as to whether to protect Asian arowana in the local ecosystem, which might be invasive in nature and cause harm to the native aquatic biodiversity. Asian arowanas have well-developed parental care, from mouth brooding to nest guarding (Greenwood and Wilson, 1998),which is a characteristic in allowing the fish to establish themselves in the local aquatic ecosystem without much harm done to the young. In addition, no natural predators of Asian arowana have been documented so far and it is believed that Asian arowanas are the top predators due to their size. These characteristics of Asian arowanas would probably make them harmful to other native species in terms of out-competition and predation. As a result, there is an urgent need for a systematic assessment on the introduced population of Asian arowana as well as the degree of invasiveness in Singapore aquatic ecosystems.
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Species Description

Though appearing to be of different colours, these four varieties are indeed one species (Kottelat and Widjanarti, 2005) because they can crossbreed with one another (Goh and Chua, 1999), producing viable hybrids with colouration traits from both parents. The most common hybrids are the Banjar Red which is crossbred between the “Super Red” and the “Green” (Goh and Chua, 1999). However, pure-breed normally fetches a higher prize in the market, resulting in many breeding farms selectively avoiding crossbreeding.
The Green Asian arowana are dorsally dark olivaceous green, silvery or golden green on the sides, and silvery or whitish on its ventral surface with longitudinal rows of dark green or dark bluish patches visible through the lateral scales. Mature fish have bright emerald colour at the postocular part of the head and the upper part of the eye. (Pouyaud et al, 2003).
In mature Super Red Asian arowana, the dorsal region is dark brown in colour. The operculum, lateral scales and fin membranes of this variety are metallic red, varying from goldish red to deep red. The Super Red variety develops their body colouration with age. The juveniles have whitish ventral surface and less intense red colouration. (Pouyaud et el, 2003).
Mature Red-Tail Golden are coloured in bright metallic gold at the operculum, the scales on the lateral flanks, the pectoral and pelvic fin membranes. The dorsal region, however, is dark in colour including the dorsal fin. The anal fin is usually reddish or brown in colour. In juveniles, the areas destined to develop golden colour start out with metallic silver (Pouyaud et al, 2003).
Mature Cross Back Golden looks similar to Red Tail Golden. They differ from the Red Tail Golden by having metallic gold crossing on the dorsal part of the body and non-reddish fins. The mature fish have relatively shorter pectoral fins compared to the other varieties and a bigger head.

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The Green variety (Curtesy of Frank Zijlmans)

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The Cross Back Golden (Curtesy of Frank Zijlmans)
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The Super Red (Curtesy of Frank Zijlmans)

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The Red Tail Golden (Curtesy of Frank Zijlmans)

Diagnosis

Asian arowana can grow up till 90cm in length (Froese, 2006). They have strongly compressed and elongated body with keeled abdomen. The gape of mouth is fairly large, oblique with a prominent lower jaw with two fleshy barbels at the tips (Mohsin and Ambak, 1983) which have been used to distinguish arowana family apart from other fish. Asian arowana have an elongated pair of pectoral fins; dorsal and anal fins located far back on the body as opposed to most fish. They have a much larger caudal fin than that of their South America relative, the silver arowana, Osteoglossum bicirrhosum, which allows for differentiation between these two similar looking fish.

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Silver Arowana (caudal fin circled)

Asian arowana have large cycloid scales with a delicate mosaic pattern (Ismail, 1989). The lateral scales are arranged in five horizontal rows from the most ventral (first level) to the most dorsal (fifth level). In addition, they have one row of dorsal scales designated as the sixth level (West and NSK, 2003). According to Kottelat et al. (1993) and Pouyaud et al. (2003), Asian arowana can be easily distinguished from the Australia relative, Scleropages jardinii and Scleropages leichardti by a lower number of lateral line scales (21-24 vs 32-35).

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Scales of Asian Arowana
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General Body Parts of Asian Arowana (Curtesy of Frank Zijlmans)
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Controversy to Species Identification

Based on the popularly adopted Biological Species Concept, all the different colour varieties of Asian arowana are indeed one species due to their ability to interbreed with one another. Despites this, several recent researches had attempted to differentiate the different colour varieties of Asian arowana into distinct species. One such research done by Pouyaud et at. (2003) adopted the Phylogenetic Species Concept and proposed three new species of Asian arowana due to many non-conspecific character states in morphology as well as Mitochrondrial DNA Cytochrome B among the different colour varieties. The original Scleropages formosus had been re-described as Green arowana and the three new proposed species are Scleropages macrocephalus (silver Asian arowana), Scleropages aureus (Red Tail Golden) and Scleropages legendrei (Super Red).
However, such claim by Pouyaud and his colleagues have received contestation. Kottelat and Widjanarti (2005) reviewed the published data and did not reach the same conclusions from the quality of the data presented. Moreover, Pouyaud and his colleagues used only one gene for DNA analysis and phylogenetic tree building, which had been criticized to be inaccurate. In order for more than one species to be recognized, a professional standard study would need to be presented (The IUCN Red List of Threatened Species, 2011). However, so far, there are relatively few scientific papers being published about the species in peer-reviewed journals (Yue et al, 2006). There is still more need to be done in order to resolve the confusion in species identification.
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Type Species Information Retrieval

Note: MZB refers to Museum Zoologium Bogoriense, Cibinong, Indonesia

The taxonomic status of Asian arowana has been debated over the recent years (refer to Controversy to Species Identification). Therefore, a study by Pouyaud et al (2003) had attempted to cross-check the original described specimen (holotype) with other specimens caught in different localities.

Original Type Species

Type species: Osteoglossum formosum (Muller and Schlegel, 1844), which was changed to Scleropages formosus in 1913
Type material: Holotype (MZB 11898)
Type locality: Barito River (Borneo, Indonesia)

Type Species as specified by Pouyaud et al. (2003)

Type species: Scleropages formosus (re-described)
Type material: Neotype (MZB 11887)
Type locality: Barito River (Borneo, Indonesia)

Type species: Scleropages macrocephalus (the holotype of original Osteoglossum formosum caught by Muller and Schlegel)
Type material: Holotype (MZB 11898)
Type locality: Barito River (Borneo, Indonesia)

Type species: Scleropages aureus
Type material: Holotype (MZB 11906)
Type locality: Siak River (Sumatra, Indonesia)

Type species: Scleropages legendrei
Type materia: Holotype (MZB 11912)
Type locality: Sentarum Lake (West Borneo, Indonesia)

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Evolution and Taxonomy

Asian arowanas belong to the family Osteoglossidae, roughly translated as “bony tongues” which refer to the presence of teeth on the parasphenoid and tongue bones (Fenner, n.d.). When it was first described, it was given the name Osteoglossum formosum due to its resemblance to the South America relative, the silver arowana Osteoglossum bicirrhosum. After various morphological and genetic studies, it was found that Asian arowanas are more closely related to the Australian relatives, the Scleropages jardinii and Scleropages leichardti, hence the genus was shifted.
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Phylogenetic Relationship among Osteoglossidaes (modified from Pouyaud et al, 2003)


In a recent study, Yue et al. (2006) had managed to sequence the complete mitochrondrial genome of Asian arowana which was found to be 16651 base pairs in length. On the whole, the mitochrondrial genome structure is very similar to that of the South America relative, silver arowana both in terms of total CG contents (46% vs 43.1%) and protein-coding genes.

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Phylogenetic Relationship between Silver Arowana and Asian Arowana (Modified from Yue et al, 2006)

It is noted that the complete mitochondrial genome is not available for both species of Australian arowana. Even though there is evidence to show that Asian arowana is more closely related to Australian arowana, the conclusion could be made more accurately by comparing the complete mitochondrial genome among the arowana species. Hence, there is a need for a further research in terms of genetic comparisons.
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References

Fenner B.,( n.d.). The bony-tongue fishes called arowanas, arapaima and featherback knifefishes in aquarium. Retrieved from http://www.wetwebmedia.com/fwsubwebindex/osteoglossiforms.htm. Last accessed on 14th November 2011

Freshwaterstingray, (n.d.). Arowana. Retrieved from http://www.freshwaterstingray.nl/Arowana/Varieties.html. Last accessed on 14th November 2011.

Froese R. and Pauly D. (2006). “Scleropages formosus” in FishBase. 2006 version

Goh W. and Chua J., (1999). The Asian arowana. Dragon Fish Industry, Singapore

Greenwood P.H. and Wilson M.V., (1998). Encyclopedia of fishes. Paxton J.R. and Eschmeyer W.N. ed. San Diego: Academic Press. pp. 81-82. ISBN 0-12-547665-5

Ismail M. Z. (1989). Systematics, Zoogeography, and Conservation of the Freshwater Fishes of Peninsular Malaysia. Doctoral Dissertation. Colorado State University

Kesava S., (2008). Molecular tools help uncover the breeding habits of expensive fish. The Straits Times Friday 3rd October 2008.

Kottelat M., Whitten A.J., Kartikasari S.N. and Wirjoartmodjo S., (1993). Freshwater fishes of Western Indonesia and Sulawesi. 259 p. Hong Kong: Periplus Edition

Kottelat M. and Widjanarti E., (2005). The fishes of Danau Sentarum National Park and the Kapuas Lake area, Kalimantan Barat, Indonesia. Raffles Bulletin of Zoology Supplement No. 13: 139-173

Kottelat M., (2011). Scleropages formosus. In:IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2

Kumazawa Y. and Mutsumi N., (2000). Molecular Phylogeny of Osteoglossoids: A new model for Gondwanian Origin and Plate Tectonic Transportation of the Asian arowana. Molecular Biology and Evolution 17(12): 1869-78

Kumazawa Y., (2003). The reason the freshwater fish arowana live across the sea. Quaterly Journal Biohistory (Winter). Retrieved from http://www.brh.co.jp/en/experience/journal/39/research_1.html. Last accessed on 14 November 2011.

Mohsin A.K.M. and Ambak M.A, (1983). Freshwater fishes of Peninsular Malaysia. Penerbit University Pertanian Malaysia, Malaysia. 284 p.

National Park Singapore, (2009). Freshwater fish species. Retrieved from http://www.nparks.gov.sg/cms/index.php?option=com_content&view=article&id=135&Itemid=128. Last accessed on 14th November 2011

Pouyaud L., Sudarto and Teugels G.G., (2003). The different colour varieties of the Asian arowana Scleropages formosus (Osteoglossidae) are distinct species: morphologic and genetic evidences. Cybium 27 (4): 287-305

Scott D.C.B and Fuller J.D., (1976). The reproductive biology of Scleropages formosus (Muller and Schlegel) (Osteoglossomorpha, Osteoglossidae) in Malaya and the morphology of its pituitary gland. Journal of Fish Biology 8(1):45-53

Stachowicz J.J. and Tilman D., (2005). What species invasions tell us about the relationship between community saturation, species diversity and ecosystem functioning. Species invasions: insights into ecology, evolution and biogeography. Sinauer, Sunderland MA.

Suleiman M.Z., (2003). Breeding Malaysian golden arowana, Scleropages formosus in concrete tanks. Retrieved from
http://aquarticles.com/articles/breeding/Suleiman_Breeding_Asian_Arowana.html. Last accessed on 14th November 2011.

The Convention on International Trade of Endangered Species of Wild Flora and Fauna (CITES), (2003). Proposed revision of Resolution Conf.9.24 (CoP12 Com.I.3); Criteria for listing on Appendix I and Appendix II; Test of the applicability of the criteria. Retrieved from http://www.cites.org/common/prog/criteria/fauna/S_formosus_US.pdf. Last accessed on 14 November 2011.

The IUCN Red List of Threatened Species, (2011). Scleropages formosus. Retrieved from http://www.iucnredlist.org/apps/redlist/details/20034/0. Last accessed on 14 November 2011.

West and NSK, (2003). General Parts of an Asian Arowana. Retrieved from http://www.arowanaclub.com/assets/AroParts/ArowanaParts.htm. Last accessed on 14 November 2011

Yue G.H., Liew W.C. and Orban L., (2006). The complete mitochondrial genome of a basal teleost, the Asian arowan (Scleropages formosus, Osteoglossidae). BMC Genomics 7:242, doi:10.1186/1471-2164-7-242


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