Danio margaritatus

Celestial Pearl Danio
Adult male
Conservation status
Not evaluated (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Superclass:	Osteichthyes
Class:	Actinopterygii
Subclass:	Neopterygii
Infraclass:	Teleostei
Superorder:	Ostariophysi
Order:	Cypriniformes
Family:	Cyprinidae
Subfamily:	Danioninae
Genus:	Danio
Species:	D. margaritatus
Binomial name
Danio margaritatus (Roberts, 2007)
Synonyms
Celestichthys margaritatus Roberts, 2007

The Celestial Pearl Danio, Danio margaritatus - often referred to in the aquarium trade as galaxy rasbora or "Microrasbora sp. 'Galaxy'"^[1] - is a small cyprinid from Myanmar. It has so far been found only in a very small area near Hopong east of Inle Lake, at an altitude of over 1,000 m (3,400 ft). Its habitat is part of the Salween basin, namely the Nam Lang and Nam Pawn rivers. Discovered in 2006, the species quickly appeared in the aquarium trade, where its small size and bright colours made it an instant hit.^[2]

Description

This is a small plump danionin with a markedly blunt snout, measuring just 1.5–2 cm standard length. The body is about three times as long as it is high. In general shape, it resembles Danio erythromicron more than any other known species.

Female

This species shows some sexual dimorphism: males have a bright blue background color (dull blue-green in females), and their fins are more brightly colored. The tail end of their bodies (the caudal peduncle) is also higher than in females. The body is sprinkled with small, pearly dots. The back is bronzy green, and the belly in females is yellowish-white. The gill covers are transparent, letting the blood-red gills shine through.

The males' fins (except the pectoral fins), which they will prominently display to conspecifics, show two parallel black lines with a bright red area in between; on the tailfin this pattern is present twice (once on each lobe) and the outer black band is vestigial. Females have a weaker version of the pattern in the tail and dorsal fins only, sometimes in the anal fin too.

Courting males develop a red belly and the flanks brighten and darken, making the pearly spots stand out even more, with the back appearing paler than the flanks and also standing out. Females in reproductive age can be recognized by a black anal spot which separates the belly color from the uniformly reddish base of the anal fin. Males have a small black pad at the edges of the lower jaw, which is absent or reduced in females. Immature fish show some indication of a striped pattern, which eventually decomposes into the pearly dots.^[3]

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  Male

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  Male

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  Female

Systematics and taxonomy

Initially the Celestial Pearl Danio was assumed to be a member of the genus Microrasbora, due to its similarity to "Microrasbora" erythromicron. Less than a year after the discovery of the Celestial Pearl Danio, it was scientifically described and given the genus name Celestichthys.^[3] In 2008 a more comprehensive study^[4] showed that the Celestial Pearl Danio was a member of the genus Danio, with Danio erythromicron and Danio choprae as its closest relatives.^[5]

Ecology

The fish lives in small ponds created by seeping groundwater or overflow from small brooks or springs. Water temperature in January was rather low (22-24°C), but as the habitat is very shallow, it would heat up quickly during hot spells and thus D. margaritatus is probably tolerant of temperatures in the low to mid 20s. Like most waterbodies in the Inle drainage, the water is slightly alkaline. The habitat is heavily vegetated with Hydrocharitaceae similar to Elodea (water weed).^[3][6]

The Celestial Pearl Danio shares its habitat with very few fish species. Namely, a Microrasbora similar to M. rubescens, a rosy loach (Yunnanilus, possibly a new species) and the dwarf snakehead Channa harcourtbutleri.^[7] The latter species presumably is the only known significant predator of D. margaritatus.^[3][8]

The species is locally fished for food to some extent; it is dried and bought as a protein source by poor people. A can of some 500 D. margaritatus sold for food fetched about 25 kyat (about 2 UK pounds/3.9 US$/2.7 EUR^[9]) before the fish was discovered for the aquarium trade.^[6]

Reproduction

The spawning behavior has significant consequences for captive breeding (see below). It appears as if the Celestial Pearl Danio is adapted to somewhat ephemeral habitat. It does not have a dedicated spawning season, nor do the females lay continuously. Rather, they produce small batches of around 30 eggs per spawning episode. The time between spawnings is unknown at present. Eggs are not strewn freely into the water, but they are not deposited in clutches to a prepared surface either; rather, it seems, that they are hidden away in vegetation as a loose batch. Courting males will seek out and try to defend a patch of dense vegetation. While pursuit swimming has been observed, it does not seem to be connected directly to the actual act of reproduction in which the male displays to a passing female, and tests her readiness with a brief chase. The pair then moves into the substrate and deposits the eggs. Other males noticing reproduction will try and follow the mating pair, either to try to fertilize the eggs with their own sperm or eat them.^[3]

At 24-25°C, the larvae hatch after 3–4 days. They are dark and cryptic initially and for about three days after hatching, they hide away between substrate and detritus and are very hard to see. They subsequently become lighter in color and start swimming freely and feeding on their own. At some 8–10 weeks after hatching, they undergo metamorphosis to adult form, and the color pattern starts to appear from week 12 onwards.^[3]

Status and conservation

Within six months of its appearance in the aquarium trade, the species had become so rare that collectors were obtaining only a "few dozen fish per day".^[6] Whilst some aquarists have managed to breed the fish successfully, almost all the fishes currently offered for sale are wild-caught. British fishkeeping magazine Practical Fishkeeping is currently asking that only aquarists prepared to breed the fish should buy any fishes they see for sale, to reduce pressure on the wild stocks by diminishing the demand for them in the UK.^[6] As the species seems adapted to living in and colonizing small, possibly ephemeral pools, it seems not very well able to withstand prolonged and intense exploitation - if the stock in all pools at one location is entirely fished off, it is unclear in how far the fish would be able to recolonize them. On the other hand, if only part of a local subpopulation is removed, it seems likely that pools from which all Celestial Pearl Danios have been removed will be recolonized with a healthy population again after one year or so.^[3]

The government of Myanmar banned exports of the fish in February 2007; as the fish were still available in numbers in the aquarium trade since then,^[1] it is unclear to what degree the ban was enforced. Since then, new populations of this fish have been found around Hopong. In the long run, this fish seems a valuable source of income for locals; by early 2007, a single fisherman could make up to 750 kyat (about 60 UK pounds/116 US$/80 EUR^[9]) from a single day's work;^[10] in 2005 the average gross domestic product per capita (PPP) in Myanmar was just around 1,700 US$.

In the aquarium

The Celestial Pearl Danio is a rather undemanding fish if its basic requirements are being taken care of. It seems quite hardy but obviously thrives best in fairly soft and slightly alkaline water at not too high a temperature - conditions that can often be met with pretreated tap water. It does not require much space, as it is not a very active swimmer, and is not a true shoaling fish, meaning it does not require large numbers of conspecifics for its well-being. In a small tank, a group of 6 individuals - half males, half females - will do well and exhibit natural behavior. They tend to be rather stationary, hovering in a peculiar position in favorite spots; males and females tend to keep separate when at rest. Altogether, their behavior again resembles Danio erythromicron more than other fish.^[3]

Tanks for the Celestial Pearl Danio should be well-planted and direct daylight may be favorable (the natural habitat is so shallow as to be well lit throughout). Water weed and similar plants should be abundant, and stones and wood to create hiding spots should be provided. It is advisable to supply the fish with a spawning mop or a dense growth of suitable plants (java moss has been successfully used). A dense tangle of natural plants for spawning has the additional advantage of harboring protists on which the fry feed initially. The Celestial Pearl Danio seems overall quite peaceful, though some fin-nipping occurs. Consequently, it cannot be accompanied with large or "bully" fish. Small, swarming danionins which require similar water conditions would be a natural choice for company, as such more active species provide nice contrast behaviorally and, being available in a wide range of colors and patterns, also make it possible to choose fish that complement the brilliant colors of D. margaritatus. Note than many danionins prefer slightly acidic water however, and that maintaining the rather high pH found across the Inle basin seems a necessary condition to keep fish from there successfully.

See also

• Danionin

Footnotes

1. ^ ^{a b} Clarke (2007b)
2. ^ Clarke (2006)
3. ^ ^{a b c d e f g h} Roberts, Tyson R. (February 2007). "The "celectial pearl danio", a new genus and species of colourful minute Cyprinid fish from Myanmar (Pisces: Cypriniformes)". The Raffles Bulletin of Zoology 55 (1): 131–140. http://rmbr.nus.edu.sg/rbz/biblio/55/55rbz131-140.pdf. 
4. ^ Conway, Kevin W.; Chen, Wei-Jen; Mayden, Richard L. (2008). "The "Celestial Pearl danio" is a miniature Danio (s.s) (Ostariophysi: Cyprin- idae): evidence from morphology and molecules". Zootaxa 1686: 1–28. http://www.mapress.com/zootaxa/2008/2/zt01686p028.pdf. 
5. ^ Fang, F., Norén, M., Liao, T. Y., Källersjö, M. & Kullander, S. O. 2009. Molecular phylogenetic interrelationships of the south Asian cyprinid genera Danio, Devario, and Microrasbora (Teleostei, Cyprinidae, Danioninae). Zoologica Scripta 38:237-256.
6. ^ ^{a b c d} Clarke (2007a)
7. ^ "harbourtbutleri" in Roberts (2007) is a lapsus.
8. ^ Clarke (2007a) mentions Danio sondhii but not Microrasbora and Channa; it is not clear on what data this information is based.
9. ^ ^{a b} Official exchange rate of January 21, 2008, as per XE.com Universal Currency Converter.
10. ^ Clarke (2007c)

References

• Clarke, Matt (2006): The next big thing: Microrasbora sp. Galaxy. Pract. Fishkeeping Version of 2006-SEP-09. Retrieved 2007-FEB-20.
• Clarke, Matt (2007a): Galaxy rasbora under threat. Pract. Fishkeeping Version of 2007-FEB-05. Retrieved 2007-FEB-20.
• Clarke, Matt (2007b): Galaxy rasbora placed in new genus. Pract. Fishkeeping Version of 2007-FEB-28. Retrieved 2007-FEB-28.
• Clarke, Matt (2007c): New populations of Celestichthys discovered. Pract. Fishkeeping Version of 2007-JUN-05. Retrieved 2007-JUN-05.
• Mayden, Richard L.; Tang, Kevin L.; Conway, Kevin W.; Freyhof, Jörg; Chamberlain, Sarah; Haskins, Miranda; Schneider, Leah; Sudkamp, Mitchell; Wood Robert M.; Agnew, Mary; Bufalino, Angelo; Sulaiman, Zohrah; Miya, Masaki; Saitoh, Kenji & He, Shunping (2007): Phylogenetic relationships of Danio within the order Cypriniformes: a framework for comparative and evolutionary studies of a model species. Journal of Experimental Zoology B: Molecular and Developmental Evolution 308B(5): 1-13. doi:10.1002/jez.b.21175 PMID 17554749 (HTML abstract)
• Roberts, Tyson R. (2007): The "Celestial pearl danio", a new genus and species of colourful minute cyprinid fish from Myanmar (Pisces: Cypriniformes). Raffles Bulletin of Zoology 55(1): 131-140. PDF fulltext

External links

• Danio margaritatus
• Danio margaritatus on the Aquarium Wiki Encyclopaedia