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 "Microrasbora" erythromicron more than any other known species.
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.
Systematics and taxonomy
Initially Danio margaritatus was thought to be a member of the genus Microrasbora and was traded as such. This was assumed because of its similarities with "M." erythromicron, which almost certainly does not belong into that genus and appears closer to Danio[4]. Less than a year after the discovery of the Celestial Pearl Danio, it was scientifically described and given the genus name Celestichthys. This once again raises the issue of where "Microrasbora" erythromicron should be placed; it was noted that including it in Celestichthys might seem plausible at first sight, but probably would not correctly represent these fishes' evolutionary relationships. In 2008 further a study including DNA testing placed the species in the genus Danio as the study discovered that it was more closely related and more similar to other "Danios" than Tyson Roberts study had shown. Tyson Roberts didn't DNA test this species in his studies.
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).
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. The latter species presumably is the only known significant predator of D. margaritatus.
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) before the fish was discovered for the aquarium trade
Reproduction
The spawning behavior has significant consequences for captive breeding. 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 and fertilize the eggs with their own sperm or eat them.
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.
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". 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. 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.
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, 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) from a single day's work; 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 "Microrasbora" erythromicron more than other fish.
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.
