Taxonomy & naming
Danio margaritatus was described by Tyson Roberts in 2007, originally in a new monotypic genus as Celestichthys margaritatus — the "celestial" or "heavenly" fish. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Danio margaritatus (Roberts, 2007), the parenthetical authority marking the reassignment to Danio after later work showed the fish to be a true danio rather than a distinct lineage.
The story is a model of the danio–rasbora confusion. When the fish reached the trade in 2006 it was marketed as the "galaxy rasbora," and many hobbyists still know it by that name, even though it is not a Rasbora at all. Roberts' Celestichthys captured how unusual it looked, but morphological and molecular study placed it firmly within Danio, alongside the zebra danio and its kin. The trade name persists, but the valid name is Danio margaritatus.
Morphology
The celestial pearl danio is a tiny but deep-bodied and rounded danio, reaching only about 1 in — one of the smaller fishes in the hobby. The body is a deep steel-blue to blue-green, scattered all over with rounded, pearly cream to gold spots arranged in rough rows, an effect that gives the fish its names. The fins are boldly patterned: the dorsal, anal and the lobes of the tail are marked with red-orange and black bars on a paler ground.
Sexual dimorphism is pronounced and useful. Males are smaller and slimmer but far more intensely coloured, with deep blue bodies, bright pearl spots and strong red-and-black fin bars; females are a little larger and rounder, duller blue-grey with less vivid spotting and paler, more orange-tinted fins, and show a small dark mark near the vent. Colour in males intensifies dramatically during display and competition.
Habitat
Danio margaritatus has a very restricted natural range in the highlands of eastern Myanmar, in the Salween (Thanlwin) drainage around the Hopong area of the southern Shan plateau, with reports from adjacent waters. Its habitat is distinctive: shallow, clear, heavily vegetated spring-fed ponds and the still margins of small streams, densely choked with aquatic plants and fed by groundwater on a limestone plateau.
This is not soft blackwater. The plateau's spring water is cool and mineral-rich — neutral to alkaline in pH and moderately hard — and, lying at altitude, it runs cooler than typical lowland tropics, broadly in the low-to-mid twenties Celsius and seasonally cooler. The fish lives among dense vegetation in shallow water, which provides both feeding cover and the thickets of plant growth it uses to spawn. Its tiny range and dependence on these particular spring habitats are central to concern about the species.
Feeding
The celestial pearl danio is a micropredator. In its vegetated spring habitat it feeds on tiny invertebrates — zooplankton, small crustaceans, insect larvae and the like — gleaned from among plants and from the water column, together with some algae and fine organic matter. Everything about it is scaled to small prey: a tiny mouth, a small body, and a habit of picking carefully through dense plant growth.
In the aquarium it needs correspondingly small foods. It will learn to take fine flakes and micro-pellets, but it does conspicuously better on small live and frozen items — baby brine shrimp, cyclops, daphnia, microworm and the like — which bring out its colour and condition. As a shy, small fish it can be intimidated away from food by larger or faster tankmates, so it is best fed in the company of equally diminutive, gentle species.
Mating
The celestial pearl danio is an egg-scattering, non-guarding spawner, but a continuous and undramatic one rather than a seasonal mass-spawner. Males are strongly territorial in miniature, displaying intensely and sparring with one another among the plants, flaring their barred fins and deepening in colour to attract females and see off rivals.
A receptive pair spawns down in dense vegetation or fine-leaved plants, the female releasing a few eggs at a time which lodge among the leaves; there is no nest and no parental care, and the adults will eat eggs and fry. Rather than producing one large clutch, a well-conditioned group spawns small numbers of eggs more or less continuously, so a planted tank of adults will quietly yield fry over a long period if the eggs are protected from the adults.
Breeding
The celestial pearl danio breeds readily and is straightforward to propagate, which — together with deliberate captive breeding — quickly relieved the early collection pressure. The simplest approach is a well-planted tank with fine-leaved plants, moss or a spawning mop, into which a conditioned group is settled; the fish spawn among the vegetation and the eggs drop into it out of easy reach.
Because spawning is continuous and the eggs and fry are vulnerable to the adults, breeders either remove eggs or moss for separate hatching, or keep the spawning tank so densely planted that some fry survive in situ. The eggs hatch in a few days, and the tiny larvae require very small first foods — infusoria or commercial liquid fry food — before graduating to baby brine shrimp. Given good live food and clean, cool, neutral-to-hard water, the species is now bred in large numbers, and most fish offered for sale are captive-raised.
In the aquarium
The celestial pearl danio is a superb nano-aquarium fish, but a small and rather shy one that rewards the right setting. It should be kept in a group — ideally a dozen or more, with several females per male so that male sparring is spread out and no single fish is harassed — in a densely planted tank where it can feel secure and display naturally. Bold or boisterous tankmates overwhelm it; the best companions are other tiny, peaceful fishes and dwarf shrimp.
Unusually for a danio, it prefers cooler, harder, neutral-to-alkaline water reflecting its limestone-plateau springs — a planted tank in the low-to-mid twenties Celsius suits it well, and it does not want warm soft blackwater. Given fine foods, gentle company and plenty of plants, it is hardy and long-lived for so small a fish, and a mature group in full colour, the males displaying among the leaves, is one of the small marvels of the planted aquarium.
Conservation
Danio margaritatus is listed as Data Deficient (DD) on the IUCN Red List, with its population noted as decreasing — a reflection of how little is firmly known about a fish from a tiny, recently discovered range. When it first reached the market in 2006 the intensity of collection from its few known pools raised genuine fears for the wild population, and in 2007 the Myanmar authorities restricted its export.
Since then the picture has improved markedly: wild populations around Hopong are reported to have recovered following reduced collecting, and the trade is now supplied almost entirely by captive breeding, which has taken the pressure off the wild fish. The underlying vulnerability remains its naturally restricted range and dependence on particular spring-fed, vegetated habitats, which are sensitive to water extraction, agriculture and habitat change on the plateau — so its long-term security is tied to those specific waters rather than to demand from the hobby.