This species was discovered in August 2006, with the first image posted by fish exporter Kamphol Udomritthiruj on the website www.petfrd.com the following month.
After a few sceptics voiced concerns that it had been computer-generated or enhanced more-detailed pictures were supplied and these, along with the ensuing media reaction, caused D. margaritatus to quickly become one of the most hyped fishes the hobby has ever known.
Initially marketed as Microrasbora sp. 'Galaxy' or 'Galaxy Rasbora' it went on sale in Singapore a fortnight later and reached the international trade soon after.
Initial prices were high for such a small fish but quickly fell as many thousands were exported in just a few months.
However by February 2007 images of the type locality with heavily-trampled margins accompanied reports that it had been overfished to near-extinction, and the Myanmar Department of Fisheries officially banned exports.
By June 2007 the authorities had conducted a survey of wetlands close to Hopong and discovered 5 additional populations, and current evidence suggests the species is not under any immediate threat of over-collection.
Demand has also reduced since it became clear the species is easy to breed and not particularly suited to the 'general' community tank, and prices have now stabilised.
It's also been subject to intensive commercial breeding and specimens with spots or blotches instead of stripes in the pelvic and anal fins or morphological deformities are now common in the ornamental trade (see images).
There also exists limited evidence to suggest it might have been hybridised with the related D. erythromicron although these fish appear not to have reached the trade.
One interesting, apparently wild caught, form has been sold as female D. margaritatus. It has no stripes in the fins plus a dark spot at the base of the caudal peduncle as in D. erythromicron, but spotted colour pattern on the body like D. margaritatus (see images) and it remains unclear if it's conspecific with either described species or not.
The official description arrived in February 2007 and the fish was placed into new genus Celestichthys by Tyson Roberts despite morphological and behavioural similarities with Danio (formerly Microrasbora) erythromicron.
Roberts suggested the two appear to be closely-related but separated them primarily on the basis of their differing colouration while remarking that D. erythromicron may be moved into Celestichthys later. The other main reason for the separation was the possibility that the pair evolved independently from different species.
Conway et al. (2008) revalidated the species based on aspects of its osteology and a phylogenetic analysis, concluding that it's related to the genus Danio and the closest relative of D. erythromicron.
They also found that the combination of apomorphies characterising members of Danio (A stripe on the anal-fin, two or more P stripes on the caudal; see below for explanation) is present in this species, and that the spotted body patterning is not unique but a derivation of the striped pattern seen in other danios.
Other morphological similarities, such as the presence of the 'danionin notch' on the lower jaw, were also recorded, and as a result Celestichthys was placed in synonymy of Danio with the authors preferring to recognise a broader concept of the latter rather than maintain the former. This situation lasted until 2013 when Kottelat revalidated Celestichthys.
The small adult size evolved via a process known as miniaturisation, characterised by sexually mature adults with a significantly reduced size of less than 20 mm0.8 in SL.
Among bony fishes cyprinids are one of the few groups in which this phenomenon occurs repeatedly with all Barboides, Danionella, Microdevario, Microrasbora, Horadandia, Boraras, Paedocypris, Sawbwa and Sundadanio species representing miniaturised taxa along with a few members of Danio, Laubuca and Rasbora.
All show a preference for still or slow-moving waters, often in nutrient-poor habitats such as forest peat swamps.
The anatomical structure of miniaturised cyprinids can vary greatly, and there are two principle 'groupings' with some species possessing intermediate features to some degree.
The first contains those fishes which though small are essentially proportionally dwarfed versions of their larger relatives, e.g., Barboides, Microdevario, Microrasbora, Horadandia, Boraras, Sawbwa, Sundadanio, Danio, Laubuca and Rasbora.
The other includes those in which anatomical development stops at a point where adult still resemble a larval form of their larger ancestor, i.e., Danionella and Paedocypris.
The latter are usually referred to as 'developmentally truncated' or 'paedomorphic' and are thought to have evolved via a process known as 'progenetic paedomorphosis', i.e., paedomorphosis brought about by accelerated maturation.
They typically exhibit a simplified skeletal structure along with species-specific morphological peculiarities such as the tooth-like projections in male Danionella dracula.
Britz et al. (2009) consider that developmental truncation may have facilitated the development of such novelties 'by freeing large parts of the skeleton from developmental constraints, dissociating developmentally linked pathways and creating a greater potential for more dramatic changes'.
In recent years it's become commonplace to refer to the stripes on the body and fins of danionins as follows:
- P stripe: or 'pigment stripe' is the central, dark, lateral stripe on the body which extends into the caudal-fin in some species. Stripes above it are numbered P+1, P+2, etc., and those beneath P-1, P-2, P-3, etc.
- A stripe: the central stripe on the anal-fin; the proximal stripe (above it) is A+1 and the distal stripe (beneath) A-1.
- D stripe: The submarginal dorsal-fin stripe.
The putative danionin grouping has undergone some significant taxonomic reshuffling in recent years following the publication of a series of phylogenetic studies.
Older, molecular, phylogenies tended to agree that it represented a monophyletic group consisting of two major clades; the 'Danio devario' group containing the larger, deeper-bodied species and the 'D. rerio' clade comprising the smaller, slimmer fish.
However in 2003 Fang conducted a more detailed study based on morphological characters which included members of other related genera, and the results suggested for the first time that the genus Danio as previously considered represents a polyphyletic grouping, i.e., not all members derived from a single common ancestor.
The genus name Devario was suggested for the larger species with Danio being applied only to the smaller fish (with the exception of the type species, D. dangila which can grow to around 89 mm3.5 in SL). Recent molecular studies by Mayden et al. (2007) and Fang et al. (2009) resulted in further changes, with the latter study considering the genus Danio to be composed of three subclades. These were subsequently split into distinct genera by Kottelat (2013), as follows:
The former species D. erythromicron, D. margaritatus, D. choprae and D. flagrans were grouped together in the revalidated genus Celestichthys Roberts, 2007. These exhibit unique body patterning consisting of vertical bars (D. erythromicron, D. choprae, D. flagrans) or light spots (D. margaritatus) and possess either very short barbels or none at all.
Kottelat (2013) left only the type species, D. dangila, in Danio, separated on the basis of its larger size and the shape of the caudal-fin, which in adults is only slightly emarginate or even truncate in shape, a feature it shares only with Tinca tinca (the common tench) among other cyprinids.
The remaining species, of which D. rerio is thought to be the most ancient, were placed in the revalidated genus Brachydanio Weber & de Beaufort, 1916. Kullander (2015) did not follow this split, because phylogenetic analyses strongly support a single Danio in the sense of Fang (2003), with Devario a separate genus. The Catalog of Fishes and FishBase both keep all of these species in Danio.
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