Scientists have suggested changing the scientific name of the Celestial Pearl danio to Danio margaritatus, after further study suggested it did not warrant a genus of its own, writes Matt Clarke.
The Celestial Pearl danio, was named Celestichthys margaritatus by taxonomist Tyson Roberts in February 2007, just seven months after it was first discovered in a remote wetland area of Myanmar.
Roberts claimed that the species, which was initially sold in the trade under the name Microrasbora sp. "Galaxy", was so different to other members of the Danio genus that it warranted a genus of its own, so Roberts erected the Celestichthys genus for the new fish.
However, research published today by Kevin Conway, Wei-Jen Chen and Professor Richard Mayden of Saint Louis University provides evidence to suggest that Roberts' original description contained some inaccuracies.
The scientists believe that the tiny fish would actually be better placed in the Danio genus and not in Celestichthys:
"Though accepting a broader concept of Celestichthys (including D. erythromicron) as the sister group to Danio could be considered a feasible option, we feel that this approach would only unnecessarily inflate the number of generic names within Cyprinidae.
"We therefore place Celestichthys in the synonymy of Danio," the authors said.
Bones and DNAConway, Chen and Mayden undertook a detailed analysis of the bones of C. margaritatus - something absent from Roberts' original description of the fish - and compared its bone structures to those of a number of related fishes.
They also performed an additional molecular study, based on the RAG1 gene, and studied its presence in 31 rasborine fishes, including five species of Danio, in order to determine the phylogenetic position of Celestichthys alongside its relatives.
The combined results provide strong evidence to suggest that C. margaritatus is a close relative of Danio erythromicron.
This similarity is something that aquarists stated when the species was first introduced, and something Roberts also mentioned in his description.
The results also placed Celestichthys and Danio erythromicron as a sister group to Danio, where they all appear to be of monophyletic origin, suggesting that they evolved from a single common ancestor.
Placement in DanioAs part of the study, the authors also redescribed the colour pattern of the fish using the Danio colour pattern terminology created by the taxonomist Fang, which now makes it simpler to compare its markings with those other so-called danionin fishes.
Fang's 2003 study of the Danio genus, which restricted the use of the name to only those Danio species previously in the Danio dangila species group, defined two characters that fish had to possess in order to be considered members of the genus.
The two features, known as apomorphic character states, were the presence of an "A stripe" on the anal fin rays (a dark stripe which extends along the middle of the rays to the tip of the last branched anal fin rays), and the presence of two or more pigmented stripes on the tail fin.
The authors state that no other genus of fish in south or south east Asia, apart from Danio, has these features and only one fish considered to be a Danio, D. erythromicron, lacks both traits.
They say that C. margaritatus possesses both an "A stripe" and has two pigment stripes on the caudal fin, characters which suggest it is actually a Danio, and not a member of another genus, as suggested by Roberts.
For more information see the paper: Conway KW, Chen W-J and RL Mayden (2008) - The "Celestial pearldanio" is a miniature Danio (s.s) (Ostariophysi: Cyprinidae): evidence from morphology and molecules. Zootaxa.