Genetic and morphological data has revealed the fish in the aquarium trade which is usually referred to as 'giant danio' and labelled with the scientific name D. aequipinnatus in the majority of older aquarium literature is actually D. malabaricus (R. Collins, pers. comm.).
These two nominal species are frequently confused but their identitities are in a state of disarray, particularly that of the latter since the drawing accompanying the original description by McClelland depicts a fish with a deeply-forked caudal-fin but completely plain colour pattern and no reference to any caudal-fin patterning or stripes on the bod.
D. malabaricus possesses a moderately deep body, 35-38 scales in the lateral series, at least 15 rays in the unpaired fins, a complex axial skeleton, a vertically-extended cleithral marking and a P stripe that extendeds to the fork of the caudal-fin, whereas D. aequipinnatus probably has a less deep body, 31-34 scales in the lateral series, 13 rays in the unpaired fins, a well-defined, rounded cleithral marking and a P stripe that probably does not extend into the caudal-fin, rather terminating at its base.
The two are also separated in a geographical sense, D. malabaricus native to the Indian peninsula and D. aequipinnatus inhabiting the Brahmaputra River system in northeastern India.
D. malabaricus is one of a group of similar-looking, deep-bodied congeners with a blue-and-yellow colour pattern alongside D. anomalus, D. assamensis, D. devario, D. ostreographus and D. regina among other nominal species.
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.
- 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.
Following Fang (2003) Devario spp. are characterised by: possession of a P stripe extending onto the median caudal-fin rays; a short maxillary barbel (absent in some species); absence of the A stripe (a less distinct, relatively wide stripe is present in some species, e.g., D. acrostomus, D. annandalei, D. xyrops); a short, wide premaxillary process (cleft in the upper jaw) with a tiny apophysis (bony tubercule) touching the kinethmoid bone; infraorbital 5 not or slightly reduced.
The current genus name has only been in general use since 2003 prior to which members were considered to belong to the genus Danio.
Older, molecular, phylogenies tended to agree that the latter 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 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, although Kottelat (2013) later restricted Danio to its type species, D. dangila, and placed most former members in the revalidated genera Brachydanio and Celestichthys. Kullander (2015), the Catalog of Fishes and FishBase do not follow that split and keep those species in Danio.
These results have largely been supported by subsequent phylogenetic analyses (e.g. Mayden et al., 2007), although Devario has still undergone a little reshuffling, particularly following Fang et al. (2009) and Kottelat (2013).
In the former study the two species previously comprising the genus Inlecypris were brought into synonymy with Devario and three species formerly included in Microrasbora were moved into the new genus Microdevario based on possession of shared synapomorphies with Devario. The existence of a monophyletic clade consisting of the genera Devario, Chela, Laubuca, Microdevario and Microrasbora was also hypothesised, a theory upheld in the more recent study by Tang et al. (2010). The genus Betadevario (Pramod et al., 2010)[4] is also nested within this grouping and is sister to Devario and Microrasbora.
Kottelat (2013) revalidated the genus Inlecypris whilst noting that phylogenetic evidence suggests the existence of two genetic lineages within Devario. The first contains the species with prominent lateral stripes, which leaves the remaining members (D. apogon and D. chrysotaeniatus) of uncertain taxonomic placement. In addition, a number of the striped species share a colour pattern comprising a midlateral stripe on the posterior half of the body with a few bars in the anterior portion and golden patches between them. These may eventually be placed in a separate genus for which the name Parabarilius is available.
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